Several factors are converging.
1. The airframe experiences the least drag ie is most efficient, at a certain AoA so flight must be maintained at this AoA
2. For given weight the aircraft needs an equal & opposite amount of lift. Increased weight --> increased lift. Can't do it by increasing AoA due (1) above.
3. This means that speed is the other variable that can be controlled to change lift. Go faster (CAS or EAS, can't remember. Logically I vote for EAS but am happy to be corrected) --> more lift, so heavier a/c can achieve the required amount of lift while staying at the most efficient AoA by going faster. In other words, for a given weight there will be a particular speed that corresponds to the most efficient AoA.
4. The lift required will result in an amount of drag being produced, proportional to the lift.
5. Thrust must be supplied to to counter the drag so more weight --> more lift --> more drag --> more thrust needed, however only *just* enough thrust must be supplied or the a/c won't maintain the speed that corresponds to the most efficient AoA *unless* the excess thrust is converted into altitude.
6. There is a limit on how high the airframe can go before high speed aerodynamic effects result in a loss of efficiency ie speed of sound effects.
At the same time engine efficiency has an effect.
If turbine:
7. A turbine is most efficient when operated near max. RPM. Trouble is that at low altitudes this produces more thrust than is required to counter drag (see #5 above) so the engine must be throttled to restrict its output with a resulting cost in efficiency.
8. For a given RPM a turbine loses output as altitude increases. At some point as altitude is gained its reduced outpoint will coincide with what the airframe needs to balance drag at the most efficient AoA. This is the optimum altitude .
9. As fuel is burnt the a/c obviously gets lighter, requiring less lift & less resultant thrust. The choice is to reduce AoA (bad, moves away from most efficient AoA) or reduce speed (good at keeping best AoA but with a catch: carries with it a reduced requirement for thrust.)
10. Second option: Reduce thrust & lose some engine efficiency? Or climb higher until thrust is reduce 'naturally'. Second option is the one that wins. Climbing also has a gain w.r.t increased TAS for a given IAS/CAS/EAS althought the reduced IAS may mask this effect.
End result: For the most efficient flight the aircraft will fly at a reducing speed along with a continuous climb profile.
edited to correct an error
Last edited by Tinstaafl; 19th Feb 2005 at 15:20.