Contents
The Aircraft You
Fly Is Showing Its Age; …Pitot-Static
System; …Airspeed
indicator; ...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; ...Touchdown Speeds; …Airspeed
and slips; …Va; …The Performance Envelope; …Structural
Speed Limits; …Best Glide by
Weight; … Speeds and Density
Altitude; …
Breguet Range Equation; …
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.
--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.
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.
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 become s 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.
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.
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 airplane’s 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.
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.
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.
Indicated airspeed:
(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.
Indicated airspeed:
(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.
Calibrated airspeed:
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.
True airspeed:
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.
Ground Speed:
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. 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 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.
Vlof Lift-off speed
Vso is the stall speed in landing configuration. This speed is for level, unaccelerated, 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.
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, unaccelerated, 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 is the maximum landing-gear operating speed.
Vle is the maximum landing-gear extension speed and
is based upon the durability of the landing gear doors.
Vlo Maximum landing gear operating speed
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
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 unstick speed will get you off the ground in ground effect but not allow climb.
UncommonV-speeds
Vmu Is the minimum unstick speed as when first lift off in a soft-field
takeoff.
Vd Design diving speed
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
Vlof
Vfc
V-n
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
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.
Vww means maximum windshield wiper operating speed.
Vll means maximum landing light extension speed. For most V-speeds, consult FAR Part 1.2
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 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 checklisted 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 glide
Slow cruise
Cruise
Cruise climb
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 aircraft’s 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 aircraft’s present capability and make the aircraft perform to that level.
The sum of an aircraft’s 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.
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° 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.
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.
Stall speed
Inverse effect on Va in turbulence
Absolute Altitude.
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 aircraft’s 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.
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 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 cruse 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-pretakeoff
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 pretakeoff 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.
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.
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.
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.
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
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.
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. I 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
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.
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. It how you teach 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.
Certification Speeds
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.
Opinion
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 no 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
Don:
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.
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