Solution
2.3.6 FTT: the Trim Shot
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Introductory Concepts 125
k k Table 2.5 Selected specifications of the Extra 300.
Item Specification
Primary function General aviation, advanced aerobatics
Manufacturer Extra Aircraft, Germany
First flight 6 May 1988
Crew 1 pilot + 1 passenger
Powerplant Lycoming AEIO-540-L1B5 six-cylinder engine
Engine power 300 hp (224 kW) at 2700 rpm
Empty weight 1643 lb (745.3 kg)
Maximum gross weight 2095 lb (950.3 kg)
Length 23.4 ft (7.12 m)
Height 8.60 ft (2.62 m)
Wingspan 26.25 ft (8.0 m)
Wing area 115.2 ft2(10.7 m2)
Wing loading 16.7 lb/ft2(81.3 kgf/m2)
Airfoil, wing root MA15S (symmetric, 15% thickness) Airfoil, wingtip MA12S (symmetric, 12% thickness) Maximum cruising speed 158 knots (182 mph, 293 km/h)
Service ceiling 17,000 ft (5200 m)
Load factor limits +10.0 g, −10.0 g
sustained inverted flight. The first flight of the Extra 300 was on 6 May 1988. Selected specifications of the Extra 300 are provided in Table 2.5.
You will be flying the aircraft solo from the aft cockpit, as required to stay with the center of gravity limits for solo flight. You will perform the trim shot FTT as a setup for another maneuver, a maximum rate aileron roll, where the aircraft will complete a full, 360∘ roll about its longitudinal axis. Your desired trim shot flight conditions for this maneuver are an airspeed and altitude of 155 knots (178 mph, 287 km/h) and 7000 ft (2100 m), respectively.
As shown in Figure 2.18, the aft cockpit flight controls include a center-mounted stick for pitch and roll control and rudder pedals for yaw control. A pitch trim lever, located on the right side of the cockpit, can be rotated up or down to reduce the control stick pitch forces to zero. A for-ward and aft moving throttle lever on the left side of the cockpit controls engine power. Cockpit instrumentation includes round analog indicators for airspeed and altitude and an electronic flight information system (EFIS) display. Flight test data is supplied to the EFIS by three onboard data sources, a global positioning system (GPS) receiver, an engine information system, and an attitude and heading reference system (AHRS). The system can provide aircraft airspeed, altitude, attitude, position data, engine information, three-dimensional linear accelerations and angular rates, and other data. The flight test data is collected at a rate of 10 samples per second, or a sample rate of 10 Hz. The primary flight test data to be collected for the trim shot FTT are airspeed, altitude, pitch and roll angles, and roll rate. The cockpit instrumentation does not include an artificial horizon, an instrument used to determine the aircraft’s pitch and roll attitude. For the setup of the trim shot in flight, you will rely on basic attitude flying, where you will set the aircraft attitude with respect to the outside horizon.
Now that you have reviewed the instrumentation and data system, you are ready to go flying.
You decide to perform your test in the early morning, when there is less chance of atmospheric turbulence. The still air will make it easier to set up a stable trim shot, and the quality of the test data will be better. You climb into the Extra 300 aft cockpit, secure your safety belts, start the Lycoming engine, and taxi to the runway for takeoff. Applying full engine power, you take off and
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Introductory Concepts 127
Figure 2.18 Aft cockpit and instrument panel of the Extra 300 aircraft (Source: Courtesy of the author.)
start the climb. After your climb is established, you set climb power, slightly less than full power.
At 7000 ft, you push forward slightly on the control stick to lower the aircraft nose and set up for the trim shot.
You adjust the nose attitude, in relation to the horizon, to maintain your constant altitude. After giving the engine 15 seconds to stabilize, you confirm that the altitude and airspeed are stable. The altitude is constant at 7000 ft and the airspeed is stable at 160 knots (184 mph, 296 km/h), faster than your desired target airspeed. Using attitude flying, you make a small nose attitude adjustment, slightly raising the nose relative to the horizon to decrease the airspeed and make a small power reduction, from the previously set climb power, to prevent the aircraft from climbing. All of these adjustments are small, giving time between them to allow the flight condition to stabilize. While you are patiently waiting for everything to stabilize, you are careful to maintain a wings-level attitude and a constant heading, with very small adjustments in roll and yaw, using lateral stick and rudder inputs, respectively. After some time, it looks like the aircraft is stable in airspeed, altitude, and attitude. You are holding some aft pitch control force to maintain this trim shot, so you slowly move the pitch trim lever to reduce this force to zero. Finally, you check your trim shot by gingerly releasing your grip on the control stick and verify that the flight condition is stable. The airspeed and altitude are stable at 155 knots and 7000 ft, respectively. You maintain this trim shot for several seconds to ensure that nothing is changing. It has taken some patience and precise attitude flying, but you have succeeded in setting up a stable trim shot for your test maneuver.
k k To execute the maximum rate roll maneuver, you apply a rapid, full left lateral stick input, trying
to avoid any pitch input. The Extra 300 has full span ailerons, extending almost the full length of each wing, so the roll is almost a blur as the aircraft rolls 360∘ around its longitudinal axis. After the aircraft rolls through inverted flight and back towards upright, you rapidly center the control stick to stop the roll. The test maneuver is complete and you descend for landing. After landing, you download the flight test data from the data system. You are pleased with your trim shot, but the data will really tell how stable it was.
The data from the trim shot and roll maneuver are plotted in Figure 2.19. The parameters are plotted on the vertical axes as a function of time, commonly known as a “strip chart” format.
Starting from the top of the figure, the plotted parameters are airspeed, altitude, roll rate, pitch angle, and roll angle. The pitch angle is the angle between the aircraft longitudinal axis and the horizon, indicating where the aircraft nose is pointing relative to the horizon. The roll angle is the angle of bank of the wings with respect to the horizon. The trim shot segment of the data is to the left of the vertical dashed line and the roll maneuver is to the right, as indicated.
170
Trim shot Roll maneuver
Airspeed, V (kts) 160
Altitude, h (ft) 150
0
−150
−300 Roll rate, p (deg/s)
6 0
−6 Pitch angle, θ (deg)
200 0
0 1 2 3
Time, 𝘵 (s)
4 5 6
−200 Roll angle, 𝜙 (deg)
8,000
4,000 6,000
Figure 2.19 Extra 300 flight data; trim shot followed by a maximum rate aileron roll. (Source: Figure created by author based on data trends in [28], with permission of Christopher Ludwig.)
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Introductory Concepts 129
Examining the data for the trim shot, the most important few seconds, preceding the maneuver, are shown. The altitude is stable at 7000 ft and the airspeed is about 154 knots, about 1 knot less than the target of 155 knots. The pitch angle is stable at about 5∘. Since the test maneuver was an aileron roll, it was critical that the trim shot start at a zero roll angle and with zero roll rate. If these were non-zero at the start of the maneuver, it would be difficult to accurately determine the aircraft roll performance. This again emphasizes the importance of the trim shot. From the data, the wings are level, as indicated by a roll angle of zero and the roll rate is zero. Based on the data, the trim shot looks correct and stable, prior to initiating the roll maneuver.
The roll maneuver is completed in less than two seconds. The roll angle is seen to go from −180 to +180 degrees, indicating a 360-degree roll. The altitude remains constant throughout the roll, while the airspeed increases, indicating that the nose was dropping in the maneuver, as also verified by the decrease in pitch attitude. A parameter of particular interest in assessing roll performance is the roll rate. As is seen by the data, the maximum roll rate was about 250 degrees per second. This maximum roll rate was obtained for a little less than one second of the two second-duration roll, due to the finite amount of time to achieve the roll rate and to recover from the roll. This review of the aileron roll shows the degree of analysis detail that can be obtained from a few, properly selected measurement parameters.