Expert: Paul Soderman - 2/25/2009

Question
I need to find thrust, lift and drag force for my physics school project

I’m using : thrust = final(mass x velocity) – initial (mass x velocity)

I have 2 masses and thrust but no velocity (I’m only using data on the airbus’s website)
So can I still use value I would find for the velocity
The same velocity will be used to find lift and drag

And do you know any websites where I could find lift/ drag coefficients for various airplanes and density of air for different height  

Answer
Hi Jane
You can find air density versus altitude by doing a google search on standard atmosphere.  Try:  http://www.digitaldutch.com/atmoscalc/

I don't know where to find lift and drag coefficients for specific aircraft, but they can be computed.  If you pick an airplane such as a Boeing 747, you can find data on the web such as at:

http://en.wikipedia.org/wiki/Boeing_747

where I found a table of aircraft weight, cruise speed, thrust, cruise altitude, and wing area (under the table).  From that you can compute the following:

Cl = L/(qs) = W/(qs)     lift coefficient

where L = W = weight in cruise (i.e., lift equals weight in cruise)
q = dynamic pressure = rho*V^2/2
rho = air density at altitude
V = velocity
S = wing area

All units must be consistent so Cl is non dimensional

Similarly

Cd = D/(qs) = T/(qs)     drag coefficient

where D = drag = thrust in steady flight

qs is as above.

The equation you are using is not quite right.  You should use Thrust-Drag = d(mv)/dt
I.e., thrust greater than drag equals change in momentum with time.  You left out time.  And it is only true for an accelerating aircraft.  I'm not sure where you are getting data on change in momentum with time.  The method I outline is much easier if we know aircraft conditions in steady flight.  

You are almost ready for a class in aerodynamics.  A little more math would help - like calculus.  Good luck.
Paul