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Guru
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A Turboprop Question

09/15/2013 4:37 PM

On turboprop aircraft, what percentage of thrust is attributable to the prop and to the turbine?

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Guru

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#1

Re: A turboprop question

09/15/2013 6:06 PM

Most (by far) is from the prop.

I don't think you will find a standard turboprop with the prop providing less than 85%.... and that is conservative.... it is probably higher for most.

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Guru

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#2
In reply to #1

Re: A turboprop question

09/15/2013 6:58 PM

Almost none. All the power is run throung the gear reduction unit to the prop with maybe5-10% going out the back as thrust.

What tinac said.

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Guru
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#3

Re: A Turboprop Question

09/15/2013 11:40 PM

dear Mr. ronseto,

I am of the view that

The thrust can be calculated (like for any other jet engine) which is given by M x V where M is the Mass of the Burnt gases leaving the engine and V is the Velocity of the Gas leaving at the exit.

DHAYANANDHAN.S

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#4

Re: A Turboprop Question

09/16/2013 2:16 AM

Almost all of the exhaust energy is consumed in turning the driveshaft, and so exhaust velocity is too low to produce meaningful thrust.

Many turboprop engines have their exhaust outlets in such configuration that rearward thrust is impossible to achieve, ie, downward or out the side as in some cropdusters etc., and this would not be the case if it were considered to be a worthwhile method of added propulsion.

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Guru
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#5
In reply to #4

Re: A Turboprop Question

09/16/2013 4:46 AM

Dear Mr.bwilko,

The olden days, petrol fuelled Aircraft Engines (Present day JET AIR CRAFT uses AVIATION TURBINE FUELS, with ADDITIVES, to avoid/reduce freezing at High Altitudes of 10,000 Metres High and temp. is -30 Deg.C or so, this is above KEROSENE Grade, but below Petrol Grade) the Engine Exhaust is considered to develop some thrust, and as you said, it will be small.

But Thrust is PRODUCED. In Thermodynamics, it can be seen, that the Max.Jet Efficiency is achieved, when the Exit Gas Velocity is twice the Air Craft Velocity. Hence the Exit Diametre is carefully calculated.

DHAYANANDHAN.S

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#17
In reply to #5

Re: A Turboprop Question

09/17/2013 8:14 PM

Not sure that any of what you have said had a jot of relevance at all to what I posted.

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Guru
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#6

Re: A Turboprop Question

09/16/2013 8:50 AM

Let me just say that if you feather the prop for zero thrust, the exhaust of a turboprop engine is NOT gonna move the aircraft an iota (or an inch, or a centimeter).

Hooker

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#7
In reply to #6

Re: A Turboprop Question

09/16/2013 9:18 AM

Right you are, a tail wind would be more thrust.

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Guru
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#8
In reply to #7

Re: A Turboprop Question

09/16/2013 10:32 AM

Absolutely!!

Which reminds me, leaving a turboprop or turboshaft aircraft running on the ground with a still tailwind can result in an overtemp in the hot section if you're not careful.

I've seen it happen!

Hooker

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#11
In reply to #8

Re: A Turboprop Question

09/16/2013 11:52 AM

That should have been "stiff" tailwind. Sorry.

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#12
In reply to #11

Re: A Turboprop Question

09/16/2013 12:45 PM

Is that as bad as a "hot start" ?

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Guru
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#13
In reply to #12

Re: A Turboprop Question

09/16/2013 1:29 PM

It can be, if allowed to persist long enough. Both situations result in the combustion chamber operating beyond safe temperatures, as determined by monitoring the EGT (exhaust gas temperature) gauge.

Depending on model, any over temperature operation that exceeds a prescribed period of time requires a hot end inspection for damage.

Hooker

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#14
In reply to #6

Re: A Turboprop Question

09/16/2013 6:57 PM

"iota" - is that the Mexican unit iof measurement?

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Guru

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#20
In reply to #6

Re: A Turboprop Question

09/21/2013 8:51 PM

Yep, it won't. My Mu-2 had two fuel loving engines and at 0 thrust turbines turning up to 70%, aircraft would sit as still as a windless night.

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#21
In reply to #20

Re: A Turboprop Question

09/21/2013 9:07 PM

Nice airplane.

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#22
In reply to #21

Re: A Turboprop Question

09/22/2013 9:26 PM

Yep

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#9

Re: A Turboprop Question

09/16/2013 11:24 AM

In the earlier days of turboprops, manufacturers always used to quote the power output and the residual thrust. As an example, this is for the Rolls-Royce Dart 10/1, an early turboprop from the 1940's-1950's with a 2-stage centrifugal compressor and 3 stage axial turbine:

3,030 hp (2,259.47 kW) estimated power - 2,750 hp (2,050.67 kW) shaft power + 750.4 lbf (3.34 kN) residual thrust at 15,000 rpm.

750 lbs. thrust was nothing to be sneezed at (!). It's very probable that more advanced turboprops extract a higher percentage of the exhaust gas energy, rendering the residual thrust negligible and not worth mentioning.

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#15
In reply to #9

Re: A Turboprop Question

09/17/2013 4:48 PM

During WWII during development of aircraft it was found that if the exhaust was directed to the rear that the top speed of the aircraft was increased by almost 20 knots. While not much it did make a difference if you had an enemy plane on your tail and you were trying to out run them. I believe that the Spitfire was the first to take advantage of this but I can't remember for sure. Even small amounts of thrust can make a difference.

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Guru
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#16
In reply to #15

Re: A Turboprop Question

09/17/2013 5:03 PM

6 mph over the stock Rolls exhaust

NACA exhaust

Hooker

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#18
In reply to #15

Re: A Turboprop Question

09/18/2013 2:59 AM

Maybe due to reducing resistance between the air and the aircraft skin after the exhaust port?

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#10

Re: A Turboprop Question

09/16/2013 11:45 AM

Typical, although some engines may direct the exhaust straight out the back.

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#19
In reply to #10

Re: A Turboprop Question

09/18/2013 8:18 AM

And some out the "front".

One of the most popular turbo-prop engines in the world is the P&W PT6A. It's a reverse flow design with a free turbine (the take off turbine is not connected to the compressor) which allows the compressor to run without turning the prop.

Good animation of engine.

Hooker

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