View Full Version : Engine performance units

June 14th, 2009, 14:20
I'm fiddling with the engine tables to simulate a turbojet more accurately.
In AAM it equates Ram Drag to (V/g)*Intake Area*Table 1507*Delta t/sqrt (Theta t).
Could someone confirm that a) the units are imperial and b) what Delta t and Theta t are?!
I'm guessing it's something to do with temperature but not sure what.

June 14th, 2009, 15:03
The notes you're referring to in AAM are incomplete. Credit Ian Kerr for the complete set of equations:

theta = (288.15-1.98*altitude/1000)/288.15
delta = theta^5.256
delta2 = delta*(1+(M^2)/5)^3.5
theta2 = theta*(1+(M^2)/5)
Gross Thrust = Static Thrust * [TBL 1506] * delta2
Ram Drag = (V/g)* Intake_Area * [TBL 1507] * delta2/sqrt(theta2)
Net thrust = Gross Thrust - Ram Drag

M = mach number
altitude is in feet
V is velocity in feet per second
g is 32.2
thrust is in pounds

June 14th, 2009, 15:56
Ta very much!
What is theta? Something to do with air pressure? I find it quite hard thinking in imperial units!