Oswalds efficiency factor

[rpilski]

While reading an article on Charles Rivers Radio Controllers website I encountered a term not familiar to me "Oswalds efficiency factor".
This efficiency factor is a measure of wing efficiency.

Could someone explain what is this factor or point me to a web site where this factor is defined and described ? I did a google search but without success.

The article I was mentioning is here:

http://www.charlesriverrc.org/articles/design/gavinbotha_optimumar.htm

Regards,
Radek

[doug928]

Oswalds eff. factor is a planform efficency factor. Elliptical and tapered wings have better efficiencies than rectangular planforms. Aerodyamic and Aero design text books will give you much more detail. Hope this helps for a simplistic explanation.

Doug

[Ollie]

See:
http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html
Induced drag is minimum when the lift distribution along the span is elliptical. When the lift distribution is not elliptical an efficiency factor of less than one in the denominator of the induced drag calculation accounts for the efficiency of the actual lift distribution compared to the ideal, elliptical distribution.

The lift distribution of a wing depends on the angle of attack, aspect ratio, taper, twist and sweepback. See:
http://aero.stanford.edu/WingCalc.html

[rpilski]

All is clear to me now.
Thanks a lot Dough and Ollie!
Radek

[Sail 'n Soar]

Induced drag is minimum when the lift distribution along the span is elliptical. When the lift distribution is not elliptical an efficiency factor of less than one in the denominator of the induced drag calculation accounts for the efficiency of the actual lift distribution compared to the ideal, elliptical distribution.

Key point to understand is that it does not take an elliptical plan form to get an elliptical lift distribution. Additionally, you can get VERY close to the ideal elliptical lift distribution induce drag with more easily constructed single or double, tapered wing planforms and with more forgiving flight characteristics.

[Ollie]

An extreme example, to illustrate Sail 'n Soar's point, is the Culver twist formula for swept back, tapered wings. In this case the twist distribution along the span is designed to produce an elliptical lift distribution. See:
http://www.b2streamlines.com/Culver.html

Scale modeler's of full scale aircraft with swept back wing planforms should find the Culver twist formula very useful in designing their scale projects.

[rpilski]

Thanks again.
If I may also add someting for others interested in the subject of this thread: Martin Simons in his "Model Aircraft Aerodynamics" describes this subject (induced drag reduction "via" planform, twist, wing tips and winglets) in very good way (IMO). His book is very good. I would consider it a must have for every modeller that wants to understand the theory behind.

Radek