All,
Some thoughts...
It would seem that a basic assumption has been made by most posters and this ignores the fact that work is being carried out by the engine, even when the aircraft is stationary.
Let’s break down the stages: -
1) Aircraft stationary, engine stopped.... no work being carried out, therefore no power. (Everyone agree?)
2) Aircraft stationary, engine started (idle)... work being carried out by engine accelerating cold stationary air in front of engine, to warmer, flowing air at rear of engine (i.e mass flow through engine) The power (horsepower /kilowatts etc) produced by the turbine is absorbed in a number of ways.
To simplify let us ignore the thermal effects and identify the main power source and two main power "sinks" for the turbine: -
Source: - Combustion produces hot gases which expand towards turbine section. Therefore, for a given mass flow rate, there is a given source of energy which we can call S1, which can be converted from mass flow to mechanical energy)
a) Power is used to drive the turbine/compressor - this is not useful work as far as the aircraft is concerned when it comes to thrust and producing acceleration, but nevertheless cannot be ignored. (Call it T1)
b) Power is used to drive the fan/turbine - if the fan was 100% efficient (assume) then the work per second (power) absorbed by the fan will be completely converted to thrust. (Ignoring flow losses etc) Call this T2.
Therefore remaining energy in gas stream exiting engine, S2 = S1 - T1 - T2.
There are two sources of thrust from the engine: -
1) The fan (T2)
2) Jet efflux from the engine core (S2)
Ignoring frictional losses within the engine (both frictional losses and transfer losses) the energy contained by these two gas streams S2 and T2 would be equal to S1-T1 i.e. power from combustion less power required to drive compressor. Also assume S1-T1 is a constant, C.
Therefore, maximum energy available for thrust is C = S2 + T2
Now hopefully, the crux…..
1) Maximum energy would be obtained when the engine is running at 100% (i.e max fuel flow rate, producing lots of hot expanding gas to drive turbine), giving C max.
2) And, maximum power (work/second) is when the gas flow must be accelerated from zero to max in engine and max exhaust gas flow is exhausting to a stationary body of gas.
These two conditions will be met when take-off power is set and the aircraft is stationary.
Point two above also explains why thrust reduces as altitude / temp increase.
Mind you, all this doesn’t help convert lbs thrust to shp!!!
Regards,
Shuttlebus.
I my attempt to explain this I may have muddied the waters some. Unfortunately, I know what I mean, but on re-reading my post I feel I have not expressed myself particularly well.