# CG for rectangular flying wing?

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Does anyone know how to calculate where the CG should be for a rectangular flying wing? I am very interested in building something like the old Scimitar series. Basically a rectangular wing on a box fuselage with only a trailing vertical stabilizer - like this:

I know it should be *somewhere* around C/4 ahead of the leading edge, but I also understand the design is very sensitive to proper CG location so I am looking for more specific info if anyone has it. The references I have found so far are for swept wings which are a different animal. The reason I am going for a rectangular wing planform is I am planning on using coroplast for the wing.

If nobody on this forum knows, I'm headed to the craft store for some foam board and balsa to "derive" it the old fashioned way :)

Thanks!

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My experience with flying wings of every type is that they need to be balanced further forward that the typical 25% MAC. Usually I start at 12% and then move slowly back, and in the end I often find myself at around

15-17% MAC. The shape and planform of the wing will also have something to do with proper CG location, so YMMV, but I would deffinately not try flying it at 25%, chances are you will be instantly out of control.
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I've built several. Use 20% - 25%. Since there's no tail providing lift, you need a slightly forward CG. Dr.1 Driver "There's a Hun in the sun!"

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1. Take planform of one half of the wing.
2. Find the cord line which divides the wing area of that half of the wing into equal parts.
3. Balance is at 27% to 30% of the mean aerodynamic cord.
4. Check the many URL's under "Flying Wings (inc Delta), Flapping Wings or Ornithopter." in particular "The Wing is The Thing = TWITT
also under "C of G & MAC. (Design and Flight Trimming etc see above.)" at Alan's Hobby Web Links
A small 12" sheet balsa model of your project can be used to test your calculations. NB most flying wings/deltas require a reflex airfoil...both elevons (or ailerons and elevators if split) should be set up 2 degrees .. optimise performance with a little experimentation on reflex settings.

regards

Alan T. Alan's Hobby, Model & RC Web Links

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I agree with your reference for rearward swept flying wings. However, for rectangular wing designs like the Scimitar, I believe the CG actually needs to be in front of the leading edge - hence the need for a fuselage or boom of some kind. The Scimitar has a fuselage, and here is a similar design where the author indicates the CG is ahead of the leading edge:

I'm just trying to find a formula or reference for how far in front it needs to be. It's a simple design, so I may just mock it up @ 1/3 scale in foam board and balsa to get the answer - and then give the glider to my girls to play with :)

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The Morris Hobbies Spinsation is a forward-swept flying wing with a rudder. It balances pretty close to 25% of MAC, as I recall.

It takes some getting used to.

-- Mike Norton

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Thanks all for the info! The Gremlin is particularly interesting :) The airplane I am working on will be a SPAD for A-class, slow, survivable combat so the Gremlin is very close to what I had in mind!

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"The CG should be no farther back than one inch from the leading edge from what I have seen."

I am reading this as he is finding the CG should not be more than 1" BEHIND the leading edge

This plane has a 4" chord with a sweep at the tip section. Without the tip sweep, 1" would put the CG at 25% MAC... the sweep moves the

25% point back a bit, but 1" is still well behind 10%.

--- Rich