Testing for Oswald factor

As long as you are involved in airplane aerodynamics, either by design or by testing or by any other means, you are surely aware of the span efficiency or Oswald factor “e”. This factor accounts for the difference of a wing’s lift distribution to the elliptical lift distribution. Literature suggests usually values of 0.8-09 or sometimes even 0.65 upwards. Various methods for estimating “e” from wing geometry parameters have been published. But what happens when coming into testing? A proposed method of deriving the “e” for prop aircraft is found in D.T. Ward’s book “Introduction to Flight Test Engineering” based on a generalized power curve. An application of the testing method resulted in really unexpected outcomes. At least for 3 different light airplanes the “e” has received values below 0.5, while in an aerobatic airplane case which was tested with various wing tips gave some values even below 0.3! Some error maybe? Values seem extraordinary, but I wouldn’t rush in such conclusion. There has been one reference before in literature of a general aviation airplane being tested for “e” with a result of 0.45, but that outcome was not commented at all. At the moment I am tending towards the effect of the tail plane produced lift contributing to a reduced overall “e”. It needs to be investigated more and maybe the analytic tests results will be interesting for a paper (time permitting…).

An example of an e calculation for an aircraft tested recently can be seen in the graph below, shoeing the approximately linear relation between the product of equivelant weight BHPew and Vew to Vew^4. The same process provides the zero lift drag coefficient for the whole aircraft.


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