That's crazy. Incredible brains at work to get that all dialed in, and to get it working without it destroying itself.
When starting a turbine engine, how do you know its started?
- Thread starter beasly
- Start date
That's crazy. Incredible brains at work to get that all dialed in, and to get it working without it destroying itself.
inigo88
Composite-lover
The PT-6 is actually a pretty interesting (to the lament of Boris Badenov ) because like I said, it's mounted backwards in the airplane and the inside of the engine is split in half.
The prop would be on the left side of this picture, and this is looking at the engine from above in a top-down view. As you can see, the central driveshaft going through the middle of the engine is split right down the middle. The blue part (Gas Generator Section) is all physically connected by the driveshaft on the right, and the red part (Power Section) is all physically connected by the drive shaft on the left, but the two halves are independent of each other.
Air travels down the duct underneath the engine all the way to the back, goes through the inlet screen at the back and works its way forward. It gets squeezed through three axial compressors and one centrifugal compressor (the blades are tapered and are more scoop shaped, like a pump). This high pressure air then gets mixed with fuel and ignited in the combustion chamber, and shot through compressor turbine. That compressor turbine is connected by the blue driveshaft back to the four compressor sections which sucked the air into the engine to begin with, and the whole process becomes self-sustaining. That's the entire gas generator cycle, and the gas generator is basically just a miniature turbojet engine... whose sole purpose is to keep itself turning.
After the hot exhaust gas is shot through the compressor turbine, it passes into the power section (the other half of the engine) and turns two power turbines before being expelled out the exhaust ducts (hence why the exhaust ducts are at the front just behind the prop). These power turbines turn the front driveshaft, which is connected to a reduction gearbox that slows the power turbine from 30,000 RPM to the 1900-2200 RPM needed to turn the propeller (for perspective, the compressor turbine spins at 45,000 RPM... so even though 15,000 RPM was lost between the compressor turbine and the power turbines, reduction gearing is obviously still needed). These power turbines are the ONLY thing that turns the prop on a PT-6, and you'll notice that the prop doesn't start to turn until very late in the start-up sequence, because you need to start the small turbojet "gas generator" first to get the hot exhaust gasses flowing through the power turbines. There's an urban legend that you can actually hold the prop stationary by hand on a PT-6 airplane during start-up, since the exhaust gasses aren't putting enough torque on the power turbines to move it. Everyone seems to know a guy who knew a guy who knew a mechanic who did it successfully, but I wouldn't recommend trying.
As for the reduction gearbox itself, it's a planetary gear system that looks a lot like what you'd find in the automatic transmission on a car. Here's what part of it looks like on the PT-6:
(Here's a great website on the PT-6 with more technical details on how it works.)
The second school of thought in turboprop engine design - and one which has a healthy cult following on this site
- is the direct-drive, as seen in the Garrett TPE-331 (the engine found in the SA-227 Metroliner and the Mitsubishi MU-2, both of which are known to be "all that is man"). They took the more straightforward approach and said lets build a small turbojet engine and bolt a propeller driveshaft to the front of it. Problem is, since the gas-generator (turbojet) runs at ~42,000 RPM, you need a hefty reduction gearbox to slow that driveshaft down to 2,000 RPM before it reaches the prop:
It's a pretty straight-forward engine. Two centrifugal compressors (the blue part) increase the air pressure as it enters the combustion chamber (red part) and turns some axial turbines before being shot out the back as exhaust. The driveshaft which allows the turbines to keep the compressor blades turning, continues straight into the gear box. The gear box looks more complicated than the Pratt since the accessory drive gears (connecting to the yellow accessories and FCU on top) mix in the same gear box responsible for the reduction gearing (on the PT-6 the accessories are geared into the gas-generator at the back and the propeller reduction gearing is in the power section at the front).
Here's a nice cutaway of the TPE-331:
Finally you can get even smaller turbine powered engines like the Rolls-Royce Allison C250, which was mainly intended for use as a turboshaft engine for helicopters (but you see it as a turboprop "prop-jet" conversion on some smaller GA planes as well). The Allison is kinda funny because it's basically built around the reduction gearbox, which hangs way down below the centrifugal compressor inlet, and is what is actually connected to the output driveshaft on a helicopter. Its turbojet "gas-producer" is a cute little itty bitty thing compared to the TPE-331.
Another cool thing about the Allison that I hadn't realized until just now staring at the cutaways is that the airflow pattern is all weird. The air comes in the centrifugal compressor at the front, then gets bent around through ducts surrounding the gearbox in the middle, and completely reverses direction into the combustion chamber where it's ignited, sent back the way it came through the turbines and bent 90 degrees up and out the top of the engine (which explains why helicopters like the Bell 206 have the exhaust duct on top. Cool!).
The Allision has two sets of axial turbines, connected to two concentric driveshafts. The N1 "gas producer turbine" shaft connects the first set of axial turbines (closest to the combustion chamber) to the centrifugal compressor via the inner driveshaft. The N2 "power turbine" shaft is hollow and surrounds the N1 shaft. It connects the second set of axial turbines to the gearbox to drive the rotors, via the outer driveshaft. This webpage has an excellent diagram and explanation of the whole engine.
Did that help at all? Also, I think beasly got more than he bargained for with this thread.
///AMG said:
Blows my mind dude...
EDIT: I found a diagram of the reduction/accessory gearbox on the Garrett. It's part of an investigation into an incident where an Australian Jetstream 31 experienced an uncontained "bull gear" failure, sending gear fragments through the gearbox casing, out the engine nacelle wall and into the FO's side of the pressure hull. Good times!
http://www.atsb.gov.au/publications/investigation_reports/2004/aair/aair200401353.aspx
The reduction gearing does indeed look very similar to an automotive automatic transmission. It has a central sun gear (green), planetary gear assembly (yellow) and outer ring gear (orange). Click the "Factual Information" tab to see the diagram.
ASpilot2be
Qbicle seat warmer
The Garrett TPE-331-10R-511C has a 26:1 gear reduction. The engine spins at 41,730RPM and the prop spins at 1591RPM.
nocturnalaviator
AI Jedi
When the computers are off that's when I fly in MANLY mode! Who needs reverse thrust when you're piloting the MANLY way. Go Garrett or Go home
scooter2525
Very well Member
[quote="Boris Badenov, post: 1926781, member: 6786", YES I'LL GET THE A/C ON AS SOON AS I CAN.[/quote]
Bingo.
Bingo.
scooter2525
Very well Member
Engine doesn't... side windows might...
scooter2525
Very well Member
Explain. Is this air pressure from a cart or APU? Would the engine spool on its own with 30 psi?
dasleben
That's just, like, your opinion, man
Pressure from the APU, a huffer cart, or the other engine (in the case of a crossbleed start). And no, the engine won't start normally without it; not sure what the minimum would be for a successful start, but anything less than 20-25 would be abnormal.
inigo88
Composite-lover
Assuming the system even lets you engage the starter with insufficient duct pressure, would you be at risk for a hot/hung start in that case?
dasleben
That's just, like, your opinion, man
Yes, that'd definitely be a risk. Depending on pressure though, it may not even spool to the minimum 15% N2 required to introduce fuel.
inigo88
Composite-lover
Boris Badenov
Fortis Leader
This is all very beguiling, but look at the internals of a TPE and look at the internals of a PT-6. TPE was built by dudes with crew cuts who manufactured planetary gears and titanium splines that somehow went THROUGH the compressor section, all in the interest of keeping the damned engine facing the Right Way. PT-6 was built by layabouts who weren't capable of figuring out how to make a turboprop work put in the Right Way. And now it's built by Cannuckistanians. I think we all know what the right answer is here.
scooter2525
Very well Member
Yeh, it's fly what pays the bills.
Boris Badenov
Fortis Leader
I can't in good conscience argue against that.
Boris Badenov
Fortis Leader
In all earnestness, though, if you get a chance, check out a TPE331 torn down. It's a tiny little thing. The important bits are maybe the size of a picnic basket, but oh so intricate. All whirling around keeping the pilot-monkeys in the air. If you're ever of a mind to make the MX guys walk a respectful 10 paces behind, take a look at a Garrett and recognize that you're basically a monkey getting bananna-pellets for pushing the right lever. The real badasses are the guys who designed, engineered, and maintained these incredible powerplants.
inigo88
Composite-lover
I think the Allison C250 takes the cake though. At least the PT-6 is shaped like a regular turbine engine, just facing the wrong way. With the Allison, they took a turbine engine facing the right way, bent it 180 degrees and then bent it another 90 degrees. All in the interest of building a gas-turbine engine around a gearbox I guess.
I was looking around to see if I could find any epicyclic planetary gear animations for GX, and I landed on this gem:
It's a real Cannuckistani (in the flesh), taking apart and refurbishing a planetary reduction gear system, inside a pneumatic turbine starter. The concept is exactly the same as what you'd find in a Pratt -OR- Garrett (although to the Garrett's credit it has more gears lol). You take a turbine spinning at tens of thousands of RPMs and somehow slow that down to a manageable level, only this time the turbine is in a starter, being turned by bleed air and turning an engine (not a propeller).
I think we've managed to come full circle: From talking about how a turbine engine starts, to how it slows down enough to turn the prop, to how the starter works to start the turbine to begin with. Who would have known, the gearbox on Boris Badenov 's beloved TPE-331 is more or less the same piece of machinery you'll find starting dasleben 's 767.
I think I'm having an a.net moment. Brb, gonna go watch the rest of this guy's youtube videos.
SteveCostello
My member is well-known.
You guys should see the reduction gear on a Nimitz-class aircraft carrier, going from the steam turbines to the output shafts. Each of them (there are four) are bigger than a full-size van, are incredibly expensive, and are extraordinarily high-precision pieces of machinery with very little tolerance for something being out of whack.
I worked at an FBO as a line attendant years ago, one of the Melin pilots showed a co-worker (fellow rampee) how to start up the engines.
Not long after, one night while working a midnight shift alone he hopped in the Melin (that was parked in the Hangar) and tested his memorization of what the Pilot showed him...this was soon followed by a call in the middle of the night to the said Pilot by a frantic Rampee asking how to shut it down...2 Garrett Turbo props dusting the Hangar in the background.
He was fired the next day.
Not long after, one night while working a midnight shift alone he hopped in the Melin (that was parked in the Hangar) and tested his memorization of what the Pilot showed him...this was soon followed by a call in the middle of the night to the said Pilot by a frantic Rampee asking how to shut it down...2 Garrett Turbo props dusting the Hangar in the background.
He was fired the next day.