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Finite Wings 
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#1
Apr1906, 03:20 AM

P: 55

If I take an infinitely long, thin wing and measure its lift curve slope I should get 2pi.
Now if I take a 1/2wing and fit 1 end to a plate and test it in a wind tunnel, it should simulate the flow around a full span wing, am I wrong? Therefore measuring lift and working out the CL values using the area of the half wing, air density, and flow speed I should get approximately the same results for CL as for a full wing (same section) twice the area and twice the aspect ratio (with no plates fitted to the ends). I've done that experiment for a NACA 0015 aerofoil section and my results are confusing me. I understand the theory and the limitations of Prandtl's lifting line theory but this doesn't seem to be helping me here. I was expecting the theoretical and experimental results to match up fairly well (looking at Prandtl's graphs they are pretty much identical). The 1/2wing I used was AR 3. Therefore it simulated the full wing AR 6. If I use AR 3 for the theoretical calculations the graphs match perfectly, but I can't do this, because the 1/2wing AR 3 was simulating a wing AR 6 with that attachment plate on 1 end. So... experimentally the lift curve slope is 3.5rad1 and theoretically it's 4.5rad1. Can anyone just point me in the direction of the reason for this massive difference? I'm guessing it's something to do with the 1/2wing with a centreline plate mount not properly simulating the flow around a full span wing, but this to me would suggest a steeper lift curve slope than theoretically predicted because there's 1 less wing tip vortex to cause more downwash over the rest of the wing. I hope someone can make sense of that  I'm sure the only reason I can understand what I've written is because I know what I'm trying to say! 


#2
Apr1906, 08:46 AM

P: 55

As is usually the case I answer my own question: the main (obvious reason) is that the theory I was using (Prandtl's lifting line theory) assumes invsicid flow and of course there are viscous effects (even if small).



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