ORIGINAL: BIGBARPER
Two identical airframes this time very very slippery, same 20lb engines as above and as per your scenario the two different exhaust gas velocities 400 & 1050 mph, I suspect that as the two airframes approach 400 mph the engine with the EGV of 400 mph is producing next to no effective (dynamic?) thrust and therefore stops accelerating. The second airframe equipped with the engine with higher EGV will continue to accelerate until either the total drag equals the thrust or until in this mythical case the dynamic thrust is zero, at I assume the EGV of 1050 mph.
Only my thoughts......
Phil.
Spot on!
Thrust is thrust but thrust is related to area only in the respect that you need to increase efflux velocity on smaller turbines to compensate for reduced area.
EG: Lets say we have a 12 square inch area efflux, our exhaust velocity is 200 mph
This produces thrust of "X"
Our smaller turbine has only 6 square inches, so if the efflux velocity stays the same the thrust = X/2 (half X)
To create the same thrust we will have to double the efflux velocity to 400 mph
Both give the same thrust (static) but once moving the lower efflux velocity loses its 'pushing power' earlier because the air it is pushing against is moving in the same direction.
So in level flight the plane driven by the lower efflux velocity cannot exceed 200mph (ignoring drag).
In the same level flight the higher efflux velocity plane can exceed 400mph (ignore drag again).
Taking drag into consideration, the 200mph version will get closer to 200mph than the 400mph version will to 400mph due to the square rule of induced drag. It will however (on a like for like airframe) exceed the 200mph version by an appreciable figure.
The acceleration of the larger efflux will initially be higher due to the larger area to ‘push against’ but it will drop off much quicker than the smaller one.
Remember, a jet is not a reaction motor such as a rocket; it has to push against something to function, (air) if that air is moving at the same speed as its efflux velocity then there is no effective thrust!
Paul