Center of gravity
05-26-2002 | 01:54 PM
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Center of gravity
I am scratch building my own ultralight and I am at the wing stage. I am going to use a PT-40 wing for its high wing loading but what I need to know is how do I find the center of gravity for the model if its scatch built and no plans to tell me where it should be? Do I use the CG point as if it where a PT-40?
Thanks
Thanks
05-26-2002 | 03:01 PM
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From: Punta Gorda, FL
CG
The best CG location depends on the purpose of the plane, its configuration and the skill of the pilot.
An aerobatic plane and a trainer will have different CG locations because the trainer needs more stability and the aerobat needs more control authority.
The CG location should be adjusted (chosen) for the desired stability. The purpose of the plane and the skill of the pilot will determine the desired stability. The stability is measured by the static margin which is the distance the CG is ahead of the neutral point. The neutral point is determined by the configuration of the aircraft. The longer the tail moment arm as a multiple of the wing mean aerodynamic chord and the larger the horizontal tail as a percent of the wing area, the more aft the location of the neutral point. To complicate matters, the effectiveness of the tail area is reduced by the turbulence of the wing wake and fuselage. Fat and unstreamlined fuselages tend to move the neutral point forward.
Similar configurations and purposes require similar CG locations.
If your tail area is 15 to 20% of the wing area and the tail moment arm is 2.5 to 3 times the average wing chord, then start with a CG at about 25% of the MAC (a safe location). Move the CG aft in small increments between flight tests till you get the handling qualities you are looking for. As the CG is moved aft, the controls will become more sensitive. When the CG is at the neutral point the plane will not have a trimmed flight speed, it will go where it is pointed and it will not recover from a gust or abrupt control input. Moving the CG aft of the neutral point will result in tuck-under, which is dangerous.
An aerobatic plane and a trainer will have different CG locations because the trainer needs more stability and the aerobat needs more control authority.
The CG location should be adjusted (chosen) for the desired stability. The purpose of the plane and the skill of the pilot will determine the desired stability. The stability is measured by the static margin which is the distance the CG is ahead of the neutral point. The neutral point is determined by the configuration of the aircraft. The longer the tail moment arm as a multiple of the wing mean aerodynamic chord and the larger the horizontal tail as a percent of the wing area, the more aft the location of the neutral point. To complicate matters, the effectiveness of the tail area is reduced by the turbulence of the wing wake and fuselage. Fat and unstreamlined fuselages tend to move the neutral point forward.
Similar configurations and purposes require similar CG locations.
If your tail area is 15 to 20% of the wing area and the tail moment arm is 2.5 to 3 times the average wing chord, then start with a CG at about 25% of the MAC (a safe location). Move the CG aft in small increments between flight tests till you get the handling qualities you are looking for. As the CG is moved aft, the controls will become more sensitive. When the CG is at the neutral point the plane will not have a trimmed flight speed, it will go where it is pointed and it will not recover from a gust or abrupt control input. Moving the CG aft of the neutral point will result in tuck-under, which is dangerous.
