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Old Jul 26, 2012, 07:33 AM
Herk
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Aileron coupled drag rudders

This is a continuation of an exploration that I began some time ago.

I wanted to experiment with CG effects on swept flying wings that had no fins, and make a comparison of the similar models with fins added.

The reports of my tests and discussions related, began on this thread:
http://www.rcgroups.com/forums/showthread.php?t=1618447
continued on this one:
http://www.rcgroups.com/forums/showthread.php?t=1619719
and this one:
http://www.rcgroups.com/forums/showthread.php?t=1654485

and included posts on several other threads.

After the first experiments I confirmed that finless flying wings needed to have a calculated static margin > 15% in order to have adequate lateral-directional stability and control. Eventually I decided to try drag rudders as a way of managing the directional issue while reducing static margin to the range of about 5% - which I consider normal for flying wings with fins.

This thread represents the final stage of this process. I will add another post or two with more details and some additional background. But I'll let this video stand on its own in this post. The calculated static margin of this model is 6%.

Note: there is a dead space part way through the video - which I left running while I retrieved the model from a landing that was out of the range of the camera. I then launched it and made another landing which I did just get into the field of view. If you can't tell - it was windy and gusty - 12 mph with higher gusts.

N9M final (7 min 20 sec)
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Old Jul 26, 2012, 08:07 AM
I don't like your altitude
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Herks,Congratulations!It looks to be very well behaved,even with the power off.Have you plans to fit the system on the composite wing you showed earlier?
Best regards Stuart
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Old Jul 26, 2012, 08:44 AM
Herk
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Virginia USA
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Originally Posted by Stupot46 View Post
Herks,Congratulations!It looks to be very well behaved,even with the power off.Have you plans to fit the system on the composite wing you showed earlier?
Best regards Stuart
Stuart - thanks - If I build another flying wing of any kind, I think the coupled drag rudder is a plus - even on a wing with fins. The plank handled much better with the drag rudders - even with all the fins and vertical tail installed. For now I plan to fly this one a bit - just for fun. It's really a very good flying model at this point.
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Old Jul 26, 2012, 10:20 AM
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Quote:
Originally Posted by HerkS View Post
After the first experiments I confirmed that finless flying wings needed to have a calculated static margin > 15% in order to have adequate lateral-directional stability and control.
Nice work Herk. I've noticed that you have been collecting data from the various planes posted on these forums and calculating the SM for each. Have you compiled that information into a spreadsheet? If so, we'd all be interested in seeing your results.

Thanks,
Kent
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Old Jul 26, 2012, 10:26 AM
Herk
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Virginia USA
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Originally Posted by Knoll53 View Post
Nice work Herk. I've noticed that you have been collecting data from the various planes posted on these forums and calculating the SM for each. Have you compiled that information into a spreadsheet? If so, we'd all be interested in seeing your results.

Thanks,
Kent
I have not done that Kent, but it's a good idea. When I get some time, I will give it a go.
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Old Jul 26, 2012, 05:05 PM
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Herk,
Excellent! Looks like you've got a winner there.
I may just have to go back to your posts on the N9M and log away specifics (dimensions, etc) for a future build to replicate your model. At this point, too many incomplete builds so I am forcing myself to finish the ones that are yet unfinished before I start another.
Best Regards,
-Eric
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Old Jul 27, 2012, 12:53 PM
Red Merle ALES
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Herk,

Very nice project. If I recall correctly to maintain a 5%sm the Drag rudders were open 20 degrees. What do you think the estimated speed change is of the model with them at the same static margin with them closed. Perhaps that's not a fair comparison as the model doesn't fly as well without them at that SM. So what speed/soaring comparison would you make without the drag rudders and say a 15%sm compared to the drag rudders and a 5%sm?

I really do like how they function! It really looks nice!

Oh, I couldn't see all of the loop but it looked real nice and round with no tendency to fall off of a wing or have a lot of yaw. Nice! I bet the lop is much easier at the 5%sm and drag brakes than without them and the 15%sm.

Thanks for sharing!
When is the kit going to be available? Ha ha

Curtis
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Old Jul 27, 2012, 01:57 PM
Herk
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Virginia USA
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Hi Curtis -- Yes the loop went very smooth. Couldn't see my attempts to stall very well either. Power on and power off it dropped straight ahead. At 6%sm I think it's a bit nose heavy.

Don't know how much drag those things add to the model. I would like to come up with a program that allows me to close and disable them at high speed and open them when I want to slow down or maneuver or land. I'm working on that.

What kit - it's flat All you need is some depron, some CF and a sharp knife
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Old Jul 27, 2012, 08:36 PM
Herk
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After following several Horten build logs I decided to conduct some personal exploration. I have been fascinated by the Hortens and their aircraft for many years. And, I am very pleased that RC Groups has established this forum.

So this thread is a summary of my experiments.

This is just background - blah blah blah - but hey - it's my thread

Lothar, cpdude, Kent, Vernon and others have, in provided me with some insight that I was lacking until they shared their building and flying experiences. Kent was the one who really got my attention - particularly during his Horten XC build and subsequent discussions with Vernon about his - titled Colibri.

What really got my attention is the very forward CG location that these aircraft require to fly and handle properly.

Nevertheless, I decided to make a very simple pseudo-Horten and do some extended stability and control experiments while varying the calculated static margin and at some point adding fins for comparison.

I have built and flown quite a few flying wing models, but only once did I attempt a finless design - It was unsuccessful, and now I understand better why.

We normally think about the importance of CG location in terms of longitudinal stability and control.

These earlier experiments (threads referenced above) indicated that finless flying wings need to be balanced quite differently - not for longitudinal stability - but for lateral/directional stability and control.

Making a complex subject dangerously simple; adverse yaw caused by aileron deflection causes the model to yaw away from the desired turn and yaw-roll coupling caused by sweep and dihedral, works against the desired bank. Horten-type wings solve this problem with wing twist and CG location such that the outer portion of the wing is lifting negative relative to the rest of the wing - so what would be adverse yaw becomes turn-favorable proverse yaw. In effect they turn adverse yaw upside down.

The down side of this is that the nose weight required can be quite large and the favorable effect of the forward CG and twist diminishes when the angle of attack increases.

Regarding my use of "static margin" as a reference: - The mean aerodynamic chord (MAC) of a wing is a phantasm; and yet it serves a useful purpose for analysis and comparison. Theoretically, and in my tests, the 25% chord point of the MAC represents the generally accepted data point for CG determination. The location of the CG, in terms of the percent of the length of the MAC, ahead (+) or behind (-) that 25% point, is Static Margin - theoretically the amount of longitudinal stability available to a flying wing. I say it's a phantasm because the real stability margin of a wing depends on a number of different factors that vary with geometry, speed and angle of attack - and some other more subtle influences. But I'm using it as a fairly stable reference point for comparison in these tests.

So – what I found is that finless flying wings fall apart when their SM gets below 15% of the MAC. The many flying wing aircraft that I have owned and flown generally do quite well with SM in the range about 5% if they have substantial fins. That means that they are lighter, handle better and do well in slow flight. They still have all that adverse yaw potential, but fins seem to damp it down to the point that it’s manageable.

How then to create a pure flying wing that has no fins, but which has acceptable flying qualities when the SM is down in that 5% range? Somewhere in all that discussion, someone mentioned drag rudders. I knew that they had been used on a number of full scale flying wings, but I doubted they would help a model.

Short story, I was wrong. In those other treads I played with the idea, and using them successfully flew an unswept plank – no sweep and no taper – and with no vertical fins with a calculated SM less than 5%.

This final test bed – the sort-of N9M – is once again truly a nurflügel. It is working so well that it is a delight to fly without need for the forward CG and with only four degrees of linear twist. I want to program this model in order to be able to close the rudders when they are not needed; yet be able to re-activate them when they are needed for slow flight, maneuvering and landing. If I am successful, I’ll report it on this thread.
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Last edited by HerkS; Jul 28, 2012 at 10:36 AM. Reason: Add pictures
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Old Jul 28, 2012, 05:09 AM
I don't like your altitude
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Herks,whilst not being at all knowledgable on the theory behind all this,it does seem to me that you have opened up a whole new envelope of possibilities with finless wings.I wish you success with the latest refinement;after your achievements so far I'm sure you'll crack it.
Regards Stuart
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Old Jul 28, 2012, 07:38 AM
Red Merle ALES
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Herk,

Wonderful synopsis. You should write for a magazine!

Curtis
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Old Jul 28, 2012, 10:15 AM
Herk
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Virginia USA
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Kent asked for a spreadsheet that includes the CG detail that I've collected.

This is the best info that I have on hand. Some may be incorrect - so if it is your model and the data has errors, let us know and I'll fix the compilation.

Also, if you have a flying wing model that's been flown and you've settled on a CG location, I'd be happy to add your data to the list.
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Old Jul 28, 2012, 11:50 AM
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This is a great chart Herk. It really tells a story.

PDF version attached for those without Excel.

Kent
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Old Jul 28, 2012, 12:26 PM
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Herk: Thanks for your thorough summary and conclusions. In an effort to create the needed proverse yaw for turns, there seems to be a few "camps".
  • Horten: lots of wing twist with forward CG
  • Vertical Fins: modest wing twist with modest CG
  • Tip dragger ( either split wing tip control surface or wingtip spoiler) modest wing twist with modest CG.

One question...... Do you have feel for which method produces the least drag?

I do not have any real idea which method might have the least drag.

The Horten appears to have wingtips that produce both form drag and induced drag.

Although the Vertical Fins add a lot of wetted surface area (form drag), those fins appear to only create induced drag when needed, which is a good thing.

Your Split Surface Drag Rudder has mostly form drag.

What I find interesting (currently) is that the Horten and Tip Draggers can adjust the amount of drag. The Horten with in-flight CG adjustment and elevon trim setting (reduced wing twist) and the Tip Dragger with programmable split.

I may have to add a servo to the Alpine Sled to get in-flight CG adjustment. If I do, it should be fun to dial the SM back to 5% when thermalling, then throw it forward to 20% if I get in trouble. Maybe that control should be on a 3 position switch.

Kent
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Old Jul 28, 2012, 01:05 PM
Herk
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Kent -- One question...... Do you have feel for which method produces the least drag?

Hi Kent - very simply - NO!

When it comes to drag, there is 1. the minimum drag at the aircraft's design point - straight ahead ideal cruising speed. (that is the speed for max L/D - Lift is constant, equal to the aircraft weight, so by definition that is the point of minimum drag)** 2. Then there is normally more drag when it is off optimum cruise - slower or faster. 3. After that, there is maneuvering drag - caused by control deflection, adverse yaw, increased lift - etc.

I honestly think that a plane with the lower SM, and with aileron coupled drag rudders that are normally closed and only open during maneuvering, would have the lowest cruising drag. But, I'm only guessing.

** caveat for any lurking professors of aerodynamics - I am aware that his is only true for level flight and shallow glide/climb angles.
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