Expert: Paul Soderman - 6/10/2008

Question
hello and thanks Paul. Ive been struggling with the math for lift for wings. Id like to build something like a dirigible motorglider using helium. Are there any simple guidlines for wing aspect ratio vs the weight of the craft?
Am I even going about this the right way? Thanks again.
tanner

Answer
Hi Tanner
I'm curious about the design of your aircraft because conventional dirigibles don't depend on wing lift, but I assume you have something different in mind. Wing lift, L, can be calculated as follows:

L = Cl q S

where Cl = integrated wing lift coefficient, a dimensionless number that depends on wing shape and angle of attack
q = dynamic pressure = rho * V^2 / 2
rho = air density
V = flight speed
S = wing area

Of course all units have to be consistent.  In cruise, the lift must equal aircraft weight for level flight.  (I am not counting lift due to helium of course.)

Aspect ratio does not enter directly into lift as long as the span is long compared to the chord.  Aspect ratio does enter into induced drag (drag due to lift) - high aspect ratio wings create less drag than low aspect ratio wings.

If you tell me more about your design and mission, I might be able to calculate some numbers for you.
Paul