Wing Tip Designs (Full Version)

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AV8TOR -> Wing Tip Designs (6/26/2003 4:46:52 PM)

I cannot find plans for an old design that I flew in the 70-80’s so I am going to try to duplicate it. This plane used an Ugly Stick wing mounted as a low wing on a rolled plywood fuselage. I am going to scale this up a tad for my YS91 and I am thinking of going with spilt wings mid mounted with wing tubes. My question is this. The Ugly Stick wing had unique wing tips if you remember them. I see a lot of 3-D fun fly planes these days with no wing tip what so ever. Basically just a last rib. What is the advantages and disadvantages of just going with no tip design or should I still use the classic Ugly Stick wing tips?

Thanks



Tall Paul -> Wing Tip Designs (6/26/2003 9:57:00 PM)

Where speeds are low, finessing the tip is pointless.
Also the Ugly Stik style merely adds span, which isn't of any particular value aerodynamically.
Shorter wings (no tips) roll quickerl
And don't look as good.
Tips aren't important on a sporter.



Dave_Gherardini -> tips (6/27/2003 10:32:03 AM)

I built my 3D planes ,all 2 of them with big tips. (long) The reason, is so when pattern style flying it helps with reducing the giant alerons effectiveness. 3D alierons are so big that when trying to fly fast the are very responsive. To responsive. When in a hover the prop blast is only covering say 30 % of the wing so Paul is correct in the fact that they dont make any differance at all in controllability. And when fun flying they dont create any advantage. Only when flying fast will tips help so if your trying to get all ranges of peformance i would recomend them.

Dave G



AV8TOR -> Wing Tip Designs (6/27/2003 4:01:27 PM)

Thanks guys



banktoturn -> Wing Tip Designs (6/27/2003 9:37:08 PM)

AV8TOR,

You can't count on much benefit from any particular wing tip, at any speed. For building simplicity, you can leave that last rib alone. If you'd like to save a little drag, and look better, carve yourself some nicely rounded tips from soft balsa. This will be almost as good as any other tip treatment you can do. The wingtip shape often gets a lot more discussion than it deserves. It's good to remember that the wingtip is a very small part of the wing, and it's not likely to have a big effect on performance.

banktoturn



Montague -> Wing Tip Designs (6/27/2003 10:18:41 PM)

BankToTurn,
Actually, a rounded tip like you describe is one of the highest drag tips you can put on, due to the wingtip vortex. The simple squared off "last rib" tip is actually one of the more efficient.

The idea of leaving the ailerons "short" so they don't reach the wingtip to reduce their effectiveness in level flight while still allowing a lot of area to be in the prop blast is an intersting one. Though you could get the same effect by tapering the ailerons as well.



AV8TOR -> Wing Tip Designs (6/27/2003 10:34:21 PM)

[QUOTE]Originally posted by Montague
The idea of leaving the ailerons "short" so they don't reach the wingtip to reduce their effectiveness in level flight while still allowing a lot of area to be in the prop blast is an intersting one. Though you could get the same effect by tapering the ailerons as well. [/QUOTE] The Tower Hobbies Uproar has aileron tips that are almost a 3” chamfer is this what you mean here?



banktoturn -> Wing Tip Designs (6/28/2003 12:33:16 AM)

Montague,

I'm not armed with wind tunnel data here, but I don't think that a rounded wing tip is especially high drag, and certainly not because of the wingtip vortex. First, this would imply that induced drag is a big component of the total drag, which is usually not the case. Second, it would imply that the wingtip shape has a big effect on induced drag, which is also not usually the case. Profile drag is likely to be a more important source of drag, and rounding the wingtip will help a little bit with that. If induced drag really is an issue, it would be silly to worry about the wingtip shape until one has increased the aspect ratio as much as is practical, and made sure the the lift distribution is approximately elliptical. These two measures are much more important for induced drag than anything done at the wingtip.

There is a widely held view that induced drag is caused by the wingtip vortex. This is not really true. Induced drag is caused by the downwash, which is due to the fact that the wing is generating lift. While the downwash can be viewed as a vortex that has its centerline near the wingtip, the air that spills over the wingtip is pretty much incidental. The reduction in induced drag that some well designed wingtips can give is due to their effect on the distribution of downwash along the span of the wing. In particular, you cannot assume that any wingtip that discourages the spilling of air around the wingtip is reducing induced drag.

banktoturn



Montague -> Wing Tip Designs (6/28/2003 2:00:45 AM)

banktoturn,
Hmm, ok, you have a point about induced vs other forms of drag.

And, in particular, I totally agree when you said this:
quote:

In particular, you cannot assume that any wingtip that discourages the spilling of air around the wingtip is reducing induced drag.

I was, in fact, making that exact leap, and I see it could be bogus. (I don't have any info to say one way or the other)

However, a rounded wingtip may or may not be "high drag", but it is inefficient when it comes to generating lift.

The air flowing around the wingtip will move the vortex on to the top surface of the wing, which reduces the "effective wingspan". This causes the wing to have to operate at a higher AOA, which will create more drag. I have no clue if it's signifigantly more though. I do know that the higher AOA moves you that much closer to stall, so changing wing tip shape can improve the stall charastics of a wing in a big way.

I wish I could quote the source, but someone did, in fact, do a bunch of wind tunnel tests with differnet wingtip shapes to look at the tip vortex, and the results are really interesting. That vortex can really get around :). If I recall, one of the better wingtip shapes from just a vortex standpoint was the "last rib" squared off tip. In that case, the vortex formed off the end of the wing instead of on the top surface.

So, I was mixing up things. Speaking in terms over overall drag, I can't say what is best. Speaking in terms of overall performance, especially acrobatics and high AOA (non-stalled), I think the round wingtip can really hurt.



Montague -> Wing Tip Designs (6/28/2003 2:10:15 AM)

AV8TOR,
I took a peak at a picture of an Uproar. That is pretty much what I was talking about, except only towards the wingtip. When I say tapering the aileron, I mean making the aileron chord smaller. I have no idea whythe Uproar has the ailerons tapered at the root as well.

In general, there are two reasons that I've run accross for tapering ailerons, to reduce aileron flutter, and to make it look cool. I'm going to guess the Uproar aileron shape is from a combination of those.

I've never flown an Uproar, nor do I know anyone who has one. But, taking a WAG, I'd say those ailerons loose effectiveness in a hurry when you start relying on propwash for control authority.

Here's a couple of quick aileron shape pictures:
First, on this plane, my ailerons were dynamically shortened. It made for some "interesting" performance.

[img]http://www.wheek.org/airplanes/repairedfloyd3.jpg[/img]

One these purple wings, you can see the ailerons have a smaller chord at the tips. This was done to fix an aileron flutter problem. It did reduce the roll rate a noticeable amount compared to straight ailerons:
[img]http://www.wheek.org/airplanes/fleetof72.jpg[/img]

On my new wings, I wanted to keep the ailerons straight, reduce tip stall, and fix aileron flutter all at once. The answer was winglets:
[img]http://www.wheek.org/airplanes/1fsa2.jpg[/img]



banktoturn -> Wing Tip Designs (6/28/2003 2:37:46 AM)

Montague,

I'm not sure what all the tradeoffs are, but if a wingtip design reduces the effective span, then it will increase induced drag. Moving the axis of the wingtip vortex out is exactly what the various wingtip designs that purportedly reduce induced drag are trying to do. If the rounded wingtip allows the vortex axis to move inboard, then it does indeed increase induced drag. The claim that I am making is that most wingtip designs have no significant effect on the effective span. Well designed winglets are kind of an exception to this. I could be wrong, but I have read the comment in several books/papers/websites that wind tunnel testing failed to verify the hoped for benefits of most proposed wingtip designs for reducing induced drag. This rings true to me. A magic wingtip that significantly reduces induced drag always seemed a lot like putting a cow magnet on your fuel line to improve fuel economy.

banktoturn



Tall Paul -> Wing Tip Designs (6/28/2003 6:47:58 AM)

Wingtip shape and its contribution to "efficiency" is probably only detectable in the very low percentages of the results.
Something where true aerodynamic efficiency is needed to get the best performance.. cross-country gliders, racing planes... could probably benefit from a design session or three.
A typical sporter which flies "erratically".. manuvering etc. isn't the type of airframe where the small contributions of wingtips or practically anything else could be picked out of the data, if such data were even possible to obtain.
.
On Montague's combat planes, I wonder how a more conventional aileron... less span, more chord, area towards the tip.. would affect roll rate.
From what I've observed, the difference in performance would be minute... but the difference in ease of assembly, use and repair would favor the current strip aileron.



Ohlsson -> Wing Tip Designs (6/29/2003 11:43:06 PM)

I actually made a work at school a few years ago wind tunnel testing wingtips. The measurements weren't all excact but i received some ok data I think...

I used a test model with a span of 20 cm and a chord of 16 cm, airfoil Goettingen 604. One problem was of course the size of my wind tunnel and the interference with the walls but they were all measured the same way...

The tips I used were: straight cut, rounded, pointed i.e. triangular surface attached orthogonally to the straight cut one and then blended to the wing with solar film (ugly stik), winglet (rounded edges) and endplate (race cars).

My measurements were made using pressure differences and, again, can of course not be compared to the big guy's tests...



Montague -> Wing Tip Designs (7/1/2003 12:02:18 AM)

Paul,
Yeah, strip ailerons are used for simplicity and durability. I'm sure you could get the same results with more "conventional" ailerons. But then you'd have to either run 2 servos or have a long control linkage in the wing. I do wonder what kind of effect a "convention" aileron would have on tip-stall tendancies. Aileron deflection (and reflex) can really affect tip-stall in high-G turns.

BankToTurn,
I wish I could remember my source and the date of what I saw. I know I saw it somewhere, but I honestly can't vouch for it's accuracy now. So, maybe you are right, and the differnet tip shapes don't really move the location of the tip vortex (other than winglets).

I agree with Paul's comment that in the majority of models out there, you won't notice a difference in most of the time.



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