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Apr 15, 2012, 07:36 PM
Registered User
Kent,

I flew my hang glider there the same day, but I think I missed you! Sorry I didn't contact you in advance, I made a last minute decision to go flying and got focused on only that. It would have been fun to see your wing from the air.

Steve
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Apr 15, 2012, 11:10 PM
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nmasters's Avatar
I would be neat to see some aerial pictures of it but I suppose it wouldn't be safe to get that close.

--Norm
Apr 16, 2012, 01:52 AM
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andrecillo76's Avatar
Hi Kent,

nice videos! Sometimes it seems like the strong curves induce the dutch-roll mode. It has a period of about 1.5 to 2 sec. Look at the first turn right after take-off at 24 sec. A yaw oscillation is clearly seen. Maybe the negative yaw you experienced is part of the the dutch-roll. It's hard to say if the wing also rolls, but usually a yaw oscillation is accompanied by a roll oscillation in a Horten (yaw-roll coupling). Reducing the dihedral could help in a next prototype.

Cheers,
Andrés
Last edited by andrecillo76; Apr 16, 2012 at 04:16 AM.
Apr 16, 2012, 09:42 AM
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Knoll53's Avatar
It's funny Norm, I always fly near the hang glider pilots. My MO is as follows. When they are at distance I'll be flying aerobatics and generally being a danger to myself and the seagulls. THEN when the HGers get close, I straighten up, fly smooth and slow with the nose into the wind. I'll fly right over to them, in this fashion, and stay far enough away from them that they could not hit me if they tried, which is quite close. The second things become too close, I dive and move out. I've never had a problem with this method. We can easily communicate as they fly by, but never a negative comment from the HGers. I too hope to get some in-air video.

Andres: I was hoping someone would notice! Thanks for your comments. That first turn at :24 seconds is a common occurrence. I'm trying to develop a flying technique to combat that behavior, but I am not always able to do so. It occurs at higher AOA. Also, the wind is not perpendicular to the hill, so turns on the left are down wind turns and turns on the right are up wind turns. Or opposite that. I don't remember which turns are which, but as a pilot, you quickly learn which side is the problem side.

I think that I have about 1.5 degrees dihedral per side. I will continue to try different elevon mixing schemes to attempt to build-in a coordinated turn. At this point, that and CG adjustment is all I can think of as available variables.

Kent
Apr 16, 2012, 11:08 AM
Registered User
andrecillo76's Avatar
Hi Kent,

dutch-roll appears in Hortens at high AOA (slow speeds), because directional stability falls below a critical threshold. A small distubance yaws the plane and there you go, the wing rolls strongly, it yaws in the other direction, etc. leading to the dutch-roll. Actually on an unmanned plane this flight mode is non-critical. If passengers are on board, you'll probably need a lot of bags . Your HXc seems not to dutch-roll during normal flight, but that tight turns are really near to being a stall .

Is the dihedral 1.5° continuous or is the more outside? Reducing the dihedral would help. BTW I've never seen a Horten with a truelly uncontrolable unstable spiral mode, which is always the argument for not reducing too much the dihedral. You could reduce dihedral at least by 0.5°. The other prossibilities involve changing the lift distribution...

Cheers,
Andrés
Apr 16, 2012, 11:47 AM
You know nothing....
Stuart A's Avatar
Hand up at the back of the class.Given Andres comments,can you enlighten me on the aerodynamic thinking behind the original design? It has more dihedral,but anhedral tips and fins at the angle change.I understand that too much dihedral is not good with a swept wing,so why would they use it and then apparently introduce the tips and fins to counteract it.
Stuart
Apr 16, 2012, 11:55 AM
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Knoll53's Avatar
Quote:
Originally Posted by andrecillo76
Your HXc seems not to dutch-roll during normal flight, but that tight turns are really near to being a stall .

Is the dihedral 1.5° continuous or is the more outside?
When turning hard, at the slopes, I pull back hard on the stick. Lots of up elevon. Being near stall does not surprise me. Maybe I can learn to ease off of the up elevator to reduce the yaw problems after the turn.

Answering your question regarding dihedral is a little hard to do. There is nothing linear about this wing. I can tell you that first I laid out the ribs at the proper angle, then adjusted their height to provide a straight hinge line at outer elevon, then modified the height again, while keeping a straight hinge line so that the LE was as straight as possible. At this point, the entire assembly was tilted 1.5 degrees for dihedral.

The images show the top surface of each rib in green. The red line connects the LE of each rib. The blue line is a reference line from the root to the tip. You can see that at the tip, the LE droops down a bit. I attempted to make the dihedral linear.

Based on your comments, I'm glad I did not go with the 6 degress +- shown on the original prototype drawing.

Kent
Last edited by Knoll53; Apr 16, 2012 at 12:06 PM.
Apr 16, 2012, 12:45 PM
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Knoll53's Avatar
Quote:
Originally Posted by Stupot46
Hand up at the back of the class.Given Andres comments,can you enlighten me on the aerodynamic thinking behind the original design? It has more dihedral,but anhedral tips and fins at the angle change.I understand that too much dihedral is not good with a swept wing,so why would they use it and then apparently introduce the tips and fins to counteract it.
Stuart
I'm no historian, but the Horten brothers were not only way ahead of their times, but also free thinkers. My guess is that they were experimenting with these design features in an attempt to solve the various control and performance issues. Also, since this is a foot launched glider, having the wings up off of the ground with dihedral is a good thing for ground handling and launch. In this case, maybe too much dihedral, which explains the tip anhedral. Just a guess. It is worth noting that the full scale HXc was never built.

Kent
Apr 16, 2012, 01:02 PM
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Knoll53's Avatar
Quote:
Originally Posted by andrecillo76
The other prossibilities involve changing the lift distribution...
I missed this on the first reading. Maybe I COULD improve things by different elevon mixing schemes.

Kent
Apr 18, 2012, 12:59 AM
Registered User
andrecillo76's Avatar
Quote:
Originally Posted by Stuart
It has more dihedral,but anhedral tips and fins at the angle change.I understand that too much dihedral is not good with a swept wing,so why would they use it and then apparently introduce the tips and fins to counteract it.
I guess they used the tip fins not only to reduce dihedral but also to increase directional stability. The inner fins prevents the stall from spreading towards the tips and avoids tumbling. The CG of the true H-Xc lies in a different position on the z-axis than in a model. Dutch-roll happens mostly at slow flight and usually a pilot hangs his/her legs down at that conditions (take-off and landing), shifting the CG downwards and reducing the dihedral effect. Also, the tendency and frequency to dutch-roll depend on size and mass distribution, which cannot be simulated in a model. So not all models that dutch-roll would do it exactly the same in a 1:1

Cheers,
Andrés
Apr 18, 2012, 08:15 AM
Registered User
andrecillo76's Avatar
Quote:
shifting the CG downwards and reducing the dihedral effect
Uuups, the second part of the sentence is of course not true I should not write before having the first coffee of the day Dihedral increases, also drag and maybe damping of the dutch-roll.

Anyway, the HXc was the third one. I'm sure Horten had a good reason for choosing such a high dihedral. Maybe he found the ability to fly tight turns for thermalling more important than a tendency to dutch-roll...

Andrés
Mar 24, 2017, 06:33 AM
Excaliburst
Hi Kent,

And thank you for a great build thread of this beutiful Horten. Some guys at NASA have come up with a very promising solution to the original Bell-shaped lift distribution and they have som interesting takes on the planform.

Apparantly they have implemented the wing in a way so that at
  • 25% span the twist is positive +0.57 deg (wash-in)
  • twisting towards 90% span, where it has the biggest twist/Wash-out of -8,97 deg,
  • and at the tip this is somewhat reduced to -8,5 deg.

Apparantly this wing configuration makes the plane pro-verse yaw stable. And after having read your continued battle with this phenomenon, I think you should read this paper.

Thank you for your inspiration

I am in the proces of designing and building a 2.8m model with this design. I am not much for too elaborate (wood) building, so I decided to go for vac-bagging the wing with foam-cores.

I attach the Scientific paper from the NASA Dryden Group
Mar 24, 2017, 10:00 AM
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Knoll53's Avatar
Be sure to start a new thread for your project. I, for one, am always interested in seeing actual flight testing that is not tied to government funding.
May 03, 2017, 01:42 PM
Donate Platelets
Kent,


The video of the maiden flight in post #62 was taken down from YouTube.


I'm getting a kick out of this thread. I saw in one of the videos a smaller Horten/BSLD wing. Was that one of your designs?


Jim
May 03, 2017, 08:00 PM
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Knoll53's Avatar
You may be thinking of a Horten wing built by FlyingDennis. For my projects, you could do an advanced search with my name and Horten in the title.


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