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About Aircraft Speeds
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The Aircraft You Fly Is Showing Its Age; Pitot-Static System; Static Air Ports; Airspeed Indicator; ...Airspeed; True Airspeed; ...Pitch vs. Power; ...Pitch for Speed; ...Airspeeds; ...V- Speeds; Uncommon V Speeds; ...V Speeds and Flaps; ...Standard Performance Profiles; ...Pitch; ...Climbs; ...Power Curve; ...Pattern Speeds; ...Variations on a Theme; Pattern Airspeed; ...Preliminaries to Airspeed; ...Cessna 150; ...Cessna 172; ...Speed Changes from Normal Cruise; ...C-150 Airspeed Exercise; ...C-172 Airspeed Exercise; ...Glide Speeds; Approach Speeds; ...Touchdown Speeds; Airspeed and Slips; Va; The Performance Envelope; Structural Speed Limits; Best Glide by Weight; Speeds and Density Altitude; ...Certification Speeds Breguet Range Equation; Excess Speed; Velocity Is another Word for Speed; Indicated vs Calibrated; Best Glide Rule of Thumb; Finding Best Glide Vref; Finding Minimum Descent Rate Vref; Airspeed Control; ...Drag of Two Kinds; ...Best Range Speed; ...Airspeed Control; ...Why the Spread between Va and Vno; ...Knowing What You Don’t Know about Va; ...
The Aircraft You Fly Is Showing Its Age
Considering the average age of general aviation aircraft you, the pilot, should expect and plan for a considerable difference between what the POH said about the aircraft when new and what will happen when it flies years later. The hazard of performance expectation and anticipation from the POH has become a continuous and ongoing flight problem.
--It is very unlikely that you will be able to lean for the performance climb and descent the factory pilot was able to obtain in your aircraft when new.
--There is no FAA effort to compare the new aircraft performance with the old aircraft performance. It is up to the individual pilot/owner to design and chart current performance tables in all the parameters.
--The exterior and interior of the new aircraft affects both appearance and performance. A crack, dent or bend can affect the damage resistance capability.
--Over the passage of years there has been an ongoing replacement of instrumentation and avionics. Hopefully, this has resulted in a net improvement, as would be the case of replacement of vacuum tubes by transistors.
--Although over the years many engines have been through TBO replacement the average aircraft is operating on a mid-time engine. The new factory engine is required to produce from 100 andl105 percent of design horsepower. There is no such requirement for any subsequent engine.
--There are no high altitude test requirements of G.A. aircraft. At high altitudes you may well be a test pilot.
--It was not until 1979 that G.A. aircraft were required to have a Pilot Operating Handbook specific to that aircraft as opposed to the traditional Flight Operation Manual for the whole model production line.
--It has not been uncommon that manufacturers would find way to fudge the performance numbers of their aircraft. When performance contests occur, some makes and models never win when held to the performance numbers of the manufacturer. (Cessna)
--Tests have shown that takeoff distances of older aircraft can be from 15 to over 40 percent longer.
--Time to climb times can be expect to require up to 50% more than book time.
--At cruise power settings you should expect to get at least 10% less than book speeds.
--The specific air range of an older aircraft (Speed and fuel consumption) can be 25% below book figures. Not only will it take longer to fly; it will take more fuel per unit of time flown.
--Any time you fly over three hours in an older aircraft you risk fuel exhaustion.
This system gives a two-tier pressure to operate aircraft instrumentation. Static pressure uses ambient air that is protected from any movement influence. Pitot pressure is a measurement of ram air against a closed tube with a small opening into moving air. There is no airflow in either the pitot or static system. Blowing into either the static air hole or pitot tube can severely damage aircraft instruments. Blocked ram air into pitot can result in zero airspeed. Blocked static air will cause airspeed to increase during climb and decrease in a descent. (Common PTS question)
Either system can be put into a failure mode by leakage or blockage. The smallest change in the integrity of an instrument or the tubing is hazardous. Most such changes cannot be visually ascertained as when I picked up a plane early one morning to ferry back to the home field after radio work. The aircraft had been parked out all night without the pitot cover. The morning was cool and sunny; the night had been slightly above freezing area wide but the aircraft was sheltered from the wind by a hangar.
Every thing was normal as I prepared to take off from a moderately short runway. A C-182-RG accelerates readily under a light load and I was airborne before I noted that the airspeed indicator was hardly reading at all. I believe that a smidgen of ice crystals was partially blocking the pitot tube. The blockage disappeared as I passed through 900 feet. What to do? Under similar conditions do a pitot heat check during preflight. It is best to keep these ports covered until readying for flight.
A blocked static port means that the altimeter needles will remain fixed except when turned by the Kollsman knob. The VSI would remain at zero. With only the pitot blocked and the airspeed indicator would operate only on static pressure. Lowering the nose would show a decrease in speed and raising the nose would give an increase indication. I have recently come across a situation where the alternate air was turned on in flight and never turned off.
When the static port blockage occurs in a situation where
there is no change in altitude, as it would on the
ground, the airspeed indicator will not be affected. Should your
altitude increase you will get progressively
lower airspeed indications. Where this problem exists, it is
best to fly by power setting only.
Static Air Ports
The most favorable static port location is one that has neither positive or negative pressure. Manufacturers have become expert at putting static ports at the best location. The pressure on the static port can be influenced by the use of flaps, a slip, skid or yaw. The static port is located to minimize such maneuver influence. Some static ports have angled holes as part of the installation to help reduce maneuver effects.
Prior to location of static ports a drogue is flown behind
the 'proof of concept' design to get an undisturbed reading to
compare with various trial static air port locations. Some Pipers
have the static air hole as a part of the pitot mast. Aircraft
with autopilot, pressurization or electronic flight instrument
systems will have independent backup static air systems to act
as back up. Some static ports are heated. The alternate air control
usually under or near the bottom of the instrument panel is designed
for use when, for whatever reason, the static ports become fouled.
The airspeed indicator uses the differences between the static pressure and the pitot pressure to display airspeed. The pitot tube takes ram air pressure (not a flow of air) from aircraft motion to drive the diaphragm of the airspeed indicator. The static air hole(s) takes the ambient pressure of the aircraft and registers this pressure on the altimeter, the vertical speed indicator and the side of the airspeed diaphragm opposite the pitot's ram air side. The IAS requires both the pitot and the static to operate. The pitot tube usually has a heater element around it to melt ice as an obstruction. It would be useless against an embedded insect. Static system failure component by component or total is nearly impossible to detect during preflight. The functioning of the alternate static source is best checked during climb and descent for detecting a partially blocked system.
The airspeed indicator is color coded to show certain ranges of flight operation. White is flap range with Vso at the slow end and Vfe the high end. Cumulative damage will occur if flaps are lowered at speed higher than this range. The green range is normal with Vs1 as the gross stall speed without flaps. Where the green meets the orange is Vno listed as the maximum structural cruise speed. The orange range is to be avoided in turbulence. The high end of the orange range is capped by the red line. All warranties of structural strength are voided when speed meet or exceed the redline. All marked color codes are based on indicated airspeeds.
The airspeed will read slightly slower when you climb and slightly faster when you go down. If the pitot tube is blocked the airspeed indicator will work like an altimeter. As altitude is gained the IAS becomes greater than would be expected. As with the ear the airspeed works best which air pressure is equalized. There is no required inspection of the pitot-static system. There is a required inspection of the static system which involves the altimeter only.
Many need to know speeds are not shown on the airspeed indicator. Va the maneuvering speed at which warranties are voided if abrupt full control movements are made. Airspeed indicators since 1976 have been standardized as to panel location and to reading in knots with mph as an inner line of readings.
--Is a pitot-static indicator that provides pitch and power information
--The redline indicates the aircraft's Vne, the never-exceed airspeed.
--Yellow is a caution range which is avoided in turbulence
--the normal operating range is green as Vno
--The top of the green shows the maximum structural cruising speed
--Clean stalling speed is at the bottom of the green arc.
--The white arc indicates flap operating range
--The top of the white is the maximum flap extension speed, Vfe
--The Vso-stall speed is the landing configuration is at the bottom of the white arc.
--The clean landing speed is at the bottom of the green arc.
--The top of the white arc, Vfe, is the maximum extension flap speed.
---Which color of the airspeed arc identifies the power-off stalling speed in =which= configuration?
Lower white = landing configuration,
Lower green = clean configuration,
---Which color of airspeed arc identifies the power-off stalling speed in a specified configuration?
Lower limit of the green arc.
Every airspeed is the end result of thrust overcoming drag. The relative movement of the plane to the earth and the air above is the end result of pilot settings of pitch and power. Airplanes need speed to fly. However, the recording of this speed does NOT use moving air; instead, it uses air pressure. You should know that moving air does not enter the pitot tube. Only air pressure is applied through the pitot tube.
The pitot tube pressure can be indicated in several ways but the most common is a differential pressure indicator that measures the difference between impact pressure and static pressure on different sides of a flexible air chamber. A movable arm is geared to the air chamber so that diaphragm movement is measured on the airspeed dial. The pitot tube measures impact pressure while the static tube measures undisturbed air pressure. A single static port has inherent errors that occur when uncoordinated flight disturbs the air at the static hole. The resulting speed measure, called indicated airspeed (IAS) is uncorrected for the plumbing installation, air density, or instrument imperfections.
Over the years the markings of airspeed indicators have remained much the same. However, the source of the markings have varied. In the 1970 airspeed indicators were usually in miles per hour and in calibrated speeds. Calibrated airspeed is indicated airspeed adjusted for installation and interment error. Generally the airspeed correction tables have indicated airspeeds that show lower and higher than calibrated speeds. This is a built in safety factor in that when you are indicating a slow speed you are not quite as slow as you think you are. When you are fast you are not quite as fast as you think you are. Calibrated airspeed should always be used to calculate the 1.3 Vso approach speed and then converted to indicated airspeed for the actual approach. 1.3 Vso is the speed to use if no maneuvering is required on final. The final authority for any aircraft is the appropriate model and year matching the airplane to the POH.
Calibrated airspeed is different from indicated airspeed in that it makes corrections for installation, density, and instrument errors. The range of difference between indicated and calibrated airspeeds are shown on a chart in the POH.
The calibration of an airspeed system is based on standard conditions of pressure and temperature. As density decreases with altitude the speed of the aircraft must be higher in order to achieve the same instrument reading. While the indicated speed will decrease with altitude due to this decreased impact pressure, the true airspeed will increase. True airspeed is available in the POH for planning purposes. True airspeed can also be calculated in flight using E6B calculations.
If wind is not a deciding factor, it is always better to fly high to obtain the resulting higher true airspeed. The calibration of an airspeed indicator is based on standard pressure of 29.92 inches and 59 degrees F temperature. Indicated and true airspeed are identical. Above sea level your true airspeed will be faster than indicated because it takes more speed in thinner air to register the indicated speed. True airspeeds are slightly faster the cooler the temperature but any increase is negated by the drag of the denser air.
Deviate from the manufacturers V speeds and you will have reduced performance in every flight regime. However, if you trust the performance figures in your POH you are an accident waiting to happen. The book figures are for a new aircraft. A ten year old plane with a mid-time engine may have a 20% performance deficit. Instead of the book we must trust our experience and judgment augmented by local experts. You could develop your own POH book for your aircraft and insert your real performance figures. The Piper pitot/static air mast is not good over the full range of speeds and gives variations of static pressure.
Instrument aircraft often have an alternate air valve that allows cabin air to replace the blocked exterior static hole. When being used, the alternate static air causes the altimeter to read high, the airspeed to read high, and the VSI to show a climb for level flight.
Several things happen as you go higher. The decreasing air density decreases drag. However, your engine power does decrease as well. That doesn't matter so much. A given power will give you about the same indicated airspeed at any altitude. Your fuel consumption also depends on the power you are producing. With a piston engine you will get about twelve horsepower for every gallon per hour of gasoline you burn.
At cruise speed you generally throttle back to something less than maximum takeoff power. Typically you will cruise at sixty-five or seventy-five percent of maximum. Near sea level this will give you a specific indicated airspeed. You will also have a specific fuel burn that corresponds to that amount of power.
As your altitude increases your true airspeed becomes greater than your indicated airspeed, increasing with altitude. You do have to advance the throttle to maintain your cruise power output. However, as you advance the throttle you fuel consumption doesn't go up because you are leaning the mixture and still producing the same absolute amount of power, even if it takes more throttle to do it. At around eight thousand feet, you require full throttle to get seventy-five percent power in a normally aspirated engine. You are still burning the same amount of fuel that you were at that power setting down lower and still seeing the same indicated airspeed. However your true airspeed is higher, so you are both going faster and getting better "gas mileage".
Once you can no longer maintain "cruise" power you will start to slow down, but when you do your fuel consumption goes down as well, so you still get good "gas mileage" and you airspeed doesn't decrease very much.
Any airplane is the most efficient at covering the ground at the airspeed where the lift/drag ratio is greatest. That is pretty much a constant indicated airspeed for you weight. The airplane will get the greatest range at the altitude where you can just maintain your "best glide" speed ( optimum L/D ratio ) at full throttle. For most normally aspirated single engine aircraft that is somewhere between ten and eleven thousand feet above sea level on a standard day.
If you look at the POH for a Cessna 182 you will see that the airspeed for best range is quite low and the altitude is about 10,500 msl. That is not necessarily the fastest ground speed, but it does give you the most distance covered. It is your best "gas mileage" flight condition.
Highest cruise speed is at the altitude where full throttle
yields your selected cruise power output. Generally a bit lower,
around 8000 msl.
Pitch vs. Power
For years old timers and the FAA have been arguing what controls airspeed and altitude. 'Stick and Rudder' pilots believe that elevator controls airspeed. The FAA demands that the elevator controls altitude with attitude controlling airspeed. The conflict has became one of theory against reality. Will an FAR soon legislate the laws of physics?
The altitude and airspeed performance is not independent of either attitude or power. The pilot is the controlling factor. What he does in the cockpit with the elevator set attitude and what he does with the throttle sets the power. One control or the other is a factor in all flight and only in some situations does one dominate the other. How well you know your airplanes flight characteristics is as essential for all flight regimes not just landings. Every aircraft has idiosyncrasies your checkout must expose you to those. Otherwise, you become a test pilot.
In cruise flight and constant speed approaches the elevator dominates attitude and altitude while power sets airspeed. The ability of the elevator to exercise this control exists when power is both variable and available. It is only when power is not available or considered as a locked constant that the elevator can control airspeed. The FAA expects you to use the vertical speed indicator (VSI) (elevators) to maintain pitch and the throttle to keep airspeed. This works for flying the instrument landing system (ILS). If the throttle is set as a constant 1500 rpm then the trimmed elevator can control airspeed. The FAA accepts this idea that elevator controls airspeed when power is constant. The trimmed elevator gives greater control over the glide path than power.
Power is a relatively coarse adjustment to the glide path. Massive reductions or increases in power produce illusions of change, especially, if airspeed changes occur simultaneously. Only by keeping the airspeed constant can the illusions of change due to power application be overcome. Mistakes, and corrections, in being high and low on approach will be an important part of your landing training. Reduction of power in increments of 100 rpm can allow slight and smoother changes in the approach path. Since the effects of power are so variable, due to inertia, most pilots chose to set power at predetermined levels and use the elevator to set the trim to give the attitude most likely to meet the airspeed sought. Why? Because it works.
We use pitch as a form of energy control. We can covert kinetic into potential and potential back to kinetic. With speed we can use the elevator control to create a climb at a cost of airspeed. With altitude we can use the elevator control to forgo altitude in exchange for airspeed.
In reality the pilot controls the situation by using a combination of elevator and power which through this combination determines airspeed and altitude. At any given moment the pilot will make a control decision between altitude or airspeed and use power or elevator, in combination, to meet the needs of that decision. Where ever the FAA does not have a recommendation as to procedure "...a coordinated combination of both pitch and power adjustments is usually required". The Flight Training handbook is being rewritten as of 1995. Hang on.
Pitch for Speed
1. Best angle climb - obstacle clearance - most altitude over distance. Full power, level wing, set wing angle, hold nose, trim
2. Best rate climb - most altitude over shortest time. Full power, level wing, set wing angle, hold nose, trim
3. Cruise climb - most distance over time + altitude. Full power, level wing, set wing angle, hold nose pitch attitude same as slow cruise with reduced power, trim
4. Cruise - Most distance over time - select altitude carefully within 3000 feet above ground. Same as acceleration for takeoff, best glide at idle, slow cruise descent at 2000 rpm, power-on landing at 1500 rpm and approach, trim. Accelerate to desired speed then reduce to power required to maintain selected speed.
5. Cruise descent - distance over time minus altitude. Reduce power but leaving trim alone will give level flight again just by replacing the proper reduction. Called economy of effort.
6. Power-off landing and approach. Not recommended unless engine is cooled down first. Run at reduced power in gradual descent.
(common usage) What the airspeed indicator shows it is a raw value. The air pressing on the pitot tube is compared with the pressure on the static port and registered on a dial as miles per hour or knots per hour. (Post-1970 aircraft give performance figures (stall etc) as indicated airspeeds. Indicated airspeed can be used to obtain calibrated airspeed and then changed to true airspeed by correcting for temperature, pressure altitude and instrument installation error.
(uncommon usage) To get valid "indicated" airspeed you must know what the instrument error of a given reading may be. Airspeed indicators allow up to 5 mph error on installation as new. Indicators tend to hang up or ratchet with age. An ever increasing amount of friction is also expected with age. This also applies to people.
Indicated airspeed corrected for installation errors. A chart for calibrated airspeed vs. indicated airspeed is in the Pilots Operating Handbook. (Performance Speeds of pre-1970 aircraft are given as calibrated speed.) The indicated airspeeds tend to be lower than calibrated airspeeds on the slow end and higher than calibrated airspeeds at the fast end. Most accurate in mid-ranges.
Indicated airspeed corrected for temperature and pressure as different from standard. True airspeed increases with altitude. True airspeed can be calculated on the E6B or in the cruise performance table of the POH. At best this is a rough estimate. GPS will give the most accurate true airspeed if the speed is determined over two reciprocal flight courses. The spread between indicated airspeeds and true airspeeds increase with altitude. Only Indicated airspeeds are used in taking off or landing an aircraft.
True airspeed corrected for wind effect gives ground speed. Groundspeed is determined on the E6B wind correction slide. Most radar services can give you a ground speed read-out. (See DME, Loran, GPS) At high altitude airports as much as 20% more ground speed will be required for takeoff. The same 20% increase in ground speed will exist at touch down.
These are the "Vital Velocities required for precision flight. Many V speeds vary with the aircraft weight and not always in the way you might expect. Va speed will decrease with weight, stall speed decreases with weight as does best glide speed and approach speed.
Va, Is the design maneuvering speed. This speed is thought of as having to do with control movement. Va, is the turbulent air penetration speed. A full deflection of the controls will cause the stall before it folds, spindles or mutilates. In normal category "clean" aircraft this load limit is 3.8 positive. Extended flaps have only 2.0 positive, don't use flaps to slow down in turbulence.. The load limit of an aircraft can be exceeded by a PIO (pilot induced oscillation) and turbulence in a seconds. Get to or below Va even if you must stall. Va is based on weight, the heavier the weight the higher the Va. Reduce the maximum gross weight Va by a percentage equal to half the weight reduction. 10% weight reduction reduces Va by 5%. I find that thinking of it as driving over country railroad tracks. The lighter the car the higher the bounce. Know your Va for gross from the POH and how to compute it otherwise. Once way to determine the approximate Va at below gross weights is to change the Va by half the percentage of weight reduction. Find the actual weight reduction below gross as a percentage of gross. Increase the published Va by half of the weight reduction percentage. A 30% reduction of weight would result in a 15% increase in Va. Don't even "think" about what happens to Va in over gross situations. Essential knowledge: Va maneuvering speed decreases with decreasing weight. Va maneuvering speed is determined by aircraft weight. Vb Is the seldom used design speed for maximum gust intensity speed.
The most likely way to bend or break an airplane is to fly at published Va for max weight when you are not at max weight. Maneuvering speed in normal category is about 1.95 of Vref stall speed, not POH stall speed. Stall speed is determined by your wing loading. As your wing loading decreases your stall speed decreases. Flying in turbulence at nearly double your stall speed means that the aircraft will stall at 3.8 g loading and not more. Any stall that occurs at a faster speed has the potential of folding some part of the aircraft structure first. At around four Gs you can expect things to break. A pilot should always know his present gross weight and fly at a speed adjustment for the Va.
At any speed below Va an aircraft will stall before exceeding limit load factor. By stalling at speeds above Va the load factor limit will be exceeded and damage will occur. Va is the maximum speed full deflection of a control will not result in damage. A different Va exists for every aircraft weight. The lower (lighter) the weight the lower the Va. Since most aircraft have POH's that list the Va of a gross allowable weight, it is important for the pilot to be aware than any flight below gross is going to be slower than the POH listed Va. gusts are not a factor is figuring Va. Manufacturers design a 50% safety factor into the aircraft to account for gusts.
Va (maneuvering speed) is a safety speed at which a deliberate stall will reduce the potential G-force damage before it occurs. Deliberately stall (unload the g forces) before damage occurs. Before starting a flight you should know what the weight of the aircraft is and what speed adjustments from the POH Va are required to determine Vref. The speed should be reduced by the square root of percent weight is below POH gross. At 70% of max weight you should fly at 85% of POH figures for all V-speeds.
When an aircraft flies at a constant speed a straight-line relationship between any increase in the angle of attack and the G-load applied. At higher speeds an aircraft can reach a higher angle of attack. With G.A. aircraft by the time the angle of attack becomes great enough to crunch the aircraft with G-load, it will stall. It will not break at speeds below Va. The aircraft that is flown in turbulence must fly at slower speeds, specifically Va or lower, so that abrupt G-loads produced by turbulence angle of attack changes will not exceed the structural capability of the aircraft.
When you fly slower you must increase your angle of attack to maintain level flight at 1-G. A 3.8 increase in G-load is about 3.8 times the higher angle required for the slow speed. The aircraft will stall before reaching this angle. Hence, the value of Va is in protecting the aircraft. Additional weight also requires a higher angle of attack for level flight at 1-G. So additional weight plus a slower speed compounds the G-load protection offered because they require a higher angle of attack. Flying lighter and faster decreases the protection. By reducing weight below gross by 2% we can reduced the Va by 1% .
In a descent from altitude it is very easy to exceed maneuvering speed. A sudden maneuver may peel the airplane apart. Airspeed is maintained with power or if you will airspeed is primary for power. Positive limit load factor for aircraft is +3.8Gs/-1.52 for normal category, 4.4 Gs/-1.76 for utility, and 6.0/-3 for acrobatic. When you consider that the 'zero' G is actually 1.0 the ability of an aircraft to take G-loads is nearly the same in either direction.
If you don't know the Va for a particular flight weight, a close approximation can be obtained by doing a stall and noting the configuration and speed at which the stall occurs. Double the stall speed and use that as your Va where you can expect a stall before destruction.
Vr Rotation speed
Allows you to use the wheel axle to raise the nose prior to lift off where you use the center of lift as rotation axis for setting climb speed. This explains the change of takeoff attitude required when you takeoff.
Vlof Lift-off speed
Vso is the stall speed in landing configuration. This speed is for level,
un-accelerated, 1-G flight and a slow deceleration to stall. It is at the bottom of the white arc of the airspeed indicator at gross weight. At lesser weights it will be slower. The change tends to be proportional, a 5% lower speed for a 5% decrease in weight. The bottom of the white arc is the stall speed for maximum landing weight at the most unfavorable but allowable center of gravity location. Depending on year of aircraft this may be either calibrated or indicated airspeed.
Vs1 Stalling speed or the minimum steady flight speed obtained in a specified configuration. Briefly: Vs1 is stall speed in specified condition
Vs 1 is stall speed in a clean configuration and is the bottom of the green arc. Vs Stalling speed or the minimum steady flight speed at which the airplane is controllable. This speed as well as Vso is for level, un-accelerated, 1-G flight and a slow deceleration to stall.
Vfe is at the top of the white arc maximum speed for flap extension. The use of lower flap extension speeds reduces the strain on the system. As aircraft age this reduction can be important.
Vf ---Design flap speed
Vlo ---The maximum landing-gear operating speed.
Vle ---The maximum landing-gear extension speed and is based upon the durability of the landing gear doors.
Vlo--- Maximum landing gear operating speed
Va---Design Maneuvering Speed is lowest speed where full abrupt movement of the controls at gross weight will produce lift not exceeding the design load limit.
Vle---Maximum speed for extended retractable landing gear.
Vlo---Maximum speed for extension and retraction of landing gear.
Vmc---Minimum control speed is lowest speed at which full control can be maintained in shallow bank
Vne---Redline speed never to be exceeded in any operation. Little or no margin for error.
Vne is the redline speed. This speed is found by diving 1.4 beyond Vno to attain Vd design diving speed. .9 times Vd = Vne. Beyond Vne you become a test pilot. Vne is a constant expressed as indicated airspeed and is not influenced by weight.
Vmo Maximum operating limit speed determined by maximum continuous power in other than level flight.
Vh Maximum speed in level flight with maximum continuous power
Vno is shown where the orange and green lines of the airspeed indicator meets. It is called the structural cruise speed at which speeds must be below to avoid damage in turbulence.
Vc is the speed range of the green arc or design cruising speed
It is used in turbulence that is different than Va; it is called structural cruise speed or Vno. Unlike Va this is shown on the airspeed indicator as the meeting point of the orange (yellow) and green. This is a speed below cruise that is recommended for rough air penetration. Vno does not offer the structural assurances offered by Va. At Vno the aircraft, as certified, should not be structurally damaged by a 35 knot vertical gust. This is not the same protection given by Va.
Vx is the best angle of climb speed which gives the greatest altitude over horizontal distance. It is used to climb over FAA trees. (50')
Vy is the best rate climb speed. It gives the most altitude over time. It is used in noise abatement situations. Vy is greater than Vx and decreases with altitude, while Vx increases, The only time they are equal is at the aircraft's absolute ceiling.
Vso Is the stalling speed or the minimum steady flight speed obtained in the landing configuration. Briefly: Vso is stall speed in landing configuration
POH stall speeds are for gross weights, reduce stall speed by half of percentage of below gross weight.
Vso is also referenced at every 10% reduction in weight gives
a %5 reduction in Vso as the full flap landing speed. This is
your over the fence speed. It is a minimum normal
final approach speed. The hydroplaning speed is within a couple
knots of this speed using 7 times (smooth) or 9 times (treaded)
the square root of the tire pressure.
Vref This is a reference speed based on Vso. We can find the short final approach speed by multiplying Vso x 1.3 + 1/2 wind gust speed.
Vmu Minimum un-stick speed will get you off the ground in ground effect but not allow climb.
For most V-speeds, consult FAR Part 1.2
Vmu Is the minimum unstick speed as when first lift off in a soft-field takeoff.
Vd Design diving speed
Vd is dive speed. is the dive speed the aircraft reached while free from ‘flutter’ of any control. FAA certification uses 262 knots or .4 Mach as the point at which compressibility becomes a factor in certification.
VDU Demonstrated flight diving speed
VFW Maximum speed for stability characteristics
Vmc Minimum control speed with critical engine inoperative
Vtoss Takeoff safety speed for Category A rotorcraft
V1 Takeoff decision speed
V2 Takeoff safety speed
V2min Minimum takeoff safety speed
Vww means maximum windshield wiper operating speed.
Vll means maximum landing light extension speed.
V is the indicated airspeed and n is the load factor expressed in g's. Indicated airspeed determines load factors not true airspeed or ground speeds
Vb--Small aircraft have no published Vb (turbulence penetration speed)
Learn to fly aircraft without airspeed indicator. Being able to land and takeoff without IAS may be a life saver.
Maximum load factor or Limit Load Factor is the point at which
the aircraft will be deformed when exceeded. 3.8 is the Minimum
Limit Load Factor for normal category aircraft. Utility category
is 4.4 and aerobatic is 6.0. The higher the load factor the higher
the stall speed will be.
V-Speeds and Flaps
--Vy decreases with flaps applied
--Vx increases with flaps applied
--Vx and Vy converge with flaps applied
--Vx and Vy separations are greatest with flaps up
--Vs is slower with flaps applied
--1.3 Vs is slower with flaps applied
--Climb rate decrease with flaps applied
--Vx can be below Vs at maximum flap settings
--1.3 Vs is slower than Vy (Behind the power curve)
--1.1 Vs is less than Vx (Short field landings)
--Sudden retraction below 1.3 results in stall
--Flap use is restricted to 2G maneuvers
---You must know these speed in complex aircraft exceeding these speeds can damage the aircraft or worse.
Standard Performance Profiles
You can develop your own standard profile for flying or landing any aircraft. There are only a few flying profiles required. They are takeoff, climb with its variations, level cruise with its variations, descent and its variations, missed approach/go-around, and landing. For each phase there are variations of configuration and airspeeds which can be on a checklist to provide constancy in procedure and performance. Once you have developed the procedures and profiles for one aircraft it is much the same process for other types. Standard profile development is needed if you expect to have IFR competency.
Fixed pitch aircraft at gross weight climb at full throttle at Vy or Vx from the POH. Simple, except that most of your flying is not done at gross. You can roughly figure that for every 10% decrease below gross that the Vx and Vy indicated speeds will decrease by 5%. For cruise speeds always let the aircraft reach the desired indicated speed before reducing power. If you reduce power before or afterwards you will begin a cycle of changing speeds, trim, and altitudes that lead to frustration. To make the required power change you must know the required power. Cycle through several climbs and cruise changes until you can anticipate your level off at 75% power by knowing where to set power and trim and the sequence required. Go through the procedure again to determine the requirements to maintain low cruise and slow flight. Follow the same process for going from level cruise, to low cruise, to slow flight and back again until you can make changes without hesitation. Make your own checklist.
By using the wing chord line as a pitch angle indicator a VFR pilot can note that there are several performance standards where all aircraft have very close to the same pitch angle. The variables being power and airspeed.
Best-angle of climb
Best-rate of climb
Only your computation of Vref can make more specific climb speeds that the POH. Regardless, Vx is slower than Vy until they meet at the aircrafts ceiling. Vy is highest at sea level and decreases with altitude. The decrease is related to the decrease in excess engine power.
Any climb in excess of Vy is in front of the power curve. The rate of climb is determined by excess power. The angle of a Vx climb is set by excess thrust. One of the reasons Vx is lower than Vy is because thrust decreases with airspeed. Vx is on the backside of the thrust-required curve and in a region of reversed command that if pitch increases you will reach the power-on stall.
Any takeoff or climb at Vx is going to test your coordination abilities. At Vx you have reduced forward visibility and an increased need for proper rudder application. Rudder will still do what the ailerons cannot. Wrong amount of rudder and a Vx stall will initiate a spin. Aileron application will, however, provide the adverse yaw needed for a spin entry. The Vx climb and stall at several thousand feet has none of the inherent dangers of one at takeoff. You cannot trust the POH speeds any low level Vx flight should be practiced at altitude first.
An airplane can land in considerably less distance than it can takeoff. even your roll to a stop is a shorter distance than your accelerate to liftoff distance. An airplane has certain design capabilities. Age and attrition will reduce original ability. Knowing this, a pilot, should be able to determine an aircrafts present capability and make the aircraft perform to that level.
The sum of an aircrafts flying energy must add together the potential energy it has in altitude along with the kinetic energy due to its movement in air. These two energies can be exchanged within limits. There is a curved relationship between the airspeed and the power required from the engine. The limit of the curve is determined by available power. The power of the engine is changed into thrust via the propeller with some additional loss of efficiency. The faster you fly the lower the thrust because the faster the air moves past the propeller the less additional kick it can provide.
The fore-mentioned curve has a front side and a back side. The dividing line is the point at which a minimum amount of power can be used to maintain a minimum airspeed in sustained level flight. The back side is a slower speed while the front side is a faster speed. At this point there is a relationship reversal between power and airspeed. On the back side more power produces a lower airspeed only when the aircraft is first slowed; on the front side more power produces a higher airspeed. At some point there is insufficient power to fly any slower. Only by lowering the nose and losing altitude can a recovery be made. One of the saddest moments of my life was during WWII when an airplane crashed near me after getting too far behind the power curve. (B-25 on one engine, Karagapur, India)
The next major phase is the descent. By using the POH Vy speed, as adjusted for weight, and adding about 30 knots we have obtained the approach speed for that weight that will give a 500 fpm descent. for the desired 3 degree approach slope at 10 miles we should be a 3000 feet. At five miles you should be 1500 feet above touchdown. At your two mile report or downwind entry you should be a pattern altitude but no less than 600 feet. Every additional drag configuration will contribute a 10 knot reduction in speed. Appropriate trim adjustments must always be made to maintain the 3-degree approach slope. If power alone is used to make a descent, you will find that 500 rpm reduction will approximate a 500 fpm descent. Pattern descent power setting begin at the numbers. From full cruise the C-150 first has carburetor heat applied and the throttle reduced to 1500. You hold heading and altitude until reaching 60 knots. The same is done with the C-172 except the power is reduced only to 1700 rpm and heading and altitude maintained to reach 70 knots.
Airspeed control begins with knowing the power setting required for any flight condition. We climb, with full throttle, not with a range of speeds, we climb at a certain speed which the POH says will get us the highest in the least amount of time. Vy is that speed. It is 65 in the C-150 and 75 in the C-172. These are gross weight speeds. At less than gross a slightly slower speed by two or three knots would be Vy. Noise abatement requirements and safety say that you should always climb at Vy. Level cruise is 85 knots in the C-150 and 100 knots in the C-172. All speeds are indicated airspeeds. The last speeds are landing speeds flown in both types of aircraft with a power setting of 1500 rpm. The C-150 uses 60 knots in all configurations for normal approaches into the flare. The C-172 uses 70 knots until final, which is flown at 60 knots.
Full power in all fixed pitch climbs is the simplest application. When leveling off the power is kept fully applied until the C-150 has reached 85 knots and the C-172 has reached 100 knots. On reaching these speeds the power is reduced to 2450 rpm for level cruise flight below 5000 feet. This is very close to 75% power. Above 5000 the rpm can be advanced 100 rpm for every additional 2500 feet of altitude. Again, this is about 75% of power. Leaning for best operation can be done at all altitudes flown with constant power.
As a student you begin your sight profile development by getting the required POH speeds. You need to write in the changes as affected by weight variations of solo and dual. Apply these changes as required by your normal operating weight. Fly the proper Vref airspeed. A standard profile works for the pilot who can fly the aircraft to the speed and performance determined to be safe.
Variations on a Theme
--For every airspeed in level flight there is a power needed.
--At every airspeed there is equal thrust and drag. Excess thrust results in acceleration. Whenever drag exceeds thrust deceleration occurs.
--Power and speed are normal in front of the power curve.
--Adding power while holding altitude will cause acceleration until thrust and drag are equal.
--A level back-side airspeed can be increased with power but then the power needed to stay there is less than that required to fly slower.
--Reducing power to fly a slower level back-side speed requires added power to maintain that speed. More power will be required for this slower speed.
--When trimmed for level flight at 75-percent power, removal
of the power will result in the aircraft going into a descent
with approximately at 10-percent increase in airspeed due to
the decrease of loading of the horizontal tail surfaces
--T-Tail aircraft have less of pitch and airspeed change with power changes only.
Inverse effect on Va in turbulence
Vx is the best climb to get over an obstacle. Vy is the best climb to attain altitude over time. At altitude increases the Vy speed decreases while the Vx speed increases. At a given altitude where the two speeds are the same we have reached the aircrafts absolute altitude.
In the landing sequence where the pilot is king, most certainly airspeed is queen. Perhaps the worst airport landing situation would be construed as taking place on a dark night to a short runway. There may be no visual horizon on a dark night. Unfamiliar runway width, length and slope can create additional illusions. In such a situation a little too fast could easily use up all available runway.
With only a slight wandering of the airspeed, any pilot's judgment of approach slope being highs or low becomes a matter of chance. Early recognition of approach slope is essential for such a runway. Only by a constant airspeed on final can a stable approach be made and recognized as such. This is true for even familiar airports but even more so for the unfamiliar airport.
Airspeed problems are a direct function of currency, proficiency and training. Only flying with another pilot who gives a critical analysis of airspeed can resolve the problem. This is a basic skill and requires a return to basic practice and instruction. With an on target airspeed, normal touchdown, rollout and braking will suffice even on a short runway.
Knowing how to compute the proper Vref approach speeds is not as important as being able to fly them. You can practice short field procedures and speeds on long runways by selecting a displaced threshold. Using only the longest available runway is not going to improve your short field skills. The use of higher than required approach speeds can develop into a habit. You are prone to equate the smooth flying touchdown to a good landing. A good landing is approached at Vref and touchdown occurs when the airplane is through flying. Firm ground contact is preferred to smooth only because it will minimize ground roll and required braking. To get a firm landing you should hold the nose in the final moments of flare so high as not to see the runway. A good technique for this is to cover the far end of the runway with the nose of the airplane. Use your peripheral vision to detect any altitude increase or loss. Holding a small reserve of power will aid in raising the nose and permit control of any slight ballooning by taking off a bit of .power..
Preliminaries to airspeed
From the very first flight we have worked, unknowingly, towards the selection airspeed in climb and descent. On the second and subsequent flights we have worked on differing level flight speeds and aircraft configuration. What we have been learning is how to perform the various elements that make up an airport pattern and a landing. The first speed used in the pattern is the rotation-speed. This speed approximates 40-knots and is the speed that allows the nose of the aircraft to be raised while still accelerating to the lift-off speed. Rotation occurs around the axle of the main wheels. Once a plane is in the air it pitches around the center of lift on the aircraft wing. This means there will be a slight lowering of the nose to set the aircraft attitude for acceleration required to climb at Vy. Vy is the preferred climb speed for greatest altitude over time.
By climbing at Vy we minimize noise and get to our level-flight altitude more quickly. Initial level flight maneuvers are done at cruise speed. Normal cruise-speed is whatever speed the airplane can maintain at 2450 rpm. Because of the relatively high power loading, pounds per horsepower, of some aircraft it is usually best to maintain full power until the cruise speed is approximated.
A critical aspect of this level-flight acceleration is to strive for accuracy in reducing power at the appropriate cruise speed. Reducing power too soon means that the plane may well take several minutes to reach cruise speed. During this time it will be necessary to hold the aircraft at altitude and make several fine trim adjustments. Reducing power too late means that power and trim must be adjusted while the plane slows to the 2450 rpm cruise-speed. The procedure for attaining and maintaining level cruise needs repeating in the first three flights both from climbs and descents. Time spent working out the required sequence of power and trim will pay multiple dividends in time saved in later flight maneuvers.
In addition to the required power timing and changes, the student should learn to turn the trim with the fingertip. The advantage of the fingertip over the pinch method of moving the trim is that the trim setting indicator can be moved more accurately.
A little known or taught engineering design feature of Cessna aircraft is the ratio of trim movement that exists in the various critical airspeeds and flap configurations. The ratios are there best in Cessnas that have 40-degree flap deflection. Those that have only 30-degree deflection have lost much of this design feature. What they gained in go-around capability and gross load was surrendered at a price.
The Cessna C-150, 172 and 182 with 40-degrees flap extensions should all be trimmed for takeoff as part of the preflight-pre-takeoff preparation. With repeated experience, trim settings can be made to account for loading differences. As soon as possible after liftoff get the plane fine-trimmed for hands-off Vy climb.
If the C-150 made a full-flap 60-knot landing, it will be trimmed for level flight without flaps. This is engineered into the aircraft to simplify student go-arounds. The pre-takeoff trim setting requires one top to bottom trim movement for takeoff and climb at Vy.
The C-150 can be leveled from a Vy climb by making a full one-turn of the trim wheel using only the fingertip catching the lowest trim button and rolling it and the wrist up as far as possible. This and the yoke will lower the nose to level. At level flight one-finger backpressure must be held to prevent the nose from dropping while acceleration to 85-knots proceeds. As acceleration proceeds toward 85-knots, the pressure must be relaxed to maintain altitude. At 85-knots the power is reduced to 2450 rpm and any necessary fine-trim applied. In ten-seconds you can be trimmed for level flight.
If the C-172 made a 40-degrees of flap, 60-knot landing, it will be trimmed for a Vy 75-knot climb after the flaps are removed. This differs from the C-150 since it is less likely to be used as a primary trainer.
The C-172 can be leveled from a 75-knot Vy climb by making a one and a third turn of the trim wheel. This amount will vary a bit according to passenger load. This trim and yoke movement will lower the nose below level unless one-finger backpressure is used to hold the aircraft level. Because of power loading, the C-172 will require longer to accelerate to normal cruise-speed. Attention must be paid to the altimeter since the acceleration may take as long as three minutes to reach 100-knots. Pressure must be relaxed very slowly throughout the acceleration to 100-knots. Any gain or loss in altitude will prolong the maneuver. At 100-knots reduce the power to about 2500 because there is a tendency to lose 50 rpm. Many of the C-172 power settings require momentum-adjustments like this. Do the C-172 exercise of climbing at Vy to level normal cruise transition until the student becomes proficient. All in-flight airspeed changes should be based beginning from normal-cruise.
Speed Changes from Normal Cruise
In the pattern the Cessna engineering inter-relationships of power setting, trim and flaps really shines. No need to fiddle or fool with adjustments. Just set, adjust and airspeed will be there. To learn to do this in the pattern it is first necessary to practice aloft.
C-150 Airspeed Exercise
From level cruise, pull C.H., reduce the power to 1500 and make three fingertip top to bottom trim turns. Hold heading and altitude and plane will stabilize at 60 knots. Bring power up to 2000 and the C-150 will be in slow-flight at 55/60-knots. Leave power at 1500 and C-150 will descend at 60 knots. Use light yoke pressure throughout to prevent oscillations.
At 1500 rpm, put in 10-degrees of flap and airspeed will slow to 50-knots. Take off one of the three previous turns of trim. Up by fingertip bottom to top and aircraft will return to 60 knots. Repeat the exercise but use the yoke to maintain 60-knots while put in the 10-degrees of flap and take off the turn of trim. C-150 will be descending, hands-off at 60 knots.
Put in successive notches of flap to 20 degrees and 40 degrees while taking off full turns of trim. Remember we initially put in three turns down (nose up) and now have removed all three. For the 20 degrees go-around we are trimmed for Vy climb. For the 40 degree go-around we must trim down one full turn.
The entire process is best initiated on only one heading and followed up with both left and right 90-degree turns for each flap setting. This exercise with all its components duplicates all the C-150 pattern maneuvers and airspeeds to the point of round-out.
C-172 Airspeed Exercise
The procedure to be followed is identical to that of the C-150 except for the procedure in setting power and airspeeds for each flap setting. From level normal-cruise, reduce the power to 1700 rpm. As the aircraft decelerates to 70/80 knots the rpm will fall to 1500 rpm. Hold heading and altitude. Put in the first 10-degrees of flap and 20-degrees of flap as with the C-150. Going to 40degrees of flap you should allow the aircraft to slow to 60 knots and do not take out the third turn of trim. You will use this as a climb setting during a no-flap go-around.
For C-172 IFR speeds, you want to know the power and trim setting that will give a desired performance. Most C-172 speeds for low cruise, climb and descent can all be done at 90-knots. Low cruise from normal-cruise will be close by reducing power to 2200-rpm and trimming down one full turn. Climb is always at full power and one full down turn of the trim wheel. Descent from normal-cruise requires (Checking on this) power to
1800 and two turns of trim top to bottom.
Every pilot should have some idea of three glide speeds. One is the one that keeps you in the air the longest, another is the speed that covers the most distance, and the third is the all-purpose combination of the other two. The two climb speeds Vx and Vy can be used as approximations since they are easily available in the POH. The combination speed is somewhere in between.
Glide speeds that use available power are recommended because it reduces the impact of excessive cooling. Once an aircraft is on a stabilized flight path with a constant power just making a change in the airspeed will change the glidepath. The no-flap approach speed is used because any faster will increase the rate of descent and the distance covered. Any slower will increase the descent rate over less distance.
The faster speed is an erroneous correction speed often used by pilots who believe that keeping the end of the runway in sight will take them to the runway. The runway can be kept in sight only by ever increasing the speed. A greatly increased descent rate can be attained but only at the cost of increasing the speed. At some point the runway will pass under the aircraft and any landing will be impossible. A tailwind causes the same visual effects and problem due to increased ground speed.
The slower speed is useful when an approach is very high. After the aircraft has maximum recommended flaps and the power is off, the slow speed is used to increase the descent rate over less distance. Five knots below normal approach speed is the standard usually set. A further decrease in speed to Vref can produce dramatic descent rates in a headwind. I recommend increasing speed shortly before flare as a precaution. The increase in speed increases the ground effect that makes a more normal flare possible.
A clean airplane gliding as the minimum sink airspeed vs. the maximum distance speed will give a half mile less distance for every 3000 feet of altitude. A glide in no-wind conditions cannot be stretched beyond the maximum glide distance. You will glide somewhat further by slowing with a tailwind. You will glide somewhat farther by adding 1/3 of the estimated wind velocity to best glide speed into a headwind. A heavier aircraft will glide at a higher speed for attaining best glide. Gliders often carry water to increase their long-distance glide capability. Stopping the propeller by slowing to a stall will make up to a 20% increase in glide distance. For every 10% that you are below gross weight you can reduce the approach speed by 5%. With GPS or LORAN and some wind data it is possible to determine the best glide speed for a specific weight and aircraft.
A couple of aspects about approach speeds have not been mentioned as I view it:
--POH figures are usually quoted for gross weight. I find that in my C-172 I can figure a Vref as low as 55 knots. Solo and lightly loaded. You should always try to know the appropriate Vref for the weight.
--Those of you using aiming points should be aware that the point is referenced by ground speed. A light and variable wind condition changing by +/-4 knots can have a dramatic effect on the landing accuracy.
--For those who choose to dive for the runway. "Slow down to get down". A C-172 will drop quite well with full flaps and 40 knots IAS. Just remember to leave enough altitude for the dive to the runway to obtain flare speed.
--At one time or another everyone should be exposed to a downwind landing. I feel the exposure is important so that the pilot will become more sensitive to recognizing the visual effects. The most difficult downwind situation occurs as night where visual effects are harder to spot. Be extra careful where a locked tetrahedron designates the runway.
The landing distance required of a given aircraft would approximate 30% of the square of the touchdown speed. A 10% increase in touchdown speed will result in over a 20% increase in landing distance. Putting some numbers to this is quite revealing. Approach at 60-knots, flare and touchdown at touchdown ground speed
Landing distance ............................10% increase in speed
60-knots 1080 feet 66-knots 1307 feet
50-knots 750 feet 55-knots 907 feet
40-knots 480 feet 44-knots 581 feet
30-knots 187 feet 33-knots 326 feet
Flying an incorrect and higher speed as Vref will require a substantial longer roll out. The idea is to fly an approach speed that will minimize float, allow sufficient elevator authority to give a nose high flare, and touchdown as slow as possible.
Pilots do not and should not be looking at the airspeed indicator during the flare. I cannot recall noting one touchdown speed when I was flying. I do like to comment on touchdown speeds of student landings. Fact is you don't need to know or see the touchdown speed. The full-stall-touchdown-speed is the slowest speed we can achieve prior to touchdown. Only the airplane knows that speed. The actual ground contact should come as a complete surprise to the pilot.
Airspeed and Slips
The airspeed indicator does subtraction in the process of indicating speed. Airspeed is the difference between the static port pressure and the pitot tube pressure.
A static source on the left side of an aircraft will indicate correctly only when the relative wind provides no ram or vacuum effect on the static port. In a slip, high or low airspeed variations will occur depending on the direction of a slip. In a slip the airspeed error cannot be predicted.
Slipping into the static port causes ram air effect on the port. Thus the difference between the static air pressure and the pitot air pressure will be less. Indicated airspeed will be less. Any time the relative wind is not directly into the pitot tube the pressure will be lower. The result is a lower indicated airspeed unless countered by the much greater effect of static port pressure. Relative wind is any wind created by motion will act opposite to the direction of motion.
Va is commonly misunderstood and seems counter intuitive. This is because we usually think of larger size and weight as resulting in a reduction in rate of climb and higher stall speeds. Va is based upon the accumulative forces acting on the airplane due to acceleration. The larger and heavier the aircraft the better it resists turbulent accelerations thus allowing a greater Va speed.
Va acts upon the entire aircraft but every aircraft has a weak link. This is a part that in its design is most likely to break under stress. The tail surfaces tend to be in appearance the most fragile. When the entire aircraft is heavy, it resists sudden changes as might occur in turbulence or maneuvers. The Japanese Zero could out turn the heavier U.S. aircraft because of a lighter weight but it stressed itself in so performing.
This acceleration of the heavier plane resulting from turbulence or maneuver will go down and so will the maximum amount of force on the aircraft weak link. The Va is engineered into the aircraft structure so that it will stall before the weak link will break.
Va is the highest speed where the design load factor of the wing will not be exceeded before the stall will occur. The stall occurs as a protection to keep the wing from breaking. Greater weights require more lift from the wing; this results in a higher load factor. Federal regulations require that the design load factor of the wing be able to withstand a load 3.8 greater than the force of gravity. Manufacturers often add an additional 50% safety factor above the FAA requirements to produce an aircraft capable of withstanding a 5.9 G-load. .
Lift increases as the square of the speed for a given angle of attack The design is predicated that the design load factor will occur at a speed that is twice the stall speed When in level flight the wing stalls the critical angle of attack and is pulling 1.0 Gs. When the wing meets both the critical angle of attack and the design load limit it has also attained the Va speed. Va speed is determined when the wing simultaneously reaches the design load limit and the critical angle of attack.
Maneuvering speed is set at that magic speed where NOTHING you can do can actually break the airplane.
Everything else you wrote I agree with. Not this, though. You can break the airplane below Va - it's just not particularly easy. Designs are based on assumptions, and the assumptions the design engineers are required to use are mostly spelled out in 14 CFR 23. It is sometimes educational to read that stuff.
Va is the stall speed at which you will stall before you pull more than the maximum positive g-loading. In a normal category airplane, it's almost always just a bit less than twice the stall speed. That's because normal category airplanes are stressed for just a bit less than 4 gees positive (3.8 to be exact) and when you quadruple the weight you double the stall speed. Easy.
So basically, below Va you can yank back on the elevator as hard as you like and nothing will break. Before you break anything you will experience an accelerated stall that may throw you into a spin - but it won't break anything. This may be scant comfort in an airplane which won't recover from a spin...
You can also go hard over on the rudder - once. See, at Va or less the rule is that when you start with zero yaw, you need to be able to go hard over on the rudder without breaking anything. But what about if you start when it's not zero yaw? You're on your own. If you go hard over once, get a yaw going, and go hard over the other way, you are not necessarily protected.
What you have to realize, though, is that the assumptions are geared around normal flying. Why in the world would you ever go hard over on the rudder in one direction, and then the other? Why would you pull up hard and go hard over on the ailerons (a great way to break spars, BTW). Well, the only possible reason is serious hardcore aerobatic maneuvering.
Since aerobatic maneuvering is not normal flight (in most cases) it is handled in a special way by the FAR's. An aerobatic category aircraft is at least +6/-4 gees, but that's not for maneuvers - that's for screwing up maneuvers! The maneuvers themselves have to be demonstrated for certification, and procedures and entry speeds published. This forces the engineering team to consider the stresses caused by each individual maneuver, verify that the structure can take it from an engineering standpoint, and then verify it with flight testing. Airplanes with the same gee ratings can have very different lists of approved aerobatic maneuvers.
Where airplanes do get broken (other than T-storms) is dog-fighting. That's when you make up maneuvers on the fly, and sometimes that ends badly. There's a wholly undeserved AD out on the older T-34's for that reason.
As long as you are flying maneuvers for which the airplane
is intended, it's virtually impossible to break anything. Screwing
up won't be enough - you have to screw up really, really badly.
Exceeding bank angle by 10-15 degrees won't be anywhere near
The Performance Envelope
Every light aircraft has a range of capability, several of which are called 'envelope'. Envelopes have a low and high end. It is the high end of the envelopes that we become most aware of as a point at which structural damage begins to occur and accumulate. Every envelope is a planned compromise of weight carried, range flown, and cost. Every pilot must know the capability of his airplane to perform inside its aircraft category (ies) as certified.
Envelopes are based upon design speeds in several areas. The most important are the low end stall speed at 61 knots for light aircraft, the cruise speed, and the dive speed. Aircraft seldom cruise at the high end of the cruise envelope and even more rarely reach the high end of the dive envelop known as the red line. These last two speeds are the parameters that form two other strength envelopes based on ability to maneuver and survival of turbulence.
Pilots are more familiar with the maneuver envelope that has an upper limit number known as Va, or maneuvering speed at gross weight. The selection of Va by the aircraft designers mean that the aircraft will not bend, break, nor spindle when maneuvers with full deflection of the controls at or below this speed. The contrarian aspect of Va is that the lighter the aircraft the lower will be the Va. Light aircraft making abrupt maneuvers below the variable Va should never be able to pull above 3.8 times the force of gravity.
Inverted, a light aircraft in normal category is supposed to only survive 1.52 Gs negative load. This means that any inverted flight is pushing the outer limit of the envelope and is capable of causing the aircraft to self-destruct from structural stress.
The gust speed envelope has a light aircraft top at close to 4.5Gs. This top is higher than the Va range of 3.8 because a gust stresses different aircraft structures. Just where and when a gust will stress the weakest point of the aircraft is a variable known only to the aircraft designers. In turbulent conditions the pilot is better off to go for the ride except for steep up/down nose conditions. Slow down and avoid maneuvering in turbulent conditions. Higher speeds will not help and are likely to exceed the aircraft's stress limits. Do not put in flaps. The structural load limit of an aircraft with flaps is only 2.0Gs. Stressing a flap can turn it into a pretzel.
The top of the dive speed envelope is marked by the red never exceed indication on the airspeed indicator. Any operation at or near the redline can cause accumulative stress and structural strain. All structural speeds are computed into the design of the aircraft by the engineering team. The maximum structural cruising speed is the low end of the yellow arc. Flight in this range during descent can easily encounter gusts or turbulence that intrude upon the never exceed speed. At 10 percent over the Vne speed the aircraft is into the maximum (test) design dive speed where control flutter speeds that can quickly cause control failure. This failure can occur before you can slow up. An elevator may be a bit out of balance; the flutter will occur a much lower speeds. Early aviation movies made this factor in the development of aircraft a plot for any number of exciting accidents.
Thus, when it comes to structural speeds, the FAA requires a safety margin over and above that indicated in the POH manual and traditionally the manufacturer adds a 50% additional factor. These factors are not speed factors, they are structural stress factors and most of these increase exponentially with speed, not linearly. Any pilot who exceeds the POH structural speed numbers is an experimental pilot flying an experimental aircraft. With the average age of light aircraft reaching 30 years, accumulative damage is almost certain to exist.
Structural Speed Limits
Storm conditions can easily stress an aircraft's structural envelope beyond regulatory limits. A pilot is required to fly within the allowable performance limits. It is a violation of the FARs to do otherwise. A speed attained in smooth air is a serious breech in turbulence.
There is a V speed beyond Vne. In fact Vne can be figured figured as 90% of Vne, called Vd. Vd is a design speed for which the aircraft is engineered for a given configuration. In flight testing the bottom of the green arc is determined as Vsl or minimum steady flight speed configured without flaps. The low end of the white arc is Vso done with flaps. Any stall speeds are advisory for a new aircraft; your speeds may vary with an 'identical' aircraft. The top end of the white arc, Vfe is a 2-G limited airspeed as are all flap extended speeds. A 60-degree banked level turn pulls 2-Gs and a 72-degree bank pulls 3.8 Gs, which is the maxim positive load allowed in normal category aircraft
Engineering practice is to design over the FAA required G-loads. Since the weakest link in the aircraft is required to sustain 3.8 and the engineered load exceeds this by about 1.2 Gs we are looking at least a 30% engineered safety factor. At 3.8 Gs no new aircraft should suffer permanent deformation. However, there is no legal warranty for fatigue stress from repeated maneuvers. Fatigue stress is accumulative. The number of times an aircraft can be stressed is an engineering guessimate. Many common and popular aircraft are known to incur fatigue stress failures. Military aircraft do carry inertial odometer counters that are used to 'age' aircraft instead of time.
Without a G-meter we had no way of knowing how must stress a turbulence jolt gives to an aircraft. There are three different turbulence forces. Vertical turbulence is up and down, lateral turbulence is a sideways thrust, and longitudinal relates to speed changes only the famous air pocket is a vertical jolt. Aircraft weight, speed, wing load per square foot, density altitude, gust impact and aircraft lift capability all combine to determine just how much the aircraft is stressed by a single incident. Each one of the foregoing items will add or subtract from the ability of the aircraft to withstand turbulence. A heavy fast aircraft is more capable than a light slow aircraft. Slow in turbulence is always better than fast. A high wing loading (Pounds per square foot) is better than a low wing loading.
The markings of the airspeed indicator are predicated about a 1 G-load factor. Normal category maneuvers are best performed in smooth air conditions. A lazy-eight could stress the aircraft beyond limits if performed in gusty conditions. In mountains where turbulence prevails you should slow to less than Va, hold a level attitude and do not chase altitude or airspeed. Limit turns to 1/2 standard rate. You should, if traveling light by 30% you should lower your turbulence speed at 15% of Va Any avoidance maneuver should be first DOWN or UP or as a last resort in a turn. This order is in terms of the structural strength of an aircraft structure. Going abruptly down is as though the aircraft suddenly gained weight. Going up puts downward pressure on the flying surfaces and is only a second choice. Any turn will add G-loads to the original maneuver. Think on it. React the way
birds do toward you.
Best Glide by Weight
Take 1/2 of the percentage below max gross weight and subtract that percentage from the POH maximum gross glide speed.
Best Glide Speed decreases as total aircraft weight decreases. The POH's Best Glide Speed is based upon aircraft gross weight. Vbg (Best Glide) speed decreases with weight. Since Vbg uses induced drag (which depends on weight) as one of the components in the determining the equation for best glide.
-- Vbg is defined as the indicated speed at which the greatest flight distance is attained per unit of altitude.
--As airspeed increases, induced drag decreases and parasitic drag increases.
--Vbg is the speed when the ratio of induced and parasitic drag is least..
--The change in best glide per 100 pounds of weight change would be determined for each aircraft but --Vbg is calculated with a CG limit of zero.
--Your glide range is determined by the L/D ratio as flown. Increasing the weight increases the speed for the best L/D ratio and increases the power required to maintain altitude. This increases the rate of descent with power off.
--The Speed : rate of descent=lift: Drag.
The ratio of speed to rate of descent is the same as the L/D
--Flying at the best L/D ratio ( best glide speed ) distance covered will be the same regardless of weight in a no wind condition. With more weight the speed and rate of descent is increased. The same distance will be covered in less time.
--The distance covered is the same regardless of weight when flown at the best L/D airspeed.
matter. Against a wind you will travel further the heavier you
are because you will fly faster with less time
for the headwind to affect the flight.
--Drag is a force measured in pounds. The moment of time when induced drag equals parasite drag is where you are getting your best glide speed.
--Induced drag decreases AT A DECREASING RATE as airspeed increases.
--Parasite drag increases AT AN INCREASING RATE as airspeed increases.
--The least drag will occur when the airspeed produces an equal amount of induced drag and parasite drag.
--This is the speed where the minimum thrust overcomes the drag. This is not the best glide speed.
http://avweb.com/articles/bootstp1/ by John Lowery:
Speeds and Density Altitude
Density altitude affects the airspeed indicator in the same way it affects the lift. power and thrust.. The indicated airspeed at rotation will be the same at high density altitude as at sea level. Because of the weight of the air molecules impacting the pitot tube. The groundspeed will be much higher at the higher density altitude when the airspeed indicator indicates the same rotation speed.
At high-density altitude, a normally aspirated engine loses power with every increase in altitude and temperature. Full throttle at 7500' density altitude is 75 percent or less. This is why you can demonstrate density altitude takeoffs at sea level with only partial throttle.
The airplane has to actually accelerate to a higher ground speed before rotation, and the acceleration is much slower because you have much less power and thrust. Explains why high altitude airports have long runways. The ground roll increases as the density altitude goes up. The rate of climb decreases, once again because you have to fly faster and you have less power and thrust.
What is effect of density altitude on Vx and Vy indicated airspeeds?
For a fixed pitch propeller airplane, as density altitude increases, Vx remains constant and Vy decreases.
For a constant-speed propeller airplane, same conditions, Vx increases and Vy decreases.
There are two ways excessive loads can be put into the airframe. The first is by the pilot using violent control techniques. The second is by turbulence acting on the plane. However, maneuvering speeds come from loads that would exceed the 3.8 load factor limits for normal category small airplanes. Va is the speed at which a full deflection of a control aileron or elevator would place a load factor on the wings of 3.8.
Certification of aircraft requires 3 other speeds to be defined Vb, Vc and Vd, These speed you won't find in the POH Vb is called the rough air speed, It is based on a 66 ft/sec vertical gust. Vb is described as the speed the pilot should slow down to if he encounters rough air. Since most of the aircraft I fly recommend Va for turbulent air I can only guess that for these aircraft Va and Vb are actually so close that the designers wanted to just give the user one number.
Vc is the never exceed speed, It is based on a 50 ft/sec vertical gust. Vc Is in some way related to the redline speed.
Vd is the dive speed; It is based on a 25 ft/sec vertical gust. Vd is the dive speed above red line without encountering any gust above 25 ft/sec.
These speeds were obtained statistically by people flying through thunderstorms and measuring gust velocities and determining the likelihood of actually encountering such vertical gusts. Therefore slowing down to Vb or Va whatever the case may be does not guarantee that your plane will not have its wings ripped off by turbulence. There is no guarantee that nature will not deal out the 75 ft/sec gust. It is only unlikely. On the other hand slowing down to Va will guarantee that you can put in any control deflection in and as long as the speed remains less than Va you will not overstress the aircraft. Hope this helps
Breguet Range Equation
(Breguet was pre-WWI French aircraft designer/builder.)
Determining factors are the lift-drag ratio, propeller efficiency, specific fuel consumption and ratio of takeoff to landing weights. You must consider that the only change in weight will be through the use of fuel.
You can get the most distance out of fuel by flying at a speed that is 1/4 above the headwind component added to the still air best speed. Use1/6 lower of the tailwind component below the best still air speed. The lower you are on fuel the slower you should fly.
Is what puts you into a condition where a sustained float likely, By definition the "float" speed is too slow to fly and too fast to land. You have a sustained exposure to a situation with minimal flexibility for selecting an option.
Velocity is Another Word for Speed
It comes from a V-g diagram. A V-g (sometimes called V-n) diagram is a plot of load factor vs. speed for a given set of conditions. Va resides at the point on the diagram that corresponds to the positive limit load factor - something on the order of 3.5g for normal category aircraft (the exact number escapes me).
There's an explanation of the V-g diagram in 'Aerodynamics For Naval Aviators'
Indicated vs Calibrated Speeds
-- negligible difference at cruise speeds
--Important at low airspeeds).
--For example, 1.2*Vso is not Vso (in IAS) multiplied by 1.2. You have to convert the Vso into CAS multiply by 1.2, and then convert it back to IAS. The difference is surprisingly large.
--Here is a 172N chart
Vso 1.1Vso 1.2Vso 1.3Vso
33 40 47 52
Multiply the IAS by the 1.x factors
Vso 1.1Vso 1.2Vso 1.3Vso
33 36 40 43
--Difference important because the incorrect method of computing 1.x*Vso under-estimates the speed.
--Puts the airplane at slow airspeeds that could be a 10% margin over stall..
Glide Rule of Thumb
--Trim full nose up
--Make Vref best glide chart for several loadings using indicated speeds
--Determine your percentage of gross weight
--Divide by two and call answer as knots
--Subtract computed knots from POH best glide speed.
--Fly minimum descent speed as being five knots below best glide speed
--Only one speed is right for aircraft and its weight
--You can't stretch a best glide
--Train/teach to deviate from best glide if too close to destination
--Minimum sink will not take your as far at best glide, but it will keep you in the air longer.
Best Glide Vref
--Use LORAN, GPS or DME toward a VOR
--At altitude slow to 2.5 knots below best glide in POH reference airspeed, ground speed and distance.
--Time descent through 2000 feet
--Divide 2000 by time taken to lose altitude
--Do exercise several times, reducing speed 2.5 knots each time finding best glide speed for your Vref.
--Repeat process as required for most usual Vref loadings.
Minimum Descent Rate Vref
--Use LORAN, GPS or DME toward a VOR
--At altitude slow to 2.5 knots below minimum descent glide in POH or to full nose up trim speed
--Time descent through 2000 feet
--Divide 2000 by time taken to lose altitude
--Do exercise several times, reducing speed 2.5 knots each time finding minimum sink rate for your Vref.
--Repeat process as required for most usual Vref loadings.
Airspeed Control (Opinion)
Which one did you notice first - the airspeed drop or the cowling in the sky? The airspeed indicator lags quite a bit behind what the plane wants to do. By the time the indicator shows you there's a problem, it's already worse than you think. If you correct until you see the right indication, you are certain to overdo it and end up with an excursion the other way. Then the cycle repeats. The lag makes it too difficult to hold a steady airspeed using the airspeed indicator. Meanwhile, you are staring inside the cockpit and not looking out for traffic...
There is a better way to do this. For a given power setting and flight attitude, the plane will stabilize at a certain airspeed after a few seconds, the same one every time. Knowing what attitude gives you Vy with full power is all you need to maintain Vy precisely without ever looking inside the cockpit. Adjusting the attitude with the yoke gives you a much more immediate response, without the lag of the airspeed indicator, and thus avoids the overcorrecting cycle described above. As a bonus, you are looking outside, so you can spot conflicting traffic.
You can use the same trick on approach for landing. When you have the right
speed, pick a bug on the windshield
which is on the horizon and keep it there. The airspeed will not budge, unless
you need to mess with the power. If you always fly in the same plane, you'll
soon memorize the right attitudes for each phase of flight. The day when your
airspeed indicator packs it in will be a complete non-event.
of Two Kinds
There are several different issues and at least two different 'powers' involved. Perhaps I can straighten out most of the confusion.
When an airplane flies it has to continually keep moving. There are two forms of drag, or resistance to that motion that you have to deal with. The obvious one is the friction drag of the airplane itself, called "parasite" drag. There is also another source of drag, called "induced" drag. Induced drag is the drag created by the process that creates lift to keep the airplane up. This drag is proportional to the intensity of the lift. The slower you fly, the more intense the lift gets. The technical way to say this is that the lift coefficient has to get larger as the airplane goes slower.
The drag of the airplane can be plotted against the airspeed. Induced drag starts real high when the speed is real slow. Parasite drag starts real low and get rapidly higher as the speed increases. The result is the total drag starts real high. As the speed increases the total drag decreases, to the point where the induced drag and the parasite drag are the same. Then at higher speeds the drag increases again at an increasing rate.
The power required to keep the airplane is the air is force times speed. The drag is a force that must be overcome by the thrust of the prop. You can also overcome this drag by coasting downhill! :-) Thrust times speed is also a measure of power.
This "power required" curve starts real high. It reaches a minimum about twenty percent above the stalling speed of the airplane. That is the speed where the LEAST expenditure of power is required to keep the airplane in the air. It is called the "minimum sink" airspeed. Above that speed the power required starts to increase again.
An airplane can store power in three forms. Speed, Altitude, and fuel. Power stored as fuel can only be withdrawn from the store through the engine and the power setting of the engine controls the rate that fuel is converted into power. Before the engine power can be used to overcome drag it must be converted into thrust. That is done by the propeller in our airplanes.
The propeller has a couple of gotchas. The efficiency of the conversion from power into thrust for a propeller goes down quite rapidly as the propeller RPM increases. For example 35 Horsepower at 1000 RPM may give you 300 pounds of thrust. At 2500 RPM you may require 85 horsepower to get the same 300 pounds of thrust. That is why most airplane engines, that drive the propeller directly, run at relatively low RPM's. If you turn them up faster, you make more power but get less thrust for it.
For the airplane to climb to a higher altitude requires an input of power that is greater than needed just to stay in the air. The "excess power" goes directly into the altitude storehouse. One horsepower will lift 550 pounds 1 foot in 1 second. That relationship directly determines your "rate of climb."
The power available from your engine also varies with the airspeed. You may have noticed that you cannot get full power RPM when you start a takeoff. Then, as the airspeed increases, the engine revs up to higher RPM and you get closer to your maximum power. The "power available" curve generally crosses the "power required" curve somewhere around the stalling speed, or a bit lower. The greater the difference between the "power available" curve and the "power required" curve, the greater your rate of climb. Both curves are rising quite steeply and are almost parallel around Vy. Vy is the airspeed where the distance from the "power required" curve to the "Power available" curve is greatest. It is generally very close to the optimum L/D ratio for the aircraft, which gives the "best glide" speed. Vx is the best ANGLE of climb. Below Vy the excess power is decreasing quite slowly, since the two curves are nearly parallel. However, the distance covered per minute goes down with the airspeed. As a result Vx is generally less than Vy. As your altitude increases your power available decreases. As a result, Vx tends to increase and Vy to decrease. At your absolute ceiling Vx=Vy.
It is all about energy. The airplanes burns energy to fly.
You can get the energy required by giving up airspeed or altitude
or by cranking up the fuel to power converter, the engine. Excess
energy goes into one of your energy stores, either by increasing
airspeed or by increasing altitude. Both have limits. I hope that
helps it all make a little more sense. :-)
Best Range Speed
John Lowrey wrote:
I finally got around to calculating how (calibrated) airspeed for best range (Vbr) varies with density altitude (it doesn't!) and with gross weight (it goes up as weight does) for a fixed pitch propeller airplane.
For a standard Cessna 172 in no headwind or tailwind, weighing 2400 pounds, Vbr = 73 KCAS at any altitude. At 2000 pounds, Vbr = 67 KCAS. The specific range figures (nm per pound of fuel) also do not vary, for a given weight, with altitude. But if your airplane is heavier, specific range is lower. The maximum is quite broad; three or four knots off the optimum airspeed, in either direction, only makes about 1% difference.
I calculated specific range (also specific endurance) figures for various airspeeds in my book (Performance of Light Aircraft), and graphed them as functions of airspeed, but I failed to there see what the altitude effect was. I wish I had.
So for best range, don't move a darned inch, vertically, unless it's to avoid some rocky obstruction in your path. But don't forget to slow down as you burn fuel. If you run into a headwind, speed up (about 6 KCAS for a 20-knot headwind, for this airplane); if you run into a tailwind, slow down.
Fixed power, nose position determines airspeed. If speed is consistently high or low use trim to give lighter touch ‘pressure’. If CFI keeps asking you to raise nose , trim, if airspeed is consistently low, trim nose down, if airspeed is consistently fast, trim nose up—Nose moves faster than airspeed indicator increases or decreases, set the nose gently.
Why the Spread between Va
Why does an aircraft have a Va at gross of 97 knots and a structrual cruise speed (where the orange and green meet) of 122 knots? The first related to control movement and the second predicated on turbulence. If the potential for damage exists at both speeds why such a wide spread between the two speeds?
I thought that: above your "Orange and green meet" (V-no) speed, there is potential for damage in turbulence even with zero control deflection, and that below Va is safe for full control deflection. Presumably just above Va, damage would occur with full deflection... at somewhat higher speed,
damage would occur at half deflection... and at V-no damage could occur with only slight deflection. ???
This is my understanding of Va vs Vno.
Va assumes a sudden change in angle of attack caused by full control deflection. The resulting load factor will not exceed 3.8G if you stay below Va.
Vno accounts for wind gusts only and does not assume full control deflection. Presumably, the effect of wind gusts is smaller than full control deflection, and as a result Vno is faster than Va.
Vno is solely due to the stress of drag forces with no G forces involved.
Ever stick your hand out the window at cruise speed? There's a lotta force
there that has to be supported.
Since Va is for control movement, it's going to incur a increase in load due to G forces. This is an increase over and above the drag forces present at Va. So, Va is loading in several ways and Vno is only loading in one way.
Don't think so. The aircraft must be able to withstand prescribed gust
velocities up to Vno so there is some load factor consideration.
Dale L. Falk
Knowing What You Don’t Know about Va
Did it ever enter your mind that there is a major governmental miscalculation or omission in the computation of a V speed used by pilots and manufacturers alike to determine the structural survivability of an aircraft? I’m talking about the Va of your aircraft. It is written as maneuvering speed (Va)is the speed at or below which you can move the controls to their fullest extend without spindling bending or breaking your aircraft.
The truth took a major structural failure of an airliner to become public. Now we know that the Va is based upon all load force to a structure to be directed against the single strongest structure. Do you ever pull back on the yoke while initiating a power on stall while keeping the wings as level as you can. You do this while well below Va because Va is something to be avoided.
You also know that the lighter the aircraft is below gross allowable the lower the Va becomes What you do not know is that Va does not include the possibility that an additional twisting load may be applied at the same time. The combination of the two loads are accumulative and are not figured into the Va.
A climbing turn, a diving turn at the POH Va can easily exceed the actual structural capability of the aircraft. You have no way of knowing where the Va is anytime you combine the vector loads on your aircraft. As an instructor I intuitively slip at slow speeds with power off. I know that I am not supposed to turn around inside a thunderstorm regardless how turbulent.
The combination load of the turbulence and the bank is what tears aircraft apart. That the Va is a far more variable speed on the low side than we are apt to recognize. No where are we told or taught that the combination of loads in a maneuver like an Immelmann, lazy eight or chandelle have an inherent hazard if performed too abruptly at high speeds. I recall being taught to enter no higher than Va in most cases.
Never a word about the validity of the Va used. Now I find out that any rolling pull-up should be two-thirds of a single axis pull-up. Thanks to inherent caution pilots, manufacturers and government standards combine to give a limit load far in excess of 3.8g for normal aircraft. Limit load means no permanent deformation. Fifty percent beyond the limit load we are told that a permanent bend without a fracture is expected. The point of fracture is called the ultimate load.
These strictures apply to metal aircraft. Metals deform elastically and return to original shape, then plastically where the deformation remains afterwards, then they break. Composite structures behave differently and break explosively. Parts of an aircraft where a failure will destroy the aircraft are designed for one-hundred percent over expected structure failure limit. The best designs have multiple structures that must fail in sequence after the first failure. There is still a great deal to learn about composites and their structural limits. One-hundred overload capacity is usually built in. At this level weight becomes a problem so weight reduction adds unknown risk.
As with a fuel valve failure to shut off, the failure of a composite to fail may be worse than the failure itself. There can be no question but that the definition of Va needs a re-definition. Not knowing just where Va is means that any maneuver may be at risk if not in doubt.
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