"RAP" Test - Rotorcraft

[faraday]

Does anyone know of a suitable organisation capable of performing a Rap Test on a Robinson R44 Helicopter?

The test has been requested by EASA as part of an STC project for the installation of crop spraying equipment.

[Genghis the Engineer]

Could I suggest being a bit more specific - there are several ways that the requirement for a "rap test" could be interpreted from your post, and a bit more detail might help people to give you a helpful answer.

G

[faraday]

I'm given to understand that the RAP test is used to identify any new natural frequencies of helicopter with the spray kit installed compared to helicopter without the kit. In order to demonstrate compliance with applicable requirements (FAR-27 & AC27-1B) the exercise should demonstrate that the new frequencies are not shifted towards helicopter rotating element frequencies (main rotor, tail rotor and others) and their harmonics. The research I have managed to conduct so far suggests that a transducer is attached to the various elements. This in turn is connected to a spectrum analyser. The attaching parts are then tapped (hammer?)and the resulting vibrations recorded by the spectrum analyser. This is beyond the scope of our normal activities hence the potential requirement for a specialised organisation.

[Pilot DAR]

Though I have not done such a test on a rotorcraft, I have gone one step further on a fixed wing aircraft, here: http://www.lakecentral.com/images/HOWboom1.jpg This testing involved deliberate excitation of the boom at varying frequencies, and observing the natural frequency and self damping following the sudden removal of the excitation. This was done by vibration transducer mounted in a purpose built excitation device, mounted in the very front of the boom. The signals were observed by oscilloscope in the cockpit. The test was repeated in every configutation, and speeds up to 1.1 Vne. I also had to do a comprehensive flight test program on this aircraft with the boom, including spinning it.

I will be testing the same survey boom design on a Hughes 500D next week. Excitation, observation of the frequencies of vibration, or a "rap" test, have not been requested by Transport Canada.

I will, of course, be observing the boom during all phases of flight test for unusual behaviour, and performing more in depth testing if that is found necessary.

If you think I could offer more detailed help, feel free to PM me. I am not, though, a helicopter expert, and have no R44 experience. I have no experience with EASA STC process which I consider positive, or helpful in this regard.

Good luck!

Cheers, Pilot DAR

[Matthew Parsons]

I've witnessed some of our engineers using a hammer with an accelerometer in it to rap external components of the helicopter. The components had accelerometers already attached for some flight test. The strike of the hammer triggered a laptop to start recording the raw data. Spectrum produced and natural frequencies were identified.

The hammer came with various heads. Each had a different frequency band that it would most excite. I believe the hammer cost $14,000.

Matthew.