Expert: Paul Soderman - 8/23/2009

Question
1-What is aspect ratio of an airplane and how does the change in aspect ratio affect the lift of an airplane?
2-What range of aspect ratio will you choose to get an advantage over other makers and why?

Answer
The lift of a wing depends on wing area.  The aspect ratio primarily affects induced drag, which you want to minimize.  The induced drag, Cdi, can be computed as follows:

Cdi = Cl^2(1+sigma)/(pi AR)

where  Cl = wing lift coefficient
      pi = 3.14159
      AR = aspect ratio = b^2/S
      b = span
      S = wing area
      sigma = a small number that depends on planform (sigma = 0 for an elliptically loaded wing, the optimum wing for minimum induced drag)

So for a given wing area, you want to maximize aspect ratio.  That is why gliders have long spans with small chords.  There is no optimum range of aspect ratios.  Larger is better, but as the span gets large other factors such as structural loading, wing bending, roll rate, and control become important.

With respect to lift, high aspect ratio brings the lift curve slope closer to the theoretical optimum of 2*pi per radian angle of attack.  This analysis is for subsonic flight.
Paul