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flylku
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Turboprop Exhaust And Thrust

Mon Feb 04, 2013 1:53 am

I was on a Dash 8 tonight and it got me wondering, are there any turboprop aircraft where the turbine exhaust adds meaningfully to thrust?
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cornutt
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 2:46 am

My understanding is that it's minimal at best.
 
ak907
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 3:03 am

Yes actually, but like cornutt said, it's not much. Usually around 5% or so. It is actually listed in the type certificate of that engine, and is included in the shaft horsepower output of the engine. They use a formula to convert thrust into horsepower, and add both together under "equivalent shaft horsepower".

For example a PT6-114A has a shaft horsepower of 675hp, and exhaust thrust of 124 lbs. Add both together using a formula and they come up with an equivalent shaft horsepower of 725hp.
 
SAAFNAV
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 4:14 am

C-130 has about 300HP from the exhaust, so it gives 4300ESHP per engine on the -15 engines.
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N353SK
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 4:20 am

I have no source, but i believe I once read that the Saab 340 gets 15% of its thrust from turbine exhaust.
 
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Aesma
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 4:28 am

The PT6 is an interesting example since it's usually installed "backwards" so I always thought the exhaust thrust was very little.
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Mir
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 5:00 am

Quoting Aesma (Reply 5):
The PT6 is an interesting example since it's usually installed "backwards" so I always thought the exhaust thrust was very little.

The exhaust is still ejected in a backwards direction, so it does contribute to the overall thrust in a very small way.

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bomber996
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 7:31 am

I remember hearing during a cockpit visit in a PSA Airlines DO-328 that the exhaust from the engine contributed to something like 25% of the total thrust of the engine. Then again, I was a lot younger than I am today and I may be over exaggerating that number.

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Starlionblue
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 12:09 pm

Quoting Mir (Reply 6):
Quoting Aesma (Reply 5):
The PT6 is an interesting example since it's usually installed "backwards" so I always thought the exhaust thrust was very little.

The exhaust is still ejected in a backwards direction, so it does contribute to the overall thrust in a very small way.

The PT6 actually reverses flow four times, as the combustors have airflow reversed compared to engine airflow. PW wanted a really short engine with a simple prop installation I guess.  
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Dufo
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RE: Turboprop Exhaust And Thrust

Mon Feb 04, 2013 5:47 pm

Official figures for GE CT7-5A installed on Saab 340A tell that the engine produces 1735 SHP and 1800 ESHP. The difference between these two numbers is the jet propulsion.
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 2:18 pm

IIRC, even piston engines, such as the RR Merlin (specifically when installed in later Spitfires), gained a not-insignificant amount of thrust from the exhaust flow. Enough to boost the top speed by exhaust stack design.
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Aviaponcho
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 2:53 pm

Hello

On the electra 501-D13's

Thrust from the propeller was 3460 Hp (8000 lbsf) and thrust from the exhaust 290 hp (726 lbsf)
So that's not insignifiant.... that's 8%
It remain to see how this evolved in new designs... as efficiency goes up, residual thrust can go down, isn't it ?
 
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 6:09 pm

Quoting aviaponcho (Reply 11):
electra 501-D13's

Wasn't the that civilian version of the Allison T-56?

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Aviaponcho
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 7:41 pm

It is yes.
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KELPkid
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 8:58 pm

As I understand it, on both the Cessna 208 Caravan and DeHavilland Twin Otter, the thrust put off by the jet pipes is not so insignificant when the aircraft is equipped with floats. That thrust basically guarantees that as long as the engine(s) are burning, the aircraft will be moving on the water (apart from putting the prop(s) in beta). On a regular (non turbo-prop) float plane, when water taxiing, putting the throttle to idle usually lets the aircraft coast to a stop, where it is only affected by wind and water currents.
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SCAT15F
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RE: Turboprop Exhaust And Thrust

Tue Feb 05, 2013 11:45 pm

Quoting cornutt (Reply 1):

Actually, residual thrust can be very significant, depending on the design.

The Pratt and Whitney T-57 turboprop produced 15,000 hp and 5000 lbs of residual thrust... that's 33%
 
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Scooter01
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RE: Turboprop Exhaust And Thrust

Wed Feb 06, 2013 7:52 am

A bit off topic, since this thread seems to be about fixed wing aircraft.

When the first Agusta Bell 204B helicopters came to Norway in the spring of '62, they were equipped with the Bristol Siddeley Gnome H.1000 engine where the exhaust exited sideways, like on this Dutch machine:

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The owners were apparently not quite happy with the performance, and the machines were sent back to Italy at the end of '63 to be re-engined with the Lycoming T-53 -with a straight-back exhaust. After the conversion I remember it was talk of a significant improvement of forward airspeed, but I cannot recall how much.

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vikkyvik
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RE: Turboprop Exhaust And Thrust

Wed Feb 06, 2013 9:13 pm

Quoting KELPkid (Reply 14):
As I understand it, on both the Cessna 208 Caravan and DeHavilland Twin Otter, the thrust put off by the jet pipes is not so insignificant when the aircraft is equipped with floats. That thrust basically guarantees that as long as the engine(s) are burning, the aircraft will be moving on the water (apart from putting the prop(s) in beta). On a regular (non turbo-prop) float plane, when water taxiing, putting the throttle to idle usually lets the aircraft coast to a stop, where it is only affected by wind and water currents.

That's just a function of your total thrust versus drag, though. Doesn't have any special dependence on the exhaust, unless I'm missing something.

I'm assuming props at idle throttle still have some forward thrust.
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Starlionblue
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RE: Turboprop Exhaust And Thrust

Thu Feb 07, 2013 12:25 am

Quoting vikkyvik (Reply 17):
I'm assuming props at idle throttle still have some forward thrust.

Assuming you mean the actual propeller here, it depends on the pitch capability of the prop. Certainly on a GA plane with a constant speed prop the prop's pitch cannot go all the way to zero so you will still have thrust.

On airline class turboprops you tend to have reverse ("beta") so you can go to zero pitch (and beyond) and thus zero thrust.
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vikkyvik
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RE: Turboprop Exhaust And Thrust

Thu Feb 07, 2013 3:52 pm

Quoting Starlionblue (Reply 18):
On airline class turboprops you tend to have reverse ("beta") so you can go to zero pitch (and beyond) and thus zero thrust.

True, but I was just talking about idle throttle, not prop pitch (assuming they're separate).
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KELPkid
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RE: Turboprop Exhaust And Thrust

Thu Feb 07, 2013 4:26 pm

Quoting vikkyvik (Reply 17):
That's just a function of your total thrust versus drag, though. Doesn't have any special dependence on the exhaust, unless I'm missing something.

I'm assuming props at idle throttle still have some forward thrust.

In a turboprop, probably not much...rememer that there is a prop hub that is constantly adjusting blade pitch to maintain propeller RPM. Unless that RPM is really, really low, the prop will have to be at a really fine pitch to allow it to spin with the engine at a relatively low power setting (i.e. ground idle or slightly above it). On a PT6, I would imagine that the condition lever is not set for flight while taxiing...which lowers the engine's idle RPM.

Quoting Starlionblue (Reply 18):
Assuming you mean the actual propeller here, it depends on the pitch capability of the prop. Certainly on a GA plane with a constant speed prop the prop's pitch cannot go all the way to zero so you will still have thrust.

A constant-speed prop, even with the prop lever at the firewall, is going to turn at fairly "normal" RPM's on the ground while taxiing, because although the prop hub might be striving to turn the prop at maximum RPM, the engine just isn't providing the power to it, and the hub has physical limits on just how fine it can make the prop pitch  
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dw747400
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RE: Turboprop Exhaust And Thrust

Thu Feb 07, 2013 10:51 pm

Quoting KELPkid (Reply 20):
On a PT6, I would imagine that the condition lever is not set for flight while taxiing...which lowers the engine's idle RPM.

It depends on the specific installation. On a King Air C90 with four bladed props, even with the condition levers at ground idle, we have enough static thrust to start rolling at all but max gross. We spend a lot of taxi time with the props in beta to keep from wearing down the brakes. I haven't flown the three bladed C90, but I'm told it has much less static thrust and thus doesn't need beta to keep it from accelerating.

I discussed this with a C-130 pilot once; he informed me that they had considerable excess thrust at idle at light weights, but at high takeoff weights the idle power wouldn't keep them moving.

Quoting vikkyvik (Reply 19):

True, but I was just talking about idle throttle, not prop pitch (assuming they're separate).

I can't speak for every installation, but all I've seen or flown use throttle to control beta/reverse. Your prop control sets RPM, your throttle sets how much power you are producing and what direction that thrust is directed.
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Starlionblue
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RE: Turboprop Exhaust And Thrust

Thu Feb 07, 2013 11:57 pm

Quoting vikkyvik (Reply 19):
Quoting Starlionblue (Reply 18):
On airline class turboprops you tend to have reverse ("beta") so you can go to zero pitch (and beyond) and thus zero thrust.

True, but I was just talking about idle throttle, not prop pitch (assuming they're separate).

Ah. Sorry I had brainfart there. You mean the thrust from the core. Well certainly since there is airflow through the engine you'd have some thrust.

Also interestingly the thrust lever goes from controlling thrust in normal range to controlling pitch in beta range.
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MHG
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RE: Turboprop Exhaust And Thrust

Sat Feb 09, 2013 11:33 am

Here´s an example of exhaust thrust:

RR Dart Mk 552 (used in final production series of the Fokker F-27).
It´s the most powerful version used in the F-27 ...

Power rating: 2330 ehp = 2210shp + 238 Kg / 525 lb exhaust thrust

Earlier versions of the RR Dart (e.g. Mk511) had only 1650 ehp = 1535 shp + 159 kg / 350 lb thrust
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ZKSUJ
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RE: Turboprop Exhaust And Thrust

Thu Feb 14, 2013 12:54 am

Quoting flylku (Thread starter):
I was on a Dash 8 tonight and it got me wondering, are there any turboprop aircraft where the turbine exhaust adds meaningfully to thrust?

The Dash 8 has about 120hp thrust from the exhaust (or jet thrust as I like to call it)   Not where near enough to power the aircraft though as previous posters have mentioned
 
DiamondFlyer
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RE: Turboprop Exhaust And Thrust

Thu Feb 14, 2013 4:54 am

Quoting Starlionblue (Reply 18):
On airline class turboprops you tend to have reverse ("beta") so you can go to zero pitch (and beyond) and thus zero thrust.

There are also piston powered aircraft that have reverse capability.

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DashTrash
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RE: Turboprop Exhaust And Thrust

Thu Feb 14, 2013 9:10 pm

Quoting Starlionblue (Reply 18):
On airline class turboprops you tend to have reverse ("beta") so you can go to zero pitch (and beyond) and thus zero thrust.

Beta and reverse are different (Dash 8 anyway). DH called "beta", "disc". I don't recall what blade angle range was in disc, but reverse was available as well bringing the blades negative.

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