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Old Yesterday, 12:20 PM
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Question
Neutral Point for Flying Wings

.....I am making a gull wing flying wing with two sweeps (42in, KFm1, Coro+Foam) as in the screen shot. A friend mentioned that static margin in addition to getting the CG right is the key to desired performance.

...while searching for more info, I came across a few calculators and methods and somehow a bit confused.

...I am listing down the method as I understand:

1. Locate MAC and MAC Distance from root.
2. Locate 25% of MAC from LE and draw a perpendicular line to the root, thus arriving at NP(??)

I have also read that an ideal static margin range is 5%-15%.

From the screen shot, the CG is at 0.41in and the NP calculated as described above is about 0.75in from LE and the MAC is 7.7in thereby arriving at a SM of 4.4%.

I am not sure I got this concept correctly as most of the tools consider the the aerodynamic center(AC) of mainplane and tailpalne and ......I lost it from here

This would help me decide the length of the fuse (tractor) and I want to keep the weight as low as possible. Some pics of the work so far.

Could anyone please shed more light here?

Thanks !
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Old Yesterday, 09:51 PM
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hello all !

I found this nice description....but I am looking at similar explanation specifically for flying wings ! does the method described in the previous post correct......or does it give the AC.....

http://www.rcgroups.com/forums/showt...3#post32038886

.....greatly appreciate all responses
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Last edited by bmblb; Yesterday at 11:04 PM. Reason: Added link
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Old Today, 12:03 AM
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I believe your SM calculation is correct: the CG is 0.31" ahead of the neutral point (25% of MAC), on a 7.7 mean chord length, so 4.4%SM. That should be a pretty good place to start for pitch stability.

I have never seen any pitching coefficient data for a KF airfoil. Most flying wings without a lot of sweep use reflexed airfoils with near zero or slightly positive pitching coefficients. If the airfoil has a negative (nose down) pitching moment, the elevons will have to be deflected up a lot to counteract the airfoil pitching moment. A lot of upward deflection on the elevons will hurt the performance, and possibly stall the control surfaces.

I'm not sure how effective your vertical fins will be for yaw stability, since they are quite far forward. Flying wings are always tricky to get the dihedral/anhedral, sweep, yaw stability, and roll/yaw coupling working together correctly. I suspect that will be your biggest challenge.

The pusher propeller should be stabilizing in pitch and yaw when it is running, so that should help a bit.

Good luck!

Kevin
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Old Today, 06:30 AM
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Thanks kcaldwel !!

as regards the fins, I intend to have 3 - one at each wing break running entire chord, and a larger one in the center with as much area possible - this should do, I guess?

also, due to the CG location, and the choice of batteries avbl (53-68g), the motor would be in the front !

......the CG, NM & SM, WL seem to be ok..... and the evelvon area is about 9-11%.

....is there anything else I have overlooked?
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Old Today, 10:11 AM
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The force on the fins will act through their 1/4MAC points. They are little vertical wings. The force acts around the airplane CG. The fins out at the dihedral break will have their 1/4MAC points very close to the CG, and therefore won't be very effective.

You can get a bit of yaw stability by having outboard fins toed-in, so that as the airplane yaws, the forward going fin causes more drag. It isn't a very efficient solution, and works better with tip fins.

You might be better with just a central fin back as far as possible. The forward swept wing with lots of dihedral, a tractor propeller, and a forward pod with side area, will be working against the fins.

I'm a bit concerned about the effectiveness of the elevons for pitch control. That should be interesting as well. Usually, on a forward swept wing, there would be separate elevators at the centre of the wing for pitch control, as far behind the CG as possible.

Should be interesting!

Kevin
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