Appendix 3-B - Rotorcraft Simulator Validation Tests

4. Control Dynamics

The characteristics of a rotorcraft flight control system have a major effect on handling qualities. A significant consideration in pilot acceptability of a rotorcraft is the "feel" provided through the cockpit controls. Considerable effort is expended on rotorcraft feel system design in order to deliver a system with which pilots will be comfortable and consider the rotorcraft desirable to fly. In order for a simulator to be representative, it too must present the pilot with the proper feel; that of the respective rotorcraft.

Recordings such as free response to an impulse or step function are classically used to estimate the dynamic properties of electromechanical 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 simulator control loading system to the rotorcraft systems is essential. The required control feel dynamic tests are described in 2.A.5. of the Table of Validation Tests of this section.

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 hover, climb, cruise and autorotation.

For rotorcraft with irreversible control systems, measurements may be obtained on the ground. Proper pitot-static inputs (if applicable) must be provided to represent conditions typical of those encountered in flight. Likewise, it may be shown that for some rotorcraft, hover, climb, cruise and autorotation may have like effects. Thus, one may suffice for another. If either or both considerations apply, engineering validation or rotorcraft manufacturer rationale must be submitted as justification for ground tests or for eliminating a flight condition. For simulators requiring static and dynamic tests at the controls, special test fixtures will not be required during initial and upgrade evaluations if the operator's QTG 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 measurements which can be found in texts on control systems. In order to establish a consistent means of validating test results for simulator control loading, criteria are needed that will clearly define the interpretation of the measurements and the tolerances to be applied. Criteria are needed for underdamped, critically damped and overdamped systems. In the 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 is not readily measured from a response time history; therefore, some other measurement must be used.

For LevelC and D simulators, tests to verify that control feel dynamics represent the rotorcraft must show that the dynamic damping cycles (free response of the controls) match that of the rotorcraft within specified tolerances. The method of evaluating the response and the tolerance to be applied is described below for the underdamped, critically damped and overdamped cases.

Underdamped Responses

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. Each period will be independently compared to the respective period of the rotorcraft control system and, consequently, will enjoy the full tolerance specified for that period.

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 residual band, labelled T(Ad) on Figure1, is ±5% of the initial displacement amplitude, Ad from the steady state value of the oscillation. Oscillations within the residual band are considered insignificant. When comparing simulator data to rotorcraft data, the process should begin by overlaying or aligning the simulator and rotorcraft steady state values and then comparing amplitudes of oscillation peaks, the time of the first zero crossing and individual periods of oscillation. The simulator should show the same number of significant overshoots to within 1 when compared against the rotorcraft 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 rotorcraft within ±10%. The simulator response should be critically damped also. Figure2 illustrates the procedure.


The following table summaries the tolerances, T. See Figures1 and 2 for an illustration of the referenced measurements.

±10% of P0
±20% of P1
±10% of Pn
±10% of A1, ±20% of Subsequent Peaks
±5% of Ad

Figure 1 - Under-Damped Step Response

Figure 1 - Under-Damped Step Response (Displacement vs. Time)

Figure 2 - Critically Damped Step Response

Figure 2 - Critically Damped Step Response (Displacement vs. Time)

6. Motion Testing

a. Motion Cue Repeatability Testing

The motion system characteristics in the Table of Validation Tests address basic system capability, but not pilot cueing capability. Until there is an objective procedure for determination of the motion cues necessary to support pilot tasks and stimulate the pilot response which occurs in an aircraft for the same tasks, motion systems will continue to be "tuned" subjectively. Having tuned a motion system, however, it is important to involve a test to ensure that the system continues to perform as originally qualified. Any motion performance change from the initially qualified baseline can be measured objectively.

An objective assessment of motion performance change will be accomplished at least annually using the following testing procedure:

  1. The current performance of the motion system shall be assessed by comparison with the initial recorded test data.
  2. The parameters to be recorded shall be the outputs of the motion drive algorithms and the jack position transducers.
  3. The test input signals shall be inserted at an appropriate point prior to integration in the equations of motion (see Figure3).
  4. The characteristics of the test signal (see Figure4) shall be adjusted to ensure that the motion is exercised through approximately x of the maximum displacement capability in each axis. The time T1 must be of sufficient duration to ensure steady initial conditions.

NOTE: If the simulator weight changes for any reason (ie. visual change or structural change), then the motion system baseline performance repeatability tests must be rerun and the new results used for future comparison.

Figure 3 - Acceleration Test Signals

Figure 3 - Acceleration Test Signals

Figure 4

Figure 4

b. Alternative Method for Motion Systems Testing

An alternative to the procedures described and specified in section3.A. and B. of the Table of Validation Tests and in subsection6.a. of this Appendix is "end to end" testing of the motion system and its associated washout, drive and servo systems. An acceptable procedure to conduct the end to end test is, for convenience, described as follows:

  1. At the point at which the accelerations from the equation of motion normally excite the motion system, including the washout algorithms, a sinusoidal input would be used to excite the motion system (see Figure5). Acceleration at the pilot station would be measured as the output. The test would be done independently in each of the six DOF and the response measured to determine frequency response. The resulting frequency response measured in each axis must comply with the following specification:

    0.5 Hz to 5.0 Hz
    1.0 Hz to 2.0 Hz

    NOTE: This procedure does not account for the correctness of the algebraic sign between input and output. Consequently, care must be exercised to ensure that the signs are correct.

  2. Motion systems demonstrated by end to end testing must also comply with the displacements delineated in section5.

Figure 5

Figure 5

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