24.2 Neural Network-Based Tuning

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As mentioned earlier, rotor tuning in four-bladed aircraft is performed by first specifying a condensed set

of adjustments to reduce vibrations, and then expanding these adjustments into a detailed set to satisfy

the track requirements. This same strategy is implemented in the system of neural networks shown in

Figure 24.3 (Taitel et al., 1995). The first network in this system, called the selection net, determines the

condensed blade adjustments (output) that will bring about a given change in vibration (input). To

eliminate vibration, the negatives of the vibration measurements from the flight are utilized as inputs to

this network. The validity of the condensed adjustments is then checked by predicting their effect on

vibration via the condensed simulation net. Theoretically, these simulated vibration changes should be

the negative of the vibration measurements from the aircraft so that their summation will be zero.

However, owing to the inexactness of the neural network models and noise, the resultant vibration will

most likely not equal zero. In cases where the resultant vibration is not within specifications (usually less

than 0.20 inches per second [ips]), the condensed adjustments may be refined by feeding the resultant

24-4 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

vibration back into the selection net. This feedback is depicted by the dashed feedback line in Figure 24.3.

It should be noted that the condensed simulation net may also serve as a diagnostic tool by indicating

behavior out of the norm. For example, an aircraft with vibrations significantly different from those

predicted by this network may suffer from defective components.

Just as with the traditional approach, once the condensed solution has been specified, it needs to be

expanded into a detailed form to satisfy the rotor track requirements. As previously mentioned, the

condensed set of adjustments may be viewed as the constraint on detailed adjustments so as to ensure

that the vibration solution is not compromised for track. Each one of these detailed sets of adjustments is

a candidate for the final rotor tuning solution, and it is left to the track net and the selection package to

determine which set of detailed adjustments provides the best tracking performance. For selection

purposes, the track net simulates the changes in track due to a candidate set of detailed adjustments, and

then adds these changes to the initial track measurements from the flight to estimate the resultant track.

The set of detailed adjustments that yields the smallest estimated track (i.e., smallest maximum blade

spread) is selected as the solution to the rotor tuning problem. The selected set of detailed adjustments is

then checked via the vibration net, which, similar to the condensed simulation net, serves as an

independent evaluator of the selected adjustments.

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