CFM engine spoolup initial reaction time

[bpcw001 28]

Hi,

I know that the CFM's spoolup as such is kind of slow up to around 40-50%, starting from which the CFM "bites" pretty neatly when throttles are advanced.

However, I was wondering about the time it takes to make a CFM sitting on idle do anything at all when the throttles are advanced.

Picture this:

CFM at idle, on the ground, now advancing throttle levers to about 20-30%, commanded N1 bugs shown accordingly on the upper EICAM display. Then, for the next 3 to 5 seconds, nothing happens before the engines finally decide to start spooling up.

Is that how the real thing works? I understand that the spoolup itself is slow, but does it really take that long for the CFM's to come up with an initial response to throttle movement out of idle?

Thanks for clarifying this.

 

 

[mopperle 4162]

Well it is a bit long, but it depends also a bit on the engine wear and it was simply done this way. Similar to the APU: we have seen there spoolup times to "AVAIL" from 1 min up to 3 min in different aircraft.

[acra24 83]

Otto

Can you clarify that please?  Are you saying that the spool up of CFM engines on the Aerosoft bus IS modelled with this delay?  Or in other words it's a normal feature?

Thanks

Andrew 

[skelsey 103]

I would say that 3-5 seconds from idle to any sort of power setting doesn't seem unreasonable.

Big high-bypass turbines respond quite slowly at idle: there's a lot of inertia to overcome. There's a reason why stabilised approach criteria require the engines to be spooled up, and this is it!

[bpcw001 28]

1 hour ago, mopperle said:

Well it is a bit long, but it depends also a bit on the engine wear and it was simply done this way. Similar to the APU: we have seen there spoolup times to "AVAIL" from 1 min up to 3 min in different aircraft.

 

Like I said: it is not about spoolup time, but about initial reaction time of the engine.

The APU may take various amounts of time to spool up to be "AVAIL", but once you press the APU start button it appears to start that spoolup process without undue delay.

Bringing the thrust levers out of idle with CFM engines first makes you wonder if the engines got the signal to spool up at all, because initially just nothing seems to happen (no N1 increase), then after some time the slow spoolup finally starts to meet the commanded and bugged N1 value.

5 minutes ago, skelsey said:

I would say that 3-5 seconds from idle to any sort of power setting doesn't seem unreasonable.

Big high-bypass turbines respond quite slowly at idle: there's a lot of inertia to overcome. There's a reason why stabilised approach criteria require the engines to be spooled up, and this is it!

Also here: please don't confuse the actual time to spool up from 0 to somewhere with the time it needs before the engine actually starts the process of slowly spooling up, starting from 0.

It is the latter time I am wondering about. I'm fine with how long it takes to reach a commanded N1 as soon as the engine actually starts spooling up.

Trying to make this clearer:

If throttles are advanced on an engine sitting at idle, I would expect a reasonably timely response from the engine in the form of actually starting the process of spooling up (which may be slow, yes).

But what I get is:

1. Advance thrust levers

2. 3-5 seconds no N1 rise (that is what I find strange)

3. then slow spoolup from 0 to commanded N1 (that part is fine)

[kaktus018 7]

What you are experiencing is basically a turbo lag. At ground idle the engine is running close to its minimum operating speed. Below that speed the engine would no longer be self sustaining. That's why you need pressurized air on start up - to "help" the turbine speed up beyond this point.

At such low speed the turbine, as well as the compressor, have a bad efficieny (axial turbines only work great for high volumetric flow rates) and is barely able to deliver any power. As you now want to speed it up, the fuel injection amount is increased, but that can only be done within certain limits (as you don't want overheating turbine blades - you need about 10 times more air than what is needed for the combustion). Now you've got a compressor that needs high power but does little use, you are limited with the energy you can provide to the engine and you've got a turbine with a bad efficiency that needs to drive your bad working compressor - all these effects cause a slow acceleration (slow build up of pressure ratio). Once a certain speed is reached, the overall efficiency increases distinctly, the volumetric flow increases, the fuel injection amount can be increased, the engine parameters get in the range of what it is designed for. This means decent power output of your turbine and a good reaction to load changes.

This lag is by the way the reason why you don't want to fly your final app with thrust idle. When you need power for a go around, the engines better be able to deliver it! Note that flight idle has a higher speed than ground idle.