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Author Topic: round v. square wingtips  (Read 865 times)
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billdennis747
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« on: November 15, 2017, 02:52:59 AM »

Has anyone, in the history of aeromodelling, recorded a difference in either performance or flight characteristics?
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mike
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« Reply #1 on: November 15, 2017, 03:30:17 AM »

Yes
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mike
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« Reply #2 on: November 15, 2017, 05:12:16 AM »

Now I've had time to look something out, I can be more helpful.

I started A/2 or F1A, the international FF glider class in 1967 with an 'Inchworm' kit.  At best it would do about 2:15.
Next was a 'Witchita' from the Feb ’68 Aeromodeller.  It had rounded tips and typical layout of models of the 60’S - I have seen Baguley and Monks designs which are very similar.  152.5mm chord (6 inches!) and a span of about 1.9 metres. That was #2.  I made another of those before thinking I could do better but went back to the same basic layout in a stretched version by #10 in 1979 -span 2019mm, 150mm root chord, smaller tailplane.  The biggest change had been the wing section which was switched to Wichita MK 15 standard (same as MK 1?) traced from the April ’71 FFn.  I then concentrated on improving the wing structure with #11-12.  I think #12 would do 3:00 off a perfect launch. Span 2019mm, AR 14.0, root chord 150mm.

Now the big step, #13, and the one which made for a 3:20 model (20 seconds better). An increase in aspect ratio and new tips - se picture attached.  The structure was similar but with slightly stiffer tips. Span 2015mm, AR 15.8, root chord 147mm.  #14 was 'identical' to #13.  Through all this, I wasn't thinning the wing, I got reduced chords by cutting the back off the shape and fairing the top down!  So the section thickness increased from 7.2% to 7.5% over #10-#16.  

The improvement due to the tip shape?  I estimate it roughly at about 5% of the 10% overall improvement.  As to flight characteristic, I didn't notice any change.

After #13/14, I concentrated on structure for #15, going to Kevar D box and carbon spars with the same planforms.  The bunt launch was also added and I was up to 3:40 ish.  More span on #16 just about got me to 4:00.

Sorry that got a bit long winded but the point is that it's not just the tip shape that matters - it certainly helps.  I haven't mentioned warps - that another story!

You can see that I worked at 'Hawkers' - look at the Hawk and Harrier tips.  (I was structures rather than aero)



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billdennis747
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« Reply #3 on: November 15, 2017, 05:22:47 AM »

Many thanks Mike. Why do I ask? I am currently flying two Coupes that are identical in every way except one has square tips and one fully rounded. Granted, I am not an expert trimmer but the round one seems to climb higher and 'looks' better - smoother flying, less-prone to upsets and generally more reliable. And it looks nicer. I'm just finishing a third and I've used rounded tips (it's a Dave White Baron Knight - the square one a My Coupe. Both with the same modern prop).
Bill
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mike
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« Reply #4 on: November 15, 2017, 06:17:05 AM »

Martin Simons covers the thinking behind my shape change in his book, Model Aircraft Aerodynamics', photo below...

You get a higher effective span - the distance between the tip vortices - for the same geometric span.
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Hepcat
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« Reply #5 on: November 15, 2017, 08:30:36 AM »

Bill,
I have never recorded any differences but I have often been lectured about it by our good friend J. O'donnell.  He had no time for those posers who used fancy raked wing tips and surely everyone should realize that simple semi-circular tips were the only ones that allowed a model to circle smoothly.  I kept my head below the parapet because It always seemed to me that keeping the tip vortices as far apart as possible was a good thing. The picture below is a recent indoor model.  The main consideration here was not aerodynamic but bringing the LE and TE together makes the surfaces much stiifer torsionally.
John.
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« Last Edit: November 15, 2017, 09:01:24 AM by Hepcat » Logged

John Barker UK - Will be missed by all that knew him.
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« Reply #6 on: November 15, 2017, 01:38:20 PM »

Why do we think that vortices leave the wing inboard from curved tips? Is it a Reynold's thing, where the lesser energy and comparatively greater viscosity of a small/slow (i.e. model) wing moving through air means that the vortex sluggishly detaches without having made it to the tip? It doesn't seem to apply to full-size aeroplanes. . .

Here are some stills of a P-47 aerobatting, with vortices more or less visible.

Stephen.


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RalphS
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« Reply #7 on: November 15, 2017, 02:02:53 PM »

Bill,
I have never recorded any differences but I have often been lectured about it by our good friend J. O'donnell.  He had no time for those posers who used fancy raked wing tips and surely everyone should realize that simple semi-circular tips were the only ones that allowed a model to circle smoothly.  I kept my head below the parapet because It always seemed to me that keeping the tip vortices as far apart as possible was a good thing. The picture below is a recent indoor model.  The main consideration here was not aerodynamic but bringing the LE and TE together makes the surfaces much stiifer torsionally.
John.


This made me smile.  JOD used to say the raked tips on my coupes were wrong.  I listened but chose to keep that style for no reason but that the models flew well.
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billdennis747
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« Reply #8 on: November 15, 2017, 02:09:40 PM »


This made me smile.  JOD used to say the raked tips on my coupes were wrong.  I listened but chose to keep that style for no reason but that the models flew well.
John also swore by underfins but I was put off by seeing him spend an hour mending one.
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faif2d
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« Reply #9 on: November 15, 2017, 02:57:27 PM »

Back in the 1960's there was an article in Model Airplane News on a rat racer called the Hopptee.  They showed 3 wing tips using smoke and the raked tip had noticeably smaller Vortices.  I have used that tip shape ever since.  All of my designs were combat models but I can not see while the same would not pertain to a free flight.  As an aside while I was working on a glide bomb at TI back in the early 2000's I made a raked foam wing tip for the wing that it used.  Our Aero guy Dick Johnson loved it and even taped it into place on a model that we had in the hallway.  He said that it should be worth about a 3% increase in range but the stealth guys hated it so it died.
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I used to like painting with dope but now I can't remember why!    Steve Fauble
mike
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« Reply #10 on: November 15, 2017, 04:27:09 PM »

"..... Here are some stills of a P-47 aerobatting, with vortices more or less visible. ....

Thanks for this evidence.

I wonder if we're looking at tip mounted smoke generators as here...
https://m.youtube.com/watch?v=SMhwf--0pcc

If this is a real tip vortex on a P-47, you have me confused!
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Prosper
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« Reply #11 on: November 15, 2017, 05:15:50 PM »

Hullo Mike, no - no smoke generators, this is just a standard pilot introduction film from 1943: https://www.youtube.com/watch?v=_NWaHlnI_LQ part 3 of 3 vids. Smoke generators weren't quite the thing back then (unless there was skywriting involved Cheesy ). I didn't find this because I was looking for contrary evidence - I have it on disc and was reminded of it by Bill's thread. I suspect the answer is a Reynold's Number thing: model-size v. man-carrying size. Or consider that the P-47 is peforming high-energy aerobatics meaning high AoA, high CL which might force the vortex right to the tip. Perhaps in cruising flight the vortex is less tip-wards. . .but on the other hand perhaps it still originates right from the tip and merely flexes inwards more - an image we've all seen; the F-18 pic illustrates this. I'm slightly sceptical that a vortex can originate from an arbitrary portion of unbroken, smoothly-curved T.E. but I'm quite ready to be put right on this.

Stephen.  P.S. I'm impressed that the smoke in your link is so close to the tips: I saw a Sea Fury on the ground once and asked the pilot what the canisters were under the wings - they were well inboard - "smoke generators" was the answer. Perhaps they're plumbed to the tips? Or perhaps again they rely on aerodynamics. . .spanwise flow rolling up into a tip vortex?
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flydean1
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« Reply #12 on: October 31, 2018, 11:02:04 AM »

For full-sized aircraft to pull streamers they have to be operating in relatively high humidity (F18 pic) and/or pulling serious G's.  I've read that it takes 4+ but who knows where it is.

As an aside, there is a file film of gun camera footage over the Pacific where someone has rolled in on a Zero fighter which after some tracer whizzed by per the camera, the Zeke pilot pulled the pole and, streaming tip vortices, pulled right out of the camera frame!  This footage appears in many documentaries and I watch for it.

The owner of the gun camera probably wisely unloaded and fled as the Zeke would be very soon on his tail if he insisted on following.
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