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# Turning flow through rotor blades

## Turning flow through rotor blades

(OP)
Greetings!

As I am not much of a helicopter guy, I thought I'd turn to the helo forum for help.

I am doing work on a vehicle with a rotor mounted at a 45 deg. angle relative to horizontal.  I have a lifting surface less than one rotor diameter down stream of the rotor, and I need to calculate the angle of attack on this surface.

I understand that there is a vector sum between the forward flight velocity and the velocity induced by the rotor, however through the research I have been doing, I see this as an inflow angle to the rotor.  What I want to know is how much the flow is turned through the rotor?

Ideally, the flow is turned the full 45 degrees, but I don't believe this to be the case in the real world.

Can anyone shed some light on this question?

Regards,
Grant
Aerospace Engineer

Replies continue below

### RE: Turning flow through rotor blades

A good start is to estimate total required lift and drag of system. Lift will result in a change in vertical momentum. Drag will result in a change in horizontal momentum. The air effected will be an approximate elliptical tube, entering the profile of the rotor assy depending on flight direction. Basically it is very unlikely that the downwash will be at 45', except at specific flight regimes.

I couldn't work out what you meant by rotor at 45'. Is this 45' forwards to create thrust and lift, or 45' rearwards to autorotate and generate lift? I assume you are really discussing a ducted fan, with a rear stator?

In either case the downwash rotor whirl will complicate matters, but fortunately is likely to be a fixed rotational velocity. Also the downwash is highly unlikely to be evenly distrubited across rotor (this has been the source of a recent interesting discussion).

Hope this helps...

Mart

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