Changing controls from data to suit evaluators/users

Question

On a helicopter FSTD we developed and tuned the controls loading model to fully match the controls force/position measurements taken in a reference aircraft provided by the customer, but during subjective evaluation the pilots requested about 30% controls force increase. To readjust the model is an easy task but then we loose any objective reference. What would you suggest to handle this situation from a formal point of view?

Answer

You do not say in which flight control axis the control forces were too low, if it was the collective then the friction adjustment might not be representative, if it was the cyclic forces, which are normally very small, we suggest you might set up two files, one to data from the Fokker (target) plots and the other to those requested by the pilots and then run with the subjective pilot forces until you have had a significant number of pilots to comment on the forces.

The normal situation is that the Fokker plots are taken without vibration and when there is vibration, as in the helicopter, then the running friction is lower and the hysteresis force loop smaller. Another issue is whether the pilots validating the results were true test pilots or those with significant experience on the aircraft type and that the a/c data was recorded correctly to provide the target values. It is not unusual for there to be variations between individual aircraft although a 30% difference is unlikely if the data aircraft had been properly maintained.

We assume that the data was collected by yourselves or by contract and was not supplied by the aircraft manufacturer. In either case it would be wise to obtain additional data test cases in order to explain whether there was any discrepancy in their values.

Having considered all of this and still being unable to resolve the issue, we would suggest that the matter be discussed with the Authority prior to submitting the QTG for validation and we cannot emphasis enough the need to discuss this issue with the relevant NAA ahead of time.

A 30% change in control force would need some very careful explanation. Its more than likely that there's an error somewhere in the model and not necessarily in the control system.