What's the difference between RR and GE engines?

[JJPrum]

A question asked in a recent cadet pilot interview.

From the airline point of view, what would you answer?

And from the pilot operation point of view, what would you answer?

:)

[Jetjock330]

GE twin spool (N1 N2, means 2 shafts) and RR 3 spool??? (N1 N2 N3 means 3 shafts)

[RAT 5]

Quite right JJ330. I suspect they will expect measured answers so suggesting RR is more expensive than GE, Hm? Was RR not the pioneer of the large bypass. Remember RB211 and L1011. It caused them to go to the wall.
In relation to the cars, same as for the a/c: GE has XYZ pounds of thrust in whatever model: RR has 'enough'.

However; I think I'd stick with 2 v 3 shafts. The interviewer may not have a sense of humour.

[tdracer]

2 spool vs. 3 spool and N1 vs. EPR are the big things.


Rolls used to turn the opposite way, but the newer GE engines "turn both ways" (the low and high spool turn opposite directions) so that parts no longer true.
Lesser but still important - newer GE engines have composite fan blades while Rolls are still titanium.

[Turbine D]

Was RR not the pioneer of the large bypass.
No, they were not. GE developed the high bypass engine and concept and patented it. It became the basis for the TF39 engine for the C-5 Galaxy military transport. The first commercial high bypass engine was developed by Pratt & Whitney. It was the JT9D for the Boeing 747. Rolls Royce (RB-211) and GE (CF6-6) engines followed in the commercial marketplace.

[pattern_is_full]

MOST RR engines are 3-spool (but not all).

I'm no expert on this, but the discussions I see say that the 3-spools are more efficient in climb and over short ranges, and the 2-spools are more efficient over long cruises at high altitudes.

From the airline PoV, one or the other might be more desirable depending on route structure (domestic vs. transoceanic).

From the pilot PoV (as jetjock says) you'd have an extra N gauge per engine to monitor.

RR Trents rotate clockwise (as seen from the front) and GEs rotate counterclockwise. At least the fans on the front. That may be related to the third spool and efficient energy recovery with three turbines - or it may just be the old "British drive on the left, Yanks drive on the right" habit kind of thing.

[Flap Track 6]

If you're talking A330 then the GE engine suffers from very poor temperature margin whilst the RR has much higher margins and longer on wing life.
There is a weight difference with 2 versus 3 spools. 3 spool is supposed to be lighter and the RR engine was the lightest on the 777, but the RB211-524 was the heaviest on the 747-400.

[FE Hoppy]

I used to fly with 4 twin spool RR engines.

Sounds like a dumb question to me.

[Turbine D]

tdracer,
2 spool vs. 3 spool and N1 vs. EPR are the big things.
Funny you bring up EPR verses N1 in thrust control, a debatable item amongst the engine producers.
Both Pratt and Rolls use EPR for control while GE maintains N1 (fan speed) gives the most control. I assume that on Pratt and Rolls engines, the pilot sets the EPR or core speed with the throttle that results in a particular fan speed.
But, GE says thrust is more accurately controlled by setting fan speed because all of the airflow is pumped by the fan rotor. The core rotor only handles a portion of the airflow. Since fan and core speeds are coupled, thrust can still be accurately modulated by scheduling core speed as a function of PLA (power lever angle). From data that can be obtained from energy balance between fan and core speed, a speed curve can be generated depicting corrected values for both fan and core.
Since speed is affected by various parameters within the engine design and external factors, GE concluded that core speed is not an optimum manipulated variable to set fan speed. Fan speed is controlled as a function of T2 (inlet temperature) which results in corrected fan speed control.

[stilton]

Yes I was surprised when we first took delivery of our 767's to see there
was no EPR gauge with the GE engines.


Is that the case with all GE turbofans ?


Great engines, they start so quickly its almost like starting your car, totally different from the RB211, also a great engine, just different.

[misd-agin]

N1 is more reliable.

Ask anyone with a P tube blockage, or restriction, if the EPR or N1 was more reliable. My company had several of them.

[tdracer]

Is that the case with all GE turbofans ?
I believe so - and CFM as well (at least all the GE/CFM that I'm aware of).
There are a number of pluses and minuses for both EPR and N1. One really nice aspect of EPR is that, below corner point temperature, Max EPR is basically constant, while N1 varies with temperature ("Corrected" N1 is constant, but then you need to apply that square root of theta term). Even with our fancy computer driven aircraft, some people still foul up the physical versus corrected N1. Thrust vs. EPR is also less affected by high humidity than Thrust vs. N1. Thrust/N1 is also very sensitive to fan damage (e.g. FOD or birdstrike), EPR less so.

On the other hand, N1 is a simple measurement, EPR much less so as misd-agin pointed out. Inlet probes freeze (we've had big problems with EPR inlet probes icing up in Ice Crystal Icing) or get blocked with all sorts of debris (we had an issue several years back with Singapore - they were getting flower pedals in the inlet probes of the 747 outboard engines :uhoh:). The exhaust pressure probes can also get plugged with carbon, and of course there is always the issue that pressure transducers can drift. Also as Turbine D notes, with modern engines the vast majority of the thrust comes from the fan, so the relationship between EPR and thrust is getting weaker.

There has been a lot of work done on a 'generic' thrust setting parameter - something that would simply go from 0 to 100 - 0 would be idle, 100 would be max rated for the conditions. Although it works great 99.99% of the time, it has some unlikely but nasty failure modes that have discouraged it's use (at least at Boeing).

[Flightmech]

GE use N1 as the primary power parameter where as RR and PW generally use EPR. I think for the original question the RR being a triple spool would be what they are looking for.

[halas]

RR = Rotating Rubbish (Trent)
GE = Good Engine (90)

:}

halas