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Old Jul 27, 2011, 09:24 AM
nsg
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Brian - is that on 4s? that setup would seem somewhat weak on 3s... for a 2m plane, anyway.
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Old Jul 27, 2011, 09:42 AM
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United States, CA, Cameron Park
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Yes, that would be on 4s.
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Old Jul 30, 2011, 08:02 AM
...just an earth bound misfit
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Quote:
Originally Posted by cpdude View Post
@Asto-No verticals. None necessary!

Brian
What is the Horten's key to achieving yaw stability? I've been farting around on a finless stryker. Mk-1 was promising but a little draggy and subject to wing rocking under some conditions, however the last two kicks at the can (Mk-2 & Mk-3) suffered from abrupt departures into flat spins.

It seems to be a complicated mix of torque, turbulence, laminar flow, cross-span flow, and CG. I'm missing something here and am stuck until I get a better understanding of the stability issues.

Hope you can provide at least a little insight into the success of the Horten designs.

Cheers.
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Old Jul 30, 2011, 10:24 AM
internet gadfly
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According to Karl Nickel, a Horten test pilot and the guy who did the twist calculations for some of the gliders, nose heaviness produces directional stability. That whole "bell shaped lift distribution (BSLD)" thing wasn't talked about until the '50s but a nose heavy swept wing will have a BSLD so, whether they used that phrase or not, it is what they were doing. Now to trim a nose heavy 'wing you need a lot of washout or reflexed elevons. You want to avoid reflexed elevons at cruise speed because elevons make a lot more drag than washout. Unfortunately the regular, what we call "linear" washout also creates more drag than it should because it leaves a big, non-lifting, dead spot in the middle of the half span. That dead spot just makes drag but not lift so your lift to drag ratio gos down but it also decreases the directional stability by moving the drag forward. You want this dead area as far outboard as possible and that's what the Horten non-linear twist is about. The math to figure it out involves integral calculus so not many people can do it by hand. People who can read German can use a program called Nurflugel. If neither one of those options are for you you can approximate their results by simply balancing the wing properly and only twisting part of the wing. To do the approximation you need two airfoils something like the MH-45 or PW-51 for the inboard 1/3 and a symmetrical airfoil for the tip. Use a spreadsheet like this one to find the total twist at the speed you want to fly. And this chart to find the CG for the amount of directional stability you want. Then cut your cores so that the airfoil is the same from the root to about 1/3 of the half span with no washout. Then all the washout gos into the outboard panel. Also limiting the elevons to 1/3 (or even a bit less) of the wing span can help maintain coordinated turns over a wider speed range. Another advantage of small elevons is that you can fit the wing with flaps.

-Norm
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Last edited by nmasters; Jul 31, 2011 at 12:23 AM.
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Old Jul 30, 2011, 10:32 AM
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United States, CA, Cameron Park
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Thanks Norm! I just knew that you would jump on this?

Brian
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Old Jul 30, 2011, 12:31 PM
...just an earth bound misfit
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I had thought that CG and washout might be the keys but it looks like the airfoil part of the equation could complicate things in my case.

My hope was to achieve a net reduction in drag. It looks like it might not be in the cards for a stock stryker core but I will continue to putter with this project for a bit longer.

Thanks for the great explanation Norm. Your posts are always informative and thought provoking.

PS: Awesome looking build Brian. Looks like fun.
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Old Jul 30, 2011, 12:32 PM
internet gadfly
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You're welcome, Brian, although I didn't contribute anything to your build. When I spotted this thread you were already pretty far along so I've just been watching. But when somebody calling himself "Dirt-Torpedo" says that he has been "farting around on a finless stryker", well, how could I not open my big opinionated mouth
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Old Jul 30, 2011, 12:54 PM
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Hi cpdude,
Very nice!! 2 Meter Horten XII Short kit, what will it include?
Thank you,
Joe
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Old Jul 30, 2011, 01:15 PM
internet gadfly
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Quote:
Originally Posted by Dirt-Torpedo View Post
I had thought that CG and washout might be the keys but it looks like the airfoil part of the equation could complicate things in my case.
The low aspect ratio has a much bigger influence than the airfoil.

Quote:
My hope was to achieve a net reduction in drag.
Not likely to happen in any case. A bell shaped lift distribution will never have less drag than an elliptical lift distribution for the same span. It just produces a better trimmed aircraft and, just as with conventional tailed aircraft, trim costs some drag. Smaller fins on the tips would work better and be less draggy than those big inboard fins were. Fences aren't a terrible idea either but what you tried looked to short. The height of the fence should be about equal to the thickness of the wing or a bit more and it needs to wrap around the leading edge at least 30 degrees. There are also some stall delaying device besides fences. My favorite one is the vortilon because it dose a similar job to a fence but doesn't make as much drag

Quote:
It looks like it might not be in the cards for a stock stryker core but I will continue to putter with this project for a bit longer.
If you're not averse to some butchery you might try diffuser tips but only as a last resort because they'll mess up any future experiments

--Norm
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Old Jul 30, 2011, 02:04 PM
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United States, CA, Cameron Park
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Hey Joe, I send you a PM.

Brian
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Old Jul 30, 2011, 02:53 PM
...just an earth bound misfit
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Originally Posted by nmasters View Post
... Now to trim a nose heavy 'wing you need a lot of washout or reflexed elevons. You want to avoid reflexed elevons at cruise speed because elevons make a lot more drag than washout. ...
-Norm
Would it be effective to use a nose heavy CG for low speed stability and then trim the CG and reflex at cruise to avoid the attendant drag?

I don't want to clutter up this thread, but the idea is relevant since it was apparently used to test the stall characteristics of the H III aircraft.
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Old Jul 30, 2011, 03:22 PM
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Hey Dirt,

I'm not sure that I understand what you're saying when you say "then trim the CG and reflex at cruise". Norm is correct...you don't want to reflex at cruise. It sounds like you are saying just the opposite.

If you want the wing optimized for for cruise then you will have the elevons up (reflexed) in slow flight and neutral at the optimized cruise speed.

I hope that makes sense.

Brian
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Old Jul 31, 2011, 03:52 AM
...just an earth bound misfit
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Brian, the thought was to have a very forward CG & lots for reflex during low speed flight, to improve stability without external aero devices, and then move some ballast aft to permit the elevons to be relaxed at cruise.

I'm assuming here that the directional stability would be improving with speed, as seemed to be the case from my flights on the Mk-1 finless Stryker.

The ballast would likely be the LiPo, on a tray with pnuematics or a lead screw actuator, possibly controlled by an airspeed sensor.
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Old Jul 31, 2011, 08:23 AM
Designing something...
United States, CA, Cameron Park
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@Dirt - My bad. I didn't realize you were moving ballast fore and aft. Sounds right then. I believe that the big (100") Klingberg wind did this too.

Brian
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Old Jul 31, 2011, 04:37 PM
Designing something...
United States, CA, Cameron Park
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Cabin and control surfaces

Well I managed go get the control surfaces and the cabin completely skinned. I still have to finish skinning both of the wing panels but I'm pretty happy with it so far.

Brian
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