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Turbine Engine Exhaust

Turbine Engine Exhaust

Turbine Engine Exhaust

Howdy all,
   Well since diffusers didn't grab your attentin, how about this?

   How should you design the exhaust stack for a turboprop to maximise cruise efficiency? Specifically PT6. Manufacturers installation manual appears somewhat inadequate. The results from their CCD program gives a value for Fn (exhaust thrust), although doesn't say whether or not this is momentum thrust or pressure thrust.


   PS Anyone got the instructions to use CCD?

RE: Turbine Engine Exhaust


In the CCD you can input different data for exhaust stubs angles and areas. As I remember, depending on how you create the inputs, you can obtain results including for exhaust pressure and temperature. If you have an application, I think its a good idea to contact P&WC, because they can provide you the instructions for CCD. If you will use another types of exhaust stubs (others than in their recommandations), you must certify them (this one of the reasons why most of designers use some of P&WC standard stubs which are already certified).

I understand that you have the CCD. Can you be more specific of what do you want ?

Sorry for my english. Its not my native language.


RE: Turbine Engine Exhaust


Thanks for your reply. We have the CCD and want to go beyond the P&WC standard exhausts to obtain maximum efficiency in a fast aeroplane.

The problems with the CCD and Pratt manuals are:
1. The installation manual tells you to use the CCD but does not give directions for its use. It is a primitive file input program where you need to know what each entry means in the input and output files. I'll bet there was a manual for its use at some time but Pratt's tech help people are so far away from the PT6 design now that they couldn't remember what the CCD was when I contacted them.

2. The results for the CCD, including the worked examples in the manual, do not make sense. They appear to only show pressure thrust and ignore momentim thrust, but because the calculation process is not explained you can't tell what is going on. The temperature given is P7, not P9, so you cant actually work out the volume, temp and velocity at the stack face.

Thanks again for your help

RE: Turbine Engine Exhaust


Complete informations about this CCD you can find in a manual called Estimated Engine Performance Program (User's Manual). Last edition was in Februrary 2002 as I remember. Try to contact again P&WC because I'm sure that they can send you a print copy (and believe me, they know everythink about that).

If you still have problems, give me your e-mail and I will try to explain you about input data in this CCD.


RE: Turbine Engine Exhaust

Well, Mr Macoovacany, howzit going?

Curious to know what the project is!  If you look at a number of separate PT-6 installations by companies such as Beech, Ayres and others, those exhausts all look remarkably similar.  Someone must have some idea what's going on there.  I would assume those are a reasonable starting point.

Even though a lot of those exhausts seems to just 'poke out and go around a bend', I can only assume that separation around the back is not nearly as bad as first thought.  I think you'll find there is a fair amount of flow entrainment caused by the exhaust flow itself.

Also bear in mind that those exhausts are 'rather' hot in flight.

Regarding your information from P&WC...  NFI!

the scarlet pumpERnICkel

RE: Turbine Engine Exhaust

If I do remember correctly PT6A (turboprop PT6) is a "wrong flow" engine.  Air intake is at rear end and the "bare" engine exhaust is at propeller side (sidewise, more correctly upward). If I am right than there is not much positive thrust unless the exhaust flow is reversed (by some exhaus pipe) increasing a momentum thrust.  

With such configuration compressor produces a negative pressure thrust while "reverse" combustion chamber or better to say "reverse" diffuser should produce positive both pressure and momentum thrust.  Unfortunatelu inverted combustion chamber exit produces again negative momentum thrust but luckily in this configuration the turbines (Gas Generator and Power turbine) produce positive pressure thrust.  Altogether I would dare to say zero but those P&W Canada guys should know the best what net thrust their  engine delivers (the "bare" engine - without that custom designed jet pipe, which is an aircraft designer responsability).

RE: Turbine Engine Exhaust


Designing your stacks for best cruise efficiency is a nice idea, however looking at the "sootless" stacks on the new Beechcraft King Airs and the aftermarket products for older King Airs, Piper Cheyennes, and Cessna Conquest I your energy will be better spent keeping the heat and soot off of the aircraft. The PT6 has little thrust potential as a reverse flow engine.  Minimize the drag off your stacks for cruise gains.

RE: Turbine Engine Exhaust

Be careful of the effects on spin characteristics of any stack you design.
Also, most of the stacks are angled to help entrain the flow into the propwash and if not done properly you can get localized heating and sooting in the most unlikely places e.g. underside of horizontal stab

Are you using the recommended air inlet anti-ice scheme with a pitot style take off from one stub and dumping into the other?

RE: Turbine Engine Exhaust

Good golly, you guys.  If exhaust stubs effect spin, then so does the radio volume knob.

The nozzle exhaust is probably not all that important in a low pressure turboprop like the PT-6.  But, as a general rule, the faster you go, the smaller the exit area should be.  So you can get some thrust from the exhaust at cruise, but if you overdo it, you will put too much back pressure on the engine during takeoff.

You really need to get the installation information from the P&W people.  It sounds like they're blowing you off because they don't take you seriously, for good reason it would seem.  Buy an engine, then they'll talk.

RE: Turbine Engine Exhaust

Beech encountered enough changes in spin characteristics on the PC9 JPATS to cause them to do some re-testing I believe.  And I guess the size of your knob would dictate if it will effect exterior airflow too :)

I agree with the previous couple of posts though, localized heating and sooting are more of a concern in the long run, particularly if you are in a "composite" world.

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