Turbine exhausts...
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TFS,
Yes, .................... Planes!!!!
Now for a more serious answer.
I guess I am to assume that you are asking about helicopters?
They are designed so as not to provide forward thrust, they are designed to diffuse the exhaust gases.
You must have a convergent duct to produce thrust, if you were to do this, in the example of a Jet Ranger, you would be forcing Red hot exhaust gases at the main rotor blades.
Therefore in helicopter engines the design is such that a divergent or parallel duct is used as the exhaust to minimise this effect.
Also, another point, is that if you were to look at where helicopter exhausts are, ie:- top, bottom, side etc..(also very few point straight aft) it would not be practical to produce forward thrust.
Forward thrust is proportional to N1 (gas flow) in a pure jet engine, whereas with a turbo shaft N1 effectively gives you more Torque, so high torque manoevers like heavy hovering would produce nearly as much forward thrust as flying at Max Continuous power. So in the hover situation you would be continually applying aft cyclic to counteract the thrust.
Sorry if this is a bit disjointed, but I think it will give you the general gist.
(Also sorry for the previous reply, I couldn't resist it!!)
Yes, .................... Planes!!!!
Now for a more serious answer.
I guess I am to assume that you are asking about helicopters?
They are designed so as not to provide forward thrust, they are designed to diffuse the exhaust gases.
You must have a convergent duct to produce thrust, if you were to do this, in the example of a Jet Ranger, you would be forcing Red hot exhaust gases at the main rotor blades.
Therefore in helicopter engines the design is such that a divergent or parallel duct is used as the exhaust to minimise this effect.
Also, another point, is that if you were to look at where helicopter exhausts are, ie:- top, bottom, side etc..(also very few point straight aft) it would not be practical to produce forward thrust.
Forward thrust is proportional to N1 (gas flow) in a pure jet engine, whereas with a turbo shaft N1 effectively gives you more Torque, so high torque manoevers like heavy hovering would produce nearly as much forward thrust as flying at Max Continuous power. So in the hover situation you would be continually applying aft cyclic to counteract the thrust.
Sorry if this is a bit disjointed, but I think it will give you the general gist.
(Also sorry for the previous reply, I couldn't resist it!!)
Last edited by HeliEng; 3rd Sep 2005 at 11:35.
No turbine is 100% efficient, therefore the gas stream - having passed through the power turbine - will result in some thrust whilst exiting the exhaust can.
I was led to believe that this could be as much as 10% of the total energy remaining rear of the compressor turbine on the T-55 Lycoming fitted on the CH-47.
I was led to believe that this could be as much as 10% of the total energy remaining rear of the compressor turbine on the T-55 Lycoming fitted on the CH-47.
33 years ago, during Huey conversion, we were told that the T53 produced about 10 lbs of thrust.
Logical, when you consider that the function of the turbine is to extract as much energy as possible from the hot gases, and leave them with enough grunt to just make it outside the engine.
A high-speed exhaust means you have wasted too much energy.
Logical, when you consider that the function of the turbine is to extract as much energy as possible from the hot gases, and leave them with enough grunt to just make it outside the engine.
A high-speed exhaust means you have wasted too much energy.
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TFS,
Asendcharlie has it precisely right, the answer is yes, some jet thrust is apparent, but it is slight. The S-76A has about 100 lbs total at high speed, 50 lbs per engine. Most helos have just a few pounds, below 50.
I was told that the record setting Lynx used all the thrust that the FAI allowed, about 500 pounds, I believe, since they had engine power they could not use due to transmission limits. They necked down the exhausts to increase the thrust and drive the engine temperatures up (at constant output torque).
Asendcharlie has it precisely right, the answer is yes, some jet thrust is apparent, but it is slight. The S-76A has about 100 lbs total at high speed, 50 lbs per engine. Most helos have just a few pounds, below 50.
I was told that the record setting Lynx used all the thrust that the FAI allowed, about 500 pounds, I believe, since they had engine power they could not use due to transmission limits. They necked down the exhausts to increase the thrust and drive the engine temperatures up (at constant output torque).
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So in the hover situation you would be continually applying aft cyclic to counteract the thrust.
Si
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Not if it is a free turbine (if it does then the RN Gazelles are thrusting into the ground as their exhausts point upwards...)
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I am interested to hear these different opinions, as it counterscts everything I have ever been taught as an engineer.
Nick,
I am curious, If my memory serves me right, I thought that the S76A had Allisons fitted. So which way does the residual thrust act in a reverse flow engine?
Nick,
I am curious, If my memory serves me right, I thought that the S76A had Allisons fitted. So which way does the residual thrust act in a reverse flow engine?
Surely it's only what happens in the jet pipe that matters? IIRC some singles have the exhaust offset laterally to give slight assistance with torque control; the AS350 and EC120 spring to mind.
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I've been waiting a long time to find out this one . . . .
Now that we area talking about the exhaust of turbine helicopters,
Does anyone know what VOLUME of air a helicopter engine moves thru it per second?
I understand different engines move different amounts at different power settings, but I have always been curious as the volume of air a PT6-3D or 250-C47 moves at around 90% N1 under given conditions?
Ideally I would like to see the number for and high power cruise, say about 90% N1, of a PT6 or 250-C20 or higher, at sea level in cubic feet or meters,
Then I want to figure out how long it would take an average human being to breathe the same air a helicoptere engine uses in say an hours flight?
I have even asked the PW guy who resides permanently at my hangar and he has said he has no idea?!
Does anyone know what VOLUME of air a helicopter engine moves thru it per second?
I understand different engines move different amounts at different power settings, but I have always been curious as the volume of air a PT6-3D or 250-C47 moves at around 90% N1 under given conditions?
Ideally I would like to see the number for and high power cruise, say about 90% N1, of a PT6 or 250-C20 or higher, at sea level in cubic feet or meters,
Then I want to figure out how long it would take an average human being to breathe the same air a helicoptere engine uses in say an hours flight?
I have even asked the PW guy who resides permanently at my hangar and he has said he has no idea?!
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Good info!
"IIRC some singles have the exhaust offset laterally to give slight assistance with torque control"
Has that exhaust pressure ever been used to assist a puller tail rotor? (So tail rotor not in exhaust stream) Might even be possible to use coanda effect to help suck in cool air, maybe a fenestron design, reducing tail rotor load further. In practice the duct losses would probably lose any advantages in tail rotor power reduction though...
"...how long it would take an average human being to breathe the same air a helicoptere engine uses..."
Might interest you to know that a cyclist's conversion efficiency is determined at 20% (athelete or not). Probably explain's why you get so hot cycling...
Mart
"IIRC some singles have the exhaust offset laterally to give slight assistance with torque control"
Has that exhaust pressure ever been used to assist a puller tail rotor? (So tail rotor not in exhaust stream) Might even be possible to use coanda effect to help suck in cool air, maybe a fenestron design, reducing tail rotor load further. In practice the duct losses would probably lose any advantages in tail rotor power reduction though...
"...how long it would take an average human being to breathe the same air a helicoptere engine uses..."
Might interest you to know that a cyclist's conversion efficiency is determined at 20% (athelete or not). Probably explain's why you get so hot cycling...
Mart
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Back to the basics, everyone. If the air is faster coming out the back than in the front, there will be some thrust. Yes, you would also have to consider density, etc. but that's minimal.
As has been stated already, the engines are designed to spin the free wheel rather than blow gas therefore the engine thrust is kept low.
As has been stated already, the engines are designed to spin the free wheel rather than blow gas therefore the engine thrust is kept low.
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MD520 has the exhaust pointing left to assist anti-torque. (Though it could only be a contributing factor, since I've seen pictures of it in development with the usual 500-style one as well.)
As for how much air flows through - don't know. If you're going to calculate it, don't forget the 250 is a 1000-horse engine - it's just that 600 of them drive the compressor.
As for how much air flows through - don't know. If you're going to calculate it, don't forget the 250 is a 1000-horse engine - it's just that 600 of them drive the compressor.
As you ask BlenderPilot
A starting point for you to consider follows and all figures are in ISA conditions.
The total weight of air going through a PT6A-60A (King Air KA300/B300) at Take-Off power is around 8.96 lb/sec. Air weighs about 2.87 lbs per cubic metre, therefore around 3.12 cubic metres or 110.18 cubic feet per second.
The PT6A-60A embodies a 4-stage axial, single stage centrifugal compressor. The less powerful PT6T-3D/PT6B-36A/B Turboshaft engines have one less axial stage so I would hazard a guess that volumetric flow is probably around 70 to 80% of the 60A.
Saturday afternoon in the sun, how sad am I????????????
A starting point for you to consider follows and all figures are in ISA conditions.
The total weight of air going through a PT6A-60A (King Air KA300/B300) at Take-Off power is around 8.96 lb/sec. Air weighs about 2.87 lbs per cubic metre, therefore around 3.12 cubic metres or 110.18 cubic feet per second.
The PT6A-60A embodies a 4-stage axial, single stage centrifugal compressor. The less powerful PT6T-3D/PT6B-36A/B Turboshaft engines have one less axial stage so I would hazard a guess that volumetric flow is probably around 70 to 80% of the 60A.
Saturday afternoon in the sun, how sad am I????????????
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But then thinking about it a bit more (laterally) the Coander Effect on the NOTAR uses jet thrust from the slots along the tail boom to provide anti-torque thrust.
So in a 'lateral' kind of way, a helicopters gas turbine does produce thrust which may (?) have a forward compoent effect in certain conditions by reducing torque?
So in a 'lateral' kind of way, a helicopters gas turbine does produce thrust which may (?) have a forward compoent effect in certain conditions by reducing torque?
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Helicopter Redeye
Actually the air coming through the slots on the NOTAR aircraft is coming from a fan inside the base of the tailboom and not from the engine exhaust itself.
Also to add some information the Arriel engine in the Astar and S76 A+ move about 5.5 lbs per second of air and produce about 33 lbs of thrust at 100%N1.
Max
Actually the air coming through the slots on the NOTAR aircraft is coming from a fan inside the base of the tailboom and not from the engine exhaust itself.
Also to add some information the Arriel engine in the Astar and S76 A+ move about 5.5 lbs per second of air and produce about 33 lbs of thrust at 100%N1.
Max
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I was looking at the diagram as I wrote it. I guess the fan is driven by the free turbine so is thrust from the free turbine in the engine.
Either way, a digression from the point of thrust from the engine as in a turbofan.
h-r
Either way, a digression from the point of thrust from the engine as in a turbofan.
h-r
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h redeye
the navy gazelle has an exhaust the same as a raf gazelle which point rearwards slightly bent , some of the army ones have a exhaust that goes straight up into the blades to stop heat seeking missiles
the flight manual for the gazelle shows a slower forward speed for the upward facing exaust
quote
Not if it is a free turbine (if it does then the RN Gazelles are thrusting into the ground as their exhausts point upwards...)
ive never seen one of these on a navy gazelle which one have you seen
the fan on the notar is driven from aft transmission not from the free turbine also there is no exaust going down the tail boom
the tail boom is composite the exaust heat would probobly melt it
steve
the navy gazelle has an exhaust the same as a raf gazelle which point rearwards slightly bent , some of the army ones have a exhaust that goes straight up into the blades to stop heat seeking missiles
the flight manual for the gazelle shows a slower forward speed for the upward facing exaust
quote
Not if it is a free turbine (if it does then the RN Gazelles are thrusting into the ground as their exhausts point upwards...)
ive never seen one of these on a navy gazelle which one have you seen
the fan on the notar is driven from aft transmission not from the free turbine also there is no exaust going down the tail boom
the tail boom is composite the exaust heat would probobly melt it
steve
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Simon853 your logic is so very backwards, it brings to question your concept of how a helicopter operates. The idea that the aircraft is a pendulum slung below the rotorhead is an ancient concept that popped up here on pprune a while back (something about the tail rotor being high means it acts at the center of the rotor head or some such thing.
The aircraft is a ball in space, and all forces act on its center of gravity, usually under the mast, and about mid point up the cabin. The jet thrust from the exhaust will try to drive the aircraft forward, so the pilot will have to pull the nose up, with back stick, to counter the thrust with some lift tilted backwards.
Think of the aircraft as a pendulum, hung from the rotor, and you will never get any of this right!
The aircraft is a ball in space, and all forces act on its center of gravity, usually under the mast, and about mid point up the cabin. The jet thrust from the exhaust will try to drive the aircraft forward, so the pilot will have to pull the nose up, with back stick, to counter the thrust with some lift tilted backwards.
Think of the aircraft as a pendulum, hung from the rotor, and you will never get any of this right!