RB211-22B Actual Thrust Produced vs N1 Speed

[Tom355uk]

Hi,

I have an interesting question. Being well aware that, like most Rollers the original RB211-22B used EPR as the primary power setting while in operations, what was the actual thrust curve like throughout the rotational envelope? I guess, in laymans terms what I am trying to say is how much thrust, in lbs (or kN, not fussy) is produced at idle, at 30% N1, 40% N1 etc up to the rated limit of 99.5% N1, in ISA SL conditions. Logic would say this is a straight line, but obviously it is not.

Anybody?

[Intruder]

Thrust is more or less proportional to fuel flow.

Also, for a single-spool engine, thrust is roughly proportionally to N1**4 (N1 to the 4th power). The different speeds of the multiple spools make the equation much more complex, but that is a starting point...

[Tom355uk]

Does this apply even to high bypass units? It makes perfect sense with a turbojet, that thrust would be proportional to fuel flow but as the LP speed increases surely that would be responsible for more of the thrust generated? I remember reading somewhere that approximately 20% thrust was generated at 40% N1 and 50% thrust was generated at 70% N1 but I was looking for some concrete figures really if available.

[wondering]

What makes a bypass even more complicated is the fact that the thrust ratio produced by the fan and the core varies with altitude. Someone told me that the fan of a bypass engine with a ratio of four produces roughly 75% of engine thrust at sea level but only 25% at cruise altitude.

[exeng]

Logic would say this is a straight line, but obviously it is not.


It is most definitely not a straight line in any gas turbine engine, be it turbo jet, bypass jet or fan jet.

[Gysbreght]

For example ...

http://i.imgur.com/CcsquvM.jpg

[Tom355uk]

Bingo! Well, if it was for the -22B :}

Are these found in the performance manuals for the aircraft (L1011 in this case) or the maintenance manuals for the engine itself?

[Gysbreght]

Tom,

IIRC the chart is a cross-plot of data from a performance manual for the aircraft.

[barit1]

A rule of thumb:

For a high-bypass machine, in the high power region, a 1% change in N1 results in a 2.5% change in thrust.

[DevX]

Sorry to be pedantic Tom355uk, but the old 211-22B used IEPR, not EPR as most modern engines do. IEPR (Integrated Engine Pressure Ratio) is a mix between P1 (fan inlet), P204P (or PF if you like), and P500 (LP turbine). :ok: