Flexible self-actuating flaps!

[Jim_Marconnet]

I ran into an interesting german site with several presentations on biometrics of low-aspect-ratio birds and specifically developing a self-actuating flap based on an understanding of certain birds' feathers:

http://www.bionik.tu-berlin.de/user/giani/vortrag/sld001.htm

Use the > button to step thru the slides and be sure to read the text (in English!) at the bottom of each slide of interest. Note especially slides 10 and 16 for the configuration they came up with that worked the best, and the resulting plots of CL vs CD on slide 11.

Based on their research and testing, it looks like a light, simple, self-actuating flap could be easily developed and built that would greatly improve a plane's performance at high angles of attack near stall speed with no pilot inputs needed.

I did some other searching on-line, and came up with nothing whatsoever else on this.

If you know of or find something or try it, please share your findings!

[rofujiyama]

hi,
Wow ,what a great observation and follow up study! Great find Jim.
Roland

[Jim_Marconnet]

hi,
Wow ,what a great observation and follow up study! Great find Jim.
Roland
Glad you like it. On that same general site there is another writeup on this subject that goes into some other aspects of it. It's in German. But I had good luck with the BabelFish translator:

http://babelfish.altavista.com/

copy and past the following into the "Translate a web page" block
http://lautaro.fb10.tu-berlin.de/user/giani/klappen/evo.html

Select "German to English" in the pull-down, and then click on "Translate".

BabelFish translates that page and even if you click on links on the page, those pages will be translated! Neat.

It cannot translate any text embedded in graphics, and it makes some humorous translation sections - but it does a lot better in comprehension than my Zero-German! :D

[rofujiyama]

Hey Jim,
Thanks for the heads up on the translater, Great Tool! Very simple to use. I like it as much as your find on the wing ,lol :D I gonna use it on the japanese sights next ,lol Its my ancestry ,but I may know about 4 or 5 words ,,lol. Thanks again, good flying :D
Roland

[saucerguy]

Tell the Prof. to hire a webmaster, it will take a day to extrapolate your rendering to produce in the real world. Good concept regardless, I'd like to see someone that puts this in the air myself as well.

[Sparky Paul]

If the surface mierely flaps due to air pressure, hinged at the leading edge of the flap, it will have no effect on the airframe, other than making some noise and some drag.
Any surface must be hard-fixed to the airframe via a horn or arm that holds it in position.
Bird's feathers are fixed to the wing via the quills. These provide the support to hold them in the position the bird commands.

[Jim_Marconnet]

If the surface mierely flaps due to air pressure, hinged at the leading edge of the flap, it will have no effect on the airframe, other than making some noise and some drag.
Any surface must be hard-fixed to the airframe via a horn or arm that holds it in position.
Bird's feathers are fixed to the wing via the quills. These provide the support to hold them in the position the bird commands.
I don't think you quite understand it. (Not sure I do completely)

If you look at post 3 in this thread and go see that part of the German web page, they go into force-deflection measurements on the different bird feathers and how the feathers in this center part of the wing are more flexy and they bend under certain flight conditions (due to air flow or pressure, not the bird commanding them to bend) forming the new wing shape. They also go into the importance of the "flap" allowing some air to pass thru it to make it move under the right conditions and not just due to ordinary pressure. Thus the silk or perforated plastic materials they experimented with.

Anyway, I'm not doing justice to the article. Suggest people read both articles at that site if interested in (trying to) understanding this. Yes, it's radical science, and thus perhaps somewhat unbelievable.

Even more importantly, I hope to see some brave RC experimenters and their results.

Thanks, everyone who has participated, for you interest and enthusiasm.

[Sparky Paul]

With the leading edge of the silk-steel framework taped to the wing, no force on the silk can get to the wing/airplane, other than the drag due to the fluttering of the silk.
If the leading edge of the silk-steel framework is hard-mounted to the wing,
the forces on the silk can then get to the airframe as lift and drag.
You can't pass a torque thru a hinge. Something has to connect the surface to the airframe.. either the wire bent down into the wing, or an actuator.

[nmasters]

They seem to be suggesting that the "reflux flap" (it looks like a big unconnected spoiler to me) acts as a dam to limit how far the reversed flow in the stall burble can propagate forward. The dam just acts to separate the flow regions ahead of and behind itself. If their theory is right most of the post stall lift that they measured is produced on the 60% of the wing that's ahead of the device and the only force acting on the flap itself is pulling it forward.

BTW this is the URL to the gateway to that series: http://www.bionik.tu-berlin.de/institut/xs2vogel

[Bg~]

Sparky,

The authors specifically mention that the cover feathers are very weak:

The constantly small rigidity of the federspitze of the arm cover explains itself by the fact that the cover feather/spring is already so soft in this range that its firmness can be no longer further reduced. The bending characteristics of this fitting with springs ILS are determined almost exclusively by the firmness of the feather/spring flag, which material-causes a certain value not fall below cannot. The feather/spring keel is here so strongly tapered that it does not go mechanically into action.

So as others have said, no forces are transmitted from these cover feathers or silk flaps, they only serve to block the stall eddy which merely maintains the lift already present on the leading portion of the wing.

[Bg~]

I found some papers on the topic:

Experimental (including glider flight test):
http://www.dlr.de/at/PortalData/2/Resources//dokumente/at/aiaa-2004-1243.pdf

flight test summary - 3.5% decrease in stall speed -> 7% increase in lift due to flaps


CFD:
http://www.cfd.tu-berlin.de/~schatz/PUBLICATIONS/potsdam.pdf

[Jim_Marconnet]

I found some papers on the topic:

Experimental (including glider flight test):
http://www.dlr.de/at/PortalData/2/Resources//dokumente/at/aiaa-2004-1243.pdf

flight test summary - 3.5% decrease in stall speed -> 7% increase in lift due to flaps


CFD:
http://www.cfd.tu-berlin.de/~schatz/PUBLICATIONS/potsdam.pdf
BG~

Thanks so much for sharing these links. They are better than the stuff I found earlier, not surprising since experimenters stand on the shoulders of earlier experimenters.

[Sparky Paul]

Last year I put a small video camera on top of one of my GP Spirit wings which had spoilers.
For the flight regime I looked at, which wasn't high alpha stuff, I got no float worth worrying about.
The large surface of the proposed silk flap would have some effect on the flight, but it would be mostly drag and vibration.

[Bg~]

They are better than the stuff I found earlier, not surprising since experimenters stand on the shoulders of earlier experimenters.

The experimental paper is done by the same people who wrote what you posted. :)

This sounded neat to me for possible use on a helicopter rotor to counteract reverse flow on the retreating side, but the paper mentions that cross flow from wing sweep removes any beneficial effect that the flaps have.

[Jim_Marconnet]

The experimental paper is done by the same people who wrote what you posted. :)


Thanks for pointing that out. What I said is true regardless. They are getting it better, optimizing further, doing better CFD analysis, making more sense, and approaching supplying sufficient information that we R/Cers could try it for ourselves on our own planes.

I found it interesting that in the AIAA paper, they address what Sparky Paul mentioned recently about his in-flight photos:

"It is useful to stress the marked difference between the movable flaps and a conventional rigid spoiler on a wing: A spoiler protrudes into the high-speed bow regime and increases the width of the wake. In this way, it increases the drag and reduces the lift.

By contrast, at high angles of attack, the moveable flap has the opposite effect, i.e. reducing drag and increasing lift. At the same time, the effective shape of the airfoil changes due to the slightly elevated flap and a lower effective angle of attack ensues.

Thus, the pressure distribution on the airfoil is adjusted in such a way that the tendency for flow separation is reduced. Consequently, the flow remains attached up to higher angles of attack and the lift of the wing is increased."

[Sparky Paul]

The spoiler is pulled open by a string, so it's free to float.
One of the ground tests I did included inverting the wing and checking the amount the spoiler would open due to its own weight.
It drooped 32 degrees.
In flight, nothing even close to that showed up.
Someone (not me) might try this silk flap thing... and give an objective report on the results.

[Bg~]

and give an objective report on the results.

:confused:

The researchers documented their efforts quite well, they have shown significant results in both the wind tunnel and flight test (through data and pilot comments). They show inflight video evidence.

Do you have a specific objection to their methods? In what way are they not being objective?

[Sparky Paul]

First, I'd like to see the wind-tunnel flow images with a flexible flap, not a solid flap held in position.
3.5% isn't worth much, compared to the cost of the installation.
The flap on the full-scale glider is not that described in the first messages, it's a solid surface, at the trailing edge of the wing, not at the high point as shown in the first sketch, which also mentions a silk surfaced wire braced flap.
The angles of attack reached are mostly impractical for anything but landing in a very small clearing.
.
Testing this in model size I'd use a couple of sheets of manila folder, cut to fit and taped at the leading edge of the flap at the high point on the wing.
Then fly the plane with video coverage to check how the pitch angle is changed between no flap and free flap.

[nmasters]

http://www.cfd.tu-berlin.de/research/flowcontrol/flaps/flaps1.html

[Sparky Paul]

bullet 4..."Optimum lift coefficient can not be realized by a self- adjusting movable flap"
.
And look at the alphas where the improved Cl goes to.
No one flies there!
A 10% improvement in Cl and Cd at those alphas contributes nothing to the flight regime where the airplane is actually operating.

[Lakc]

bullet 4..."Optimum lift coefficient can not be realized by a self- adjusting movable flap"
.
And look at the alphas where the improved Cl goes to.
No one flies there!
A 10% improvement in Cl and Cd at those alphas contributes nothing to the flight regime where the airplane is actually operating.

For that airfoil and in those operating areas ill bet your right.

Why do birds use that high AOA operation for? My guess is mainly STOL operations, birds dont always get a long straight branch to land on. :)

So this might have benefit on ultra low drag/lift airfoils that would require a high angle of attack to provide lift at low speeds.

[nmasters]

look at the alphas where the improved Cl goes to.
No one flies there!

What? Are you telling me that your plane doesn't nest on a 3" outcropping :rolleyes: :p , like an average pidgin? Heck, I've seen swallows build their nests apparently glued to the cliff face. Talk about a full stall landing! They swoop up from a few feet below the nest to expend all their forward momentum and plop down right on the edge of the nest. It's just like landing on a telephone wire but with a big rock in the way so there's no margin for error.

--Norm, keeping bird's feet warm

[kak8]

I don`t know much about aerodynamics, but this kind of spoiler looks a lot like the Allegro or Bubble Dancer spoiler. I am soon an owner of both, but could someone tell me how and when to use the spoiler in the real life? And why? - if it can be used for something else than slowing down.

[Sparky Paul]

Dr. Drela says he uses his for 90 degree dives to make the spot at end of the time period.
He's much braver than I am! :)