What determines spool up time?
 

Hello all,

I've had a chance to be in the jumpseat and have observed that the spool up time required for takeoff is significant. I've observed the 737NG, A320(IAE), as well as the RR 757 and GE 767.

I know that by regulation, it has to spool up in a certain time, but my question is, what causes the noticeable lag in engine response when the levers are pushed up? On my plane (E145) the engines are said to be high bypass, but the acceleration is much faster than I've observed on the CFM56.

In a nutshell, what factors determine how fast the engine can accelerate?

Surge margin, mostly.

Thanks. Can you elaborate a bit please?

On the 788 it is to gradualy increase the forces on the Fan, AKA thrust ramping

What about inertia?

Can't be bothered to answer that. :O

Mass of the rotating assembly.

Next time you do a walk round PDC reach in and spin the fan. The big engines take a bit of an armful.

JT8s on the old 737s could be spun by your little finger.

Just a thought. :ok:

what is the best fuel ratio for max thrust?

acceleration checks normally call for 8 secs or less from flt idle to max thrust.

Spool-up time is only limited in the approach/landing phase, not take-off, certification requirements dictate a set interval (8 seconds) on approach, which is why certain aircraft have an approach idle setting higher than normal flight idle...

inertia affecting surge margin. limited by accel cams or FADECs

work it out :)

Max rate as per lomapaseo, but obviously the acceleration can be slower if the thrust levers are move manually. Some aircraft / engines require a slower manual acceleration for takeoff in high crosswinds due to intake distortion which could adversely affect the surge margin.

Max rate / timings are determined in the air at approach speed. The thrust value used to determine initial GA performance is that achieved after 8sec; AFAIR max thrust has to be achieved within 13 sec.

Engine acceleration characteristics vary with engine design, and with perception of the primary thrust instrument; - N1 vs EPR, N2, N3, etc. Usually it is the engine core acceleration which is the critical control parameter.

The short answer:

At a given speed, the compressor can only pump a certain exit pressure. More precisely a certain pressure ratio Pout/Pin.

And since the turbine nozzle is a choke point, varying the temperature there will affect the pressure,which reflects back to the compressor Pout.

So, more fuel (more temperature) causes the compressor to work harder - and the compressor at any given point can only stand so much of this before it will burp, back-flow, surge, stall... Let alone the incremental hardware distress from thermal shock. :eek:

So - advancing the go-handle quickly DOES NOT give you unlimited fuel. You get enough to start acceleration, and the acceleration lets the MEC/PMC/FADEC know that now a bit more fuel is permissible, and the cycle continues until you reach TO or whatever desired thrust.

Just to clarify a common misconception: cert requirements do not DICTATE any specific spool up time, and certainly don't dictate 8 seconds.

What they dictate is that you can only take credit for the thrust you achieve after 8 seconds for a go-around type scenario; if it takes, say, 9 seconds to get to max thrust, that just means that your GA thrust for perf calcs is a bit lower than the TO thrust (where spool up isn't a concern)

In practice, good design means not "wasting" performance so you try to make sure you have all the thrust you want for GA. But if it isn't limiting to only use, say 95% of TO thrust for GA, and it's going to cost money or perf to have a higher approach idle, the correct decision would usually be to accept the lower idle and the reduced GA thrust for perf calcs.

Thanks gents. I think barite gave me the answer I was looking for. If I may re-state his answer, to make sure I'm understanding…

If you feed the compressor to much fuel at once, you run the risk of a compressor stall. In order to minimize the risk, the FADEC limits the fuel flow to a gradual increase.

Do I have that right? If so, 2 more questions:

Why would too much fuel cause a surge/stall? I thought those were due to airflow disruptions within the core.

In theory, if we eliminated the FADEC, would the engine accelerate rapidly if we slammed the levers forward?

Check Airman

When you add fuel to the burner "too fast", you get a condition in the burner called "Thermal Choking" - in short it starts limiting the amount of airflow that the burner can flow. That in turn back-pressures the compressor and can lead to compressor stall/surge.

In the pre-FADEC days, the hydromechanical controls contained complex sets of cams, levers, bellows, etc. that controlled how much fuel could be flowed to the burner, depending on things such as burner pressure and high rotor speed (N2 or N3). The old hydromechanical controls were basically complex mechanical computers. When we first went to FADEC, the FADEC software basically just mimicked what those cams and levers did. As the FADEC systems became more sophisticated, the accel/decel fuel flow limits have become more sophisticated, with more inputs and variables. But the basics haven't changed.

The modern engines are least responsive at or near idle - and the lower the idle the less responsive they tend to be. Minimum ground idle is typically set as low as possible for a number of reasons including minimizing stopping distances. As a result engines can be very lazy to accelerate from ground idle. Once the engine is up to a mid-power condition, throttle response is typically quite good.

ah the 3D cam! brings back nightmares!

Ah the memories of a fueling tube letting go in front of an engine and the pretty effects after it gets ingested.

Thanks guys. Slowly a clearer picture emerges. My E145 uses the AE3007 engine (a variant of which is used on the Citation X), and the response to lever movement is almost immediate. Would I be correct in assuming that it's less prone to thermal choking because of a more advanced design of the compressor/burner sections? Are other relatively new engines (GENx for example) also slow to respond?

The latest generation of very high bypass engines (such as the GEnx) are very lazy off of minimum ground idle. Tiny core, really big fan - it takes a while to get that fan spinning.