Appendix 4 to AMC1 FSTD(H).300  Vibration platforms for helicopter FSTDs

Decision 2012/011/R

(a)     The role of vibrations in pilot cueing

(1)     Motion feedback in rotary wing aircraft has a wide bandwidth of frequencies and amplitudes consisting of cues ranging from large sustained accelerations up to high frequency vibrations generated by the rotor harmonics. Vibrations on helicopters, in addition to creating a harsh operating environment, provide pilots with rotor dynamic feedback critical to his/her ability to control the aircraft. Normal and abnormal flying conditions are therefore sensed by the pilots through the vibration levels/amplitudes and are integral to helicopter flying. Rotor malfunctions/conditions such as icing or damage are rapidly identified subjectively by sensing the increased vibration levels and change in characteristics.

(2)     The FSTD training environment should subject the pilot to high fidelity and realistic levels of vibration in order to enhance the transfer of training. Vibrations, when accurately simulated and harmonised with visual and sound system cues, ensure that the pilot develops proper control strategies while experiencing representative workloads.

(3)     Three characteristics of the vibrations must be accurately reproduced to create an authentic flying environment and stimulate pilots with representative aircraft vibrations: the trends, the axes and the levels of vibrations. For example, the vibration trends will inform the pilot that the helicopter has entered a transition stage between hover and low speed level flight. Helicopter vibrations are multidimensional, that is, they are perceived as occurring in more than one degree of freedom at a time. Simulating combinations of X, Y and Z vibrations has demonstrated to be significant for pilot training. Accurate reproduction of vibration levels provides subjective information on the stresses that certain manoeuvres exert on the helicopter.

(b)     Limitations of using a 6-degree-of-freedom motion system to reproduce vibrations

(1)     The simulation of vibration cues for rotary wing aircraft as produced by a conventional six-degree-of-freedom (6-DOF) motion system is limited. While most motion systems are capable of reproducing vibrations, the dynamic range of helicopter vibration amplitudes and frequencies (3 Hz - 50 Hz typically) exceed the limited bandwidth capability of synergistic motion systems (typically 0 Hz - 10 Hz in the vertical axis and lower in the longitudinal and lateral axes).

(2)     Moreover, the application of representative vibrations to the entire simulator structure may adversely impact the life span of some simulator components such as the visual system.

(c)      Advantages of a dedicated 3-degree-of-freedom vibration platform

(1)     To augment the performance of a 6-DOF motion system and achieve accurate reproduction of vibrations while minimizing stresses on the simulator structure, it is proposed that the motion cueing frequency bandwidth be separated in two. Dedicated cueing devices would then be assigned to reproduce each specific frequency range. The lower frequency range is used to drive the motion system and the higher frequency range, with the majority of the vibration information, is used to drive the vibration platform.

(2)     Two solutions may be used for simulating the vibrations:

(i)      A vibration platform consisting of a 3-DOF system tailored for vibrations and installed under the cockpit as illustrated in figure 1. This system combines high bandwidth, independent driving axes (to avoid crosstalk) and high stiffness.

(ii)     A vibration platform consisting of a 3-DOF system to make the seats, the controls and the main instrument board vibrate independently from the cockpit. This solution decreases the moving mass relatively to the payload and therefore minimises the risk of resonance.

Figure 1: An Example of a 3-degree-of-freedom cockpit vibration system