Appendix 4-B - Flight Training Device Validation Tests
4. Control Dynamics
The characteristics of an aircraft flight control system have a major effect on the handling qualities. A significant consideration in pilot acceptability of an aircraft is the "feel" provided through the cockpit controls. Considerable effort is expended on aircraft feel system design in order to deliver a system with which pilots will be comfortable and consider the aircraft desirable to fly. In order for a training device to be representative, it too must present the pilot with the proper "feel"; essentially that of the respective aircraft.
Recordings such as free response to an impulse or step function are classically used to estimate the dynamic properties of electro mechanical systems. In any case, it is only possible to estimate the dynamic properties as a result of only being able to estimate true inputs and responses. Therefore, it is imperative that the best possible data be collected since close matching of the control loading system to the aircraft system is essential.
For initial and upgrade evaluations, it is required that control dynamic characteristics be measured at and recorded directly from the cockpit controls. This procedure is usually accomplished by measuring the free response of the controls using a step or pulse input to excite the system. The procedure must be accomplished in take-off, cruise and landing flight conditions and configurations.
For aircraft with irreversible control systems, measurement may be obtained on the ground if proper pitot static inputs are provided to represent airspeeds typical of those encountered in flight. Likewise, it may be shown that for some aircraft, take-off, cruise and landing configurations have like effects. Thus, one may suffice for another. If either or both considerations apply, engineering validation or manufacturer rationale must be submitted as justification for ground tests or for eliminating a configuration. For devices requiring static and dynamic tests at the controls, special test fixtures will not be required during initial and upgrade evaluations if the operator's ATG shows both test fixture results and the results of an alternate approach, such as computer plots which were produced concurrently and show satisfactory agreement. Repeat of the alternate method during the initial evaluation would then satisfy this test requirement.
5. Control Dynamics Evaluation
The dynamic properties of control systems are often stated in terms of frequency, damping and a number of other classical measures which can be found in texts on control systems. In order to establish a consistent means of showing test results for control loading, criteria are needed that will clearly define the interpretation of the measurements and the tolerances to be applied. Criteria are needed for both underdamped and critically and overdamped systems. In case of an underdamped system with very light damping, the system may be quantified in terms of frequency and damping. In critically damped or overdamped systems, the frequency and damping are not readily measured from a response time history. Therefore, some other measurement must be used.
Tests to verify that control feel dynamics represent the aircraft must show that the dynamic damping cycles (free response of the controls) match that of the aircraft within 10% of period and 10% of damping. The method of evaluating the response is described below for the underdamped and critically damped cases.
Two measurements are required for the period: the time to first zero crossing (in case a rate limit is present) and the subsequent frequency of oscillation. It is necessary to measure cycles on an individual basis in case there are non uniform periods in the response.
The damping tolerance should be applied to overshoots on an individual basis. Care should be taken when applying the tolerance to small overshoots since the significance of such overshoots becomes questionable. Only those overshoots larger than 5% of the total initial displacement should be considered significant. The results should show the same number of significant overshoots to within one when compared to the aircraft data. This procedure for evaluating the response is illustrated in Figure1.
Critically Damped and Overdamped Response
Due to the nature of critically damped responses (no overshoots), the time to reach 90% of the steady state (neutral point) value should be the same as the aircraft within +10%. The training device response should be critically damped also. Figure2 illustrates the procedure.
Alternate Method for Control Dynamics
One aircraft manufacturer asserts that adjusting a control loading system for column releases may introduce an unnecessary error for normal pilot commands away from neutral. Instead of free response measurements, the system would be validated by measurements of column force as a function of hands on column rate.
For each axis of pitch, roll and yaw, the control shall be forced to its extreme position at two distinct rates. One that achieves maximum deflection in approximately two seconds and one that achieves maximum deflection in approximately one second. Tolerances on the total force shall be the same as for the static check with the additional requirement that the dynamic increment be in the correct sense relative to the static force level. Where flight configurations influence the feel forces of the controls, these tests shall be conducted at a typical taxi take-off cruise and landing condition.
TC is open to alternative means such as the one described above. Such alternatives must, however, be justified and appropriate to the application. For example, the method described here would not likely apply to other manufacturers' systems and certainly not to aircraft with reversible control systems. Hence, each case must be considered on its own merit and on an ad hoc basis. Should TC find that alternative methods do not result in satisfactory simulator performance, then more conventionally accepted methods must be used.
Figure 1 - Under-Damped Step Response
Figure 2 - Critically Damped Step Response
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