I have to correct myself on one thing.
Actually the temperature of the air entering the engine is not much of significance. More of significance is the density (or density altitude) and the required thrust setting.
About high speed flight:
The higher the speed of sound, the lower your airplane' s machnumber.
You and dodo are saying the same thing, it' s just a matter of interpretation.
If M1 = 1500km/h and your speed is 1000km/h, you are flying Mach 0.83
If M1 = 1200km/h and your speed is the same, you are flying Mach 0.67
The simple explanation for high altitude cruise is that the air being less dense, you have less parasite drag and less total drag.
You can fly faster for the same amount of drag.
For high altitude transonic flight you will add the lower machnumber which means that you will suffer less from the effect of the compressibility of air, shockwave-induced drag.
You can fly faster for the same amount of drag.
By flying faster for the same amount of drag, you fly faster for the same amount of thrust.
There are other things but I need to add this one to get you thinking.
Gravity decreases by a bit over 0.3% at 10km from sealevel.
The apparent mass of a 500 ton A380 will decrease by 1.5 tons.
Last edited by fly_antonov; 5th Mar 2010 at 12:54.