jet engine spool up

[mstram]

ppl, piston prop flyer here only :)

I've read about the problem of spool up time on jet engines, and how it affects flying an approach and landing.

How much quicker do the newer engines spool up than jets of 5,10 or even more years ago?

Do turbine prop planes behave similarly to the jets with regard to spool up ?

Mike

[lomapaseo]

ppl, piston prop flyer here only :confused: :confused:

Is this a private question where only the above may answer?

[timmcat]

I think mstram is informing us that he is a PPL non jet flying individual.

[mazzy1026]

Oooh - bit the dust there mate :eek:

[lomapaseo]

I think mstram is informing us that he is a PPL non jet flying individual.

Ah ha, the signature section slipped to the top of the post:O

[Old Smokey]

Hi Mstram / Mike,

As you've said, you're familiar with the spool-up times of old. In that era we were well aware of the often quoted "7 Deadly Seconds" that it took to spool up from idle to full thrust. Airline training was VERY VERY paranoid (with good reason) in pounding this information into pilot's brains, and instituting minimum N1s on approach below certain altitudes etc.

Even on the older engines of the B707 / DC-8 / B727 etc. era, spool-up was quite lively in the upper range of RPM, as a 'ball park' figure, from about 70% N1 onwards. It was the eternal waiting from idle to about 70% N1 that was the killer (sometimes literally).

On the modern high bypass engines of my experience (CF6, CFM 56, RR Trent) spool up from idle is much much faster, with acceleration at the top end of the speed range about the same as before. This is a bit subjective, I never timed it, but then, nor did the airline operators either, because no longer was anyone concerned sufficiently to publish spool-up times, or quote minimum altitudes for idle thrust (Not withstanding good operating policy of establishing stabilised approach criteria). This has come about by the use of much improved surge / bleed valves, and in particular, re-scheduling the Fuel Control Unit to a Flight Idle setting much above Ground Idle. I've had an FCU (or is it a FCU) fail to ground mode in flight, and the acceleration qualities were just as lousy as in the old days. On the 'down side' of this high Flight Idle, is that the aircraft can be a real b!tch to get down if you're a bit hot and high, and that pilots are now much more adventurous in accepting low altitude sink rates, knowing that a rapid thrust response can save their skins.

Turbo-prop acceleration depends on the type of engine. There are 2 types, variable RPM engines, like the RR Dart, or constant speed engines like the Allison (P3, Electra, C130) or Garret (Metro, Jetstream etc.).

Acceleration for the variable speed engine comes about by 'fining off' the propeller as the fuel flow is increased (much like the piston engine / constant speed prop). Rapid power increases in the RR Dart could cause the prop to fine-off excessively quickly, resulting in huge drag increase just when you needed power. The throttles had to be carefully advanced, making sure that torque increased as RPM rose.

On the other hand, constant speed engines (literally that, the engine is always at the same speed) are a pilot's dream, the power is available as quickly as you can push the power levers up, there's no acceleration required - Instant Response!

All a bit subjective - I speak for the engines I've operated, others may tell a different story.

[mstram]

O.S,

Thanks for the great reply !

How / what causes the FCU to switch from "ground idle" to "flight idle" ? Is it simply the position of the thrust levers (detent ?), or is there some kind of ground/air 'sensor" mechanism ?

Is the Pratt PT6 a variable or constant speed turbine ?

All my jet "flying" has been in flight sims :) It seems that 50-60% N1 is a common setting for approaches. That seems to actually be similar to piston RPM settings, i.e. 60% of a 2500 rpm C172 = 1500, which is approx what I'd be using when landing. Coincidence ?


Mike

[Old Smokey]

Hi Mike,

Ground to Air through the quite traditional Weight on Wheels gear squat switches.

The PT6 is a Free Turbine, the turbine driving the propeller is unattached to the other compresser / turbines. Variable speed, but without the 'Nasty' acceleration tendancies of single shaft Turbine / Compresser / Propellers such as I described for the RR Dart. Undoubtedly the PT6 is one of the greatest turbo-prop engines ever built, I don't think I'll find much argument here.

For the jets I've flown, 50-60% N1 seems a bit low, more typically about 70% N1 would seem normal. Other types would vary of course. For the Turbo-Props, Allison / Garret are constant speed, PT6 can be made so on approach, at least for the prop whilst the 'gas generator' turbine assembly speed varies as required for power demand. Even the Dart had a 'funny' constant speed zone at 11,000 RPM, which was typical for approach, the problem was that when you needed to go to more than about one third power, you were back into the variable speed zone between 11,000 and 15,000 RPM (This varies a bit for various marks of the engine, but the principal was the same, e.g. 10,400 RPM Vs 14,500 RPM for earlier versions).

Again Mike, this only applies to what I've flown.

Regards,

Smokey

[mstram]

OS,

Thanks again for the additional info.

Maybe one of these days, I'll get to try the real thing.

Mike

[earnest]

". . .I never timed it, but then, nor did the airline operators either, because no longer was anyone concerned sufficiently to publish spool-up times, . . ."
Timing spool up is part of the certification air tests.

[Old Smokey]

Fully understood earnest, the point was that the operators considered the spool-up times so short that they no longer warranted publication to crews. There is a huge amount of certification / testing results not worthy of publishing to crews.

Sorry for my poor use of English in the original post.

mstram / Mike, I have a hunch that you will make it to your aspirations. Your line of reasoning and presentation is not unlike that of the professionals. Keep on keeping on!

Regards,

Smokey

[catchup]

<How / what causes the FCU to switch from "ground idle" to "flight idle" ? Is it simply the position of the thrust levers (detent ?), or is there some kind of ground/air 'sensor" mechanism ?>

On most airbuses switchover from ground to flight idle is linked to the flaps-extension.

regards

[Old Smokey]

Sorry - Flap / Slat position and / or gear squat switches, depending on who built the beast.