Flying Qualities Specifications defines good high angle of attack characteristics in terms that are qualitative rather than quantitative. The airplane must exhibit adequate stall warning, and in addition the stall must be easily recoverable. The definitions of high angle of attack characteristics will differ for the various classes of aircraft; but with respect to fighter aircraft, a pilot should not have to worry about loss of control while flying within his useful manoeuvre envelope.
The cautious approach starts with good pre-planning. Discussion with the appropriate engineering team of the predicted stall characteristics, and development of the most promising recovery technique for each stage of the stall, including possible post-stall gyrations is a necessity. In marginal cases, a suggestion for further wind tunnel testing or other alternative investigations might be warranted. The most favorable loading and configuration to be used in the initial stages must be determined. Stall and spin practice in trainer aircraft will enhance pilot performance during any out-of-control situations that might develop. If pitch-up or other control problems seem remotely possible, the first runs should terminate early in the approach to the stall and the data carefully examined (on the ground) for trends such as lightening or reversal of control, excessive attitudes, or sink rates. Advancing this data systematically on subsequent flights and avoiding the mistake of suddenly deciding in flight, because things are going well, to take a bigger step than planned is a necessity.
Stall characteristics must be evaluated in relation to their influence on mission accomplishment. Thus, both normal and accelerated stalls must be performed under entry conditions which could result from various mission tasks. However, prior to evaluating stalls entered from these conditions, a more controlled testing approach should be employed. This approach allows lower deceleration rates into the stall and lower pitch attitudes at the stall, thereby reducing chances for “deep-stall” penetration without adequate build-up. After the controlled stall investigation, if stall characteristics permit, simulated inadvertent stalls should be investigated under conditions representative of operational procedures
The easiest and safest approach to controlled stall testing is to divide the investigation into three distinct parts:
- Approach to the stall. During this phase of the investigation, adequacy of stall warning and retention of reasonable airplane controllability are the primary items of interest. Assessment of stall warning requires subjective judgement by the pilot. The general flying qualities of the airplane should be investigated. Longitudinal, lateral, and directional control effectiveness for maintaining the desired attitude may deteriorate significantly during the approach to the stall. Loss of control about any axis such as uncontrollable pitch-up or pitch-down, “wing drop,” or directional “slicing” may define the actual stall.
- Fully developed stall. During this phase of the investigation, the primary objective is to accurately define the stall and the associated airplane behavior. The stall should be will-marked by sane characteristic, such as pitch-up or pitch-down or lateral or directional divergence. Control effectiveness as evidenced by the pilot’s ability to control or induce roll, pitch, or yaw should be evaluated in the stall, if airplane behavior permits this to be done safely.
- Level Flight path Method.
- Curved Flight path Method
- Stall recovery. During this phase of the investigation, primary items of interest are the ease of recovery (the pilot’s task), general flying qualities during the recovery, altitude required for recovery and the determination of an optimum recovery technique. The recovery is started when the stall or minimum steady speed has been attained. For a conventional stall this is indicated by the inability to maintain the desired load factor -usually a sudden break is apparent on the cockpit accelerometer. During initial investigation, the stall recovery procedures specified in pertinent publications should be utilized and the ease of effecting recovery evaluated. If no procedure has been developed, initial recovery must be accomplished with a ‘preliminary” technique formulated from all available technical information. As experience is gained, various modifications to the recovery procedure should be made until an optimum procedure is determined.
A deep stall is an out-of-control flight condition in which the airplane is sustained at an angle of attack well beyond that for as while experiencing negligible rotational velocities. It may be distinguished from a Post Stall Gyration (PSG) by the lack of significant motions other than a high rate of descent. The deep stall may be a fairly stable manoeuvre such as a falling leaf, or it can be characterized by large amplitude angle of attack oscillations. For an aircraft to stay in a deep stalled condition, significant oscillations must be limited to the longitudinal axis. Lateral and directional control surfaces are either stalled or blanked out. Depending on the pitching moment coefficient, recovery may or may not be possible.
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