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Turboprop Exhaust And Thrust  
User currently offlineflylku From United States of America, joined Apr 2006, 829 posts, RR: 0
Posted (1 year 10 months 3 weeks 4 days 23 hours ago) and read 6379 times:

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?


...are we there yet?
26 replies: All unread, showing first 25:
 
User currently offlinecornutt From United States of America, joined Jan 2013, 338 posts, RR: 1
Reply 1, posted (1 year 10 months 3 weeks 4 days 22 hours ago) and read 6337 times:

My understanding is that it's minimal at best.

User currently offlineak907 From United States of America, joined Mar 2012, 42 posts, RR: 0
Reply 2, posted (1 year 10 months 3 weeks 4 days 22 hours ago) and read 6327 times:

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.


User currently offlineSAAFNAV From South Africa, joined Mar 2010, 289 posts, RR: 1
Reply 3, posted (1 year 10 months 3 weeks 4 days 21 hours ago) and read 6276 times:

C-130 has about 300HP from the exhaust, so it gives 4300ESHP per engine on the -15 engines.


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User currently offlineN353SK From United States of America, joined Jun 2006, 833 posts, RR: 0
Reply 4, posted (1 year 10 months 3 weeks 4 days 21 hours ago) and read 6271 times:

I have no source, but i believe I once read that the Saab 340 gets 15% of its thrust from turbine exhaust.

User currently offlineAesma From Reunion, joined Nov 2009, 6959 posts, RR: 12
Reply 5, posted (1 year 10 months 3 weeks 4 days 21 hours ago) and read 6262 times:

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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User currently offlineMir From United States of America, joined Jan 2004, 21876 posts, RR: 55
Reply 6, posted (1 year 10 months 3 weeks 4 days 20 hours ago) and read 6229 times:

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.

-Mir



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User currently offlinebomber996 From United States of America, joined Aug 2005, 395 posts, RR: 0
Reply 7, posted (1 year 10 months 3 weeks 4 days 18 hours ago) and read 6187 times:

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.

Peace   



AVIATION - A Vacation In Any Town, I Own Nothing
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 8, posted (1 year 10 months 3 weeks 4 days 13 hours ago) and read 6073 times:

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.  



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineDufo From Slovenia, joined May 1999, 811 posts, RR: 4
Reply 9, posted (1 year 10 months 3 weeks 4 days 7 hours ago) and read 5942 times:

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.


I seriously think I just creamed my pants without any influence from any outside variables.
User currently offlineSpacepope From Vatican City, joined Dec 1999, 2990 posts, RR: 1
Reply 10, posted (1 year 10 months 3 weeks 3 days 11 hours ago) and read 5580 times:

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.


The last of the famous international playboys
User currently offlineaviaponcho From France, joined Aug 2011, 644 posts, RR: 9
Reply 11, posted (1 year 10 months 3 weeks 3 days 10 hours ago) and read 5558 times:

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 ?


User currently offlineSAAFNAV From South Africa, joined Mar 2010, 289 posts, RR: 1
Reply 12, posted (1 year 10 months 3 weeks 3 days 7 hours ago) and read 5460 times:

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

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

Erich



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User currently offlineaviaponcho From France, joined Aug 2011, 644 posts, RR: 9
Reply 13, posted (1 year 10 months 3 weeks 3 days 6 hours ago) and read 5416 times:

It is yes.
Powering the electra


User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 14, posted (1 year 10 months 3 weeks 3 days 4 hours ago) and read 5377 times:

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.


Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineSCAT15F From United States of America, joined Feb 2007, 402 posts, RR: 0
Reply 15, posted (1 year 10 months 3 weeks 3 days 1 hour ago) and read 5316 times:

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%


User currently offlineScooter01 From Norway, joined Nov 2006, 1214 posts, RR: 8
Reply 16, posted (1 year 10 months 3 weeks 2 days 17 hours ago) and read 5222 times:
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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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Photo © Paul K



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.

Scooter01



"We all have a girl and her name is nostalgia" - Hemingway
User currently onlinevikkyvik From United States of America, joined Jul 2003, 10349 posts, RR: 26
Reply 17, posted (1 year 10 months 3 weeks 2 days 4 hours ago) and read 5048 times:

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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User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 18, posted (1 year 10 months 3 weeks 2 days 1 hour ago) and read 4994 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently onlinevikkyvik From United States of America, joined Jul 2003, 10349 posts, RR: 26
Reply 19, posted (1 year 10 months 3 weeks 1 day 9 hours ago) and read 4853 times:

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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User currently offlineKELPkid From United States of America, joined Nov 2005, 6428 posts, RR: 3
Reply 20, posted (1 year 10 months 3 weeks 1 day 9 hours ago) and read 4843 times:

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  



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlinedw747400 From United States of America, joined Aug 2001, 1265 posts, RR: 1
Reply 21, posted (1 year 10 months 3 weeks 1 day 2 hours ago) and read 4777 times:

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.



CFI--Certfied Freakin Idiot
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 22, posted (1 year 10 months 3 weeks 1 day 1 hour ago) and read 4758 times:

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.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineMHG From Germany, joined Dec 2004, 796 posts, RR: 1
Reply 23, posted (1 year 10 months 2 weeks 6 days 14 hours ago) and read 4593 times:

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



I miss the sound of rolls royce darts and speys
User currently offlineZKSUJ From New Zealand, joined May 2004, 7110 posts, RR: 12
Reply 24, posted (1 year 10 months 2 weeks 2 days ago) and read 4361 times:

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


25 DiamondFlyer : There are also piston powered aircraft that have reverse capability. -DiamondFlyer
26 DashTrash : 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 availabl
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