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Bill's CG

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PostPosted: Tue Aug 14, 2018 4:01 pm    Post subject: Bill's CG Reply with quote

On the day this aircraft was weighed, the Center of Gravity worked out to 32% of chord.

We first measured the chord both for the wing section with the aileron, and then the section without the aileron.

Our decision on which wing chord to use for CG purpsoes was based on the fact that the wing makes more of its lift toward the center of the wing than it does at the outboard section. This is why all the textbooks have what is called the "elliptical shaped lift distribution". The amount of lift it makes gets smaller the further outboard you travel along the wingspan.

However, on a Kolb, the outboard half of the wing has more area, because there is an 11 or 12 inch aileron there. So you could make an argument that the Kolb will have a little more lift outboard than a J-3 Cub, and as such it will not have as smooth and elliptical of a lift distribution as a textbook J-3 Cub wing.

But it still will make a little more lift on the inboard half than it will on the outboard half, because the lift HAS TO taper down to zero at the tip itself.

So the AVERAGE of the wing lift will be a little bit more centered on the inboard half of the wing than it would be on the outboard half.

So we used the inboard section of the wing as the "chord" when we determined what the "percentage of chord" was, because a little more of the lift is made there.

This is also the conservative safe thing to do, because even if we were wrong, it would be an error on the safe side. To explain, iIf we had a CG of 32% as measured on the inboard chord, but we should have been measuring the outboard chord instead, the same balance point on the wing surface (X number of inches behind the leading edge) would wind up being a slightly smaller percentage of the outboard chord.

Now, 32% of the inboard chord is still well forward of the maximum 37% as specified in the Kolb plans. Again, even if we plotted this out on the wrong section of wing, it would have represented LESS than 32% of the correct chord.

Tomorrow we are going to go through the actual numbers again, and do the math again, and make sure we are including all the fuel, and my "new and improved" body weight.

But at this stage it seems unlikely that the aircraft was WAY OUT of the safe CG range.

Bill Berle  - safety & performance upgrade for light aircraft           - winning proposals for non-profit and for-profit entities

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