Rotorcraft Stability: Explaining Spiral Instability in Aerospatiale Puma SA330

[n69eil]

Hi,

I have been working on a report about the stability of the Aerospatiale Puma SA330,, and shown that it has an inherent spiral instability during large bank angles.

I later found that this was due to coupling between the state matrix derivatives: Omega.aCosTheta, N’v and N’w.

Anyone any idea how to explain this?

 

[FredGarvin]

Oh man. You are going to make me pull out my very thick and old helicopter dynamics book.

IIRC, that means that there is going to be a large yaw induced at the large bank angle causing the nose to pitch down and thus the spiral. I am not sure, due to the notation, if this is due to a loss in the tail rotor effectiveness or not. I would imagine that the yaw would be in the opposite direction of the main rotor rotation if this were the case.

I really will have to look this up and see what I can find.

 

[n69eil]

Hi,
thanks for reply...

also just noticed I made a mistake (N'v should be N'u) which kind of makes the problem nit more tricky

It has been seen that the vertical stabilizer is quite ineffective for small perturbations (sideslip).

It is the nature of these derivatives, and how they couple is confusing, especially for Nu as I can't find information on it.

 

[FredGarvin]

Well, I did some searching in the literature that I own. I came up with nothing that covers the coupling that you are asking about. It may be there but I certainly did miss it if it is.

I would highly suggest that you post your question on Eng-tips.com. They have a forum specifically for rotary wing engineering. There are a lot of knowledgeable people in the business there. I am sure someone there can help you out. Sorry it took me so long to get back in touch with you. If you get an answer I'd like to see if you can post it here as well.

http://www.eng-tips.com/threadminder.cfm?pid=6