Stuart Walker
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| posted on 1/12/06 at 11:26 AM |
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Aeroplanes and Aspect Ratios...
Does anyone know about wing design?
Doing a project on a HALE (High Altitude Long Endurance) aircraft. I've determined that:
Di x AR = 70
(Induced drag and Aspect ratio)
So now I'm trying to decide on an aspect ratio value. It doesn't need very much manouverability, and wants to use as little fuel as
possible, and its really light (200N GTOW), so I can use a fairly high one...
But the question is, what's a good number? About 30 like a glider, or can I go even higher? 40?
(40 gives a span of 10m and 25cm av. chord)
Any thoughts greatly appreciated!! 
Cheers,
Stu
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bigandy
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| posted on 1/12/06 at 11:47 AM |
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Generally, the higher the aspect ratio, the more efficient the wing. there are a few exceptions, and as you are probably aware, all design choices in
aircraft design are compromises 
I suspect the only reason modern gliders don't have very high aspect ratio wings, is mainly down to structural issues. Making a very high
aspect wing (big span, small chord, and more than likely a thin profile/section) can become a right pain in the botty to make it strong enough, in
terms of spanwise stiffness, torsional rigidity etc. You also start getting complex aeroelasticity as you increase the span of wings, especially if
you cannot keep the rigidity up. This structural limit is often the overriding factor in wing design, as the aspect ratio goes up, the comes a point
where the structure weight starts to rise, and it becomes too heavy, depsite the slight aerodynamic advantage of increasing the AR.
If it is just a paper exercise on the concept design, then the structural issues of designing a very high aspect ratio wing, can be put to one side,
but if you want to do a proper job, then you need to bear it in mind, as it is rather important IMHO!
Hope that helps a bit.
Cheers
Andy
PS. It's been a while since I had to recall any of my Aero engineering degree stuff! Makes a change from drawing stuff on a computer!
Dammit! Too many decisions....
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Stuart Walker
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| posted on 1/12/06 at 12:52 PM |
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Thanks a lot Andy, very useful. Glad it made a change for you!
It is a paper exercise, but I do need to consider the structural practicality, so can't have 50m wingspan! I've decided to keep it a bit
lower and go for AR=20.
Right, off to uni to model it now!
Thanks again.
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bigandy
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| posted on 1/12/06 at 01:11 PM |
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Glad I could help a little!
Is it for a University project? Just wondering.
cheers
Andy
Dammit! Too many decisions....
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Stuart Walker
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| posted on 1/12/06 at 01:42 PM |
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Yep, 4th Year Mechanical Engineering module project.
Cheers
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BenB
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| posted on 1/12/06 at 01:58 PM |
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Yup. AR depends upon purpose. Compare something like a Cap232 with a glider. Weeny AR with huge engine vs huge AR with no engine!!!
Personally I'd go for the Cap232 any day.....
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clutch_kick
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| posted on 1/12/06 at 03:14 PM |
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Interesting stuff. Is this actually getting off the ground, or paper only? just how high altitude are you planning?
Bigandy pretty much said it all, however if this is going to be a working model then, you must also start taking into consideration, what sort of
propulsion and fuel you will be using.
Go to http://www.micropilot.com/ for the Flight guidance, and you can visit
Wren Turbines (UK Based) for the engine, they make a lovely turboprop engine, excellent for high altitude/low
speed.
I can help you with a few more links to companies, since I was interested in starting out my own UAV business, but if you google it, you'll
pretty much find more up to date info.
[Edited on 1/12/06 by clutch_kick]
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meany
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| posted on 1/12/06 at 06:26 PM |
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what you also need to take into consideration is the wing profile/section.
this does not necesarily continue the same all through the length of the span, it is quite often for the profile to change from the root to tip of the
wing.
i reckon Burt Rutan ir your man.
[Edited on 1/12/06 by meany]
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Stuart Walker
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| posted on 2/12/06 at 12:08 AM |
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Thanks guys, its a fairly simple project really (all theoretical unfortunately  ), so it's pretty much sorted now... Interesting stuff though!
Also quite pleased as I managed to design an aerofoil myself which performs better than the one the computer program I'm using came up with
Thanks again for all the info., I'll do some more background reading on engines etc. tomorrow.
Cheers,
Stu
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