# Ellipse size

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Hello,

I have more of a geometry question, rather than CAD question. I need to determine the size of a spar hole for an airplane wing. The wing will be cut out of foam with a hot wire. There is a round spar hole in the wing that runs from the root to the tip on each wing half. A carbon fiber rod goes through it. Seems easy, but the wing is swept back 35 degrees, so round hole will actually be an ellipse is when it exits the root and tip. The hole needs to be 5mm. The ellipse will be 5mm tall, but how do I figure out how wide?

Thanks!

Brian

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I would draw it out. Basically, it appears that you have two 5mm wide objects crossing at 35 degrees. So draw them. The ellipse will be 5mm high but will be as wide as the intersection of the two.

I drew the ellipse by clicking on the two intersections for the major axis, and then inputting on the keyboard 2.5mm as the minor axis.

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or simply calculate major axis =>> 5/cos35

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Thanks Eldon. You're exactly right. The width has been right in front of me the whole time.

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Thanks Paul. Can you please explain how to use cos?

Thanks!

Brian

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If you are doing this in 3D then I would simply place a rectangle at the desire angle.

The width of the rectangle should be 1/2 the drill size.

Then revolve the "drill" and subtract.

No calculations required.