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 | | From: | Craig Fink | | Subject: | Computing performance increase due to CG shift? | | Date: | Thu, 16 Dec 2004 18:08:42 -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.
You should double the 3-4 lbs, if the tail is lifting down by 4 lbs, the
wing must have and additional 4 lbs of lift to counteract the downforce on
the tail. So it should be 6-8 lbs. Also, add in the 11 lbs reduction in
overall weight. So, it should be 17-19 lbs of reduced lift.
Craig Fink
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