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Old Jul 02, 2004, 11:31 AM
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Cross country glider using Drela airfoils.

I posted this at the Yahoo Allegro site as well:


I have been thinking of building a cross country ship for a while now
and I am planning on using the AG40 to AG43 series with a 4 or 4.5
metre span.

Carbon capped spar, Kevlar skin 4 tapers per side and a large cross
section fuselage I have drawn up.

Mark or anyone else care to comment on the suitability of these
sections for this task?


T.D.
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Old Jul 03, 2004, 02:46 PM
AustinTatious
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Hurst, Texas, United States
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only comment I can offer is that after flying a Drela airfoil, it is amazing how responsive it is to camber changes... It really moves out when you set it to reflex, but when you drop down to camber settings, it jsut turns into a whole new airplane and floats around.

Having never done X-c ... Take what I say now with a grain of salt.. I suspect that these airfoils would be a great choice.. you can thermal in camber settin to get max altitude as fast as possible before say the thermal burn s out.. then go to reflex and haul arse to the nex thermal, then repeat...

my .02
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Old Dec 08, 2004, 10:50 PM
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Joined Sep 2002
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So far, I've learned a few hundred things about XC airfoils. Out of 100 pilots, there are 100 different choices. Everyone learns to fly a specific way, and you need to match your airfoil/wingloading to your flying style.

One paper says stay away from traditional sailplane airfoils- E205, S3021, etc and go with S6060, Rg15 others say go with thickend E205's for floatability.

Some say dont go for anything that is considered "unproven" makes me wonder who decides to jump off and test the untested Go ahead- make the tests- lket us know how it turns out- maybe we'll all be flying drelas next year

Dean
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Old Dec 09, 2004, 09:32 AM
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The Drela foils sound interesting for XC, but I'd look at them a bit first.

They are optimized for very lightly loaded planes, which may not be appropriate for XC. I've flown XC with light planes, and they can work if you aren't in a hurry and there isn't any wind, but going to higher wing loadings is much more stylish these days.

Drela foils are typically optimized for a particular Reynolds number, some of his best are aimed at very low Reynolds numbers. XC airfoils operate at higher Reynolds numbers.

They are very thin - the spar doesn't have much depth, meaning that you will have to be very careful to get the spar strength to handle 150 inches of span.

All that being said, I'd probably ping Dr. Drela and see if he might be interested in collaborating on an XC design. Actually, I'd try and talk him into doing the whole thing, but be happy to take whatever he was willing to do.

You should also keep in mind that XC is probably the best test of piloting skills that exist in RC soaring. It ain't the plane, it's the pilot. The worlds record holder and pretty much the undisputed king of XC is Joe Wurts. He likes a "big dumb airplane".

On the other hand, there aren't many XC designs out there - should be room for improvement!

happy trails - Rob
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Old Dec 09, 2004, 10:51 AM
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David Layne
Tracy, Ca
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Xc

Having flown some XC, I would agree with Rob, you need to go for airfoils suited to higher Reynolds numbers and thick enough to give the spar strength needed for a large span aircraft. My personal favorites are the Quabeck airfoils, they were designed for camber changing and have a very wide speed range. I would use a 1.5% camber, 9-10% thick at the root and transition it to about 75 thickness at the tip with maybe 2-2.5% camber. This would give you the speed you need, but with camber changing it will thermal quite well. At the higher Reynolds numbers that these large, heavy models fly, high cambered airfoils are not necessary or even desirable. They will slow you down when you need to step out and go fast. It's not unusual to be chasing you plane at 60-80mph.


David Layne
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Old Dec 09, 2004, 12:09 PM
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David knows the Quabeck pretty well. He was the manufacturer of the Saturn series of planes a few years ago, they did quite well with the Quabeck airfoil. I can remember him launching and zooming forever with them.

His numbers sound like a good cut at the 'foil, although I'm pretty sure he means 7.5% thickness at the tip.

There are a couple schools f thought on camber changing. One school likes the camber changing profiles, with flaps and ailerons. SBXC is the prime example.

The other school likes the "big dumb plane" approach. Basically a very large Gentle Lady. Poly birds are typically easier to see and more stable. If you do actually fly cross country the plane sometimes gets to fly itself while it disappears behind treelines or into clouds.

The airfoil would be different for each of these approaches. After having had a few cross country planes of both styles I've come to the conclusion that there is a lot to be said for simplicity.

happy trails - Rob
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Old Dec 09, 2004, 01:00 PM
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It's been a long time since I last flew cross country but one thing I do remember. Speed greater than you can drive is useless. I remember dropping flaps to slow the model down to the maximum speed we could drive on the back roads used for the course.
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Old Dec 11, 2004, 09:51 AM
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8% is pretty thin!
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Old Dec 11, 2004, 01:18 PM
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Quote:
Originally Posted by ChuckA
It's been a long time since I last flew cross country but one thing I do remember. Speed greater than you can drive is useless. I remember dropping flaps to slow the model down to the maximum speed we could drive on the back roads used for the course.
.
How about upwind?
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Old Dec 13, 2004, 09:47 AM
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A Dr. Drela 'Ne "Supra" Ultra' ( 25% linears, 100 Dia. pod and low Wt. 50% and ballast 4.5 lead). It would be very nice for a cross country plane.
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Old Dec 14, 2004, 03:06 PM
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Exaclty what I was thinking Ollie, something along the lines of a scaled up Supra.

Rob and David,

I've built a few wings with the Drela airfoils from 60" to 132" on my own designs and they respond to camber extremely well and excel in just about any conditions, one interesting thing is that they also respond well to ballast. I think a CC ship using these airfoils will be an excellent bet.

With a properly designed carbon capped spar the 8% thickness is not really a worry.

I thought about using the HQ2.5/9 - 2.5/8.5 - 2.5/8 which I like a lot but the Drela sections have been so good to me that i think I will stick with them.


T.D.
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