Basic Gas Turbine theory.

[Steve76]

I probably should know, but while searching the Commodore 64 hard drive installed in my cranium I failed to recall why there is a 2nd peak during the start of a turbine engine.

The allison C30 (god bless its oil discharging soul...) has the 2nd peak at 34% N1.

Its been a while since the BGT exam so whats the reason?

Cheers!

[NickLappos]

Not gas turbine theory, just the behavior of the fuel nozzels, which have a poppit valve that alters the gas flow as the fuel pressure builds. It give you one peak at low N1 (25%ish) and another late in the start (40% ish).

[Thomas coupling]

C'mon Steve - everyone knew that!:8

Nick, dare I ask, what were you doing up at 4am?

[Hilico]

Nick works in a different time zone - he's about six hours behind GMT, so it's actually 10pm where he is!

Come on, Thomas, I thought everyone knew that!

[Steve76]

:ok: Thankyou again Nick.

Isn't it nice that you can put out a question at 2130hrs local time and awake the next morning at 0630hrs to the answer?:cool:

[Thomas coupling]

Touche..Hilico:ok:

[TIMTS]

I have heard about a time limit between engaging the starter and the time the engine should be running self sustained. Somewhere in the ballpark of 40sec. (Allison 250)
Can anyone explain why and how?

[DBChopper]

Er, would this be a good time to ask what a poppit valve is?

By the way, it's lunchtime as I'm posting this.

:E

[Thomas coupling]

Timts:
If one exceeds the time limit, it is indicative of the starter not changing over (dropping off line) to a generator, and it may well burn out / catch fire because it is running too fast!
The caption is the deciding factor (starter caption dissappears /or Gen caption dissappears dependent on layout of panel.

DB: A popitt valve is one that cleans up after itself, as in:

There's a good poppitt.:D :D :D

[TeeS]

TIMTS

I have not got a Bolkow manual in front of me at the moment but I am fairly sure it states that the time limit is to initial light up.

TC

I am not convinced about the outcome of the starter/genny sticking in starter mode. The engine is driving the starter at normal speed for a generator so mechanically it shouldn't be a problem. There is no 'load' as such on the starter so I can't see it would be drawing a large current. However with the engine now driving the starter I assume it would be producing a 'back emf' ie trying to act as a generator (but through different windings!) would this in its self be enough to cause a problem. My school boy physics doesn't go deep enough to convince me! Any genny experts out there care to answer that one?

TeeS

[NickLappos]

A poppet valve has a spring loaded check valve function, so as the pressure changes, the valve actuates. The force of the spring determines the pressure it activates at. A typical hot water heater has a poppet safety valve, for example. the spray nozzels in the 250 series have a poppet that changes the spray pattern and volume during the start.


The time limit for most engines is actually kind of arbitrary, as it is a measure of how healthy the start is. Long time usually means the flame pattern is bad, (dirty nozzles?) and things are not normal.

There is a starter motor limit, but usually it is a problem only if there is no lightoff, and the starter could overheat itself. The light starter motors that helos use are not beefy enough to absorb all the heat of a continuous cycle at full rated power, but they don't have to be if we respect their duty cycle ourselves. That causes the on one minute-off one minute-on one minute-off 30 minutes timings that most flight manuals have.

[rotormatic]

The fuel control of the Allison 250 series engine controls the temps during start...not the fuel nozzle..

The engine output is controlled in two normal stages by the pilot. First, the engine must be started and increased to a speed range where the power turbine governor has control. This is accomplished by actuation of the gas producer fuel control throttle lever through the airframe control linkage which ends at the twist grip.

The throttle twist grip has three basic positions: CUT OFF, IDLE, and FULL OPEN. When the twist grip is moved from the CUT OFF to IDLE during engine starting, the gas producer fuel control automatically meters fuel as a function of compressor discharge air sense and N1 rpm.

Engine light off, acceleration, and idle stabilization are a function of the gas producer fuel control only.

[NickLappos]

rotormatic,

Of course the fuel control controls the start, otherwise we could leave it home. It would be hard for Steve76 to work the twist grip, because he has an S-76, and the engine lever is on the cockpit overhead. Not really much of a wrist action there, rotormatic.

In the specific question Steve asked, the fuel nozzel in the c30 engine is the cause. As fuel pressure rises, the spray pattern is changed by the nozzel, using a spring loaded poppet valve. This causes a pause in the temp rise, creating the double peak he noted.

The lever/twist grip has considerable adjustment range near idle, allowing the pilot to control the temperature with the lever. this sets a spring pressure on the Pc bellows in the fuel control, and that allows adjustability during the start acceleration.

[John Bicker]

Could be completely wrong, but from memory the dip between the peaks is actually a deceleration spot to get past a speed where there could be a surge or stall. It's very much the same on all the family from experience with 18,20,20B,20R,30 and 47. Fast and hot is best for the engine.

Can't remember if the V-slot duplex nozzle really is changing during the start cycle and somehow doubt that this is really responsible for the start peaks. Methinks it's the start notch valve (Bendix).

Any blackhanders here with more recent to avoid pilot conjecture?

Can't imagine why this would ever be in a BGT exam - although then again!

[sprocket]

The second peak from about 30 - 35% N1 (Bendix system) and occurs at the completion of the start derichment cycle in the FCU.

As John has stated, the derichment in the start cycle keeps the engine away from surge and high TOT. (15 to 30% N1.)

The basic acceleration schedule takes over from about 30% N1 allowing more fuel to the nozzle.

Nick: I'm trying to come to terms with your explanation of the fuel nozzle causing the 2nd peak. At this point I have to disagree.

The FCU initiates the fuel flow, the nozzle allows it. Also it was my understanding that the secondary flow through the nozzle occured at a higher power setting, but not during starting. If a peak occured as the secondary fuel started flowing then why does the temp reduce when the engine obtains idle RPM?

[NickLappos]

sproket has it right, I think. Part of the start derichment is the closing of some of the spray nozzle holes when the fuel pressure gets high enough to actuate the nozzle. This kind of action adds to the detailed start scheduling that takes place in the fuel control, where, as rotormatic has said, the fuel control maintains a ratio of compressor discharge pressure (Pc) to fuel flow (Wf) as a function of gas generator speed (Ng). The carefully sculpted relationship of these three values keeps the engine from stalling, going lean hot, or stagnating during the start. This same set of measurements governs the accel and decel while the engine is running. The engineers call it the "Woof-PeeCee" ratio.

The bleed valve comes into the picture further up, where the accel from idle to power range needs stall protection, and even then, only in low temp situations (for the C30).

All this claptrap is a running advert for FADEC.