SPECIFIC FLIGHT TEST DATA GENERATION AND ANALYSIS ASPECTS

Flight test data have the following roles to play [1-3,6]: (1) To substantiate that the aircraft meets certain specific stability and control parameters. (2) To correlate these data with the wind tunnel or analytical data. Based on the flight test data some adjustments can be made; these adjusted wind tunnel (WT) or analytical data can be used to extrapolate the flight data to other regions of the flight envelope where the flight tests are not performed because of the limitation of safety and cost. (3) To adjust and substantiate the mathematical models used in flight simulators, for which the time histories and stability and control derivatives are used for comparison, validation, or upgradation of the math models. (4) To aid the design of a future aircraft. Flight test data generation and analysis aspects are highlighted in the following section [2,3,8].

7.3.1 Longitudinal Axis Data Generation

The stabilizer angle to trim the aircraft exercise is determined to establish trim characteristics, to obtain the effect of configuration or thrust changes on stabilizer trim, and to determine stabilizer effectiveness. The aircraft is trimmed using the stabilizer with zero elevator deflection. The trimmed state is attained when the aircraft can fly hands-off in a steady unaccelerated flight mode. The stabilizer angle required to trim is plotted as a function of lift coefficient for low speeds and Mach number for high speeds for a range of CG configurations and thrust settings. Various desired effects are then discerned from these plots.

For a range of flight conditions and configurations, the aircraft is mistrimmed with the stabilizer and retrimmed with the elevator, keeping the thrust constant and maintaining the unaccelerated flight. The effectiveness of the elevator can be obtained by plotting the elevator-stabilizer curves as a function of CG.

In conducting a maneuvering flight the purpose is (1) to evaluate the handling characteristics such as pullup and wind-up turns (WUTs), (2) to determine pitching moment/lift characteristics, and (3) to determine elevator hinge moments.

The static (speed) stability test is required to determine aircraft’s static stability and to determine the neutral point of the aircraft (Appendix A). The aircraft is trimmed at one speed, and the elevator deflection and stick force required to fly at another speed in a straight flight are measured. Keeping thrust constant the aircraft is flown at all the speeds.

It is important to investigate controllability and handling characteristics of the aircraft and basic lift and pitching moment characteristics in the high angle of attack range.

The primary interest in the stall exercise is the ability to recover from a stalled condition. Some characteristics related to stall conditions are (1) it occurs at the critical AOA, (2) at/after this AOA, the lift coefficient does not increase with an increase in AOA, (3) this special point on the lift curve also corresponds to a point on the power curve, (4) if the lift coefficient is small, the aircraft has to fly at a higher speed to support the weight, (5) the lift coefficient has a maximum and the airspeed a minimum—this minimum airspeed is the stalling speed, (6) normally it is not possible to sustain the flight below the stalling speed, and (7) the drag coefficient is high in the stalled regime and hence it needs a lot of power to maintain the level flight, but with constant power the rate of climb decreases. Mainly there would be an uncommanded and easily recognizable nose down pitch and a roll motion that cannot be readily arrested with normal use of the controls. One can recover from the stall by reducing the angle of attack using normal control by the elevator. The stall
maneuvers are flown with varying entry rates. The effect of thrust level is also studied. The 1g stall speed is the speed below which 1g level flight is impossible. The minimum speed is the lowest measured speed during the stall maneuver. The stall entry rate is defined as the slope of the line between 1.1 * minimum speed and the minimum speed. During the closing phase of the stall maneuver, the control may be moved forward to maintain a constant deceleration rate and to reduce the large pitch angle changes.

It is well known that ground effect, which results from a change in the flow field in the presence of the ground, can significantly affect the takeoff and landing characteristics of the aircraft. The effect of ground proximity on the stability and control of the aircraft should be evaluated as it has operational implications and also is very important for validation of flight simulators [4,5]. This is because a large part of the pilot training in the simulator is related to approach and landing tasks. The simulator models should be checked properly in these flight phases. The aircraft is flown along the runway at progressively lower heights for a range of speeds. The height is measured with radar altimeter and the ILS is used for an approach. The autopilot is disengaged before entering the ground effect, and the aircraft behavior is observed in the ground effect with hands off the controls. The aircraft is trimmed for the approach and when it enters the ground effect the body angle is kept constant using the elevator control until the touchdown. The data from the ground effect and the reference data from the out-of-ground effect are compared and the increment in elevator deflection is determined. Then, the change in the pitching moment due to the ground effect is computed. This change is caused by a change in the angle of attack, a change in the Cm , and a change in thrust to keep speed constant. Based on this analysis, the update of the simulator models can be made if needed.

The tests for establishing the capability of the elevator to lift the nose wheel off the ground during the takeoff phase of the flight are also made. With the forward CG, the stabilizer is set at the recommended position and the elevator is fully pulled up some time before the rotation is expected. The rotation speed is determined when the body angle starts increasing, i. e., the pitch rate is positive. The liftoff point is determined when the main gear lifts off the ground.

The dynamic characteristics of the aircraft are assessed by performing the short – period and phugoid maneuvers and generating the data by giving doublet or 3-2-1-1 input commands to the pitch stick, as discussed in Section 7.6 (Figure 7.3). The SP mode is critical since its period can approach the pilot’s reaction time. To determine

3At At

2At At

FIGURE 7.3 Specific input types: doublet, 3-2-1-1, and pulse. (At is the unit step width in seconds.)
the stability and control derivatives of the aircraft, the data from these flight test maneuvers are processed in parameter-estimation SW (Chapter 9).