ORIGINAL: ptxman
In the real world of building wings we always have to truncate the trailing edge of flying surfaces to some amount relative to the typical airfoil theoretical end point coordinate (x=100, y=0). Yet we use the polar data which assumes this perfect profile. It seems that in 'airfoil intensive' pursuits like racing, sailplanes, high performance etc, every effort is made to minimize truncation & extend the profile as much as possible & of course keep it rigid. The thinner the airfoil profile, the harder this is to achieve assuming a minimum acceptable TE thickness.
My question is, where on the airfoil polars would this truncation show up as a degradation? An overall inneficiency loss spread out over the Cl/Cd curve? Or more weighted at high AOA & therefore reduced Cl max? See what Im getting at? Do fat airfoils suffer less than thin airfoils? Is there a magic rule of thumb like "if you achieve less than 95% of the theoretical chord, the polars cannot be trusted & you need to use a fudge factor"? Assuming a truncation is inevitable, would it be better to radius the edge or leave it square?
ptxman,
Why do you say truncation is inevitable? If performance is really important, you could choose to use harder wood for your trailing edge, and just make it sharp.
If the trailing edge is blunt, the flow will separate. If there are sharp corners (a squared-off trailing edge), the flow will separate right at the corners. If it is rounded, the flow will stay attached partway around the curve, and then separate. The main effect will probably be slightly higher drag. If you look at a plot of upper and lower surface pressure toward the trailing edge, you will see that the pressure difference is getting pretty small in that area. Since the amount of area lost is small, and the pressure difference is small, the amount of lost lift would probably be negligible. Note that if the flow would have separated upstream of where the trailing edge is truncated, then truncating the edge would make almost no difference.
I don't see any reason to intentionally blunt the edge. Sand it as sharp as you can. A few dings won't be a big problem, and if they are, use harder wood. It certainly doesn't weaken the structure to leave the sharp edge on there, and it may save you a little drag.
banktoturn