Flight Test Maneuvers and Database Management

7.1 INTRODUCTION

Flight testing of an aircraft is required for several reasons [1-6]: (1) to test aircraft subsystems in the defined flight test envelope, (2) to validate flight control laws (for fly-by-wire inherently unstable/augmented aircraft), autopilot performance, and to prove core and new technologies, (3) to ascertain the ability of the aircraft to perform conventional and mission-specific maneuvers, (4) to establish the operating envel­ope, initial operational certification, and gradual expansion of the flight envelope based on the flight test results of the previous flights, (5) to demonstrate the compliance with civil aviation/military rules/specifications as per their requirements or advice, (6) to generate the data for future work in the design and development of a new aircraft or modification of the tested configurations for future upgradation, (7) to evaluate the ground effects on low-level flying, (8) to estimate aerodynamic deriva­tives for validation of the (predicted/wind tunnel) aero database used at the design stage of the aircraft, and (9) to evaluate pilot-aircraft interactions/handling qualities.

Flight tests are carried out at various stages of the aircraft modification and for confirmation of these modifications. The generation and establishment of the (updated) aerodynamic database via flight tests is a well-recognized method in an aircraft development program. A reliable and accurate estimation of aerodynamic derivatives from flight test data requires that certain modes of the aircraft are excited properly [7]. For example, it will not be possible to obtain accurate estimates of Cm and Cm if the longitudinal short-period mode is not sufficiently and properly excited, and hence the choice of an input form and shape of the signal is a very important aspect. From the foregoing, it is very clear that an integrated system approach is also important for successful accomplishment of the flight tests.

For a flying aircraft, controlling and monitoring its speed and altitude are very important; however, as one can realize, the aircraft controls, input mechanisms, and connected control surfaces are not designed or mechanized inherently to do this task quite independently. To perform a certain flight maneuver, most often and perhaps always a complex sequence of movements of a few, if not all, controls (surfaces) is required to be carried out. The concept of aircraft energy and its management is the most important aspect in understanding the requirement of flying an aircraft at a certain altitude and speed. The energy cannot be created (the law of conservation energy), but it can be converted from one form to another. The aircraft energy can be represented in the form of Quad-E [8]: (1) kinematic energy acquired as a result of flight speed, (2) potential energy acquired as a result of the aircraft being at a certain altitude, (3) the fuel’s chemical energy, and (4) thermal energy due to the exhaust gases, where applicable. An appropriate interplay of these energies can be advanta­geously used to understand flight maneuvers. The airspeed and altitude can be exchanged as a change in mechanical energy. Some important exchanges of energy are [8] (1) in a climb maneuver, the fuel is used (burnt) to meet the drag force and attain the altitude, (2) in cruise there is no maneuver and hence the fuel is consumed only to meet the drag resistance and there is not much change in airspeed and altitude, (3) in a sudden pullup the aircraft ascend to a new altitude and the airspeed decreases, (4) in a sudden pushover maneuver the altitude is sacrificed and the airspeed is gained; and not much fuel is spent, (5) in an initial portion of the takeoff roll the altitude is not changed but the airspeed is gained—fuel is spent, (6) in gliding the altitude is sacrificed to meet the drag force, and (7) in flare the airspeed is sacrificed to meet the drag force, without muchl change in the altitude and engine power. It is emphasized here that the altitude and airspeed are very closely inter­related. Reference [8] gives very interesting conversion factors, as a rule of thumb: (1) for a loss of 1 knot of speed, one can gain an altitude of 9 ft and (2) for a 1 ton aircraft, the climb by 6300 ft would spend 1 gallon more fuel. The energy exchange guidelines are as follows:

• If at low altitude and low speed, then there is not much energy, and hence more power is required to be added to gain the altitude.

• If at low altitude and with high speed, the energy might be sufficient; moderate stick pull might be required.

• If at high altitude and with high speed, there is too much energy and it might be required to reduce the power.