>Yes, but also .... what's happening (for a fixed wing span) is a trade off >between wing loading and induced drag. Re (at least for DLG chords and up) is >a variable but not the dominant one. > >As you increase aspect ratio (reduce chord for a fixed wing span), induced >drag goes down but wing loading goes up. The wing loading will eventually >degrade your sink rate more than the aspect ratio improves it. Although the wing >weight scales with area, the total weight does not (fuselage, tail surfaces, >ancillary equipment). Re will hurt here as well (both Cd and Cl) but it's >primarily the loading (except perhaps at very small chords). > >On the other end, lower aspect ratio gives a lower wing loading but >increased induced drag (Cl^2/(Pi * AR).
Dave, you should know that for a fixed wing span, you need to be talking about total drag, not a drag coefficient, because you are changing the wing area when you change the aspect ratio. Total induced drag depends on the span only (for a constant weight and flight speed). Changing aspect ratio with a fixed span does not affect induced drag (if we ignore that the weight of the structure changes). Induced drag is proportional to the span loading squared (the formula below is obtained by manipulating the one above Cdi=Cl^2/(Pi*AR)): D = (W/L)^2/(Pi*q), where q = rho*V^2/2 Therefore with a fixed span, you can easily find a wing area that gives you the best L/D ratio (parasite drag = induced drag). But that maybe still not the best design depending on your objectives. The main trade-off variables are min flight speed (affects turn radius), best L/D ratio, and high speed performance, all of which are directly affected by the wing area. Obviously, reducing the weight and parasite (fuselage, tail) drag allows to improve all of the criteria because the optimum wing area becomes smaller. Comments and corrections are most welcome. Regards, Oleg. RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED] Please note that subscribe and unsubscribe messages must be sent in text only format with MIME turned off. Email sent from web based email such as Hotmail and AOL are generally NOT in text format
