Expert: Paul Soderman - 2/27/2009

Question
Hi it’s me again, thank you for the formulae. However, I don’t know the speed or the time,  so could I use:

a – Acceleration, T – thrust; W – weight; m –mass;  
Ratio = thrust / weight = a / g

a = Tg/ W
T – D = ma
D = T – ma

Q1 can I use the value of the Engine thrust range = T
Q2 which mass should I use:
a)   the difference between = Maximum takeoff weight and Maximum landing weight
b)   or the sum = Maximum zero fuel weight + Maximum fuel capacity


The lift equation L = 0.5*Cl*rho*v2*A
Q3 Do we use area of the wing   (as lift is mostly generated by the wings) and do we use 2 wings or just the one ?  

Answer
Jane
I'm not sure what problem you are trying to solve.  If the aircraft is in cruise, acceleration is zero. If the aircraft is accelerating, you can use

a = (T-D)/m
where m = W/g

But this is a little tricky because drag increases as the speed increases, so a goes down accordingly.  You could solve the equations in small time steps, but do you know drag or acceleration or drag coefficient ?

For cruise you could use 0.9 Maximum takeoff weight because some fuel is burned to get to altitude.  The wing area listed on the Wikipedia site is total wing area - use that number (5500 ft^2).
Paul