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Author Topic: leading edge and airfoil questions  (Read 815 times)
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justwantin
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« on: September 27, 2011, 05:57:08 AM »

I have been looking on the net and have not found an answer yet so here it goes.

Having come back to HLG after a very long time away I note that leading edges are now curved and trailing edges straight. Pretty much the opposite of when I was a kid building gliders. A good example of this is George Perryman's Whistler plans from 1965 with straight LE's on wing and stab but his Little Whistler (plans not dated but much more recent) published in the Star Skippers Newsletter have curved LE's and straight TE's.

I can see one reason for making a cureved LE on the wings when considering the wing shaping method in post #30 of the wing shaping thread below. It would be much easier to achieve a uniform taper to the wing tip all other things being correct.

However, is there an aerodynamic reason behind the change in profile.

Also I note that the airfoil shown in Dan G's superseep pics almost appear to have a hard break at the airfoil high point as opposed to a "soft" curve as is often shown in wing cross sections. I expect this helps the achieve a more uniform shape but is there an advantage to this type of wing shaping over the more gradual curve?
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Tapio Linkosalo
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« Reply #1 on: September 27, 2011, 07:12:38 AM »

First reason: swept-forward TE tends to draw the tip vortex inside (towards the fuselage), therefore reducing effective wing span and wing efficiency. Keeping TE straight keeps the vortex "out".

Second, and more recently, designers have started paying attention to the ill effect of reducing RE-number (due to reducing tip-span of tapering wing) decreasing the tip efficiency. On the other hand, taperin the tip towards the tip would be good in order to achieve elliptical lift distribution. However, it turns out that sweeping back the tip keeps up the effective width of the tip and hence RE-number, while area-wise you still get the reduction towards elliptical area distribution. For this reason some designs go even beyond just tapering the LE, some wings aven have crescent-shaped tips (well, not HL, but larger models such as RC gliders and pylon racers).

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-John-
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« Reply #2 on: September 28, 2011, 02:36:45 PM »

The above response strikes me as a very good explanation.

As far as the use of the sharp high point (Supersweep type)Airfoil on HLG's, it looks like a combination of that which has proven to work well with that which is light with that which is easy to construct. This was all worked out well before my time. I still don't know if the sharp high point is critical to good performance. So far I've found differing opinions on the subject. The leading edge design on these airfoils may play a more important role.

I hope other folks will chime in.
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justwantin
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« Reply #3 on: September 28, 2011, 04:24:54 PM »

There seems to be much more to it than I imagined. I've done some isearch since reading the first response yesterday. Enough to come to a decision that I'm not in the league that worries to much about RE numbers at the moment.

That being said I understand why there has been a change to tapered TE and it appears to me after one build that an airfoil with a sharp high point is easier to shape no matter how good my eyeballing.

However, I would appreciate any other comments offered since they are all helpful in understanding what I'm building. Things seem to have gotten complicated while I was away.

When I was a kid we just pinned out a shape off the plan, rounded the top of the wing, glued it together, painted on some red stripes (allways red) and went down the park and started tossing them. Some flew, some crashed, every now and then one caught a thermal. Who cared? It was summer, no school and we didn't have to come home till the street lights came on.
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PZLP11C
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« Reply #4 on: October 20, 2011, 12:00:38 AM »

When I was a kid we just pinned out a shape off the plan, rounded the top of the wing, glued it together, painted on some red stripes (allways red) and went down the park and started tossing them. Some flew, some crashed, every now and then one caught a thermal. Who cared? It was summer, no school and we didn't have to come home till the street lights came on.

Yep, exactly my experience. When the street lights came on we moved over to the Tennis Court and flew our indoor rubber rog or several of us would try to re-kit our .049 c/l models on the pavement!! Finally one of the older brothers or a Dad would hunt us all up and send us to our homes for the night. Ah the heady scent of acetate glue and dope mixed with good old thermal dreams....
« Last Edit: October 20, 2011, 03:02:49 AM by Ratz » Logged
lincoln
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« Reply #5 on: April 28, 2012, 11:13:40 PM »

Some of the practical Reynolds number effects aren't all that esoteric or minor. Originally, real airplanes used thin airfoils because that worked better in the wind tunnels. But it didn't necessarily work better full scale. Difference was the Reynolds number.  Older scale models often used thicker airfoils, but that doesn't work very well on models. If you made a small ff glider using the same airfoils as a large rc glider, it wouldn't work very well. Especially the older, somewhat thicker airfoils that worked kind of ok on rc models but not on little 20 inch gliders. Main idea is that as you get slower and smaller, airfoils should be thinner.

I'm now going OT, but the following paper shows just how wacky things can get when you optimize for particular Reynolds numbers. If you feel like it, skip the technical stuff and look at the optimal shapes (the light lines, not the dark one) starting on page 8. Design regime 1 might correspond to a large model of a dirigible, and goes up from there.

http://www.iag.uni-stuttgart.de/luftfahrzeugaerodynamik/paper/sanfrancisco_6_97_lutz.pdf
There seems to be much more to it than I imagined. I've done some isearch since reading the first response yesterday. Enough to come to a decision that I'm not in the league that worries to much about RE numbers at the moment.

snip
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