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Author Topic: D box wings and tip shapes  (Read 375 times)
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billdennis747
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« on: March 08, 2019, 10:52:14 AM »

Experts please. Hitherto, I have built my Coupe wings with parallel chord and square or simple rounded tips. My advisor tells me a better layout is a tip curving backwards, as in Bob White.
Unfortunately, I´ve been giving it some thought. If I make a D box to suit, it will have a short parallel section, curving back to nothing at the the end where the rest of the tip will attach (how, I´ve no idea), When I build the wing and pack up the rear of the D for undercamber, this will raise the LE towards the tip, introducing washin, surely? And the same would happen with the fashionable straight taper to the LE. So what´s what? I need a hotline to Johns Barker and O´D.
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RalphS
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« Reply #1 on: March 08, 2019, 11:20:11 AM »

I am not JB or JOD but I think that an easy way out is a cambered building board rather than packing up here and there. (The camber to be the same as the underside of the wing.)  Local packing for wash out as required if you think that you need it. 
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billdennis747
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« Reply #2 on: March 08, 2019, 12:42:34 PM »

Thanks Ralph. If my concerns are real, then surely the only answer is a cambered board with built-in washout?
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RalphS
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« Reply #3 on: March 08, 2019, 03:14:05 PM »

A drawing or photo of what you have in mind would help. 

My solution to swept back tips is shown in the FF Quarterly Special Coupe Edition from some time back.  I didn't think D boxes were necessary then although there was a lot of carbon involved. I don't think JOD ever considered swept back tips and took me to task over my approach to them - all of JOD's wings were parallel chord as far as I can remember.  I can't remember John Barker using anything as heavy as a D box on any of his models.  I was disappointed when I went to the FF Nats last year to see the lack of progress in Coupe design although I didn't see Phil Ball's models. 

You only have to fly five 2 minute rounds then get out a bigger model for the fly-off. Grin
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duration
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« Reply #4 on: March 08, 2019, 03:58:41 PM »

Bill;

A couple of thoughts:

Use the D-box for the main panels and some lighter, warp-resistant structure for the outer panels.

Use a two-piece wing for ease of transport. Then make both main panels flight. Rig up some simple way to shim trailing edge on one side up or down for desired wash. You might even consider using a timer-activated wing wiggler for first few seconds of climb. There was a real simple setup in AeroModeller some years back. I've used on several Coupes and it worked.

If you build on a constant-chord under-cambered jig, the easy way to get was-out in tips is to skew the tips forward slightly---much better than packing under t.e.

Louis
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lincoln
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« Reply #5 on: March 08, 2019, 06:50:07 PM »

First off, I wouldn't call myself an expert. I'm not even a Coupe flyer. However, I found your problem interesting.

When you say "as in Bob White", is the Bea Coupe a good example?
https://outerzone.co.uk/plan_details.asp?ID=8790

I'm guessing your d-box is some sort of thin carbon sheet molded to a c-shape, with the spar as the back side? If so, it seems to me that you could cut the d-box back and then join it over appropriate ribs. You might have to supply a bit of leading edge in some material or other. The ribs, leading edge, and trailing edge would have to be packed up to maintain the correct incidence. For the last little bit, you might just sculpt some highload 60 (or Rohacell) to a swoopy shape. I broke a tip on a moldie RC sailplane and replaced it that way, glassing it over for a bit of strength. Not sure if that fits in a Coupe weight budget or not, but your tip should be much lighter since it's much smaller and thinner.  Alternatively, you could  just use a short foam tip.

If your d-tube is wood, there might be something useful in how Mark Drela did it on his Allegro-Lite and Bubble Dancer, at least if he didn't fudge the tip assuming everyone would figure it out. If he did, I think there's at least one on line discussion group. It used to be on Yahoo, but I haven't looked in ages. Anyway, you can find info on these designs at:
http://charlesriverrc.org/articles.htm
I realize these are much  heavier than a coupe, but maybe you can do something similar, but thinner, smaller, and lighter.

It occurs to me that if you find some really light c-grain, you could use shaped balsa tips similar to the wing construction of the Apogee, which you can also find at the link above.

I suspect that you don't need to maintain as much camber all the way to the tip.

A final thought is that if you have a cambered board, you could run the tip out off the end, using a custom board you sanded yourself.

Don't expect any improvements to be obvious unless you glide your model from the top of NASA's vehicle assembly shed (with the door closed), and use a stopwatch. OTOH, even the results of a weak thermal ought to be obvious.
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billdennis747
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« Reply #6 on: March 09, 2019, 03:06:51 AM »

Lincoln, yes that´s the one. I can see that the open structure means Bob White had the LE flat to the board, eliminating my ´washin´ (incidentally, I wonder why he used so much downthrust - I have never needed any)

Thanks for all the responses. I used a wooden D box and was very impressed with the stiffness. However, it´s clear that messing about with tip shapes is all about ´marginal gains´and I need to concentrate on the big things, like reducing drag, props, building close to weight, winding it up properly and chucking it into a thermal

Thanks again
Bill
PS since going to R/L trim, I don´t use any warps at all, and nothing bad has happened
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Buster11
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« Reply #7 on: March 09, 2019, 02:47:21 PM »

This may give the purists seizures. When building swept LE F1A wingtips using a tapered chord D-box and with a cambered building board, I lay the plan (which I draw on Mylar so it can be reversed) onto the board so the LE is un-swept. This automatically provides wash-out, and by drawing the full-chord airfoil and then moving a tip-chord one on it you can see exactly how much to skew the tip Mylar to give the amount of wash-out you need.

The trendy curved tips are made from a few laminations of balsa that are glued inside the outer ends of the D-boxes. Seems to work.
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