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 | | From: | Don Stauffer | | Subject: | Computing performance increase due to CG shift? | | Date: | Thu, 16 Dec 2004 08:44:40 -0800 |
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> My club has a Piper Arrow with an aftermarket 3-blade prop. We're
> thinking of putting the original 2-blade prop back on. The 2-blade
prop
> weighs 11 lbs less than the 3-blade one.
>
> Ignoring for the moment the aerodynamic attributes of the prop
change,
> and the gross weight change, is there any easy way to calculate the
> performance increase due to the aft CG shift? Moving the CG aft
means
> less down-force on the horizontal stabilizer. I'm guessing 11 lbs on
> the nose means 3-4 lbs on the tail to balance it out.
>
> The proponents of the prop swap claim it will improve performance
> because of the aft CG. My guess is that while the change will
certainly
> be an improvement, the magnitude of the effect will be so small as to
be
> insignificant. I'd love to work some numbers and see how it comes
out.
One quick and dirty procedure would be to estimate the drag reduction
by dividing the 3-4 pounds by an estimate of the L/D of the stab. Say
it were 10/1. That says the drag reduction would be about 0.3 to 0.4
pounds.
Now, estimate thrust horsepower (75 % of max, say). Now, one can use
the binomial theorem here, because we are talking about small changes
to a non-linear quantity. Ratio the 0.3 to 0.4 to the total drag
estimated from thrust horsepower. Now, since thrust effects airspeed
by square, take one half of that and use that to ratio new airspeed.
Seems to me that would give you a quick and dirty first estimate.
Don
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