Barrowman

Help, please, with Barrowman’s equations. On page 28, looking at the vertical center of pressure on a fin, how does he take into account for the shape of the fin? DeltaXf is found by

Given a for instance of m = 2, A(root) = 5 and B (Tip)=3. Clipped delta or trapezoid design. . M(A+2B) (A*B)

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+ ( 1/6 x -----------)

3(A+B) (A+B)

If the wider part of the fin is at the top this makes sense. Clipped delta fin shape. If the wider part of the fin is at the bottom it doesn’t. If it is a trapezoid shape the vertical center of pressure would seem to be half-way down A.

Width does not matter on the center of pressure on the fin, but doesn’t width matter on the center of pressure over the whole rocket?

Open Rocket and Rocksim must do something similar but how they do it is hidden from the user.

If we use 1/3 instead of 1/6 we get better answers. For a square, rectangle, ellipse or proper trapezoid we get the center of pressure being half way-down. For M = A, a proper delta shape, we get 2/3 of the way down. This works. For other clipped delta designs we get a reasonable answer. For swept designs or irregular trapezoids we need to use something instead of 1/3. But we need to change the formula for different shape fins.