Oswald Efficiency Factor

[Guest BobL]

Does anyone know exactly what this is? How does it affect aerodynamics? If it's not an extremely complicated mathematical operation, how is it calculated? And most importantly, how does it affect the FS parameters?Thanks in advanceBobL

[Guest Firestriker]

Bob,As I understand it, the oswald_efficiency_factor (e) has something to do with with lift vs. drag and is always a figure less than 1. I find that in most cases 0.7 is a good starting point.If you do a web search for "Oswald Efficiency Factor" you might find an exact definition and how it is calculated. I can not find anywhere where this information is availble for various aircraft. If some one knows it would be nice if they would share the info.

[Guest BobL]

LouThat helped a little. Here's the definition from one of the results."The Oswald efficiency factor, e , accounts for the fact that, because aircraft design is a compromise, no wing or airplane is as efficient as is theoretically possible. The theoretical maximum value for the Oswald efficiency factor is one. The smaller the value of e the less efficient the aircraft. The Oswald efficiency factor affects the effective induced power required, i.e., the power required associated with the production of lift. Typical values for light aircraft are from 0.5 to 0.8. Based on flight test results, the Bonanza, with gear and flaps up, i.e., clean, has an Oswald efficiency factor of approximately 0.56 to 0.65. Extending the gear and/or flaps has some effect but not a large one on the value of e"So, using a little extrapolation, the manner in which the .air file or aircraft.cfg would use it would be like some type of global scalar that could be used to make your aircraft more or less efficient in the terms of increased lift produced, decreased induced, parasitic, and profile drag produced, airspeed produced for any given wing design/engine combination, etc. It also stands to reason that a smooth, low profiled design (like the C310, Bellanca Viking or Piper Aerostar) would have a higher 'e' (say 0.8 to 0.9) than a high profile aircraft such as the Piper Aztec, C337 or Rockwell 112/114 (say 0.3 to 0.5) with mid-range aircraft (C-210, Bonanza, et al) hitting in the middle 0.5 to 0.6 range specified above.Am I on point here or have I missed something? I wonder if there are any experienced FDE guys who might comment on this? I'm working on tweaking a C310R and seeking to understand how this variable is used and what effect I can expect to see from it.ThanksBobL

[Guest]

BobL,The following comes straight from my book titled:Model Airplane Design And Performance, by H.L.Chevalier, P.E.Professor Emeritus of AeroSpace Engineering, Texas A&M University.The paragraph describing what Oswald Effeciency Factor is on page 68,where the sub heading is titled: Wing Planeform."The effeciency of the wing in producing lift for the smallest amount of drag is dependent up on the planform shape. If the center portion of the wing produces too much lift, the drag due to lift in the center will be large compared to the tips. If the wing tips produce too much lift, the drag at the tips will be too high.The most efficient distribution is where the drag due to lift is the same across the span. For the drag due to lift to be the same across the span the downwash velocity must be the same across the span. For the downwash velocity to be the same, the lift distribution must be elliptical. This is the most effecient wing. The mathematical wing efficiency, e, has a numerical value of 1.0 (or 100%).There are several ways to design for an elliptical lift distribution.One is to design an elliptical planform. Another is to design a planform with taper."Then the chapter goes into various planes such as Spitfires and P47 Thunderbolts that used elliptical planforms. And the various ways one can use tappering as well as elliptical forms (thickness of various areas of the planform etc..) to build efficient wings.Within this chapter of the book, he goes into Drag due to Lift where we call it the coefficient of drag due to lift.........which is represented mathematically as:C (sub DI) = C (sub L) / pi AR ewhere CL is the lift coefficientpi is of course 3.14......AR is the Aspect Ratio of the wing and e is the Oswald effeciency factor.Now what is not brought out, is the many factors such as parasitic surface drag, skin friction drag, pressure drag and other phenominal that can be envistaged and measured that contribute the the effieciency factor of the wing. One must delve into fluid mechanics in order to obtain a real picture of the various forces operating on a poly surface such as a wing. Bottom line is.........why don't you go to your local public library and get a book on Aerodynamic Design or one such as mentioned here that gives one a over all picture of how one can design a airplane.At any rate, I hope the above quotated passage in some way increases your knowledge as to what the Oswald number indicates.cheers, grb:-)