Entire post edited because, after 20minutes lying in bed, I realised I had been typing complete b*lls.
chorned - you're right. What I'd got wrong was that sea level density x IAS SQUARED = actaul density x TAS SQUARED. Air mass flow through the engine does decrease with altitude, even at constant IAS.
But I've thought of a simpler example.
My B737, at sea level, has an IAS of 250kts, giving a fuel flow rate of (a guess) 1,750 kg/hr per engine. At 35000' and 250 kts IAS, the drag is constant (I'm sure that bit's right!), so thrust is the same, but fuel flow per engine is more like 1,100 kg/hr per engine. The engine speed is higher, so closer to the design point (as enicalyth mentions), and thermal efficiency is better (as I have), so the same thrust is produced even though the air flow (and thereby the fuel flow, keeping the fuel/air ratio the same) is slightly less.
Now, the REAL efficiency lies in the fact that at 35000 and 250 kts, my TAS (and therefore GS, still air) is nearer 450kts. So for our (slightly) reduced fuel flow/thrust, we've flown nearly twice as far - accountants happy, esso not so.
Please tell me this isn't completely wrong, I'm on earlies tomorrow & need to be able to sleep!
Last edited by Gary Lager; 3rd April 2006 at 21:44.