RE: Re: [indoor construction] VP or VD

From: John Barker <john.barker783_at_ntlworld.com>
Date: Wed, 25 Oct 2006 12:55:32 +0100

I have no practical experience of VD propellers on high class indoor
aeroplanes but I think the following basic aerodynamic stuff may be a help.

The standard expression for propeller torque is Q = kq. rho. n^2. D^5 where
kq is the torque coefficient, rho is the air density, n is the rotational
speed and D is the diameter. If the torque coefficient and the air density
are taken as constant for our purposes it can be said that Q is proportional
to n^2.D^5 or that n^2 is proportional to Q/D^5.

Now assume suffix h is applied for the high torque and large diameter and
suffix b for the low torque and diameter then:
n^2 = Qh/Dh^5 and n^2 = Qb/Db^5
Now as we want n to be constant through the flight we can equate the right
hand sides and rearrange to give:

Db = Dh.fifth root(Qb/Qh)

A typical torque curve has a peak about 5 times higher than the cruise so
that Qb/Qh is 0.2 and the fifth root of this is 0.725. If the propeller
diameter required at high torque is 20” then the diameter required at low
torque is 20 x 0.725 which is 14.5”.

I think in practice, and certainly with a two position movement, you would
probably ignore the first few second of excess torque and work on a high/low
torque difference perhaps 2.5. In this case, if the high diameter is still
assumed to be 20” then the low diameter would be 16.7”

John Barker - England


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Received on Wed Oct 25 2006 - 04:55:56 CEST