A real life example
ypj, thanks but I could not open the link but am curious..
CJ - As you suggested, I looked a bit further on the net. Lets look at some approximate numbers to illustrate the point.
Take a JT9D - I found some Static sea level figures (not verified but believable):
Fan airflow: 1248 lb/sec @ 885 ft/sec (=270m/s)
Turbine exhaust: 247lb/sec@1,190 ft/sec (=363m/s)
Thrust 43,500 lbs.
Fan contribution to thrust: approx 3.5 x turbine exhaust contribution (by applying mass flows & velocities) i.e fan thrust is dominant.
Lets bolt this JT9D onto a heavy airliner and take off. We will apply the incorrect notion that forward airspeed must be subtracted from the engine exhaust velocity.
The captain figures out that he needs to rotate the plane at 130kt (67m/s). He also knows that his airspeed in the controlled airspace is 250kt (129m/s). He also has a barber pole on his ASI at 350kt (180m/s).
Does our captain know that when he reaches his rotate speed his trusty JT9s have already lost almost a quarter of the thrust they had when he first applied power at the start of his take off run (270m/s-67m/s)? And he has not even left the ground!
And does he know if he now loads his plane up so that Vr is increased to 175kt (90m/s) he will have lost a full third of his static thrust at a time when he really needs that thrust? Aha, easy answer - as your take off weight increases, REDUCE your rotate speed to make more thrust available!!
But wait, it gets worse. He accelerates to 250kts, maintaining (near) sea-level due to traffic congestion. Shucks, now the JT9D is only producing one half of its static thrust (270m/s-129m/s). And he has not even begun his climb yet. He asks, "Gee, with the air-resistance at 250kt, is there ANY surplus thrust left to climb with?
And could he get to barber-pole speed (180m/s), where his thrust will have now diminished to only one third of the static thrust?
The more I research this thing, the more I see it has been discussed - I think it is an old chestnut. The great Wiki has an entry which supports your view CJ but it is debated at length on the discussion tab. I think the vast majority are comfortable with the incorrect but intuitive view that forward airspeed directly reduces thrust. And, yes, I am ignoring the host of highly complex issues of air behaviour in the inlet ducts and other jet engine performance aspects about which I know nothing.