How Do Turboprop engines work

[bob_albertson]

Now that I have found where all the cool guys hang out, I am so happy and unbored (is that a word ).

Anyways, I'm reading all that I can on these types of engines. A lot of what the AFH has to say is Greek to me without some explanation of what the other things do.... I like to have a complete picture of the system.

So, Fixed shaft (direct drive) vs. Split Shaft (free turbine): On a fixed shaft engine they say the engine is made of a combustion chamber (I understand that one, gee who wouldn't ), transition liner, and turbo plenum. That is all they say about the last two parts of the engine.... They don't even have a good diagram about a transition liner and turbo plenum. Google isn't getting much in the way of descriptions for me. The way most things seem, those two must have a more used slang term??

If an engine failure occurs, they say that the compressor creates a huge amount of drag, and as a secondary aid to prevent being stuck with the prop in the low pitch position... some manufacturers incorporate a NTS system (Negative Torque Sensing). I'm looking for a little clarification on what they said.... Is it just ram air flow through the engine that keeps the compressor spinning during an engine failure? If the propeller is windmilling, it is because of the compressor and not aerodynamic forces on the propeller blades?

For the direct drive engines, they said that power changes are made by increasing fuel flow and propeller blade angle rather than engine speed.... But, isn't it fuel flow (condition lever) that controls engine RPM? I was a little cornfused because of the relationship of fuel flow and RPM since the fuel affects the engine speed (RPM)?

The split shaft engines.... they have a centrifugal compressor... So does that just mean that N2 is a measure of the air pressure at the gas producer (turbine?) against the chamber walls? Does that engine also incorporate a transition liner?

I can follow the air's progress through each compressor, but the book doesn't do a good job of explaining the reason why the compressed air reverses direction twice. I am assuming it is caused by the turbine that drives the compressor, sending the air towards the second turbine that drives the prop, and that compressor sending it back to the other one more time before the air makes it out to the exhaust.

Which engine produces more exhaust thrust?

..... and that happened

Bob

 

[bbchien]

Bob, on fixed shaft engines, fuel flow produces more power which is harnessed by steeper blade pitch. In power failure, the windmilling blades are DRIVING a compressor, which means mondo drag. The engine idles at say 90% of N1.

On split shaft engines, autofeather (NTS) is far less critical. Many King Airs don't have it. I think you're looking at a PT6 diagram, but not all split shaft engines have the exhaust (and therefore the turbine) at the front. It just weighs less on the PT6 to have the driven turbine (driven by the expanding gases) right next to the blade shaft. It avoids having to make the compressor around a HOLLOW shaft. There's not much more to it than that.

 

[wabower]

I picked up a wall-art diagram of a TPE-331 in Phoenix last month, but can't lay hands on it. I'll send it to you if/when I find it. Other than that, I'll defer Garrett info to those better equipped to answer the questions.

The PT-6 compressor is centrifugal for the final stage only, to fill the combustion chamber with air prior to ignition that occurs after introduction of fuel. The double switchback was explained to me as Pratt's method of shortening the engine dimension so it would fit on a wing and allow better tankage configuration. But I have no evidence other than FSI instructors for this theory.

N1 (also known as NG) is gas generator speed, N2 is prop speed. Gas generator typically turns about 37k rpm, power turbine about 33k which is reduced by gearbox (15:1) to provide max prop speed that varies between 2,000 and 2,200 on most King Airs.

Dunno about residual thrust, but my guess is that the single-shaft should produce more. Neither is enough to fly the plane, so I'm not sure it matters, except for the amount of beta (flat or reversed prop blades with no power increase) range deflection required.

 

[Dan Thomas]

So, Fixed shaft (direct drive) vs. Split Shaft (free turbine): On a fixed shaft engine they say the engine is made of a combustion chamber (I understand that one, gee who wouldn't ), transition liner, and turbo plenum. That is all they say about the last two parts of the engine.... They don't even have a good diagram about a transition liner and turbo plenum. Google isn't getting much in the way of descriptions for me. The way most things seem, those two must have a more used slang term??

If an engine failure occurs, they say that the compressor creates a huge amount of drag, and as a secondary aid to prevent being stuck with the prop in the low pitch position... some manufacturers incorporate a NTS system (Negative Torque Sensing). I'm looking for a little clarification on what they said.... Is it just ram air flow through the engine that keeps the compressor spinning during an engine failure? If the propeller is windmilling, it is because of the compressor and not aerodynamic forces on the propeller blades?

For the direct drive engines, they said that power changes are made by increasing fuel flow and propeller blade angle rather than engine speed.... But, isn't it fuel flow (condition lever) that controls engine RPM? I was a little cornfused because of the relationship of fuel flow and RPM since the fuel affects the engine speed (RPM)?

The split shaft engines.... they have a centrifugal compressor... So does that just mean that N2 is a measure of the air pressure at the gas producer (turbine?) against the chamber walls? Does that engine also incorporate a transition liner?

I can follow the air's progress through each compressor, but the book doesn't do a good job of explaining the reason why the compressed air reverses direction twice. I am assuming it is caused by the turbine that drives the compressor, sending the air towards the second turbine that drives the prop, and that compressor sending it back to the other one more time before the air makes it out to the exhaust.

Which engine produces more exhaust thrust?

..... and that happened

Bob

Most turboprops will feather their propellers to prevent the wind's turning the engine and creating drag.

The "direct drive" engine, as you call it, is the fixed turbine. It has an idle RPM pretty close to it cruise RPM. As the prop's pitch is made to increase, more fuel is needed to maintain the RPM, and vice versa.

Some engines reverse the flow internally to shorten the engine physically. And some, like the Allison/Rolls Royce C250, have air going in all sorts of funny directions.

"Free" turbines are the split-shaft affair. They're easier to start because the gas generator section is all that has to turn; the prop, its gearbox and turbines can sit still until the exhaust gases wake them up.

Pratt & Whitney Canada PT-6 free turbine engine:

Allison/Rolls C250:

Garret TPE 331 fixed turboprop:

N1 and N2 refer to percent of max RPM for the two sections of a free turbine. Here's a classic turbofan with the two sets of compressors and turbines:

Dan

 

[taters]

typically on a garrett there is about 20% residual thrust out the back, they truly are "jet props"... designed to basically run at 90-92% power with the condition levers in flight idle and about 60% on the ground with the condition levers in ground idle, as stated above the props (controlled by power levers) take a "bite" when thrust is needed....Even with the Throttles at idle in flight the engine will still produce about 22% of max power (measured in torque) so you must slow down then go down , also TPE's can be expensive in a hurry if you jack up the start, the props must be in 0 pitch or the drag is too much for the starter...BAM!
I have limited experience with the PT-6 's and its been a while but I'm pretty sure I remember they have minimal "thrust" out of the stacks.

 

[Greg Bockelman]

Man there are a lot of parts in that Garrett gear box.

 

[bob_albertson]

Wow, thank you everyone!!! That really helps clear those things up for me. I don't have any previous experience with these engines, and I'm trying to work from scratch, solo for now. Their operation seems simple enough, but the whole system interaction is the type of information I always love.

Another quick question I could use a little clarification on... The gas producer moves the turbine which moves the compressor so the cycle is continuous -- The airflow is used to move the turbine so the turbine can move the compressor and draw air in to start the cycle again?

I was told this was gonna be an interview question, but I always like to dive into this stuff
Bob

 

[wabower]

Yes, and the exhaust from the perpetural-motion machine flows through the power turbine that turns the prop on the PT-6's

Wow, thank you everyone!!! That really helps clear those things up for me. I don't have any previous experience with these engines, and I'm trying to work from scratch, solo for now. Their operation seems simple enough, but the whole system interaction is the type of information I always love.

Another quick question I could use a little clarification on... The gas producer moves the turbine which moves the compressor so the cycle is continuous -- The airflow is used to move the turbine so the turbine can move the compressor and draw air in to start the cycle again?

I was told this was gonna be an interview question, but I always like to dive into this stuff
Bob

 

[bob_albertson]

Great, that information helps a lot!!!

And thanks to everyone for taking the time to post. I really appreciate it!!

I was told the interview answer they are looking for is just basic information.... But, why wait to learn as much about the different engines. I'm an engine kinda guy, and find it hard to not read all that I can about these.

Just a Gee Whiz: Does anyone have a diagram of a PW150A?

 

[wabower]

Looking at the diagram kinda helps explain why they are so noisy, doesn't it? All that air being squeezed through that little hole can't be anything else.


Bobby, two little-known (and perhaps less cared-about) factoids re. PT-6's:

1. Only ~25% of the air that comes into the engine is used for combustion. The rest is for used for internal cooling.

2. Everybody talks about the gearing system that is necessary to slow the power section rpm to acceptable prop speed, but few think about the gearing that is necessary to reduce NG shaft speed to acceptable speeds for the accessories that are all (except for the prop governors, torque sensors and metal detector) mounted on the the accessory case at the aft end of the compressor section. Fuel control unit, starter-generator, oil pump, tach generator, various other gadgets located there.

Man there are a lot of parts in that Garrett gear box.

 

[Mxfarm]

Now that I have found where all the cool guys hang out,

That was kinda sexist....



Just jerkin' your chain Bob.

marc

 

[Teller1900]

Where are you interviewing that you need to know about the PW150A? I have been flying behind two of them for almost two years now. I have a great color diagram at home, I'll scan it as soon as I get back either tomorrow or Friday.

As far as latent thrust from the exhaust, you get a little bit but not enough to really rely on it for anything. With the PT-6-67D our SHP was something like 1279, while the ESHP (accounting for jet thrust) was 1305.

If all they ask is "How does the engine work" then give them the simplest answer possible: suck, squeeze, boom, blow, all at once.

 

[Dan Thomas]

Bobby, two little-known (and perhaps less cared-about) factoids re. PT-6's:

1. Only ~25% of the air that comes into the engine is used for combustion. The rest is for used for internal cooling.

That applies to pretty much all turbines. Power is controlled via fuel flow, just as in a diesel. There's lots of oxygen left over. In the turbine, extra air is used to form cooler cushions over all the hot stuff like combustion cans and turbine and stator blades. They'd burn out quickly otherwise.

The compressor absorbs 60 - 80% of the power produced by the turbine section. What's left is available thrust or torque, as applicable.

Dan

 

[bob_albertson]

That was kinda sexist....



Just jerkin' your chain Bob.

marc

Doh, I may have overlooked that one

Did you see that.... Smack.... wall ..... oops I walked right into it

 

[bob_albertson]

Where are you interviewing that you need to know about the PW150A? I have been flying behind two of them for almost two years now. I have a great color diagram at home, I'll scan it as soon as I get back either tomorrow or Friday.

As far as latent thrust from the exhaust, you get a little bit but not enough to really rely on it for anything. With the PT-6-67D our SHP was something like 1279, while the ESHP (accounting for jet thrust) was 1305.

If all they ask is "How does the engine work" then give them the simplest answer possible: suck, squeeze, boom, blow, all at once.

I'm going with the simple answer, but wanting to learn about that engine and how all of them work in general. I hope to be flying behind some turbine within the next 6 months or so. Whether it's a turboprop or a turbojet I am clueless. But, I feel better answering questions about turbojets than turboprops because I've had the opportunity to look at some of the older high bypass type engines. I've not had any training or time behind a book for turboprops. I like to start with general stuff, then compare specific engines to see how small system changes can affect the engine..... or in some cases how a nearly completely different design can accomplish the same job

I hope to interview at Colgan Air.... Chuck told me to call once already, but because of a family illness I couldn't. He has sent me a few more e-mails to say "we are still considering you", and the last one was "please keep your info updated we are tracking your progress". Still haven't heard anything about an interview though, LOL. But, that is the interview I am trying to prepare for.

Bob

 

[Teller1900]

1. Only ~25% of the air that comes into the engine is used for combustion. The rest is for used for internal cooling.

Don't forget about flame-centering. As it was explained to us, think about a candle placed in the middle of a soup can. If there are no holes, the candle will go out. If there is only one hole punched in the side of the can, the flame will burn hotter toward that side eventually burning the can. However, if there are two holes, the flame will tend to stay centered and the heat will be distributed more evenly. Better still, if there are just over 16,000 holes (like in the burner can of the PT-6), it will burn even better and cooler.

Bob, I probably won't be able to get to my diagram of the 150A until late tomorrow, but I'll get it to you ASAP.

 

[wabower]

The proof is apparently in the pudding, as the burner cans on Pratt's are relatively cheap to maintain and seem to last a long time.

Don't forget about flame-centering. As it was explained to us, think about a candle placed in the middle of a soup can. If there are no holes, the candle will go out. If there is only one hole punched in the side of the can, the flame will burn hotter toward that side eventually burning the can. However, if there are two holes, the flame will tend to stay centered and the heat will be distributed more evenly. Better still, if there are just over 16,000 holes (like in the burner can of the PT-6), it will burn even better and cooler.

Bob, I probably won't be able to get to my diagram of the 150A until late tomorrow, but I'll get it to you ASAP.

 

[Steve]

suck squeeze blow and go

just like any other infernal combustin engine....

 

[Teller1900]

Hey Bob, here's the diagram for the PT150A. It's a pretty simple, straight forward engine; it's almost as good as the PT-6.

 

[bob_albertson]

Hey Bob, here's the diagram for the PT150A. It's a pretty simple, straight forward engine; it's almost as good as the PT-6.

Thank you sooooo Much!!!

Hopefully, pretty soon, Chuck will send me the e-mail about calling him.

Are you with Colgan Air? If so, are you at EWR or IAH?

Thank you, everyone, for all of the help!!!

Bob

 

[Dan Thomas]

Man there are a lot of parts in that Garrett gear box.

Naw, that's a simple engine. Here's an exploded view of an axial compressor, just the compressor, for a turbojet engine. Each rotor stage has a hub on the shaft, and every blade is a separate part that slides into the hub before it's placed on the shaft. And every stator blade for every stator stage is a separate piece, fitted to the housing. How many parts are there, I wonder? And that's just the compressor!

Every rotor blade must match a certain weight. The whole thing rotates at high speed and any imbalance would cause serious vibration.

Here's a picture of a turbine blade. Note the cooling air holes on the surface to keep the hot gases off the metal. All those drillings, in every rotor and stator blade in the hot section, in really tough metal. Note, too the "fir-tree" root of the blade; that slides into a matching place in the hub. The compressor blades are similarly mounted.

And a picture of the blade mounting arrangement:

Complicated? You bet. Now you start to understand why these things cost so much.

Dan

 

[taters]

thats a really nice view....to the poster good luck @ Colgan.