Cg
05-19-2003, 04:13 PM
#2
Cg
The easiest thing to do is to measure back from the leading edge of the wing about 1/3 of the wing cord, and balance it there (providing this is a conventional plane).
Fly it. Bank the wings 45* and cut the power. Does it nose down right away? = nose heavy.
nose up? = Tail heavy.
It should maintain a level flight for a few seconds before nosing down.
Fly it. Bank the wings 45* and cut the power. Does it nose down right away? = nose heavy.
nose up? = Tail heavy.
It should maintain a level flight for a few seconds before nosing down.
06-15-2003, 01:18 AM
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Cg
If the plane design is fairly symetrical, you can fly it up right then inverted and see the response. You should need very little imput to keep it level inverted or no input for an AFT CG point.
06-15-2003, 03:23 AM
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Cg
I think these tests would be true as long as there are no thrust angle problems built into the 'system' that would influence the flight characteristics. If the plane had severe downthrust or left thrust you can imagine how that would throw off any tests.
For sailplanes (which have no engines thus no engine thrust to worry about and no change in fuel weight because fuel is not consumed) they sometimes use what they call a 'dive test' to come 'somewhat close' to finding the 'natural center of lift point' of the airfoil at a given speed and angle relative to perfectly horizontal flight. Again this means there is no influence from engine misalignment and weight changes.
If you can determine the thickest part of the wing then that will be very close to the point of the largest lift of that wing at that distance from the fuselage. In most cases this is somewhere between 33% and 40% depending on the airfoil. But 1/3 back from the leading edge of a standard rectangular wing planform is safe and experimenting from there with small changes in CG would be appropriate. If your wing planform is drastically different from a standard rectangular planform other methods must be used to find a safe beginning CG point. For example an F-86 has wings that really rake back so the CG for the plane would not be 1/3 of the cord back from the root leading edge as you already guessed.
As for the tail pushing down, this would only occur if there is too much noseweight, thus the need for opposing force to keep the nose up. If there was too much tailweight the tail would have to lift to compensate. If there was neither too much nose weight nor too much tail weight there would be no tail force (horizontal stabilizer force) needed for level flight. If we reached this harmony then the center of gravity of the plane has been 'set' right under the center of lift of the wing. And this balance point is what we were originally trying to determine. This is actually the basis of the 'dive test' I mentioned. If you need more information on the dive test you can ask about that in the sailplane section. If is very useful and easy to perform but not 'perfect' in all respects.
For sailplanes (which have no engines thus no engine thrust to worry about and no change in fuel weight because fuel is not consumed) they sometimes use what they call a 'dive test' to come 'somewhat close' to finding the 'natural center of lift point' of the airfoil at a given speed and angle relative to perfectly horizontal flight. Again this means there is no influence from engine misalignment and weight changes.
If you can determine the thickest part of the wing then that will be very close to the point of the largest lift of that wing at that distance from the fuselage. In most cases this is somewhere between 33% and 40% depending on the airfoil. But 1/3 back from the leading edge of a standard rectangular wing planform is safe and experimenting from there with small changes in CG would be appropriate. If your wing planform is drastically different from a standard rectangular planform other methods must be used to find a safe beginning CG point. For example an F-86 has wings that really rake back so the CG for the plane would not be 1/3 of the cord back from the root leading edge as you already guessed.
As for the tail pushing down, this would only occur if there is too much noseweight, thus the need for opposing force to keep the nose up. If there was too much tailweight the tail would have to lift to compensate. If there was neither too much nose weight nor too much tail weight there would be no tail force (horizontal stabilizer force) needed for level flight. If we reached this harmony then the center of gravity of the plane has been 'set' right under the center of lift of the wing. And this balance point is what we were originally trying to determine. This is actually the basis of the 'dive test' I mentioned. If you need more information on the dive test you can ask about that in the sailplane section. If is very useful and easy to perform but not 'perfect' in all respects.
06-15-2003, 11:55 AM
#7
Cg
Here's a link that will give you a graphical scientific method for finding the proper balance point on practically any wing.
http://home.earthlink.net/~pjburke1/cg.htm
Then you, too, can be an aerodynmasis...arrowdymnai....errodymnai...xxxxxx
Airplane designer
http://home.earthlink.net/~pjburke1/cg.htm
Then you, too, can be an aerodynmasis...arrowdymnai....errodymnai...xxxxxx
Airplane designer
