>If you're designing a wing with a flat-bottomed airfoil, it's >convenient to create the effect of washout by thinning the airfoil >as you go out from the root; this reduces camber and lowers the angle >of attack at which CL = 0. So for example Mark Drela's Allegro 2M >uses an 8.7% AG35 at the root, tapering to the 7.0% AG38 at the tip; the >camber goes from 4.4% to 3.5%. The zero-lift alpha of the AG38 is 0.7 >degrees less than that of the AG35, so a wing whose bottom is flat from >root to tip will have in effect 0.7 degrees of washout.
Not to be picky, but AG38 has higher zero lift AoA (more positive). Reducing camber increases zero-lift AoA (makes it more positve). But I understand what you mean perfectly. > The lift distribution is a function of planform, airfoils and washout, >and so is the CL distribution. If the planform is elliptical, you >can (in theory) get the same CL all along the wing with no washout. >The whole wing will stall at once. You are confusing a Cl of a wing section at a given AoA with its ability to go to a (much) higher Cl (Clmax) or more importantly to sustain high AoA without stalling (max AoA). Aerodynamic twist has the same effect as geometric twist on the lift distribution of a wing at a given AoA. But their effects on the MAXIMUM AoA of a wing and its stall behavior are not necessarily the same. Reducing airfoil's camber is equivalent to reducing its AoA if the wing has the same angle of incidence at the root, so in this respect aerodynamic washout is equivalent to geometric washout. This affects the lift distribution - reduces Cl towards the tips. At the same time reduced camber GENERALLY will reduce the Clmax of the airfoil (at least that is my understanding) and also the maximum AoA. If the maximum AoA is reduced and there is no geometric washout, then reduced camber (aerodynamic washout) does not do any good for preventing tip stall, because the AoA margin is reduced in this case. Now my question was: does reducing camber of an airfoil (SD7080 in our case) reduce its Clmax and the corresponding AoA, and if yes, then the usage of aerodynamic washout is questionable. Here are the results of running X-foil for sd7080 (2.5% camber) and modified sd7080 with 2% camber. It is not a real experiment, but still gives a good idea of what to expect. The modified sd7080 (2%) had Clmax at about AoA=11.6-11.7deg, while the original sd7080 had Clmax at about AoA=11.7-11.8deg. Almost no difference, although the trend is to smaller max AoA with reduced camber. As Gavin already said, it is probably best to have an airfoil with the same camber but shifted max thickness point. Simply changing the camber will most likely not improve tip stall resistance. Ok. I am stepping off my soapbox now. I already wasted 2 hrs of work time today only... Sorry for dragging this for so long. Let's get back to our regular RCSE program - trashing vendors, ads, etc. :-) Oleg. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
