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ovrpowrd727
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Engine Spooling

Mon Dec 01, 2008 6:05 am

Hey there fellow members, just a thought... What makes the difference in spooling time for engines of today as compared to the engines of 'yesterday'? Some engines power up nearly instantly yet others have a delay...Why are pilots so careful with throttle movement speed? referring to jets mostly i know the reason for piston props but any feedback is welcome
 
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Starlionblue
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RE: Engine Spooling

Mon Dec 01, 2008 6:22 am

Modern turbofans, especially the larger ones, have big fans. These add a lot of mass to the rotating bits compared to old fashioned turbojets and low-bypass fans. This means more inertia. Thus it takes relatively longer to spool them up. On the other end of the scale, vastly improved design and engine management systems shorten spool-up time. The latter optimize fuel flow and so forth for every part of the operating range.

Throttle care: As I understand it, a modern digitally engine doesn't really require a lot of care on the throttles. You could slam them from idle to full and the system would do all the careful adjustments needed "behind the scenes". In the old days, throwing the throttles around was probably not such a good idea. It could lead to surges.

One reason I can think of for care is that pilots wish to be "ahead of the airplane". It may be better to have well planned throttle movements and adapt to aircraft behavior changes that occur as a result (this may take several seconds) before you move the throttle again. This is only my speculation though.
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ovrpowrd727
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RE: Engine Spooling

Mon Dec 01, 2008 6:44 am

any specifics to maybe the GEs, P&Ws, RRs or any others i may not have covered?
 
pilotpip
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RE: Engine Spooling

Mon Dec 01, 2008 7:15 am

Starlion hit the nail on the head about modern engines. FADEC makes sure we don't cause any limitations to be exceeded. I can rocket the thrust levers up as fast as i want. On an old engine there were a ton of different things you could do them up to and including overspeed and overtemp. On takeoff, I throw the levers to the stop and take my left hand off (captain puts his/hers on until v1) and we just verify that everything is in the green and the N1s match up.

There are a lot of things that go into making an engine more efficient and faster to spool up. Among them are FADEC, adjustable inlet guide vanes help too.

Another factor in the "spool up" feeling is mass. I saw in the other thread that you're a student pilot. When you pour the power on in whatever trainer you're flying it takes a second to feel any acceleration. You're flying a 2000 pound aircraft. Imagine how much more power it will take to move a 200,000 pound aircraft.

The amount of time from idle to full power is dictated by regulation (don't recall what time that is) and that is one of the reasons you have a "flight idle" as well as "ground idle" on the ground the engine will idle at a lower power than in the air. Embraers also have a "flight idle icing" setting when the anti ice is on because of the extra bleed air requirement to heat the wings. I'm sure other aircraft can do this but the only ones I know and the only ones I've flown are the 145 and 170 respectively.
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vikkyvik
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RE: Engine Spooling

Mon Dec 01, 2008 8:05 am



Quoting Pilotpip (Reply 3):
The amount of time from idle to full power is dictated by regulation (don't recall what time that is)

Far as I remember, isn't it about 6 or 7 seconds?

I also seem to remember reading that on some airplanes, when powering up for takeoff, the pilots first advance the levers to some low N1 setting above idle, and let the engines stabilize there before setting full takeoff thrust? Or was that on pre-FADEC aircraft?
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tepidhalibut
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RE: Engine Spooling

Mon Dec 01, 2008 11:30 am



Quoting Vikkyvik (Reply 4):
Far as I remember, isn't it about 6 or 7 seconds?

It used to be in JAR-1 that the engine had to be able to accelerate from ADC (Acceleration Datum Conditions) to 95% of rated thrust in under 5 seconds. (Some engines don't meet this, but have those longer accel times agreed with JAA .)

The timings are met at sea level conditions - at high altitude accelerations have to be a lot slower. (A Slam accel at 30,000 ft might take 20 seconds.)

Rapid accels, however, should be avoided. Because of thermal effects, the accel can result in heavy wear between static and rotating parts, so can impact engine efficiency.
 
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fr8mech
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RE: Engine Spooling

Mon Dec 01, 2008 12:58 pm

The short answer is air management. Controlling the air through the engine via VSV/VBV/bleed valve manipulation is critical to rapid accel and decel. FADEC makes it happen a lot more accurately than pure hydro-mechanical controls.
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PhilSquares
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RE: Engine Spooling

Mon Dec 01, 2008 1:11 pm



Quoting Vikkyvik (Reply 4):


I also seem to remember reading that on some airplanes, when powering up for takeoff, the pilots first advance the levers to some low N1 setting above idle, and let the engines stabilize there before setting full takeoff thrust? Or was that on pre-FADEC aircraft?

Even on FADEC/EEC aircraft, most SOPs have some guidance that the thrust levers will be advanced to an intermediate thrust setting to allow the engines to spool up at the same rate. Once stabilized then takeoff thrust is selected, normally through the autothrottles.

What this ensures is the engines spool up at a fairly uniform rate. Thus, you are minimizing any directional control problems that could occur if you went right from idle to the calculated takeoff thrust setting.
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pilotpip
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RE: Engine Spooling

Mon Dec 01, 2008 7:11 pm

You know, I forgot all about the intermediate. We go to approx. 40% N1, then advance to full.

I haven't flown since the end of August. Furloughs are fun...
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Rj111
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RE: Engine Spooling

Mon Dec 01, 2008 9:19 pm

Depend on the airline and aircraft i guess but i heard it was 70%, check they are stable, then max thrust.
 
Goldenshield
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RE: Engine Spooling

Mon Dec 01, 2008 9:27 pm

The relevant wind also plays a part during spooling for takeoff. While this has been minimized over the years as inlet designs have gotten better, there is always a chance of some of the vanes stalling if the wind flows across the inlet in certain ways. Therefore, many manufacturers reccommend a maximum power setting below X knots prior to accellerating to takeoff power.
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Jetlagged
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RE: Engine Spooling

Mon Dec 01, 2008 9:58 pm



Quoting PhilSquares (Reply 7):
What this ensures is the engines spool up at a fairly uniform rate. Thus, you are minimizing any directional control problems that could occur if you went right from idle to the calculated takeoff thrust setting.

The crucial thing is to ensure that SBVs/VBVs have all closed and any VSVs have opened up enough. If the surge bleeds remain in the idle (open) position that engine will accelerate much slower than the others. Thus you stand the throttles up, wait for everything to stabilise and then can be fairly sure the remaining acceleration will be even.

Quoting Fr8Mech (Reply 6):
The short answer is air management. Controlling the air through the engine via VSV/VBV/bleed valve manipulation is

Hydro-mechanically controlled engines have an ACU (Acceleration Control Unit) which limits the fuel scheduled during acceleration, usually as a ratio between fuel flow and burner pressure. The ACU WF/PB limit varies with governor RPM amongst other things. The ACU allows more or less carefree throttle handling, so it is possible to slam the throttle from idle to full power.

If the engine accelerates too fast it can surge, causing excess EGT and possible damage or fire. The very early jet engines had very primitive fuel control units, so it was up to the pilot to move the throttles slowly to prevent surging.

There's also a DCU, but this is there mainly to maintain a minimum fuel/air ratio to prevent flameout during a slam deceleration.
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speedracer1407
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RE: Engine Spooling

Tue Dec 02, 2008 2:18 am

Topics about passengers experiencing dramatic takeoffs appear weekly on this thread. And I'm fairly convinced that much of the "drama" from a passenger's point of view has to do with the spool-up time.

In my experience as a passenger, FADEC equipped engines can spool from idle or a low "stabilizing" setting to takeoff power almost instantly; probably little more than a second or so.

As a passenger, I've experienced a quick spool-up at least once on every modern plane equipped with FADEC I've flown on in recent years (A320, 738, EMB170, ERJ145), and each time was memorable because of the sudden whack of thrust, which made the takeoff experience feel far more dramatic than the slow spool-up that seems to happen most of the time, regardless of aircraft.
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speedracer1407
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RE: Engine Spooling

Tue Dec 02, 2008 2:33 am

Woops, I meant to ask a question on that last post:

I've been flying more frequently on AA MD80s lately, and I've noticed that, more often on these flights than other carriers/aircraft, takeoff spool-up seems to come in three distinct stages. First, a low-power setting presumably to stabilize, followed by a big increase in thrust, followed again a few seconds later by another increase to takeoff thrust.

My assumption is that JT8-Ds don't have any sort of digital or electronic fuel scheduling thingy, so the process is to a) stabilize at some lowish setting that's well above idle, b) advance to an approximation of just-below takeoff setting, c) fine-tune to the actual takeoff setting; thus three distinct increases in power. Perhaps there's something more to it.
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SpeedBirdA380
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RE: Engine Spooling

Tue Dec 02, 2008 5:12 am

As this topic is about engine spool ups someone once told me Rolls Royce engines spool up faster than other engines because of their three spool stage design compared to two spool stage engines.

Is this true?
 
dl757md
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RE: Engine Spooling

Tue Dec 02, 2008 5:15 am



Quoting Speedracer1407 (Reply 12):
In my experience as a passenger, FADEC equipped engines can spool from idle or a low "stabilizing" setting to takeoff power almost instantly; probably little more than a second or so.

As a passenger, I've experienced a quick spool-up at least once on every modern plane equipped with FADEC I've flown on in recent years (A320, 738, EMB170, ERJ145), and each time was memorable because of the sudden whack of thrust,

FADEC engines do not spool from idle to takeoff power in a second or so. As stated above acceleration from flight idle to 95% takeoff is about 6 seconds. During takeoff you're going from ground idle to takeoff power which takes somewhat longer than flight idle to 95% takeoff power. The reason you feel the sudden whack of thrust is because the thrust increases more dramatically at higher enggne speeds than at lower. The graph of engine speed vs. thrust would look something like that of the tangent function. Basically as engine speed increases thrust increases more quickly for a given increase in engine speed. Say your engine accelerates from idle 64% N2 to 74% N2. Thrust might only increase from 1000 lbs to 2000 lbs. The same engine accelerating from 89% N2 to 99% N2 would do so more quickly and thrust would increase from say 15000 lbs to 35000 lbs.

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Max Q
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RE: Engine Spooling

Tue Dec 02, 2008 5:16 am

The JT8D'S on the MD80 series are purely hydromechanical in their fuel control, they also have the slowest spool time of any jet engine I have flown.
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dl757md
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RE: Engine Spooling

Tue Dec 02, 2008 6:22 am



Quoting Max Q (Reply 16):
The JT8D'S on the MD80 series are purely hydromechanical in their fuel control, they also have the slowest spool time of any jet engine I have flown.

I respectfully disagree. Per the AMM the JT8D-219 must go from approach idle to 95% takeoff in no more than 5.7 seconds at std temp. The CF6-80c must do the same in no more than 5.8 seconds. In my experience the JT8D routinely does this with room to spare while the CF6, although no less succesful in passing the test noramally does so with less room to spare. In other words it spools slower as do all of the other large turbofans I've worked with - PW2000/4000, RB211, Trent, GE90, CFM56, and V2500. This is just my experience from having performed dozens of timed acceleration tests on each of these engines.

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Max Q
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RE: Engine Spooling

Tue Dec 02, 2008 7:21 am

You may very well be correct, we only had the -217 version.

You also mention your timed tests are done from approach idle, there is a big difference in acceleration time from approach idle or ground idle.

Spool up from ground idle took a loong time.
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Tristarsteve
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RE: Engine Spooling

Tue Dec 02, 2008 2:01 pm



Quoting Max Q (Reply 18):
You also mention your timed tests are done from approach idle, there is a big difference in acceleration time from approach idle or ground idle.

Spool up from ground idle took a loong time.

What we must remember is that there is a legal requirement that the engine can spool from approach (or flight) idle to TOGA in 6 secs. All jet engines do this.
The engine manufacturer will set up the controls to meet this requirement.
Ground idle is something else. This is set as low as possible to save fuel. The gap between ground idle and flight idle can be significant.
Some aircraft will accelerate the engines to FI when flaps are extended. Some don't.
The RB211 on the Tristar had a significant spool up time from ground idle to flight idle. Most crews are trained to stand up the throttles first, before selecting t/o power. This is to avoid a swing as one engine accelerates quicker than the other. On a mechanical FCU, adjustment of the GI setting can make quite a change to the GI to FI accel time. But this time is not a required parameter, as long as the engine goes from FI to TOGA in the 6 secs then it is not a problem if the GI to FI time is a bit slow.
 
tdscanuck
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RE: Engine Spooling

Wed Dec 03, 2008 12:01 am



Quoting SpeedBirdA380 (Reply 14):
As this topic is about engine spool ups someone once told me Rolls Royce engines spool up faster than other engines because of their three spool stage design compared to two spool stage engines.

Is this true?

The N3 spool should accelerate faster because of it's lower inertia. However, the total rotating inertia (all spools) of a RR is higher so I doubt it makes a huge difference...since almost all the thrust is coming from N1, it's the N1 spool-up time that's going to be perceived by the crew.

Tom.
 
VC-10
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RE: Engine Spooling

Wed Dec 03, 2008 4:01 pm

During the TO roll on Concorde the #4 Engine is (was) limited to 88% N1 at speeds below 60 knts. This was because the vortices rolling off the wing leading edge caused engine vibration due to the fact these rolled in a ACW direction, opposite to the engine rotation direction. Once above this speed the limit was electronically removed.

The throttles also had to be slammed forward to ensure the throttle master switches were operated. If the throttles were moved in a less forceful manner,the relevant systems may not respond in the expected manner.
 
vc10
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RE: Engine Spooling

Wed Dec 03, 2008 6:11 pm



Quoting VC-10 (Reply 21):
The throttles also had to be slammed forward to ensure the throttle master switches were operated. If the throttles were moved in a less forceful manner,the relevant systems may not respond in the expected manner.

Hello Big Brother,

That is not quitte true. The reason that the throttles were moved forward rapidly was to ensure that they accelerated on the Fuel Control Units limit rather than pilot throttle movement limit. This was critical as the whole noise abatement calculation was based on the engines accelrating at the FCU limit and thus taking a certain time to get to full power.

At all other phases of flight the throttles were handled in the normal more sedate fashion

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CanadianNorth
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RE: Engine Spooling

Mon Dec 08, 2008 2:16 pm

As a related topic, I was wondering the other day are there any large turbofans (or pure jets too) that make use of bleed ports such as those found on helicopter engines (T-53, Allison 250, etc)? The purpose of them is basically to unload the compressor for faster acceleration which is often needed in a helicopter, but I've never heard of such an installation on a fixed wing. Anyone heard of one?


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Jetlagged
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RE: Engine Spooling

Tue Dec 09, 2008 12:39 am



Quoting CanadianNorth (Reply 23):
As a related topic, I was wondering the other day are there any large turbofans (or pure jets too) that make use of bleed ports such as those found on helicopter engines (T-53, Allison 250, etc)? The purpose of them is basically to unload the compressor for faster acceleration which is often needed in a helicopter, but I've never heard of such an installation on a fixed wing. Anyone heard of one?

Surge bleed valves improve surge margin so allow faster acceleration. They are common enough on all jet engines, sometimes variable area, sometimes fixed. Older JT9Ds had a number of such valves which opened under certain conditions, such as starting, at low rpm and in reverse thrust. They opened during starting to offload the HP compressor and make it spool up quicker. The bleeds opened in reverse thrust to improve stability and prevent surge. Later JT9Ds (-7Q onwards) had variable bleed valves rather than a number of fixed ports.
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CanadianNorth
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RE: Engine Spooling

Tue Dec 09, 2008 1:49 am



Quoting Jetlagged (Reply 24):
Jetlagged

Exactly what I wanted to know, thanks!


CanadianNorth
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ex52tech
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RE: Engine Spooling

Thu Dec 11, 2008 4:45 am



Quoting Jetlagged (Reply 24):
Older JT9Ds had a number of such valves which opened under certain conditions, such as starting, at low rpm and in reverse thrust. They opened during starting to offload the HP compressor and make it spool up quicker. The bleeds opened in reverse thrust to improve stability and prevent surge. Later JT9Ds (-7Q onwards) had variable bleed valves rather than a number of fixed ports.

Just to add a little here, the JT9-7A had an added set of high stage bleed valves (4.0 bleeds) that bleed air off of the last stage of the HP compressor. They were not used on any other JT9s, found to not be necessary.

The operation of the 3.0 and the 3.5 bleed valves on the JT9s preceding the -7Q was referred to as "tandem bleed" operation, open or closed. Then as you mentioned the -7Q and subsequent models had modulating bleeds.

The 3.0 bleed valve was located after the LP compressor, and before the HP compressor. The 3.5 bleed valves were for unloading the HP compressor. The numbers 3.0 & 3.5 designate the station on the engine where they are located. I know you know all this, but maybe Canadian North doesn't,...... just trying to help him understand.

The CF6-50s used bleed system similar to the 3.0 on a JT9, but did not use any HP compressor bleed valves, they used more stages of variable stator vanes, to control flow through the HP compressor.
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