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Author Topic: Where is your plane's Center of Gravity (balance point)?  (Read 223 times)
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Little-Acorn
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« on: November 12, 2017, 02:08:34 PM »

In most (non-Wright-Stuff) planes, the center of gravity (CG) is near the middle of the wing. Sometimes a little ahead of the midpoint, sometimes a little behind..

One Wright Stuff competition plane I saw (a plane that flew beautifully, 1 min 30 sec flights with a 1/16" motor and 1,200 winds), had its CG one inch BEHIND the trailing edge of the wing. From what I've studied in Aerodynamics, a plane with the CG that far back, couldn't possibly fly well, and would likely nose up as soon as it was launched, come to an almost complete stop in midair, and then fall to the ground within a few seconds of being launched. But this one didn't do that, it flew in slowly ascending circles until it was 30 ft. up, circled there for a while, then gradually circled down to land, a beautiful flyer.

When I measured it, the CG was 1 inch behind the trailing edge. I couldn't believe it, and measured it again, looking for mistakes, a thumb on the scale, everything. Nope, that was exactly the CG position. And the plane flew very well.

I still haven't figured that out.

I set up my own experimental plane that way. Same weight, same size lifting stab, same length, and I adjusted it until the CG was one inch behind the trailing edge. Wound it up and launched it, and it nosed up, stalled, and fell, all in a few seconds.

Why doesn't my plane fly like that other one did?

Where is the CG on your plane(s)? And how do they fly?

(Is my frustration obvious?)  Huh
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Hepcat
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« Reply #1 on: November 13, 2017, 07:05:08 AM »

Little Acorn,
Your frustration is as nothing compared to the frustration of people on this forum who would be delighted to help you if you could give some plain (plane) facts. You tell us you set up your plane: same weight, same size lifting stab, same length. Same as what? And how did you adjust the CG?  Do I gather that you wound it up and launched it without a test glide?
An early simple check to see if you CG is at least in the Ball Park is to multiply the wing area by the distance of the wing quarter chord to the CG and to multiply the tailplane area by the tailplane quarter chord to the CG and if the two answers are similar then the CG position is probably useable.
John
 
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leop
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« Reply #2 on: November 13, 2017, 09:28:34 AM »

John's calculations describe a location called the neutral point. For indoor duration airplanes that use a lifting stab (most indoor planes but almost no full size airplanes or indoor gliders) this is the location that balances the wing and stab lifting forces so that there is, theoretically, no pitching (tendency of the nose to go up or down) moment  except the pitching moment of the wing and stab).  BUT, our indoor planes will not be pitch stable if the CG is located at the neutral point.  The optimum CG is usually located at a point behind the wing quarter chord location that is some fraction (less than one) of the distance between the wing quarter chord location and the neutral point.  With such a location, the stab inclination (and lift) can be decreased (compare to the wing - the difference in angle being the decalage) and pitch stability regained (compensating for the individual pitching moments of the wing and stab airfoils). 

Thus, the CG location is in front of the neutral point.  The more in front, the more stable the plane.  But this stability lessens flight times so the game is to move the CG rearward until the stability is just enough.   World Championship caliber F1D planes have a CG location from 50% to 75% (depends on the design) of the neutral point distance behind the wing quarter chord location. For F1D's (and other indoor duration planes), the decalage is often increased or the CG is moved forward when the air is rough as such a condition requires more stability so that the model will recover well from upsets caused by bad air.

BTW, we use the quarter chord of airfoils as that is the theoretical (and, most often, real) point where the airfoils' pitching moment (tendency to twist) stays that same at all angles of attack (incidence).

LP
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Hepcat
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« Reply #3 on: November 13, 2017, 12:06:55 PM »

Thank you Leopold for amplifying my reply and apologies to Little Acorn for allowing my frustration to lead to a 'snippy' answer.  I did think of going further on Neutral Point and pitching moments but as the machine you admired had a CG Behind the trailing edge I thought it might be confusing things even more.
John
 
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