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Jun 30, 2015, 12:20 PM
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bmblb's Avatar
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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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Jun 30, 2015, 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.....

https://www.rcgroups.com/forums/show...3#post32038886

.....greatly appreciate all responses
Last edited by bmblb; Jun 30, 2015 at 11:04 PM. Reason: Added link
Jul 01, 2015, 12:03 AM
solastagia
kcaldwel's Avatar
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
Last edited by kcaldwel; Jul 01, 2015 at 12:18 AM.
Jul 01, 2015, 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?
Jul 01, 2015, 10:11 AM
solastagia
kcaldwel's Avatar
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
Jul 01, 2015, 08:06 PM
Ft Lauderdale
Number_6's Avatar
Hey, what's up?

I would use a pusher motor if the 'physics'
work out.

Not a fan of tractor motor repairs.....
Last edited by Number_6; Jul 01, 2015 at 09:13 PM.
Jul 01, 2015, 11:54 PM
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elevon location - inward or outward panel?


Quote:
Originally Posted by kcaldwel
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
......thank you !

1. As regards the elevons - which would be a better location -inward OR outward?

(elevons on inward panel wont need servo extn, tips remain stronger, besides getting longer 'coverage')

2. So, fins at extreme tips running entire chord would be suitable + a center fin?

3. Pusher with a longish nose is the way to go !
Jul 01, 2015, 11:56 PM
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Quote:
Originally Posted by Number_6
Hey, what's up?

I would use a pusher motor if the 'physics'
work out.

Not a fan of tractor motor repairs.....
thanks Number_6.....to be honest, I am yet to fly a 'tractor plane...just that getting the CG right is sometimes tricky.
Jul 02, 2015, 12:00 AM
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some pics


some pics
Jul 02, 2015, 07:00 AM
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The CG and yaw stabilty issues can be worked out fairly well/easily with the good advice from the others.
The greatest challenge on a wing with multiple bends (including the bend at the root, which gets the most stress) is most likely the bracing issues - as the bends make the bracing a lot more challenging.
The dilemma is this: If the bracing is too light, it will flutter itself apart with a lot greater likelihood than something with no bends and a straight spar. Yet, if the bracing is strong enough to mitigate the flutter, it can end up being pretty freakin' heavy and/or ugly.
Jul 02, 2015, 08:41 PM
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Quote:
Originally Posted by nuteman
The greatest challenge on a wing with multiple bends (including the bend at the root, which gets the most stress) is most likely the bracing issues - as the bends make the bracing a lot more challenging.
The dilemma is this: If the bracing is too light, it will flutter itself apart with a lot greater likelihood than something with no bends and a straight spar. Yet, if the bracing is strong enough to mitigate the flutter, it can end up being pretty freakin' heavy and/or ugly.
.


thanks nuteman - I realised this from my previous build (40 hours of building time - and 6 mins of flight - it felt SO strong ) - quite devastating.

....as a matter of fact, as soon as I finished the wing, I did a test launch - farily decent - but it landed at the root and - snap - in half

So, I reinforced the gull wing at the root and the wing breaks in addition to the'braces' initially included - with additional brace, hot glue and tape, it is much stronger now !
Jul 03, 2015, 03:46 AM
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Quote:
Originally Posted by bmblb
.


thanks nuteman - I realised this from my previous build (40 hours of building time - and 6 mins of flight - it felt SO strong ) - quite devastating.

....as a matter of fact, as soon as I finished the wing, I did a test launch - farily decent - but it landed at the root and - snap - in half

So, I reinforced the gull wing at the root and the wing breaks in addition to the'braces' initially included - with additional brace, hot glue and tape, it is much stronger now !
There is a fairly reliable way to test the strength before flights of any sort:
1. Support the plane with chairs under the wings, each of them being 2/3 of the way from the centerline of the aircraft (if they're too close to the center then the test will not validate the bending moment issue).
2. Pile up books on the centerline of the plane. The weight of the books should be equal to 5 times the weight of the airplane (yes 5, that's not a typo).
You have now simulated a plane under a 6G load (1G for the plane itself and 5Gs for the books).
If it holds up to that, you have a fair chance in the air.
Believe me, you WILL pull at least 6G maneuvers in the air, if only inadvertantly.
Of course, if you really want to yank the plane around with very little worry, you can do the same test with 8 or 9 times the weight of the plane in books
Jul 04, 2015, 10:16 AM
Registered User
FYI It's possible to have too much stability, and that will make it difficult to maneuver. It looks like you have dihedral for lateral stability, and sweepback for yaw stability. This will be a very stable bird.
Jul 06, 2015, 01:21 AM
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Quote:
Originally Posted by Novacat2005
FYI It's possible to have too much stability, and that will make it difficult to maneuver. It looks like you have dihedral for lateral stability, and sweepback for yaw stability. This will be a very stable bird.
Thanks Novacat.....its almost done and will maiden in a few days.

reports soon


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