-----Begin Original Message----- Date: Fri, 24 Sep 2004 09:06:12 -0400 From: Jay Hunter <[EMAIL PROTECTED]> To: [EMAIL PROTECTED] Subject: What are airfoil polars and how do I read the charts? Message-ID: <[EMAIL PROTECTED]>
I have Profili and it lists polars as one of its great features. What are they, how do I read them, and what do they tell me? Thanks, Jay Hunter ----- End Original Message----- Jay, unfortunately you have asked a question which will take several text books to explain. Many people (aeronautical engineers etc) go to colleges just to better understand airfoil polars... *smile* I will try to give a *SHORT* explanation without going into lengthy details: An airfoil polar represent the aerodynamic characteristics of an airfoil. This is usually represented as a graph showing lift vs. drag coefficients, lift vs angle of attack, and lift vs pitching moment. An airfoil is placed in a wind tunnel, and lift, drag, pitching moment and angle of attach measurements are taken at various wind speeds and calculated as the local Reynolds number. As a general rule of thumb (gross exaggeration) you want to minimize the drag for a given lift coefficient. As is the case in aeronautics in general, there are always tradeoffs. When an airfoil has a good lift vs drag value at one point, it may be bad at another, etc. etc. etc. The "trick" is to find an airfoil which has the desired characteristics at the important phases of flight for which it is intended to be used. E.g. a trainer airfoil might have a high lift coefficient, "higher" drag coefficient (to make it fly slower or come down faster), low pitching moment and benign stall. But a F3B racer might have a lower lift coefficient (a high lift coefficient is not needed as much as it flies faster)but a very low drag coefficient at "racing" speeds. In another example, with a tailless flying wing you want very little moment so that the wing does not tuck under at flying speeds... etc etc. etc. Makes sense? Regards, Cameron Ninham Email: cpn66 at hotmail dot com 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.
