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Old Jan 21, 2013, 12:03 AM
DanielFerrando is offline
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Question

modeling help


hey everyone. I've been reading the book model aircraft aerodynamics because i would like to design a wing and I can't find an explanation on how to calculate the differences in wing design other than angle of attack. I cant imagine that the shape of the wing is irreverent.
so how do I use what is presented in the book to 'test' a new design?

sorry if im not being clear.
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Old Jan 21, 2013, 03:46 AM
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Are you talking about a flying wing (i.e. no fuselage, tail etc) or the wing for a conventional aircraft ?

Steve
Old Jan 21, 2013, 07:32 AM
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a conventional aircraft
Old Jan 21, 2013, 08:23 AM
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"Model Aircraft Aerodynamics" by Martin Simons contains a great deal about wing design. Chapters 7, 8, and 9 are about airfoils, if that's what you mean by "the shape of the wing". If you mean the planform of the wing then it's mainly chapters 5, 6 and 11 that are relevant. They all include plenty of relevant calculations.

If you're having trouble understanding them perhaps you need to ask some more specific questions.

Steve
Old Jan 21, 2013, 09:26 AM
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greg
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Planform shape affects stall characteristics.

chord dimensions affect Reynolds Number and drag (see Quantitative Analysis)

greg
Old Jan 21, 2013, 11:45 AM
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thanks slipstick and greg.
I meant the airfoils. What do i do with the drawing of the airfoil? how can I find out how the fluid behaves at different velocities? if it causes bubble separation and etc
Old Jan 21, 2013, 11:59 AM
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greg
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i think most of what your asking is captured by airfoil polar diagrams showing the lift and drag for a particular airfoil at various AOA and Reynolds Numbers (RN). RN allow are a function of velocity, as well as length. (again, see Quantitative Analysis).

see SuperGee II, designed by Dr Drela at MIT. You may also be interested in designed you own airfoils, see Xfoil
Old Jan 21, 2013, 12:44 PM
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Xfoil is a tool which predicts airflow resonably well at lower reynolds numbers. It's pretty much become the standard for evaluating airfoils for use with our model aircraft when you simply don't have a multi million dollar wind tunnel to play with...

As I understand it much or all of the Xfoil ability has been folded into XFLR5. And XFLR5 goes much further and provides the ability to examine whole wings and airframes. I've dabbled with XFLR5 a little but I'm far from even competent with it at this point. But there's no doubt it'll do whatever you can ask.

http://www.xflr5.com/xflr5.htm

Daniel, much also depends on what you want to do with the model. If you are looking at speed range and efficiency you'll quickly find yourself designing towards a glider like airframe based on the "gains" you see as you shift your numbers to follow the improvements you see from the runs. But there's more to it than this if you are after a model which is intended to be as efficient as possible but still do other things. This is where the nasty word "compromise" rears it's ugly head. For example roll rate and pitch rate is very important to aerobatic models so a long and skinny wing isn't the best option for such a design. So keep some potential practical realities in mind as you work with the program.
Old Jan 21, 2013, 01:39 PM
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wow thanks guys =D I played a little with xfoils and it seems to be more or less what i wanted and I'm downloading xflr5 to play with it a little as well. One thing that i cant get is what the program does to give me it's results like how it plots the viscid Cp vs cord length or Re. I want to find out how to do that by hand to better understand how it all works.

BMatthews to be honest I'm looking to model a boat (replica of the ijn yamato).I'm looking into model planes to learn how aerodynamics work to then move into hydrodynamics since they are both part of fluid dynamics I don't think there will be much of a problem.
Old Jan 21, 2013, 10:59 PM
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Here's a pretty good description of the methods by the author himself: http://web.mit.edu/drela/Public/papers/xfoil_sv.pdf

I'm not familiar with hydrodynamics, but compressible vs. incompressible flow are two totally different animals. I would be surprised if any analytical method could be common for both diciplines. In addition, a boat moves on the surface of water and all kinds of wave phenomena will be a significant factor.


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