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Dumb question, perhaps

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H.Evan'sRV7A

Well-Known Member
Joined
Jun 2, 2011
Messages
191
Location
Grosse Pointe, MI
I have seen many drag polars, especially in CAFE reports, which show induced drag based on the weight and the span. They also compute total minimum drag = thrust based on weight and sink at V for L/Dmax. I get that and I get why they should be equal. And, of course, at V for L/Dmax induced should equal parasite and thus induced should be exactly half of total drag.

But, conventional aircraft with the tail in the back are said to be lifting about 10% more than the weight in the front and pushing down about 10% in the back. The formula for induced drag is lift^2 divided by an expression that boils down to [q * span^2 * Pi * a fudge factor they call e]. Weight and lift are not the same. And to make it worse, there should be some induced drag from the tail (not much but some). How to reconcile this? Are they looking at e (span efficiency) wrong? Something else?

Thanks!
 
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