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Does a propellor add side area?

Old 01-02-2012, 10:27 PM
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Default Does a propellor add side area?

When I converted a perfectly stable pusher design into a tractor layout it exhibited very marginal lateral stability with almost constant 'fish tailing'.

It was a relatively large folding prop so had to be mounted quite far forward to clear the wing although the nose of the fuselage had a very small side area.
It required nearly 100% increase in fin area to achieve even a modest level of lateral stability.

The degree of the effect of prop side area surprised me.

It took some serious Googling before I found a paper release by NACA in 1945 that attempts to actually quantify this effect.
The mathematics are quite beyond me but the author summarises it as being equivalent to a fin mounted in the inflow area of the prop of a size that depends on the angle of yaw and the number of blades.

This would certainly explain why a fin set up that was entirely satisfactory with a pusher prop mounted aft of the wing became laterally unstable when the prop was moved well ahead of it.

I suppose the question is does the loss of efficiency of a prop mounted in the wings turbulence out weight the extra fin area required with a tractor prop mounted forward in clear air?
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Old 01-03-2012, 01:38 AM
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Yeah, a prop has an effect similar to vertical (or horizontal) area, but only when it's running. The reason for this is that anything that deflects airflow creates 'lift'. Deflecting airflow to create lift is most often thought of as something done with a wing but the inflow of air into a prop disk is also deflected by the spinning prop. This effects longitudinal as well as directional stability.

The precise math is beyond me but the effect is basically proportional to the thrust the prop generates. When creating no thrust there is no 'area effect'. So as you found; pusher props mounted aft of the CG are stabilising but tractor props mounted ahead of the CG are de-stabilising (while they are running). Cut the power and the effect goes away so it's a risky business to reduce fin area and rely on the stabilising effect of a pusher prop to 'top-up' stability.

Tractor installations are (from what I've read) usually thought of as more efficient overall, for most applications.

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Old 01-03-2012, 01:51 AM
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PS.. You can see this effect in some full size aircraft that were through their life fitted with more powerful engines and larger props, they often grew larger tails to compensate. The best example off the top of my head is the Spitfire; compare the vertical stabiliser size of the early Spits against that of the later Griffon engined variants, the vertical stab must be close on twice the area.
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