The created vortexes of low aspect ratios are huge at low speed. These huge vortexes influence a certain area of the very short span. This leads to bad span efficiency.

I hope to show a way to resolve this problem.

Many of you have already seen winglets. Some of you have seen experiments using "feathers" (I haven't found any official name, so I call them feathers and will do so for the rest of the text).

How do I hope they work? So, under the wing there is a positive pressure and above the wing there is a negative pressure. The positive pressure will of course leak towards the negative pressure. This happens along the tip of the wing. Wingtips have a certain form and according to that form a certain length of space where the air can leak to the other side of the wing. The shorter this space the more force the leaking air will have (I don't mean that high aspect ratios (= long wings with short airfoils) have larger vortexes then low aspect ratios) . Imagine you push a bucket of water over that was standing on a table. At the table edge each centimeter or inch will get a certain amount of water over it. If you pushed the bucket towards the short side of the table each centimeter or inch will have to swallow more water than if you pushed it to the long end of the table.

Now, how do you increase the wingtip edge length? You could do that in two ways. The first way is to increase length of the tip chord. But that would be difficult in construction. The wingtip would get more lift than the root chord. This could leads to extra forces on the spar at the root. The second way to increase the length of the wingtip edge is to deform the wingtip. By making featherlike extensions on the tip you made that edge longer. There are more centimeters or inches along the wingtip.

A view on the underside of the wingtip. The air leaks between the feathers, but not all the air can pass. So some of the air will take the way along the feathers.

I hope that this will reduce the force of the leaking air. If it does it will also reduce the force of the vortexes. And that would make that the airplane has a better span efficiency.

A second thing about the feathers that could help reduce the vortexes is the use of several leaks instead of one leak. Let me explain. An unfeathered wingtip has one major leak, the entire wingtip. A feathered wingtip can have more leaks. I don't mean that the leak power is greater, but that between each feather you will have a leak. If you only have one leak, this certainly will lead to a vortex. But two leaks next to each other disturb each other in creating a vortex. Just try it out in water. Making quick circles with one finger in the water leads to a vortex. Now try doing this with two fingers. Just try to create two vortexes next to each other. It will be more difficult. So when you make experiment, make sure to use several feathers (definitely more than 2).

There is one thing that also needs to be tested by models. Do the feathers need to be installed on the top of the wingtip (as seen in the drawings) or must they be placed on the bottom? I cannot imagine what the difference will be, but I am sure there will be a difference.

One mistake you may not make is the next. Do not turn one of these feathers into an aileron (= control area which creates a roll maneuver). The spaces between the ailerons will lead to leaks at unwanted places. That is why I drew a kind of V-tail. I wanted to cooperate all control areas into this tail. Kind of a combination off elevons (= combination of elevators and ailerons) and a vertical tail. I didn't have an opportunity to test it. But I leave that fun to those who want to try out this idea into some models.

Feel free to use this idea in your models. Please report me any good or bad flight results.

  Some people mentioned to me that there already is such a design. It is called WingGrid and you can see it at http://lsmacwww.ethz.ch/phil/winggrid/index.html. The glider has rectangular "feathers". An article tells that the results were promising.

I was glad to see that my thought about these vortex-reducing feathers actually works (imagine a man dancing while singing "it works, it works, it woooorks").

I got these 3D-pictures from Romain Schoenenberger. He called them "quickies". I call them beauties.
It looks like a jet version of the Birdwing with a proned pilot.
Click the thumbnail to see the larger picture

 I got this remark from Serge Krauss: "like your bird-wing (feathered) idea, but believe that it would change the nature of the low-aspect-ratio plane. The advantage of high CL from high angles of attack comes from the vortices rotating the air stream back over the wing to re-attach the flow over a great part of its (otherwise stalled) area. It may be that in the case of ARUP shapes, with round trailing edges, the vortices actually meet at the aft center wing. So if you interrupt these large vortices, you may raise the CL and lower the CD at lower angles, but prevent the wing from reaching those very high CL's at high angles of attack by reintroducing the stall."

I replied: “I like to react on your comment on the Birdwing-idea. OK, I see now that the stall speed will increase due to a lower CL max. I even did find on the new site of Winggrid that the test plane had a higher stall speed. So, it proofs that you are right. Anyway... I did not doubt your remark. You know your stuff. 

One thing you might not know: an American is working on testing the idea with models. He did build a small glider and found it "remarkably stable". I forwarded your remark and he replied that he still is interested in the idea. Just love the guy.    :) He hopes to develop the idea into a powered ultralight. He hopes to get an ultralight with a larger speed range than the current ones. 

So, the wing will stall earlier. But the low aspect ratio could be built lighter than an airplane with the same wing area and a conventional AR (I think). Wing loading gets lower. Lift needed to take-off is less. Could a higher speed (higher than the pure-very-low-aspect-ratio-plane-stall speed) and a lower Cl result in the same stall speed as the conventional airplane? If the stall speed can be kept the same, one would benefit from the other pros of the concept. 

Other pro's are: 

• Ease in construction (all part (ribs for instance) are bigger and easier to handle) 
• The construction is better crash survivable 
• The hanger can be smaller 
•  ... And the looks are better.   :) "

His reaction was: "Sounds like a good plan to me! I hope to learn the outcome of the experiments. That the Horton "wingless" seemed to work well with its "tip" plates indicates that you may be on a good path."

In the August 2001 edition of the magazine Kitplanes I found a article by Howard Levy about the Marsden Skylark. The Skylark has two winglets on each wingtip. At the first flight the Skylark had no winglets, quickly one was added, soon a second was added. The first winglet is placed at about 1/3 of the tipchord, the second is canted at 45° at is placed at the rear of the tipchord. David Marsden (of Edmonton, Alberta, Canada) mentions that the use of the second winglet showed a further improvement of further about 50% over the single winglet. The vortex gets split into three parts.

This shows again that maybe my Birdwing-idea can be used in planes with more normal aspect ratio. 

If somebody can get me into contact with David Marsden I can learn more about his winglets and ask his opinion about my Birdwing. All extra info will be placed in the site.

A tip to all: those that did visit the site of Winggrid, should visit it again. They now have a small, motorized glider built to test the winggrid. This time the winggrids have a fixed position and the winggrids end in a vertical connection.

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