Simple Question - or not?
Hi,
I am new to cfd and my question to the members of this Forum seems to be pretty simple: What shape has an airfoil with maximum lift when drag is abolutly not relevant? I am searching for this shape for a long time and even thougt about building a small windchannel to get an answer. Perhaps the answer is extremly simple, but perhaps it is an extremly complicated airfoil consisting of many pieces (e.g. a grid of some kind...) Let me get more specific: The air foil has to have max. lift. The angle of incidence does not matter, air speed does not matter (within "normal" ranges like 5 m/s up to 300 m/s). So is there any Software out there that could help me? Is there a simple answer? |
The airfoil needs to be really thin to obtain maximum lift.
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Hi Raj,
thanks for your fast answer! So a thin airfoil (probably at a high angle) gives maximum lift? How did you find out that fast? :) I always thought the answer would be much more tricky... |
Thin airfoil is not necessary gives high lift: it could stall at small aoa. The answer is really tricky. I would rather said: strongly cambered airfoil of an optimal thikness (depending on range of Re number) to delay stall.
There is a technique developed by Eppler and Seilig for a rough estimate of airfoil shape with desired characteristics and there is an excellent free software called "x-foil" by Mark Drela from mit (or its version with good GUI called XFLR-5) that has direct and inverse airfoil design tools. You can see e.g. http://smartsonde.org/Project/Vehicl...A-2137-802.pdf http://www.desktop.aero/appliedaero/...lsections.html Truffaldino |
I might suggest perusing "Theory of wing sections: including a summary of airfoil data" by Abbott and Doenhoff. This should help you to understand how camber and relative thickness play a role in airfoil characteristics.
To give you an idea of what is possible, it is said that the 737 wing has a maximum lift coefficient of about 3.2--and very low drag at low angles of attack. |
P.S. XFOIL, mentioned by Truffaldino, is a great tool!
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With blown/ or bl suction foils one can reach even twice of this. |
Hi,
thanks for all answers so far. @swetkyz: 3.2 sounds a little small; so what would the lift coefficient be when the angle is at its maximum? @truffaldino: I tried xfoil allready - but as you said it is only possible to calculate single foils. What you mean with "blown/ or bl suction"? Is this the technique of sucking in air on top of the wing? I think multi-foil wings will be my choice; probably the give max lift (when calculated properly). Again THANKS A LOT for your help!!! |
You may find "Simulation and Optimization of Flow Control Strategies for Novel High-Lift Configurations" interesting if you are willing to work with slats and flow control. This paper is by Meunier, and it was published in the AIAA Journal in 2009.
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You can take a look at a book by Bryan Thwaites on incompressible aerodynamics. He has examples of wing with bl suction up to 30% thick with Cl up to 6 or 9. As for max Cl without suction devices, Antonov-70 has max Cl=7.2 with slats and flaps only: you can look http://www.youtube.com/watch?v=PdqMHJJDiFM. As for software: there is a fast software MSES/MISES by Mark Drela (this is a multiple airfoil x-foil). But it you have to ask M.Drela for acopy. You could also try Fluent, of course, if you are at large reynolds numbers and flow is almost fully turbulent: Just use S-A turbulence models with multiple airfoils. In general, when you are at low Reynolds numbers, PDE based CFD software like Fluent cannot handle transition and separation bubbles (in spite of a lot of publicity of new sst transitional models) without a huge user input and comparison with experiments. |
I have also found a nice video on youtube on bl succion airfoil used by J.Y. Cousteau as a sail which has 5 to 6 times max Cl of an ordinary sail:
http://www.youtube.com/watch?v=X5E0HDBM4xA |
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