nocturnalaviator
AI Jedi
CAN YOU HEAR ME NOW?!?!!! Holy hell that must've been loud! Ouch, good thing it didn't jump the locks...
When starting a turbine engine, how do you know its started?
- Thread starter beasly
- Start date
CAN YOU HEAR ME NOW?!?!!! Holy hell that must've been loud! Ouch, good thing it didn't jump the locks...
Autothrust Blue
Welcome aboard the Washington State Ferries
The PW118 has atoned for this atrocity, as air flows through that engine in the 'correct' direction. (As in, it obeys the Principle of Least Astonishment.)
Tommay85
Well-Known Member
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.
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.
I now understand why the Garret's gear reduction is so freaking loud!!!!
Polar742
All the responsibility none of the authority
GEnx says "RUNNING" when the engine is doing so. It'll take care of every start abnormality as well.
CF6, one must wait for the red line on the EGT indication to disappear. We must actually ensure the N1 is turning and oil pressure is there. An atrocity I say.
FADEC überalles.
CF6, one must wait for the red line on the EGT indication to disappear. We must actually ensure the N1 is turning and oil pressure is there. An atrocity I say.
FADEC überalles.
Tommay85
Well-Known Member
Given what cars were doing for DECADES before any of the stuff you describe, everything should have electronic engine management by now. God, the guys that make TSO stickers would be RICH!
Polar742
All the responsibility none of the authority
Then again, the Rolls-Royce Dart was the other end. 2 centrifugal compressors, 14.5 ft Dowty Prop. For some reason it used an electric starter, so you only had so much time to get it started.
1) Pray the props were on the locks.
2) Push the huge button to get the starter turning.
3) Introduce fuel, watch it light and accelerate until it hangs;
4) Use the fuel trim switch to add fuel until it looks like it's going hot. Stop adding fuel.
5) Repeat 3 & 4 until it: reaches idle (fuel trim is 100%); hangs; or catches fire
1) Pray the props were on the locks.
2) Push the huge button to get the starter turning.
3) Introduce fuel, watch it light and accelerate until it hangs;
4) Use the fuel trim switch to add fuel until it looks like it's going hot. Stop adding fuel.
5) Repeat 3 & 4 until it: reaches idle (fuel trim is 100%); hangs; or catches fire
ppragman
FLIPY FLAPS!
This. I want to fly this - at least once, and only so that I can understand how much of a pain in the ass that must be.
Capt. Chaos
Well-Known Member
I got a chance to ride up front on Air North's HS-748's back when they did CYXY PAJN runs and they were set up with autostart.
ppragman
FLIPY FLAPS!
I remember that thing in JNU making an ungodly amount of noise at the customs ramp.
Autothrust Blue
Welcome aboard the Washington State Ferries
Let us all thank Pratt and Whitney for the superior free power turbine design.
inigo88
Composite-lover
I forgive this comment only because your PW118A points in the correct direction.
In any case, that story about the Rolls-Royce Dart on the HS-748 is insane! Gives one a new appreciation for modern FCUs, digital engine control and FADEC. She looks like a pretty damn cool engine though, in spite of what a PITA it must have been to deal with:
Polar742
All the responsibility none of the authority
The Dart I flew was a -10 on a YS-11. IIRC it was just a Whittle with a cople changes and a gearbox. The engine was simple, the fuel control was simple (which is why we had fuel trimmers and had to spin it up). The prop and controls were insane though. No prop controls inside the cockpit, except to unfeather, and move mechanical locks. It had water-meth though.
The first iteration of the TPE331 was very similar, with water-meth even. Then they added SRL and TTL and even APR.
On the upside, water-meth is sketchy.
The first iteration of the TPE331 was very similar, with water-meth even. Then they added SRL and TTL and even APR.
On the upside, water-meth is sketchy.
z987k
Well-Known Member
Learning my new fuel system today, on our -10 model TPE's we've got a fuel trim because they were not able to do it automatic for some reason. The start is the same but minus the 3rd step, as the 10% switch puts fuel in auto-magically.
Polar742
All the responsibility none of the authority
The TPE-10s installed on the notorious J31 had the enrich button, but that was the last vestiage of old school manual starts. If using the manual start sequence, you'd use the button, otherwise it would flash on and off on its own durning a normal start sequence.
There seem to be a mind boggling number of configurations for that engine.
There seem to be a mind boggling number of configurations for that engine.
ppragman
FLIPY FLAPS!
In every way hard. I came back to flying "big bore" pistons, and it's substantially more labor intensive - pretty much all the time. Gotta be way more gentle on the engines, gotta figure out each one's individual idiosyncrasies to a greater extent than a turbine airplane, you don't have excess power available to you pretty much whenever you want... the list goes on and on. On the flips side, I find there to be something slightly more "romantic" about flying piston engine airplanes around - they make noises, they talk to you, they require a gentler touch, but they're not as easy as turbines.
