6-1 CHAPTER 6. EXPANDING THE ENVELOPE SECTION 1. GENERAL
6.1 Objective. Move from a known flight environment to an unknown flight environment using a series of planned and carefully executed steps.
6.1.1 Before beginning the next series of test flights, the FAA highly recommends that the aircraft undergo the same condition inspection that would normally only be done annually at this time. The FAA strongly recommends that the builder and/or pilot take the time to inspect the aircraft because within the previous 10 hours, the test pilot has subjected the aircraft to a “shakedown cruise.”
6.1.2 During the inspection, check the torque paint marks on the engine mounts, propeller bolts, and landing gear. Double check the flight control hinges and rod end bearings for attachment and play. Check all cable installations, cable tension, and control travel, and complete all the standard inspection and maintenance items. This inspection also should include checking the oil and fuel filters for metal or other forms of contamination.
6.1.3 Even if there have been no indications of CO contamination, perform another CO test using the floodlight procedure—review Chapter 2, Section 7—or an industrial CO test meter. There is a strong possibility that operational vibration and landing stresses may have opened new paths for CO to enter the cockpit.
SECTION 2. HOURS 11 THROUGH 20
6.2 O bjective. Focus the next 10 hours of flight testing on stall speed, best rate of climb speed, best angle of climb speed, and slow flight. The FAA recommends that stall speed tests be conducted with the aircraft’s fuel tanks full.
Note: Stall speeds, best rate of climb, best angle of climb, best glide speed, and various slow flight maneuvers are best and most safely performed with the aid of an AOAI. The installation, calibration and proper use of such a device is highly recommended.
6.2.1 As with any unknown, approach flight testing slowly, incrementally, and follow the flight test plan. The stall test objective is to verify the aircraft conforms to the expected responses for this particular design in this flight regime. Acquire a thorough description of how the aircraft is expected to behave near, at, and after stall. This provides a reference point to compare against the flight test results. To improve safety and reduce the possibility of spins, the aircraft should be tested with a forward CG loading. Starting the stall tests at 8,000 feet above ground level (AGL) is good for most homebuilts, but it depends on the aircraft type. Make clearing turns and stabilize the airspeed and altitude.
The first full stall should be conducted with power off, no flaps, and gear up if applicable.
After clearing the area, reduce the airspeed to 1.3 times the predicted stall speed and trim.
Note 1: Make sure the engine’s idle speed is properly set; faster idle speeds result in higher nose-up attitudes and lower indicated stall speeds.
6-2 Note 2: Don’t trim within 10 knots of stall.
Note 3: Some clean, high performance aircraft may not have any noticeable pre-stall buffet. The actual stall may be abrupt and violent with a large amount of wing or nose drop.
6.2.2 The preferred pre-stall and stall behavior is an unmistakable warning buffet starting lightly about 5 to 10 mph/knots above the eventual stall speed, growing in intensity as the aircraft slows down.
6.2.3 The desired stall characteristics should be a straight forward nose drop with no tendency for roll or pitch up. This docile and forgiving behavior implies a stall that has started at the wing root and progressed smoothly outboard. The buffet warns the pilot about separated airflow over the wings and or tail in the form of the buffet. The ailerons will continue to operate in the attached air flow until the aircraft’s stall speed is reached and the wing stalls.
6.2.4 Use the same procedures as on the first flight. Secure cockpit items and use carb heat.
Begin by stabilizing in level flight at 15–20 knots above the predicted stall speed. When stable, make a control input in each axis; roll, pitch and yaw. The inputs should be just enough to generate about 3–5 degrees of aircraft response, then move the controls right back to neutral (usually a 1–2 second input pulse).
6.2.4.1 Then watch for the aircraft response. In roll, is there any accompanying adverse yaw? Does the aircraft stop rolling when the input is released to neutral? In pitch, does the aircraft return to the previous attitude? Is there any tendency for the pitch to continue to rise after releasing the stick? Does the pitch attitude continue to oscillate? The same questions apply in the yaw axis.
If all is as expected, slow 3–5 knots and repeat the process.
6.2.4.2 At each incremental airspeed point ensure that you still have nose down control authority, since that is your most important recovery input. Make note of any changes in response as the aircraft gets slower (it is normal for the aircraft to be more sluggish, but it should still respond positively). Make note of any warning cues. Watch closely for any uncommanded motions. At the first sign of any uncommanded motion (nose rise, or nose slice, wing rock, or nose drop), recover the aircraft by lowering the nose, adding power and increasing speed. Uncommanded motions are likely linked to being at a higher AOA than previously flown. The most correct initial response is to lower that AOA with a nose down input. Even if the uncommanded motion is a roll off, the quickest way to stop it is most likely a pitch down, not a countering roll input. After the airspeed increases and the aircraft still continues to roll, use opposite rudder to stop it with ailerons neutralized.
6.2.5 If there are no surprises down to 3–5 knots above expected stall speed, then continue to a complete stall by applying backpressure on the stick/yoke to slow the airplane down at
6-3 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.
6.2.5.1 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 misrigging or some inadvertent wing twist or asymmetry.
6.2.6 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.
6.2.7 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.
6.3 Stalls.
6.3.1 Po wer -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.
6.3.2 Conducting Power-O n 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.
6.3.2.1 T he 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
6-4 keep the flow of higher velocity air over the inboard, or root, section of the wing despite the higher AOA. This allows the root portion of the wing to continue flying after the wing tip stalls, dropping a wing.
6.3.2.2 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.
6.3.2.3 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.
6.3.2.4 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 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 AOH.
Figure 6-1. Climb Airspeed and Altitude Graph
6-5
6.3.3 B est Rate of Climb Speed Tests. To determine the best rate of climb for the aircraft, the FAA suggests the following procedures:
6.3.3.1 P erform 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.
6.3.3.2 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.
6.3.3.3 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 6-1. The airspeed that shows the greatest gain in altitude is the aircraft’s best rate of climb speed (VY).
Figure 6- 2. Best Rate of Climb Speed Graph
6-6
6.3.4 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 6-2) outward to a point on the rate of climb airspeed curve. The airspeed that the line intersects is the best angle of climb airspeed.
6.3.5 S low 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.
6.3.5.1 You should do the tests with and without flaps. Start the tests at an 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.
6.3.5.2 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.
6.3.5.3 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
6.4 Objective. Determine the aircraft’s stability limits and range of control.
6.5 General. You should 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.
6.5.1 F irst. Perform another complete inspection of the aircraft, including oil changes and fuel system filter checks.
6.5.2 S econd. 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).
6.5.3 A ll 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
6-7 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.
Figure 6-3. Static Stability
6.6 Definitions.
6.6.1 P ositive Static Stability. When an aircraft tends to return to the state of initial equilibrium position following a disturbance.
6.6.2 Neutral Static Stability. When an aircraft remains in equilibrium in a “new” position, following a disturbance from an initial equilibrium position.
6.6.3 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 6-3 for a g raphic of these three stability states.
6-8 Figure 6-4. Time
6.6.4 Dynamic Stability. The time history of the movement of the aircraft in response to its static stability tendencies following an initial disturbance from equilibrium. See Figure 6-4.
6.6.5 Test for Static Longitudinal Stability.
6.6.5.1 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.
6.6.5.2 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.
1. 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.
2. If it requires a still greater “pull” force to maintain this lower airspeed, the aircraft has positive static longitudinal stability.
3. If at either test points, no “pull” force is required to maintain the reduced airspeeds, the aircraft has neutral static longitudinal stability.
4. If either of these test points requires a “push” force to maintain the reduced airspeed then the aircraft has negative static longitudinal stability.
6-9
6.6.5.3 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.
6.6.5.4 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.
6.6.5.4.1 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.
6.6.5.4.2 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.
6.6.5.4.3 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.
6.6.5.4.4 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.
6.6.6 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 (VA) or any other airspeed limitation.
6.6.6.1 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
6-10 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.
6.6.6.2 At no time should there be a tendency toward a force reversal, which could lead to an overbalance condition or a rudder lock.
6.6.6.3 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.
6.6.6.4 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.
6.6.7 S piral 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: F riction in the aileron control system can completely mask th e inherent spiral characteristics of the airframe.
6-11 SECTION 4. A WORD OR TWO ABOUT FLUTTER
6.7 Objective. Understand the causes and cures of the condition known as flutter.
6.8 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.
6.8.1 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.
6.8.2 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.
6.8.3 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.
6.8.4 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.
6-12
6.8.5 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.
6.8.6 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.
6.8.7 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.
6.8.8 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.
6.8.9 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 check 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 they believe they 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.