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All-moving wing tips

Old 07-20-2003, 07:37 AM
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a088008
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Default All-moving wing tips

I have this idea floating around upsairs.

I'm in the process of designing my next aerobatic plane and want to incorporate the best possible aileron configuration.

What I'm thinking of doing is having all-moving wing tips similar to the all-moving tail surfaces found on modern jet fighters such as the F-15 Eagle, etc.

My rational is that, with all-moving wing tips (say, 1/6th of the wing span), less movement is required to produce the desired lift differential (resulting in a roll) accross the entire wing. Also, the roll moment is large since the lifting force (positive or negative) is at the wing tip, unlike convetional ailerons which change the camber of the wing along the length of the wing that has the aielron attached.

The all-moving wing tip would also be more effective and efficient since the wing tip's camber is not modified when moving.

An added, adventageous, side-effect might be that the span-wise flow (outward) would aid in the efficiency and effectiveness of the all-moving wing tip.

What I'm unsure of is what an all-moving win tip would do to the tip vorticies and how these would affect the effectiveness and efficiency of roll control. Also, what would the snap characteristics of such a configuration be?

What are your thoughts, suggestions?

-Q.
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Old 07-20-2003, 05:27 PM
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Tall Paul
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Default All-moving wing tips

Go for it!
I built up, almost to flight stage such a plane, but the other differences make me less eager to complete it.. (A reverse delta)
Thing to watch for: Use a STRONG servo!
sidenote:
We used similar things but very small, about 12 inches square, on the wingtips of the L-1011 to excite structural oscillations in flight. VERY effective way to do that!
Old 07-21-2003, 03:31 AM
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Default All-moving wing tips

I've only ever seen two wingerons planes and they were both gliders. Now these both used full span pivoting wings but in both cases the amoung of movement required was very slight to achieve strong roll rates. So perhaps you're onto something here. But I have to add that I think you'd be better off with a longer percentage of the span. 1/6 means it would be a very short span section. I'd shoot for 1/3 of a wing panel. Or perhaps that's what you meant when you said 1/6 span? 2 x 1/6 equals the outer third of each panel?
Old 07-21-2003, 04:22 AM
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Default All-moving wing tips

Originally posted by BMatthews
Or perhaps that's what you meant when you said 1/6 span? 2 x 1/6 equals the outer third of each panel?
Yip. That's what I meant. 1/3 of each panel should do it.

-Q.
Old 07-21-2003, 04:26 AM
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Default All-moving wing tips

Originally posted by Tall Paul
Go for it!
Thing to watch for: Use a STRONG servo!
I'm going to be using one servo per wing and they will be metal-gear types.

I need to crunch the numbers to find out how much torque is needed, though. I'm hoping to use the smallest servo I can get away with since they will be near the wing tip. I also want to minimize the amount of weight at the wing tips so that roll inertia is not a complicating factor.

-Q.
Old 07-21-2003, 05:29 AM
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Default All-moving wing tips

Shoulda mentioned in the last post. If you put a pivot at the 25% chord point the loads on the servo will be minimized. But flutter may become a bother with such large surfaces. To avoid that ballast the leading edge so the balance point of the tiperon is on or ahead of the pivot rod.

Now ballast sounds counterproductive so you'll want to ensure the rear portion of the tiperon is as light as possible so the leading edge can be kept quite light as well.
Old 07-21-2003, 06:01 AM
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a088008
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Default All-moving wing tips

Originally posted by BMatthews
Shoulda mentioned in the last post. If you put a pivot at the 25% chord point the loads on the servo will be minimized. But flutter may become a bother with such large surfaces. To avoid that ballast the leading edge so the balance point of the tiperon is on or ahead of the pivot rod.

Now ballast sounds counterproductive so you'll want to ensure the rear portion of the tiperon is as light as possible so the leading edge can be kept quite light as well.
Great info on the ballast. Thanks!

I'm going to put the pivot at 30% (or aerodynamic center for the airfoil) or there abouts, and make sure that all connections are "slop" free to eliminate flutter.

-Q.
Old 07-21-2003, 02:08 PM
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banktoturn
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Default All-moving wing tips

a088008,

I think you will gain a little in efficiency just by not having the gaps you would have with ailerons. On the other hand, increasing camber when you want to increase CL is actually better than increasing AOA, so you don't really gain by avoiding the camber change. You might also give back some efficiency at the junction where the tip panel pivots. Overall, my armchair assessment is that the overall drag will be similar to that of a conventional aileron system. I could easily be wrong about that.

Regarding flutter, I don't think it matters where the center of mass of the tip panel is; weighting the leading edge per se will not help you. You can 'tune' the system by changing the overall moment of inertia, but the weighting the leading edge is only one way to do that. It is true that pivoting near the 25% chord point will minimize the torque required. Rather than going back to 30%, I would recommend that you go forward to around 20%. You want the aerodynamic center of the panel to be behind the pivot, for stability.

Good luck, and post pictures,

banktoturn
Old 07-21-2003, 02:42 PM
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Default All-moving wing tips

Originally posted by banktoturn
a088008,
Rather than going back to 30%, I would recommend that you go forward to around 20%. You want the aerodynamic center of the panel to be behind the pivot, for stability.

Good luck, and post pictures,

banktoturn
You make a good point about pivot stability. 20% it is then.

Thanks for the support. I certainly will be posting pictures! I'll attach them to this thread once I've got the design fleshed out.

-Q.
Old 07-21-2003, 04:14 PM
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Default All-moving wing tips

Just a thought but perhaps it would be worth making up a test panel and trying it on a bracket outside your car window. The pivot bearings could be mounted on ply sub ribs so the pivot can be moved fore and aft from the 20 to 30 % points in a search for the aerodynamic center. The "strain guage" would be your hands on the angle lever. Move the pivot point until you notice it self stabilizes but has a fairly consistenly low effort to change the angle of attack over a 20 to 30 degree range.

I'm thinking of a 2x4 L that locks to the passenger door to hold the test panel out about 2 or 3 feet from the car, a shaft of metal tubing with some plastic bearing type pillow blocks on the arm of the L and a cheapo 4 rib wing panel with Monokote. The sub ribs would have slotted holes to let the pivot axle tube move back and forth and then tighten in place. A quiet road and a 1/2 dozen runs would settle where you want the pivot and give you an idea of the servo loads.
Old 07-21-2003, 04:38 PM
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banktoturn
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Default All-moving wing tips

BMatthews,

Great idea.

banktoturn
Old 07-21-2003, 06:30 PM
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Ben Lanterman
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Default All-moving wing tips

Us old aero types arn't the stick in the muds you might think, we have thought of the wing tip roll device and other things many times. If you don't see things used on airplanes today they have probably proven to be less than desireable. After all it has been 100 years and many wars to make the technology mature.

Many years ago I worked up some studies on this subject. I have since lost them but the basic result is that if the aileron chord is much more than 30 percent of the wing chord the increase in effectiveness is not worth the weight to accomplish it. The lightest structure was one that did not have the pivot in the wing tip. Most of the time it is possible to obtain all necessary rolling moment with conventional ailerons - any comanded roll that you can't control is pretty well wasted anyway.

Those are basically the reasons you don't see these kinds of tip devices used on airplanes of any kind today.

Just because the F-15 uses a differential tail does not mean that the differential tail is better for our model use - just that it was a good addition to the ailerons for the F-15 over it's required flight envelope. The all moving tail is there because there are flight conditions (mach number, angle of attack, etc.) at which the ailerons are not effective and yet roll is still required.

For instance think of the F-15 flying at 25 degrees angle of attack. The tail is deflected about that angle leading edge down. (I don't remember the exact numbers, getting old) At this angle of attack the ailerons have very little roll power but the horizontal is basically parallel to the airflow. This gives a roll moment capability when nothing else is working.

There are high mach numbers (dynamic pressures) where the flexibility of the wing makes the aileron less effective. Here the differential tail can be useful.
Old 07-22-2003, 02:34 AM
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Default All-moving wing tips

Originally posted by banktoturn
BMatthews,

Great idea.

banktoturn
I agree. I'll construct such a "moving wind tunnel" for pivot and load tests.

-Q.
Old 07-22-2003, 02:44 AM
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Default All-moving wing tips

Originally posted by Ben Lanterman
For instance think of the F-15 flying at 25 degrees angle of attack. The tail is deflected about that angle leading edge down. (I don't remember the exact numbers, getting old) At this angle of attack the ailerons have very little roll power but the horizontal is basically parallel to the airflow. This gives a roll moment capability when nothing else is working.
I was thinking of exactly such a situation, where the wing has stalled and normal ailerons become much less effective, or worse, one works and the other does not e.g. snap roll. I would like to maintain roll control deep into a stall, or when airspeed is low (as in a spin). I'm thinking of using exponential rates to achieve the proper roll control at high, cruise, landing and stall speeds.

-Q.
Old 07-23-2003, 03:27 AM
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Default All-moving wing tips

I'm not the picturesque aerodynamic citizen but I'd like to affirm:

Low moment of inertia. Keep your moment of inertia close to the CG and low as possible.

Beyond that, I think your research is beyond me : )

Good luck with your experiment!
Old 07-23-2003, 05:42 AM
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a088008
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Default All-moving wing tips

Originally posted by MTEXX
Low moment of inertia. Keep your moment of inertia close to the CG and low as possible.

Beyond that, I think your research is beyond me : )

Good luck with your experiment!
I agree. High relative inertia can make a CAP feel like a C130.

Thanks for the well wishes. I'll reciprocate with pictures and flight test reports.

-Q.
Old 07-23-2003, 06:56 PM
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Default All-moving wing tips

You may want to research a family of light planes built in Germany in the 1920's and 30's. The early Klemm's used such a configuration for roll control.

Mr Ed
Old 07-24-2003, 12:15 AM
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Default All-moving wing tips

Actually, the closer that you put your control surfaces to the tip, the LESS efficient they will be (less lift per unit area).

The reason for this is due to the 3D flowfield at the wing tip. If you consider the lift on a 3D wing (sorry, i don't have a figure to attach), the lift goes to zero at the tip.

Your getting air flowing from the bottom of the wing spilling onto the top of the wing... killing the pressure differential that allows for lift.

What does this mean for your tip control surface? Your going to need a lot of area, and a lot of servo to move that area, for the same effectiveness as a conventional aileron.

DON'T let this quash your idea! Your definitely going to want to use a cut-off tip, or better yet, an end plate. Try it! Why not!

Ken - www.litkoaero.com
Old 07-24-2003, 04:28 AM
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Default All-moving wing tips

Originally posted by KenLitko
Actually, the closer that you put your control surfaces to the tip, the LESS efficient they will be (less lift per unit area).

The reason for this is due to the 3D flowfield at the wing tip.
Ken, I'm certainly aware of the tip flow and take your point. I do however feel that the large surface (relative to an aileron) would provide more roll force due to airflow over the tip. However, as you correctly state, it would be less efficient due to 3D effects. I do feel that it would be more efficient (for a given length at the tip) than an aileron.

Another thought was to put end-plates on the extreme tip to reduce the amount of tip flow from the bottom, and around the tip, towards the top of the wing thereby keeping some of the pressure difference.

I'll be building and flight testing the design so I will certainly give some feedback on how it works in practice.

-Q.
Old 07-24-2003, 05:14 AM
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Default All-moving wing tips

Originally posted by Mr_Ed
You may want to research a family of light planes built in Germany in the 1920's and 30's. The early Klemm's used such a configuration for roll control.

Mr Ed
I did not find anything on the Klemm aircraft, but here is a interesting technical paper from NASA http://techreports.larc.nasa.gov/ltrs/PDF/tp3258.pdf

It basically coupled conventional ailerons and tiperons. It achieved roll control DEEP into a stall of 70 degrees AoA!!!
Old 07-24-2003, 05:27 AM
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Mike James
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Default Science!

I love all these experiments. Maybe something unexpected will come from it, good or bad. Keep going with your idea, and let us know the results.

What I'm dying to see is a model-size and applicable version of what NASA is working on with their Mission-Adaptive Wing technology. Wouldn't it be great if the skin of our planes could just move the way it needs to move, without any control surfaces at all? No gaps, no mechanical linkages, more options... (Really just more sophisticated, effective wing warping)

I don't expect to see us applying that right away, but it seems like it might be "the" concept.
Old 07-24-2003, 05:30 AM
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Default All-moving wing tips

Originally posted by a088008
....It basically coupled conventional ailerons and tiperons. It achieved roll control DEEP into a stall of 70 degrees AoA!!!
NOW you've got my attention...

I'm assuming that it uses gross throw angles to achieve this. Something like the 20 to 25 degree throw on some modern elevon tail surfaces like Ben was describing.
Old 07-24-2003, 06:55 AM
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Default All-moving wing tips

Originally posted by BMatthews
I'm assuming that it uses gross throw angles to achieve this. Something like the 20 to 25 degree throw on some modern elevon tail surfaces like Ben was describing.
It was actually positive/negative 30 degrees.

An interesting thing to note is the configuration of the tiperons (There is a picture of it in the document). The "hinge" line was not span-wise as with a conventional aileron, but "swept forward" by 30-45 degrees. This is not quite what I'm proposing to do, but the effect is similar. I have thought of also "sweeping" my hinge line forward, but I first want to see what a perpendicular configuration yields.

-Q.
Old 07-24-2003, 07:12 AM
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Default Re: Science!

Originally posted by Mike James
I love all these experiments. Maybe something unexpected will come from it, good or bad. Keep going with your idea, and let us know the results.
You and me both. This stuff keeps me up late at night (usually until sometime early the next morning - see the time on this post ) and daydreaming during the day.

Originally posted by Mike James
What I'm dying to see is a model-size and applicable version of what NASA is working on with their Mission-Adaptive Wing technology.
I have also been thinking about that from time-to-time. The crux of the matter is computer control. Even the modern computer radios we fly with pale in comparison to what would be required to "warp" a wing in a adaptive way. However, the mechanics might not be as big a problem since there are synthetic "muscles" that would do the job nicely. If I remember correctly, the synthetic "muscle" is a plastic polymer that contracts or expands in reaction to an electric current. Your only problems would be to provide sufficent power to the "muscles" and construction of the right type of flexible wing.

-Q.
Old 07-24-2003, 10:36 AM
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Default What about WrightBrother Kitty-Hawk

The wright brother's plane had no ailerons. Their roll control was provided by warping the wings. (My history is shakey, but I believe I got the concept right). Essentially, they had cables running from tip-to-tip of the wings, and by tensioning the cable, the wing warped, and changed the lift characteristics of the wing.

Seeing your idea on the tiperon, I am wondering if the warperon would be more useful - no hinging, controls can be in fuselage, (weight low at tips), and other benefits.

Just a thought

gus

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