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Help! Scratch build CG?

#3
I usually assume that the CG should be a bit in front of a point that is 1/4 of the root chord length, measured back from the leading edge at the root. Basically, take the root chord length (distance from the leading edge to the trailing edge at the root of the wing), then measure from the leading edge and make a mark on the wing that is 1/4 of that distance. your cg should be a little bit in front of that mark.

For a maiden flight, I usually start with the CG a little bit more than a little bit in front of that point, just to be safe. If you are nose-heavy, you can usually still fly the plane enough to adjust and land. If you err on the other end of the spectrum, you will know very quickly, and your plane will be very difficult if not impossible to fly.

Scientifically, what you want is for the CG to be just in front of the aerodynamic center of the aircraft. The location of the AC is affected by the shape of the fuselage, wing, and tail. However, for most wings, the AC can be fairly well approximated at 1/4 chord.
 

Tench745

Well-known member
#5
I usually assume that the CG should be a bit in front of a point that is 1/4 of the root chord length, measured back from the leading edge at the root. Basically, take the root chord length (distance from the leading edge to the trailing edge at the root of the wing), then measure from the leading edge and make a mark on the wing that is 1/4 of that distance. your cg should be a little bit in front of that mark.

For a maiden flight, I usually start with the CG a little bit more than a little bit in front of that point, just to be safe. If you are nose-heavy, you can usually still fly the plane enough to adjust and land. If you err on the other end of the spectrum, you will know very quickly, and your plane will be very difficult if not impossible to fly.

Scientifically, what you want is for the CG to be just in front of the aerodynamic center of the aircraft. The location of the AC is affected by the shape of the fuselage, wing, and tail. However, for most wings, the AC can be fairly well approximated at 1/4 chord.
This sounds like a simple way to have a nose-heavy plane on anything but a straight, constant chord, or forward swept wing.
 
#6
This sounds like a simple way to have a nose-heavy plane on anything but a straight, constant chord, or forward swept wing.
It's my methodology, and it's worked for me. It's an easy approximation that doesn't require making a bunch of measurements or calculations. Most of the wings i've built have been trapezoidal, and I usually only have to make small adjustments after the maiden to get a more ideal CG.

To each their own.