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Old Jan 10, 2011, 08:58 PM
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Free Flight Airfoils?

Dear Forum,
I am designing a slow fly park flier. I have always been fascinated with the highly undercambered free flight airfoils. Do they offer any advantage for this application as compared to, say, an AG36?
Thanks!
Mike
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Old Jan 11, 2011, 12:37 AM
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Freeflight airfoils are usually designed to maximise duration. That often results in the highly cambered airfoils that you are thinking about. The advantage is they produce good lift so fly slow and will stay up on minimum power. The disadvantage for RC use is they are very much optimised for 'single speed' flying.. They get very draggy if you try to fly faster which could be a pain on an RC model if you are trying to fly into a headwind for instance.

So if you want a plane to fly slowly, stay up on minimum power have maximum duration, but windy weather flying and speed are not important, then highly cambered freeflight type airfoils could be just what you want.
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Old Jan 11, 2011, 06:26 AM
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What he says, plus I have found them a bit more vicious in the stall.

BUT for a very low powered park flier that isn't gong to do inverted, they really can bring the flying speed down. Its a wing with flaps extended basically.

The are also a pain to cover ad somewhat fragile, due to thin ribs.. so the normal method to get slow flying speed is to use an enormous wing..in fact a large, rather low aspect ratio, wing is a good way to go as it has highish drag as well. That means the speeds are slow AND the glide angle s steep, which is useful when landing in tight spots surrounded by trees...
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Old Jan 11, 2011, 08:34 AM
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What they said.

But also size is a factor - at low Reynolds numbers you can easily over do the camber, even if the profile is thin.
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Old Jan 12, 2011, 05:13 AM
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I would say that a GWS slow stick is a good example of a highly cambered wing and what it can do.
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Old Jan 12, 2011, 03:52 PM
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Quote:
Originally Posted by SargeNZ View Post
I would say that a GWS slow stick is a good example of a highly cambered wing and what it can do.
I'd say it was a good example of what a lot of low aspect ratio wing can do, irrespective of camber..
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Old Jan 12, 2011, 04:34 PM
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Low aspect ratio also keeps the Re number up because of the wider chord...
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Old Jan 12, 2011, 05:40 PM
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Quote:
Originally Posted by vintage1 View Post
so the normal method to get slow flying speed is to use an enormous wing..in fact a large, rather low aspect ratio, wing is a good way to go as it has highish drag as well.
Is that because a high aspect ratio wing's lift would suffer from very low Re-effects due to its small chord, compared to a low aspect ratio wing of same area? The loss of lift due to the tip vortex-induced lower section AOA near the tips would not be so dramatic in comparison?
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Old Jan 12, 2011, 06:24 PM
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Thats more or less true.

The Slow Stick has an aspect ratio of 4.2
The span is 46.3" and the chord is about 11".

Even at a stall speed of 9mph thats Re76k. Not so low really. At AR=8 you'd get Re38k. That's getting a bit awkward...

The real reason for its slow-ability(?) is the incredibly low wing loading: 3.4oz/ft2! Thats about a third the wing loading of a 'normal' trainer of equivalent wing span.

Low aspect ratio allows more area, giving lower wingloading plus the greater chord. If you want to fly slow it's the way to go!

You wouldnt want to go much lower than AR=4 however. For AR's above 4-5 the CL_max doesnt differ too much from infinite span. Below AR=3 and max lift starts to suffer.

A reduction in CL_max means an increase in stall speed. Low Re can have a significant negative effect on CL_max however!

All in all it's a well designed model - with the right airfoil!
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Old Jan 12, 2011, 06:57 PM
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Mike, are you willing to give up most of the medium speed option to enhance the lower speed side and limit yourself to flying in 5 mph or less of wind? If you are then there's some nice options out there for a good parkflyer.

And I'd like to believe that they would fly a lot better than the cheezy molded meat tray Slow Stik wing.

The airfoils used on the modern FAI class free flight models are actually not all that bad at speeding up yet fly very slowly. Not as slowly as some of the banana like airfoils used by A2 Nordic gliders of the past but not bad at all.
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Old Jan 12, 2011, 08:16 PM
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Hi B!
Do you know which airfoils they use? I found a video of an F1A glider, and it looks awesome. http://www.freeflight.org/Illustrated/video/index.htm
I found a website for F1A. It has some plans, but none of the airfoils are labeled:
http://www.f1a.info/models/plans/f1a...hort_Model.gif
http://www.f1a.info/models/plans/f1a/
Certainly something better than the lunch tray wing is possible!
Vingage: The flaps extended wing is a great comparison!
I have CompuFoil.
Thanks!
Mike
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Old Jan 14, 2011, 06:59 PM
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There were lots of them that you may not find in the usual databases at this point.

The Benedek 7457 was one that I used on the two F1A models that I built back in the 70's. One was a faithful copy of Peter Allnut's Adagio design and the second was the same planform but with simple V dihedral. That airfoil worked very nicely for the roughly 8 inch chord flying slowly.

Others that I know work well at Coupe d'Hiver size are the Benedek 6356. An even thinner airfoil that makes it more suitable for even lower reynolds numbers.

The airfoil I used for a Coupe of my own design from the 90's that did very nicely for me (but you STILL need to find thermals) is the one in Profili's database labled "SOKOLOV". I assume it is a Russian or east European airfoil used to good effect in international meets. But it sure did well for the two models that I used it on.... At least for the glide. I used it on a P30 but tried to go too far with the design. I made it a low aspect ratio which made it fly SOOOO slowly that it would just get bumped away from most thermals instead of flying into them. So apparently too much of a good thing, as in wing area for a lighter loading, can backfire on you at least in super light free flight models. The thing was dyno-mite on days with lift that was vapourous though. It would center in and ride whisps that other models could barely see and that would only slightly slow down their descent.

Other superb airfoils from that time were the Ritz airfoils used by some of the old Nordic class and the Kaczanowski GF-6.

Basically there isn't a huge amount to recomend one over another. They are all thin, have sharp'ish leading edge radii and LOTS of camber. Many of them respond well to using two or three turbulator spars in the upper portion between the LE and the main spar. And one more to support the covering behind the spar would not be amiss either. As for spars the optimum way to deal with such thin airfoils is thin flat spruce or carbon spar caps joined with inbetween sheet webbing to produce a definite I beam that is as thick as the airfoil shape will permit.
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Old Jan 19, 2011, 12:18 PM
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Blacksburg, VA 24060 USA
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Re: Slow Stick Wing and Undercamber

I've found the Slow Stick wing to be well behaved at slow speeds, as when landing with the nose down a bit. My Slow Stick, with scratch-built fuselage, weighs 19 oz.

I built ailerons into my SS wing, using the outlines as indicated. They are a bit small, but work well enough for level flight. I tried doing a roll and scared myself into the next decade . The serious undercamber does not like rolling to inverted .

The ailerons don't work well with the nose high, at low speed, so rudder is necessary to maintain heading. When it stops flying, in this condition, the stall is manageable.

Since I don't fly it in windy conditions, I can't verify its handling.

One other item: A thin under-cambered wing may have a tendency to twist a bunch of washout at high speed ("high" being relative). The SS wing will do that if the nose is tucked down a bit at full speed. Suddenly, it's diving very dramatically .

To counteract this, I braced the SS wing with struts, which reduces the problem at most speeds. I've never tried diving the airplane straight down, nor do I intend to .

Jim R.
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