42 5/24/95AC 90-89A
b. Climbs and descents to preselected altitudes.
(monitor engine performance)
c. Airspeed indicator in-flight accuracy check NOTE: After each test flight, ALL DISCREPANCIES must be cleared before the next flight. The aircraft also must be THOROUGHLY INSPECTED prior to the next flight.
NOTE: It is recommended that all flight test maneuvers be preceded with two 90 degree clearing turns to ensure that the flight test area is free of other aircraft.
3. GEAR RETRACTION.
a. Before the gear is retracted in flight for the first time, it is advisable to put the aircraft up on jacks and perform several gear retraction tests, including the emergency gear extension test. These tests will determine if, in the last three hours of flight testing, any structural deformation or systems malfunctions have occurred. In addition to the gear retraction test, the pilot/chase pilot/ground crew should use this time to review the aircraft’s kit/ designer instructions and emergency checklist procedures for malfunctioning gear and plan accordingly.
If at any time the aircraft has suffered a hard landing or side loading on the gear during flight testing, the aircraft and its gear should be tested for operation and condition on the ground.
b. The first gear retraction test should be conducted with the aircraft flying straight and level at or above 5,000 feet AGL, over an airport or emergency field. The airspeed must be well under the maximum landing gear retraction airspeed. When the gear is being retracted, note if there is any tendency for the aircraft to yaw, pitch, or roll. Record what changes to the aircraft’s trim are required to maintain straight and level flight. If there are no adverse flight reactions or system malfunctions, cycle the gear several times. When satisfied with the straight and level gear retraction test, try an emergency gear extension but only if this is practical.
c. With the gear extended, slow the aircraft to 1.3 times the pre-determined stall speed, stabilize, lower the flaps to the take-off position, trim, and maintain straight and level flight.
d. Simulate a normal takeoff by increasing rpm to full power. Raise the nose 3 degrees, trim, and then retract the gear. Observe the following: aircraft reaction, such as pitch or roll; length of time for gear to retract; trim requirements;, and the time necessary to establish a 1,000-foot climb before leveling off.
e. Practice a simulated takeoff several times to ensure that the aircraft’s response is predictable and the gear retraction system is mechanically reliable.
4. CLIMBS AND DESCENTS. The purpose of these tests is to monitor engine performance and reliability. The pilot should start the test only after the aircraft has been flying straight and level for a minimum of 10 minutes to stabilize engine oil pressure and temperatures.
a. Engine oil pressure and temperatures must be kept within the manufacturer’s limits at all times during these tests. High summer temperatures may place restrictions on the flight test program because both oil and cylinder head temperatures will increase 1 degree for each 1 degree increase in outside temperature.
(1) Climbs. Start the first climb at a 15 degree climb angle, full power, at a predetermined designated altitude (e.g., 1,000 feet). Maintain the climb angle for 1 minute. Record the engine temperatures and pressures. Reduce power, stabilize the engine temperature, and repeat the test. For the second climb test, the Flight Test Plan should call for increasing the climb time -- record the results. When satisfied that an engine cooling problem does not exist at this climb angle, repeat the tests using steeper climb angles until the pilot has reached 15 degrees or encountered an engine manufacturer’s limit or a 5-minute climb period at full throttle has been reached.
(2) Descents. Should begin above 5,000 feet AGL with both the engine temperatures and pressures stabilized.
(i) The test pilot should use carb heat and clear the airspace below him before starting the descent. The first descent should be at a shallow angle, at low rpm and last for 30 seconds, not exceeding 1.5 times the estimated stall speed of the aircraft. During long, low power descents, the pilot must be on the alert for too rapid cooling of the engine usually identified by a signifi-
43 cant drop in oil and CHT temperature. If a noticeable drop occurs, increase the engine rpm and reduce the angle of descent. If not corrected, the repeated rapid cooling of the engine may cause thermal shock to the engine cylinders and eventually cause cylinder head cracking or seizure.
(ii) Conduct each test as before, but increase the time by 30 seconds until limited by the engine manufacturer’s restrictions or 5-minute descents are reached. Record temperatures, pressures, altitudes, and airspeeds data for climbs and descents for addition into the aircraft’s flight manual.
5. AIRSPEED IN-FLIGHT ACCURACY CHECK. The following procedure for airspeed calibration is offered for evaluation:
a. A measured course should be chosen with readily identifiable landmarks at each end. The landmarks should be a known distance apart, and the length of course should be at least 1 to 2 miles long.
b. The pilot must fly a precision course maintaining a constant altitude (e.g., 1,000 feet), constant airspeed, constant magnetic heading, and constant engine rpm. The pilot must record the temperature, altitude, indicated airspeed and the time over each landmark for both directions. The average of these speeds is the ground speed of the aircraft. An E6B computer will convert the temperature, altitude, and ground speed into True Indicated Airspeed for the tests.
NOTE: The difference between the E6B computer readings and the aircraft’s ground speed readings is the error in the instrument and the error caused by the installation of the system in the aircraft.
c. The airspeed calibrations runs should be made several times in opposite headings for each of the selected airspeeds the pilot wants to check.
Such accuracy test runs should start at the lowest safe airspeed and work up to cruise speed using 10 mph/knot increments.
d. Most errors will be found at the low end of the speed range due to the angle of the pitot mast to the relative wind and/or the location of the static ports. Recently, amateur-builders have been using Global Positioning Satellite (GPS) hand held receivers to check airspeed accuracy.
NOTE: Flight testing of all amateur-built aircraft is restricted to a flight test area. If a pilot must run additional tests on the aircraft that require more airspace, he should notify the FAA District Office that issued the aircraft’s operating limitations and request a change to those limitations. If a pilot is found to be operating an EXPERI- MENTAL AIRCRAFT in violation of the aircraft’s Operating Limitations, the FAA may take certificate action.
e. If the aircraft has retractable gear or flaps, test the accuracy of the airspeed indicator with the gear/flaps up and down.
f. Record all the data in order to prepare an airspeed calibration table for the flight manual.
44 5/24/95AC 90-89A
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45 CHAPTER 5. EXPANDING THE ENVELOPE ‘‘Checklist! Checklist!! Checklist!!!’’ Jim Byers, Flight Instructor/Examiner SECTION 1. GENERAL
1. OBJECTIVE. To move from a known flight environment to an unknown flight environment using a series of planned and carefully executed steps.
a. Before beginning the next series of test flights, it is highly recommended that the aircraft undergo a ‘‘Condition Annual’’ inspection as identified in the FAA Operation Limitation the amateurbuilder received with the special airworthiness certificate. It is strongly recommended that the builder and/or pilot TAKE THE TIME to inspect the aircraft because within the previous 10 hours, the aircraft has been subjected to what can be referred to as a ‘‘shakedown cruise.’’
b. 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 in addition to completing 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.
c. Even if there have been no indications of CO contamination, perform another carbon monoxide (CO) test using the floodlight procedure (see chapter 1, 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 ‘‘Fly Scared!’’ Admiral Jack Ready, U.S.N.
1. OBJECTIVE. To focus the next 10 hours of flight testing on the following: stall speed, best rate of climb speed, best angle of climb speed, and slow flight. It is recommended that stall speed tests be conducted with the aircraft’s fuel tanks full. (CG).
a. As with any unknown, approach slowly, incrementally, and follow the FLIGHT TEST PLAN.
To improve safety and reduce the possibility of spins, the aircraft should be tested with a forward CG loading. Start the stall tests at 6,000 AGL. 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: Do not trim within 10 knots of stall.)
NOTE: 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.
b. 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.
c. 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. This gives an early warning to the pilot in the form of the buffet from separated airflow over the wings and or tail. The ailerons will continue to operate in the attached air flow until the aircraft’s stall speed is reached and the wing stalls.
d. Begin by using the same procedures employed on the first flight. Secure cockpit items and put on carburetor heat. Decelerate slowly at 1⁄2 MPH/knot a second. Make small control inputs, keep the ball centered, and note the aircraft’s reaction.
46 5/24/95AC 90-89A
e. Let the aircraft stall and recover immediately, with stick forward and increasing power.
Note the stall speed.
f. Practice the same stall sequence several times at 1⁄2 mph/knot speed deceleration rate to determine the power-off, one 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 the ball in the middle, try a series of stalls with flaps up and then flaps down with a faster rate of deceleration. Do not exceed the deceleration rate expected in normal operations.
2. 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.
It is not advisable to jump straight from idle to full power with the resultant large changes in pitch attitude, torque reaction, and slip stream effect on the wing and tail.
b. Conducting Power on Stalls. It is recommended that the aircraft be stabilized 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 in the center 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 (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.
(2) Tip stalls usually do not give advance warning and will almost invariably result in some 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 to reduce power, exercise prompt application of full opposite rudder combined with 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 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.
47 FIGURE 6. Climb Airspeed and Altitude Graph
c. Best Rate of Climb Speed Tests. To determine the best rate of climb for the aircraft, the following procedures are suggested:
(1) Perform the tests in smooth air, free from thermal activity. Select an altitude (e.g., 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 one minute time check. At the end of 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.
(3) The airspeed that shows the greatest gain in altitude is the aircraft’s best rate of climb speed (Vy).
48 5/24/95AC 90-89A FIGURE 7. Best Rate of Climb Speed Graph
d. Best Angle of Climb Speed Tests.
(1) Best angle of climb speed can be found 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 4) outward to a point, on the rate of climb airspeed curve. Where both lines touch, draw a line straight down to the airspeed leg of the chart.
(2) The airspeed that the line intersects is the best angle of climb airspeed.
e. Slow Flight Test.
(1) For added safety, the slow flight tests should be performed at 6,000 AGL or higher to allow room for spin recovery. THE PRIMARY PURPOSE
OF THESE TESTS IS FOR THE PILOT TO BECOME FAMILIAR WITH THE AIRCRAFT’S
HANDLING QUALITIES AT THE MINIMUM
GEAR UP/DOWN AIRSPEEDS AND POWER SETTINGS.
(2) The tests should be done with and without flaps. Start the tests at an airspeed of 1.3 times (X) 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.
(3) Maintain 5 mph/knots above the previously determined stall speed. This figure is the initial slow flight airspeed. Practice with each flap setting, noting its affect 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 AIR- CRAFT AT GROSS WEIGHT to determine the actual slow flight airspeed and stall speeds.
(4) 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.
49 SECTION 3. HOURS 21 THROUGH 35: STABILITY AND CONTROL CHECKS ‘‘A superior pilot uses his superior judgement to avoid those situations which require the use of superior skill.’’ Old Aviation Proverb
1. OBJECTIVE. To determine the aircraft’s stability limits and range of control.
2. GENERAL. Before attempting to satisfy the requirements of Federal Aviation Regulations § 91.319 Aircraft Having Experimental Certificates:
Operating Limitations and declaring that the aircraft is controllable throughout the normal range of speeds, two things must be done.
a. Perform another complete inspection of the aircraft, including oil changes and fuel system filter checks.
b. 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.
FIGURE 8. Static Stability
3. DEFINITIONS.
a. Static Stability: (positive) is when an aircraft tends to return to the state of initial equilibrium position following a disturbance.
b. Static Stability: (neutral) is when an aircraft remains in equilibrium in a ‘‘new’’ position, following a disturbance from an initial equilibrium position.
c. Static Stability: (negative) is when an aircraft tends to move further in the same direction as the disturbance that moved it from the initial equilibrium position (figure 8).
50 5/24/95AC 90-89A FIGURE 9. Time
d. Dynamic Stability: is the time history of the movement of the aircraft in response to its static stability tendencies following an initial disturbance from equilibrium (figure 9).
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.) 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.
(i) If it requires a ‘‘pull’’ force, pull a little further back on the stick and stabilize the airspeed at approximately 20 percent below the initial cruise trim speed.
(ii) If it requires a still greater ‘‘pull’’ force to maintain this lower airspeed, the aircraft has
POSITIVE STATIC LONGITUDINAL STABIL- ITY.
(iii) If at either test points, no ‘‘pull’’ force is required to maintain the reduced airspeeds, the aircraft has NEUTRAL STATIC LONGITUDINAL STABILITY.
(iv) If either of these test points require a ‘‘push’’ force to maintain the reduced airspeed then the aircraft has NEGATIVE STATIC LONGITUDINAL STABILITY.
(2) 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.
(3) After confirming the aircraft has positive STATIC longitudinal stability, the pilot can check for positive DYNAMIC longitudinal stability (short period). 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.
(4) 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 guard it). The aircraft with positive dynamic longitudinal stability will oscillate briefly about the trim attitude before stopping at the trim attitude position.
(5) To test the aircraft for positive DYNAMIC longitudinal stability (long period), 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.
There is no need 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) 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.
(7) An aircraft with ‘‘NEUTRAL’’ dynamic longitudinal stability (long period) 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 are to determine if the aircraft can demonstrate a tendency 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.
(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 AFL. 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.
52 5/24/95AC 90-89A SECTION 4. A WORD OR TWO ABOUT FLUTTER ‘‘Stay up on the edge of your seat.’’ Scott Crossfield, Test Pilot
1. OBJECTIVE. To understand the causes and cures of the condition known as flutter.
2. DESCRIPTION. Flutter in an aircraft structure is the result of 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 flags 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 with a control hinged to it (e.g., an aileron). 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 take-off speed. At low airspeeds, however, structural
53 and aerodynamic damping quickly suppress 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 ( 1⁄4 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 difference are the masses involved are much smaller, the frequencies much higher, and there is less feed-back 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.
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.