Surly the ultimate limiting factor is turbine temp. If you put too much fuel in before the compressor has had time to spool up then things will over temp.

The way I understand it, it's both the surge margin AND spool innertia, that play part:

- the surge margin (as explained so neatly in previous posts) limits the excess power that can be used to accelerate the engine

- with the limited excess power available, it takes more time to accelerate a bigger fan than a smaller one.

I guess you can improve the former a bit by designing a more advanced compressor, but there's not much to be done about the latter, especially if efficiency dictates designing a small core/large fan combination.

Because of relatively small fan in your Allison engine, the core has an easier task of accelerating the fan, than in a GEnx...

dixi188To quote an old book title on my shelf, "It all depends".

A modern high-efficiency compressor might be quite happy at steady state conditions (TO, cruise etc) but have little tolerance for a transient over-fueled/over-pressure condition. It stalls, and when it stalls, it's like a Harvard stalling with crossed controls - only more violent. :} :{

OTOH, a brief overtemp by itself might not cause immediate big damage, but if the operator expects to keep that engine on-wing for many thousand hours and cycles, o/temp is definitely to be avoided.

Much depends on pilot technique. Take off procedure in the 737 includes firstly opening the thrust levers to approximately 40% N1 to ensure spool up is equal. The FCTM recommend no longer than two seconds after the N1 reaches 40% then select TOGA. Otherwise the take off performance is compromised.

Is it possible you are mistaking the hesitation at 40% N1 as part of the total spool up time when it has got nothing to do with it?

My recollection is that from idle of 23% N1 to 40% N1 there is up to three seconds tolerance between any engine in rate of spool up with equal thrust lever angle. However, from 40% N1 to take off power using autothrottle the tolerance between engines is only one second assuming both thrust levers advance equally.

The reason for the 40% N1 figure is that if TOGA was used from idle of 23% N1 and one engine took the full three seconds of allowable tolerance, while the other engine accelerated normally to high thrust, the aircraft would likely become uncontrollable in a very short time and in danger of leaving the runway centreline at a high angle.

Correct
 

Checkairman,

Your observation is correct - take two 767's, one with GE CF6 engines, one with P&W 4060's. Both engines very similar in max thrust etcetera, otherwise they could not be an engine of choice on that one type of aircraft.
When spooling up the engines at the start of the take-off run, the P&W's spool up MUCH quicker off the idle RPM than the GE's.
Also, in flight, at the end of an idle descent, you have to anticipate that the GE's come off the idle RPM much slower than the P&W's.
Once a medium RPM has been established, acceleration is comparable.

This all is not a result of pilot technique - with both engines you start the take-off roll by setting the initial (off idle) thrust manually, then you engage autothrust, which runs thrust up to take-off setting. The initial manual setting CAN be done as a slam, to 1.1 EPR on P&W or to 70% N1 on GE. The actual acceleration will be regulated by the EEC's.

From what I gather from engineers, the GE's are a bit more sensitive to stall, perhaps their stall margin is engineered a bit tighter, resulting in slightly more max thrust than the P&W's and somewhat better fuel efficiency. Downside apparently, very sluggish initial acceleration.

Thanks. That makes sense...

As does this. I kind of figured the big fan would take longer to accelerate. I just never expected the relatively small CFM56 to take a long time as well.


Nope. I know that setting thrust is a two step process. I was looking at the command arc vs the N1 pointer. In both instances, the N1 pointer had a considerable lag, thus my question.

EMIT: Never been in the jumpseat of a PW powered plane. I'll pay attention if I am though.

Thanks all, for enlightening me.

Yet another factor:

The core engine may be capable of relatively fast acceleration, but if the acceleration is controlled in the traditional way - via a 3-D cam - one engine may wind up faster than the other, with possible Vmca or Vmcg issues arising. :eek:

And so FADEC can now be programmed to follow a "N-dot" logic, requiring the fuel rate be limited to that necessary to just meet cert accel requirements, and no more. Advantages: No yaw as thrust is advanced, and lower peak temperatures in the turbine.

(N-dot is the rate of change, in rpm/second, of rotor speed)

Just read that RR is developing a large turbine engine with variable pitch fan blades, may be to compensate for the extremely large fan blades we are seeing and the need to keep spool time down. No doubt efficiency is the largest goal.

Look to car technology. . .the original Porsche Turbo from 1974 . . lets go. . .Uh ? . . . .what happened . . .Ooops, why we left the road backwards, versus, smaller more efficient Turbos , VTG (variable turbine geometry ) & now, instant thrust when you ask for it.

An aero engine is pretty much the same, on a larger scale (or a turbo the same on a smaller scale ? ) so, lots to learn from the auto industry ( & vice versa )

Density altitude also plays a role. Boeing just put out a bulletin vis a vis 737's something about a potential for lag. Haven't read it yet as I'm on vacation.

check airman
 

I would hazard a guess that the much smaller total thrust increment on the AE engine between idle and full power maybe a factor, a smaller fan will be easier to spin up.

I once flew a 737 which seemed to have quite a high taxi speed (but we had several different variants 300,400,500) after 3 sectors i discovered that the ground idle CB had been inadvertently pulled by the F/0 seat belt shoulder reel strap, learnt something from that !!

QUOTE:
Just read that RR is developing a large turbine engine with variable pitch fan blades, may be to compensate for the extremely large fan blades we are seeing and the need to keep spool time down. No doubt efficiency is the largest goal

1) 'R-R might develop....' Nothing in concrete yet. ;)

2) Variable pitch will be to optimise fuel burn, any other benefits will be a bonus. :ok:

I'm not an expert on engines, but IMHO it's not the inertia of the fan that governs the spool-up time. The fan is driven by a turbine that is driven by the power generated in the gas generator, and the surge margin of the gas generator does not allow it to spool up faster.

The inertia of the spools determines how fast they will accelerate for any given increase in torque on the turbines.

The surge margin determines how much of a torque increase can be applied without the compressors stalling/surging.

The turbine temperature limits determine how much extra fuel can be added without damaging the turbines.

The FADEC limits the rate of increase in fuel flow to prevent surge/stall of the compressors and overheating of the turbines.

They are all in it together.

captplaystation, variable geometry - both the fan and the hot section - has been a wet dream of aircraft turbine engine designers for decades (variable geometry compressors have been SOP for a long time). The problem has been cost, weight, and especially reliability have not been there to support it. The biggest problem is reliability - if the variable geometry on that automotive turbocharger fails, the piston engine will probably still run well enough to drive it to the dealer. If that happens on an aircraft turbine, your best case is probably in inflight shutdown - and it can easily lead to an engine overspeed, and it's not like you just pull over to the side of the road when it breaks :=. On a car, one failure per 100,000 miles might be acceptable, on a commercial airplane one failure per 100,000 hours would be totally unacceptable. Hence aircraft turbine designers MUST be more conservative.
Gysbreght, it's both. As the fans have gotten bigger relative to the core, the accel characteristics from min idle have gotten much slower, and those big fans start out slower to boot. For example the CF6 typical ground idle was ~60% N2 and ~28% N1. The GEnx ground idle is more like 70% N2 and N1 is ~21% N1. When you advance the throttle on the GEnx, N2 goes up fairly quickly, but it takes a long time to get to 40% N1. From 40% to 100% the CF6 and GEnx are pretty similar.

Excellent, easy to follow explanation.

Dave Reid,

I agree. Since my last post didn't add anything I've deleted it.

Variable pitch fan, should we discount the torque resistance as a result of air flow loading on large fan blades? Seems to me propulsers could accelerate faster with a lower pitch on the blade at the compression rates RR is going for, then lay the torque on a higher pitch fan that produces most of the thrust gradually. I understand this tech has been of interest for years, more moving parts more complications. The higher compression values/lower tolerances is the big challenge in the core compressor/turbine sections.

Isn't a Turbo-Fan just an up-market Turbo-Prop, designed to look sleek for the SLF ?

Looks have (almost) nothing to do with it.

The Turboprop moves a large mass of air at relatively exit pressure, low velocity. Great low-speed (takeoff) performance, and efficient cruise at moderate airspeed. Limited in max speed, however.

Turbofans use the case and shroud and fan stators to achieve higher pressure ratio, higher exit velocity, mostly benefiting high altitude cruise.

BTW: taking the turboprop to the logical extreme, turning the shaft to vertical with a very large low-speed prop, results in a VTOL machine such as a helo.

You pays yer money, you takes yer choice. ;)