Par 5-2 Page 63 roughly 1 knot/mph per second. Stall warning should occur about 5 knots/mph before the stall itself. Do not depend too much on an uncalibrated stall warning system.
(1) As the airplane decelerates, ensure that the airplane requires an increasingly heavier pull force. If the force lightens or changes to a push force, abandon the test. Once you get to the stall, initiate recovery and record the altitude required to return to level flight as well as the stall speed. Also make note of aircraft responses during the recovery such as wing rock, secondary stall or any uncommanded motions. All these observations need to be compared in your post- flight analysis to the expected behavior written down before the test.
NOTE: Some airplanes reach the up elevator stop before the wing stalls.
This is acceptable as long as the elevator has the authority to flare the aircraft at its maximum landing weight with the forward most CG allowed for that weight. Also, at the stall, many airplanes tend to roll toward one wing or the other. A properly designed and rigged airplane will be able to maintain the wings within 15 degrees of level with normal aileron inputs.
Make sure you’re not cross-controlling the airplane (the slip-skid ball is centered). If you need excessive aileron or rudder inputs to keep the airplane straight and the wings level, abort the test. On the ground assess the airplane for miss-rigging or some inadvertent wing twist or asymmetry.
f. Practice the same stall sequence several times at a ½-mph/knot-per-second deceleration rate to determine the power off, 1-G stall speed. Practice the same stall series with flaps, starting with the lowest setting first and working slowly to the full flap configuration.
Record the findings.
g. After exploring the stall and recovery behavior in a slow deceleration with ball centered, try a series of stalls with flaps up and flaps down, with a faster deceleration. Do not exceed the deceleration rate expected in normal operations.
5-3. STALLS.
a. Power-On Stalls. As before, use the same procedures moving from the known to the unknown. Increase power incrementally and run a stall test at each new power setting until full power is reached. We do not advise jumping straight from idle to full power with the resultant large changes in pitch attitude, torque reaction, and slipstream effect on the wing and tail.
b. Conducting Power-On Stalls. The FAA recommends that you stabilize the aircraft in level flight at low cruise power. The power-on stall is reached by slowly increasing the power to the desired power setting. The pilot then steadily increases the pitch attitude until the aircraft experiences the stall buffet. Remember to keep the ball centered until the onset of the stall buffet.
(1) The power-on stall may be more likely to cause a wing drop than one at idle. This is due to torque reaction and because the propeller slipstream tends to keep the flow of higher velocity air over the inboard, or root, section of the wing despite the higher angle of attack. This allows the root portion of the wing to continue flying after the wing tip stalls, dropping a wing.
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(2) Tip stalls usually do not give advance warning and will almost invariably result in severe wing drop. These stalls are more likely to result in a spin, even if the controls are not mishandled. If the spin does not develop, considerably more height will be lost in the recovery than if the stall had been straight-ahead nose down.
(3) If the pilot yields to instinct and tries to correct the wing drop with aileron, it could result in a spin. Since a sharp wing drop could be regarded as the onset of spin auto-rotation, the recommended corrective action is reducing power, full opposite rudder, and lowering the nose to the horizon or below. Take care to avoid this situation until the aircraft’s spin behavior has been tested.
(4) Perform the same sequence of events for power-on stalls as power-off stalls, unless limited by the designer’s instructions. Record all findings in a log if not automatically recording on an electronic PFD for the aircraft’s flight manual.
NOTE: Aircraft with retractable gear will have to go through a separate series of slow flight and stall checks with gear extended, with and without flaps. Record the different stall speeds for each configuration in the aircraft’s flight manual.
FIGURE 7. CLIMB AIRSPEED AND ALTITUDE GRAPH
c. Best Rate of Climb Speed Tests. To determine the best rate of climb for the aircraft, the FAA suggests the following procedures:
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(1) Perform the tests in smooth air, free from thermal activity. Select an altitude such as 1,000 feet AGL, as a BASE attitude. Use a heading 90 degrees to the wind, and for the best results, reverse the heading 180 degrees after each climb test.
(2) Begin a full throttle climb well below the predetermined BASE altitude and stabilize at a preselected airspeed approximately 15 mph/knots above the predicted best rate of climb speed. As the aircraft passes through the base altitude, begin a 1-minute time check.
(3) After 1 minute, record the altitude gained. Descend down below the base altitude.
Decrease the airspeed by 5 mph/knots and run the test again. After each succeeding test, the pilot should decrease the airspeed by 5 mph/knots until reaching an airspeed that is 10 mph/knots higher than the stall speed of the aircraft. Record the airspeed and altitude gained for each climb on a graph similar to Figure 7. The airspeed that shows the greatest gain in altitude is the aircraft’s best rate of climb speed (Vy).
FIGURE 8. BEST RATE OF CLIMB SPEED GRAPH
d. Best Angle of Climb Speed Tests. You can find the best angle of climb speed can by using the same chart developed for the best rate of climb tests. Draw a line (tangent) from the zero rate of climb feet per minute (see Figure 8) outward to a point on the rate of climb airspeed curve. The airspeed that the line intersects is the best rate of climb airspeed. The airspeed that the line intersects is the best angle of climb airspeed.
e. Slow Flight Test. For added safety, the slow flight tests should be performed at 6,000 AGL or higher to allow room for spin recovery. These tests primarily show the pilot the aircraft’s handling qualities at the minimum gear up/down airspeeds and power settings.
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(1) You should do the tests with and without flaps. Start the tests at airspeed of 1.3 times the stall speed of the aircraft. Once the aircraft is stabilized and maintaining its altitude, reduce the airspeed by 5 mph/knots. Maintain the altitude. Keep reducing the airspeed until approaching a stall.
(2) Maintain 5 mph/knots above the previously determined stall speed; this is the initial slow flight airspeed. Practice with each flap setting, noting its effect on the aircraft’s performance. If the aircraft has retractable gear, test in all gear and flap combinations. These tests will have to be run later in the flight-test program—but with the aircraft at gross weight—to determine the actual slow flight airspeed and stall speeds.
(3) Remember, to help reduce the possibility of unplanned stalls in slow flight configurations, avoid bank angles of more than 5 degrees. When all the test data has been evaluated, and if the aircraft is equipped with a stall warning horn or indicator, set the stall warning at 5 mph/knots above the aircraft’s highest stall speed.
SECTION 3. HOURS 21 THROUGH 35: STABILITY AND CONTROL CHECKS “A superior pilot uses his superior judgment to avoid those situations which require the use of superior skill.” Old Aviation Proverb
5-4. OBJECTIVE. Determine the aircraft’s stability limits and range of control.
5-5. GENERAL. You must do two things before declaring that the aircraft is controllable throughout the normal range of speeds per part 91, § 91.319, Aircraft Having Experimental Certificates: Operating Limitations.
a. First. Perform another complete inspection of the aircraft, including oil changes and fuel system filter checks.
b. Second. Carry out a close examination of the stability and control characteristics of the aircraft. Stability and control checks will be centered around the three axes of the aircraft:
longitudinal or roll axis (ailerons), the lateral or pitching axis (elevators), and the vertical or yaw axis (rudder).
c. All Tests Need a Starting Point. The starting point for stability and control checks is called the state of equilibrium. An aircraft is said to be in a state of equilibrium when it experiences no acceleration and remains in a steady trimmed condition until the force or moment balance is disturbed by an atmospheric irregularity or by pilot input.
Par 5-5 Page 67 FIGURE 9. STATIC STABILITY
5-6. DEFINITIONS.
a. Positive Static Stability. When an aircraft tends to return to the state of initial equilibrium position following a disturbance.
b. Neutral Static Stability. When an aircraft remains in equilibrium in a “new” position, following a disturbance from an initial equilibrium position.
c. Negative Static Stability. When an aircraft tends to move further in the same direction as the disturbance that moved it from the initial equilibrium position.
NOTE: See Figure 9 for a graphic of these three stability states.
Par 5-6 Page 68 FIGURE 10. TIME
d. Dynamic Stability. The time history of the movement of the aircraft in response to its static stability tendencies following an initial disturbance from equilibrium. Refer to Figure 10.
e. Test for Static Longitudinal Stability.
(1) This test should be done first. All tests should be conducted with the aircraft in the forward of center CG. Climb to at least 6,000 feet AGL and trim the aircraft for zero stick force in straight and level flight at low cruising speed.
NOTE: Do not retrim the aircraft once the test has begun.
(2) Apply a light “pull” force and stabilize at an airspeed about 10 percent less than the trimmed cruise speed. At this reduced airspeed, it should require a “pull” force to maintain the slower speed.
(a) If it requires a “pull” force, pull a little further back on the stick and stabilize the airspeed at about 20 percent below the initial cruise trim speed.
(b) If it requires a still greater “pull” force to maintain this lower airspeed, the aircraft has positive static longitudinal stability.
(c) If at either test points, no “pull” force is required to maintain the reduced airspeeds, the aircraft has neutral static longitudinal stability.
(d) If either of these test points requires a “push” force to maintain the reduced airspeed then the aircraft has negative static longitudinal stability.
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(3) Repeat another series of static longitudinal stability tests using a “push” force on the control stick. At an airspeed 10 percent above the trim cruise speed, the control stick should require a “push” force to maintain the airspeed. If a “pull” force is required, the aircraft has negative static longitudinal stability.
WARNING: If the aircraft exhibits negative static longitudinal stability, seek professional advice on correcting the problem before further flight.
(4) After confirming the aircraft has positive static longitudinal stability, the pilot can check for short-period positive dynamic longitudinal stability. First, trim the aircraft to fly straight and level at normal trim cruise speed. With a smooth, but fairly rapid motion, push the nose down a few degrees.
(a) Quickly reverse the input to nose up to bring the pitch attitude back to trim attitude. As the pitch attitude reaches trim attitude, release the stick but keep your hands close to it. The aircraft with positive dynamic longitudinal stability will oscillate briefly about the trim attitude before stopping at the trim attitude position.
(b) To test the aircraft for long-period positive dynamic longitudinal stability, begin from trimmed, straight and level flight. Without re-trimming, pull (or push) the stick to a speed about 5 mph/knots off trim and release the stick. You don’t have to stabilize at the new speed. Expect the aircraft to oscillate slowly about the trim airspeed a number of times before the motion dampens out. If there is significant friction in the control system, the aircraft may settle at a speed somewhat different from the original trim speed.
(c) If the amplitude increases with time, the dynamic longitudinal stability is negative or divergent. This is not necessarily dangerous as long as the rate of divergence is not too great. It does mean, however, the aircraft will be difficult to trim and will require frequent pilot attention.
(d) An aircraft with long-period neutral dynamic longitudinal stability will continue to oscillate through a series of increasing/decreasing airspeeds and never return to the original trim airspeed.
f. Lateral-directional Stability Control Tests. Lateral (dihedral effect) and directional stability tests show if the aircraft tends to raise the low wing in a sideslip once the ailerons are freed. They also determine if the rudder is effective in maintaining directional control.
CAUTION: This test may impose high flight loads on the aircraft. Do not exceed the design maneuvering speed or any other airspeed limitation.
(1) To check lateral and directional stability, the aircraft should be trimmed for level flight at a low cruise setting and an altitude above 5,000 feet AGL. Slowly enter a sideslip by maintaining the aircraft’s heading with rudder and ailerons. The aircraft should be able to hold a heading with rudder at a bank angle of 10 degrees or the bank angle appropriate for full rudder deflection. The control forces and deflection should increase steadily, although not necessarily in constant proportions with one another-in some cases, rudder forces may lighten until either the rudder or the ailerons reach full deflection or the maximum sideslip angle is reached.
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(2) At no time should there be a tendency toward a force reversal, which could lead to an overbalance condition or a rudder lock.
(3) Release the ailerons while still holding full rudder. When the ailerons are released, the low wing should return to the level position. Do not assist the ailerons during this evaluation.
(4) To check static directional stability, trim the aircraft at a low cruise setting above 5,000 feet AGL. Slowly yaw the aircraft left and right using the rudder. Simultaneously, the wings should be kept level by using the ailerons. When the rudder is released, the aircraft should tend to return to straight flight.
g. Spiral Stability. This is determined by the aircraft’s tendency to raise the low wing when the controls are released in a bank. To test for spiral stability, apply 15 to 20 degrees of bank either to the left or right, and release the controls. If the bank angle decreases, the spiral stability is positive. If the bank angle stays the same, the spiral stability is neutral. If the bank angle increases, the spiral stability is negative. Negative spiral stability is not necessarily dangerous, but the rate of divergence should not be too great or the aircraft will require frequent pilot attention and will be difficult to fly, especially on instruments.
NOTE: Friction in the aileron control system can completely mask the inherent spiral characteristics of the airframe.
SECTION 4. A WORD OR TWO ABOUT FLUTTER “Stay up on the edge of your seat.” Scott Crossfield, Test Pilot
5-7. OBJECTIVE. Understand the causes and cures of the condition known as flutter.
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5-8. DESCRIPTION. Flutter in an aircraft structure is an interaction between aerodynamic inputs, the elastic properties of the structure, the mass or weight distribution of the various elements, and airspeed.
a. To most people, the word “flutter” suggests a flag’s movement as the wind blows across it. In a light breeze, the flag waves gently but as the wind speed increases the flag’s motion becomes more and more excited. It takes little imagination to realize if something similar happened to an aircraft structure, the effects would be catastrophic. The parallel to a flag is appropriate.
b. Think of a primary surface—a wing—with a control, such as an aileron, hinged to it.
Imagine that the airplane hits a thermal. The initial response of the wing is to bend upwards relative to the fuselage.
c. If the center of mass of the aileron is not exactly on the hinge line, it will tend to lag behind the wing as it bends upwards.
d. In a simple, unbalanced, flap-type hinged control, the center of mass will be behind the hinge line and the inertial lag will result in the aileron being deflected downwards. This will result in the wing momentarily generating more lift, increasing its upward bending moment and its velocity relative to the fuselage. The inertia of the wing will carry it upwards beyond its equilibrium position to a point where more energy is stored in the deformed structure than can be opposed by the aerodynamic forces acting on it.
e. The wing “bounces back” and starts to move downward but, as before, the aileron lags behind and is deflected upwards this time. This adds to the aerodynamic down force on the wing, once more driving it beyond its equilibrium position and the cycle repeats.
f. Flutter can happen at any speed, including takeoff speed. At low airspeeds, however, structural and aerodynamic damping quickly suppresses the flutter motion. But as the airspeed increases, so do the aerodynamic driving forces generated by the aileron. When they are large enough to cancel the damping, the motion becomes continuous.
g. Further SMALL INCREASES will produce a divergent or increasing oscillation, which can quickly exceed the structural limits of the airframe. Even when flutter is on the verge of becoming catastrophic it can still be very hard to detect. What causes this is the high frequency of the oscillation, typically between 5 and 20 Hz (cycles per second). It will take but a small increase in speed (¼-knot or less) to remove what little damping remains and the motion will become divergent rapidly.
h. Flutter also can occur on a smaller scale if the main control surface has a control tab on it. The mechanics are the same with the tab taking the place of the aileron and the aileron taking the place of the wing. The biggest differences are the masses involved are much smaller, the frequencies much higher, and there is less feedback through the control system. This makes tab flutter more difficult to detect. The phenomenon known as “buzz” is often caused by tab flutter. Since flutter is more prevalent at higher speeds, it is not recommended that the flight-test plan call for high speed runs within 10 percent of red line.
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i. What can be done about it? Having described how flutter happens, the following suggestions should help reduce the possibility of it happening to the amateur-builder’s aircraft:
(1) Perform a mass balance of all flight controls in accordance with the designer/kit manufacturer’s instructions.
(2) Eliminate all control “free play” by reducing slop in rod end bearings, hinges, and every nut and bolt used in attaching flight controls.
(3) Ensure that all rigging and cable tension is set accurately to the design specifications using a calibrated cable tensiometer.
(4) Re-balance any flight control if it has been repaired, repainted, or modified in any way.
NOTE: If the pilot experiences flutter, or believes he did, reduce power immediately, and land as soon as possible. Do not attempt further flight until the aircraft has been thoroughly inspected for flutter-induced damage. This inspection should include all wing/tail attach points, flight controls, their attach points/hinges, hardware, control rods, and control rod bearings for elongated bolt/rivet holes, cracks, (especially rod end bearings) and sheared rivets.
SECTION 5. SPINS “Go from the known to the unknown -slowly!” Chris Wheal, Military Test Pilot
5-9. OBJECTIVE. To determine if spin testing is required.
Par 5-9 Page 73 NOTE: All FAA spin tests for type certification require a spin parachute attached to the aircraft. Even though amateur-built aircraft have no such certification requirement, use of a spin parachute during testing should be considered.
5-10. CAUTION.
a. If the manufacturer/designer of the aircraft has not demonstrated satisfactory spin characteristics and safe recovery, avoid all types of high angle of attack flight testing and placard the aircraft: “spins prohibited.”
b. If the prototype aircraft has satisfactorily demonstrated spin recovery and the builder’s aircraft is identical to the prototype aircraft, the pilot may confirm the aircraft will recover promptly from inadvertent spin entries. Further tests to prove that the aircraft will recover from a fully developed spin (three turns or more) are not necessary unless the aircraft is designed for, and will be routinely flown in, aerobatic flight.
c. During all spin tests, it is strongly recommended that the pilot wear a parachute and that a quick release mechanism to jettison the canopy or door be installed. If the pilot is unable to exit the aircraft because of the design restraints, it is recommended that intentional spins not be conducted even though the design has successfully demonstrated spin recovery.
d. If any modifications or alterations have been made to the airframe’s original design or configuration (e.g., adding tip tanks or fairings), it is not safe to assume that the aircraft still has the same spin recovery characteristics as the prototype aircraft. Spins in a modified aircraft should not be attempted without consulting a qualified test pilot and/or flight-test engineer.
e. The pilot who conducts the spin tests should have experience in entry into and recovery from fully developed spins, preferably in makes and models similar to the aircraft being tested. If the pilot needs additional experience, aerobatic training with an emphasis on spins from a qualified instructor is highly recommended.
5-11. PLANNING THE FLIGHT. At this point, nearly all the preparatory work for spin testing has been accomplished. Planning the next flight should be identical to planning for the first flight through stalls. IT IS EXTREMELY IMPORTANT THAT THE CENTER OF GRAVITY OF
THE AIRCRAFT IS AT THE FORWARD CG LIMIT AND ANY BALLAST USED SHOULD BE SECURELY ATTACHED TO THE AIRCRAFT.
a. The aircraft should be tested with landing gear (if applicable) and flaps in the up position. The pilot’s minimum entry altitude for these tests should be no less than 10,000 feet AGL with the cockpit secured.
NOTE: The following procedure is one way, but not the only way, of conducting a spin test and executing a recovery. Non-conventional aircraft may require significantly different spin recovery control applications. The pilot should evaluate these procedures and determine if they are compatible with the aircraft before attempting any spin testing.
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b. The basic technique used to get a clean spin entry is to continue to reduce airspeed at about a 1-mph/knot-per-second rate in level flight, carburetor heat on, and the power at idle.
(1) As the aircraft stalls, APPLY FULL RUDDER in the desired spin direction, followed immediately by full aft movement of the control stick keeping the ailerons neutral.
(2) The transition from a horizontal to a vertical flight path takes approximately three or four turns and is referred to as the incipient stage of the spin.
(3) During the incipient spin, the dynamic and inertia forces have not achieved equilibrium. Many aircraft can recover from the incipient spin phase, but may not be able to recover from a steady spin.
(4) The normal spin recovery technique is to apply full rudder opposite to the direction of yaw (check the turn needle). Move the control stick smoothly and fairly rapidly forward towards the instrument panel until the rotation stops.
(5) Quickly center the rudder and ease out of the dive. Do not attempt to pull up too rapidly because the structural limits of the aircraft can easily be exceeded, or the aircraft can stall again. Recover from the first deliberate spin after a half a turn.
(6) If the aircraft is not built for aerobatics, no further spin testing is required. It is recommended the instrument panel be placarded “SPINS PROHIBITED.”
(7) If further spin testing is required, it is strongly recommended the services of a professional flight-test pilot be used.
SECTION 6. ACCELERATED STALLS “Does it pass the Common Sense test?” U.S. Air Force, Thunderbird
5-12. OBJECTIVE. To further explore the stall characteristics of the aircraft.
a. An accelerated stall is not a stall reached after a rapid deceleration. It is an in-flight stall at more than 1 G, similar to what is experienced in a steep turn or a pull up.
NOTE: Do not attempt this or any other extreme maneuver unless the designer or kit manufacturer has performed similar tests on a prototype aircraft identical to the amateur-builder’s aircraft.
b. The two standard methods for accelerated stalls are the constant G (constant bank) and constant speed (increasing bank). Most preferred of the two is the constant bank method in which the airspeed is decreased and the angle of bank is held constant, until the aircraft stalls. It is the most preferred because the potential violence of any accelerated stall is largely governed by the increasing g load and airspeed.
c. Performing recommended constant bank method. First, review the techniques described for wing level stall tests card and apply them here as well. If you are not spin-qualified