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.
6-13 SECTION 5. SPINS
6.9 Objective. To determine if spin testing is required.
6.10 Caution.
6.10.1 If the manufacturer/designer of the aircraft has not demonstrated satisfactory spin characteristics and safe recovery, avoid all types of high AOA flight testing and placard the aircraft: “spins prohibited.”
6.10.2 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.
6.10.3 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.
6.10.4 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.
6-14
6.10.5 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.
6.11 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 (CG) OF THE AIRCRAFT IS AT THE FORWARD CG LIMIT AND ANY BALLAST USED SHOULD BE SECURELY ATTACHED TO THE AIRCRAFT.
6.11.1 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.
6.11.2 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, carb 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.
6-1 SECTION 6. ACCELERATED STALLS
6.12 Objective. To further explore the stall characteristics of the aircraft.
6.12.1 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.
6.12.2 The two standard methods for performing 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.
6.12.3 Performing recommended constant bank method. First, review the techniques described for wing level stall tests on the flight test card and apply them here as well. If you are not spin-qualified and comfortable, hire a professional test pilot for these tests. Start with a light aircraft weight and forward CG. The amount of fuel in the tanks while performing the stall test should be enough fuel for 1 hour minimum. Follow the procedure below:
1. Normal takeoff and climb to 8,000 feet AGL.
2. Verify engine readings in the green.
3. Trim airplane to 1.5 times estimated stall speed.
4. Set the flaps (and landing gear) appropriate to test.
5. Power to idle (carb heat if applicable).
6. Establish—and maintain—a coordinated 30-degree bank turn.
7. Decelerate at 1 knot/mph per second.
8. Note positive stick/yoke force: it should increase.
9. Note control reversal: abort if evident.
10. Note speed at pre-stall buffet.
11. Recover, climb to starting altitude, and continue series until complete.
12. At stall, note indicated airspeed, pitch changes, roll direction, and amount.
13. Recover, climb to starting altitude, and continue series until complete.
14. You should have a detailed description of the expected behavior in an accelerated stall available (either from the kit manufacturer or through design analysis) prior to beginning the test. The objective is to verify (or not) that the behavior of your aircraft matches the expected behavior.
6-2
6.12.4 Make separate test flights to evaluate the airplane’s performance at different weights and CG locations. Start with lighter gross weight and forward CG locations. Each test series includes six runs, left and right coordinated turns with the indicated flap settings. If your airplane has retractable gear, run the series with the gear up and down.
6.12.4.1 For each stall test fly the aircraft at a safe altitude (8,000 feet AGL or as appropriate for aircraft type, in smooth air), trim the airplane to 1.5 times the predicted stall speed, and set the flaps as required for the test (this should be slower than maximum flap-extended speed (VFE)).
6.12.4.2 Apply carb heat (if required), pull the power to idle, and establish a coordinated turn in a 30-degree bank; ensure the slip/skid ball is centered.
Reduce speed to 10 knots/mph above the straight and level stall speed for that flap setting. Then decelerate at 1 knot/mph per second.
1. As the airplane slows, make sure it requires an increasingly greater stick/yoke pull force. If the force lightens or changes to a push force— abort the test. This may indicate an aft CG location or insufficient elevator authority. Either of these may cause the nose to pitch up at stall.
2. As the airplane slows, normal control inputs should maintain the 30-degree bank attitude and nose position. When you feel the pre-stall buffet, note the speed, roll level, and recover to straight and level flight.
Note: Performing accelerated stalls in the airplane is a personal decision that should be based on the wing drop it exhibited in your wings-level stall tests. Accelerated stalls usually exacerbate this roll, especially when you’re turning to the direction the wing drops. Even a properly rigged airplane may roll up to 60 degrees into the turn, or up to 30 degrees in the opposite direction. If the possibility of a roll to 60 degrees is deemed unsafe—do not perform this test.
7-1 CHAPTER 7. PUTTING IT ALL TOGETHER: 36 HOURS TO —————?
SECTION 1. MAXIMUM GROSS WEIGHT TESTS
7.1 Objective. To develop aircraft performance data across the weight and CG ranges.
7.1.1 Up until this point, all tests have been performed well below the test aircraft’s maximum gross weight, with the possible exception of single seat aircraft designs. A complete series of flight tests at maximum gross weight from stalls, rates of climb, angles of climb, stability, retraction tests, slow flight, through accelerated stalls should be investigated.
7.1.2 These tests should demonstrate that the aircraft has been successfully flown throughout the CG range, and will operate in and at the full range of aircraft weights from minimum to full gross weight. The findings should be documented in the AOH.
7.1.3 Each phase of the testing should be done slowly, incrementally, with the same careful attention to detail that should characterize all the flight testing.
7.1.4 Increases in the aircraft weight should be done in a series of steps. Usually, 20 percent increments of the maximum payload (e.g., sandbags, lead shot) are added in the aircraft to simulate passengers or baggage weight. The pilot should carefully weigh and secure the ballast. A new W&B and CG location should be worked for each new increase in weight. Stop testing when the aircraft’s maximum gross weight is reached.
7.1.5 The testing up to this point has been done at, or near, the forward CG limit. During these tests, the CG should be slowly, but progressively, moved aft between each test flight.
Limit the change to the CG range to about 20 percent of the range. Again, the pilot should weigh the ballast and work a new W&B for each flight. With each CG change, the aircraft longitudinal static stability and stall characteristics should be carefully evaluated by using the same technique discussed earlier. Stop testing when the designer’s or kit manufacturer’s aft CG limit is reached.
7.1.6 If the aircraft develops either a neutral or negative longitudinal stability problem, or the aircraft displays unsatisfactory stall characteristics at any CG location being tested, STOP FURTHER TESTING!
7.1.7 These tests should confirm the designer’s aft CG limit or establish the last satisfactory aft CG location. If the aft CG range is not satisfactory, consult with the kit manufacturer, aircraft designer, or a flight test engineering consultant.
7.1.8 The pilot should avoid the temptation to take a live ballast weight up for a ride for three reasons:
7.1.8.1 The aircraft has not been proven safe for the higher gross weights.
7.1.8.2 The pilot and passenger are at great risk. It is a sure sign the pilot has become complacent and sloppy in their flight test program.
7-2
7.1.8.3 In most cases, unless otherwise approved by the FAA, the pilot will be in violation of the Operating Limitations for Phase I flight testing. If this is discovered and determined to be an intentional and/or reckless deviation from the regulations, legal action would be appropriate (refer to FAA Order 2150.3, FAA Compliance and Enforcement Program).
7.1.9 P ilots should ensure that the added ballast weight in the cockpit is secured. A seatbelt over some sand bags will not stop the weight from shifting and getting loose in a cockpit.
The last thing a test pilot needs is a 20-pound lead-shot bag free in the cockpit during a climb test, a landing, or a spin. Tie each weight down individually, and cover all the weights with a cargo net.
7.1.10 Ensure the ropes/nets and airframe attach points are strong enough to take the added load.
Make sure the passenger seat can take that much localized weight safely.
7.1.11 The maximum gross weight test results should be recorded in the flight manual. If there are any changes to the stall speed initially marked on the airspeed indicator, it should be changed to reflect the aircraft stall speed at maximum gross weight.
SECTION 2. SERVICE CEILING TESTS
7.2 Objective. To determine the highest altitude at which an aircraft can continue to climb at 100 feet per minute (service ceiling).
7.2.1 Pilots who wish to determine the actual service ceiling of their aircraft are offered the following suggestions:
7-3
1. Ask the responsible Flight Standards office to amend the Operating Limitations to permit a climb to the aircraft’s service ceiling, if that altitude is above 18,000 feet.
2. Contact the local Flight Service Station (FSS) or ATC facility, and reserve a time and airspace to make the test.
3. Install a transponder (refer to part 91, § 91.215) or obtain an authorization from ATC.
4. Install a portable oxygen bottle or use supplemental oxygen (refer to § 91.211), if plans are to go above 12,000 feet. (Recommend the pilot becomes familiar with the symptoms and cures of hypoxia and hyperventilation.)
5. R eview the engine manufacturer’s mixture leaning procedures.
6. Maintain communications with an air traffic facility at all times.
7.2.2 The climb to the aircraft service ceiling should be made in a series of step climbs during which engine performance, temperatures and pressures are recorded. At the slightest indication of engine performance or aircraft control problems, the pilot should terminate the test and return to the airport.
SECTION 3. NAVIGATION, FUEL CONSUMPTION, AND NIGHT FLYING
7.3 Objective. To ensure all the small but important aspects of flight have been tested and found reliable.
7.3.1 T he Magnetic Compass. The magnetic compass should have been checked for accuracy prior to the first flight. However, the addition and removal of equipment, changing of wire bundle routing, and other airframe modifications may have affected the accuracy of the instrument. The following recommendations are offered:
7-4
7.3.1.1 The magnetic compass can be checked for accuracy by using a compass rose located on an airport, or using a hand held “master compass.” The master compass is a reverse reading compass with a gun-sight mounted on the top of it. With the aircraft facing north and the pilot running the engine at 1,000 rpm, a second individual standing 30 feet away facing due south “shoots,” or aligns, the master compass with the aircraft’s centerline. Using hand signals, the pilot aligns the aircraft with the master compass. The pilot then runs the aircraft engine up to approximately 1,700 rpm to duplicate the aircraft’s magnetic field and reads the compass.
Note: Conventional gear aircraft builders will have to position the magnetic compass in a straight and level position for this test. Raise the tail or mount the compass level with the horizon.
7.3.1.2 If the aircraft compass is not in alignment with the master compass (start at north), correct the error by adjusting the north/south brass adjustment screw with a non-metallic screwdriver (can be made out of stainless steel welding rod, brass stock, or plastic) until the compass reads correctly. Go to the reciprocal heading (south) and remove half the error. On the east/west headings, use the other brass adjustment screw to make the corrections using the same procedure that was used to correct the north/south errors.
7.3.1.3 Check again for errors at each cardinal heading. Record the last readings and prepare a compass correction card. The maximum deviation (positive or negative) is 10 degrees on any one heading.
7.3.1.4 If the compass cannot be adjusted to meet this requirement, install another one. If the new compass is not available, try a different location in the cockpit, away from all ferrous metals and electrical bundles.
Note: A common error that affects the compass’s accuracy is the mounting of magnetic compass on/in the instrument panel with steel machine screws and nuts rather than brass fasteners.
7.3.1.5 If the aircraft has an electrical system it is recommended that two complete compass checks be made, one with all electrical accessories on
(e.g., radios/navigation lights), and one with all electrical accessories off. If the deviation in level flight is more than 10 degrees on any heading with the accessories on, make up a separate compass correction card that shows the magnetic heading with the equipment on.
7.3.1.6 Record the findings in the AOH and create a compass correction card, mounting it near the magnetic compass in the cockpit. Make two cards; one with radios on and one with radios and non-essential electrical accessories off.
7.3.2 Very High Frequency (VHF) Omni-Di rectional Range (VOR) Check. The best guide to check the accuracy of the VOR onboard equipment is the VOR Receiver Check found in the Aeronautical Information Manual (AIM), available online at
7-5 https://www.faa.gov/air_traffic/publications/. The following is an abbreviated summary of the VOR procedure in the AIM.
7.3.2.1 F or a ground test of the VOR, you should use a VOR Test Facility (VOT). To use the VOT service, tune the VOR receiver to the VOT frequency, normally 108 MHz. With the course deviation indicator centered, the omni-bearing selector should read 0 degrees with the To/From indicator showing “from,” or the omni-bearing selector should read 180 degrees with the To/From indicator showing “to.” The maximum bearing error should never be more than 4 degrees.
Note: You can find the VOT facilities closest to the flight test location in the FAA’s Chart Supplements, available at https://www.faa.gov/air_ traffic/flight_info/aeronav/digital_products/dafd/, or by contacting an FSS.
7.3.2.2 Fo r an airborne test, select a prominent ground point along the selected radial, preferably more than 20 miles from the VOR. Maneuver the aircraft directly over the point at a reasonably low altitude.
1. Note the VOR bearing indicated by the receiver when over the ground point. The maximum permissible variation between the published radial and the indicated bearing is 6 degrees.
2. If the aircraft has dual VORs, the maximum permissible variation between the two receivers is 4 degrees.
7.3.3 F uel Consumption. The amount of fuel consumed is a good indication of how much the engine is working for each rpm produced. For a new or recently overhauled engine, the fuel consumption should improve each flight hour until the engine finishes its break-in period (i.e., after approximately 100 hours of operation).
7.3.3.1 T o determine the aircraft fuel consumption, lay out a racetrack course with 8- to 10-mile legs. If the aircraft has one fuel tank or cannot switch tanks, do the following: Determine the approximate fuel burn to reach 1,000, 3,000, 5,000, 7,000, and 9,000 feet of altitude. With full tanks, climb to 3,000 feet and run the racetrack course for half an hour at 55 percent power.
7.3.3.2 Land and measure the fuel used by dipping the tanks with a calibrated fuel stick, or by adding measured amounts of fuel to the tank until the tank is full.
Subtract the approximate fuel burn to altitude, and multiply the remainder by two to get the fuel burn per hour.
7.3.3.3 The tests are much easier and the results more accurate if the aircraft has two independent fuel tanks. Take off on one tank and switch to the opposite tank at the test altitude. At the completion of the test, switch back to the first tank;
land and measure the amount of fuel added in both tanks and multiply the quantity by two to get the amount of fuel used per hour.
7-6
7.3.3.4 Run the same test at 65 percent and 75 percent power at the same altitude, using the same procedures. Move up to the next altitude and run the same tests.
7.3.4 Night Operations. You must conduct night operations during Phase II in accordance with the aircraft’s FAA operating limitations and should limit operations to normal climbs and descents, such as rates of 500 feet per minute, pitch angles of less than 5 degrees, straight and level flight, and coordinated turns of no more than 20 degrees of bank angle.
However during Phase I there are some checks you should perform prior to accomplishing night operations in Phase II.
7.3.4.1 Some night testing should be accomplished on the ground during Phase I. For example, the testing should:
1. Determine that the electrical load of lights, pumps, instrumentation, and avionics does not exceed 80 percent of the aircraft’s charging system capacity.
2. Determine that the cockpit instrumentation lighting is adequate. Cockpit instrumentation should also have been tested for reliability of operation during daytime flights.
3. Ensure that the pilot has at least 30 minutes of night-time practice taxiing the aircraft. The pilot needs this practice to familiarize the pilot with a different operating environment. Do not exceed engine operating temperatures during taxiing.
4. Determine that the position and brightness of instrument panel lights, anti-collision strobe lights, and rotating beacons will not adversely affect the pilot’s night vision.
7.3.4.2 Once Phase I flight testing is complete the main concern for initial night testing during Phase II should be the availability of a horizontal reference, such as a bright moon or artificial horizon.
7.3.4.3 Before every night flight, place a reliable flashlight with fresh batteries and a set of flight test plan procedures on board.
7.3.4.4 The first actual night flight should be conducted with a single pilot and no passengers on board.
7.3.4.5 A suggested night flight test plan is a series of takeoffs and landings and traffic pattern entries and exits. The tests should begin while there is still enough light to read a newspaper and transition to true night flying. The actual night flight will consist of an evaluation of the effectiveness of the taxi/landing light system during taxi, takeoff, and landing. The pilot should note any glare on the windshield or light flicker on the instrument panel.