Turbine exhausts...

[TheFlyingSquirrel]

Do turbine exhausts provide any forward thrust in any capacity on any machine ?

[HeliEng]

TFS,

Yes, .................... Planes!!!! ;) :p ;)







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!!)

[Hilife]

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.

[Ascend Charlie]

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.

[NickLappos]

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).

[Simon853]

So in the hover situation you would be continually applying aft cyclic to counteract the thrust.


But you'd need forward cyclic to counter the tendency of the exhaust to push the pendulum forward, wouldn't you?

Si

[helicopter-redeye]

Not if it is a free turbine (if it does then the RN Gazelles are thrusting into the ground as their exhausts point upwards...)

[HeliEng]

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?

[Droopy]

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.

[BlenderPilot]

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?!

[Graviman]

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

[Matthew Parsons]

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.

[Hilico]

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.

[Ian Corrigible]

Blenderpilot,

As it happens, I ran the numbers for an Allison C47 a couple of months back. At 90% Ng, I came up with 4,300 CFM / 121 PSI.


I/C

[Hilife]

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????????????

[helicopter-redeye]

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?

[maxtork]

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

[helicopter-redeye]

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:)

[md 600 driver]

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

[NickLappos]

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!

[Ian Corrigible]

While today's NOTAR system uses a stand-alone fan, Bell's propulsive anti-torque system (PATS) concept, proposed for UCAR and being considered for MAPL, would use PT airflow. PATS would add an LP fan to 'wrap' the PT stream with a cooler airflow, resulting in higher mass flow and lower exhaust temps.

I/C

[BlenderPilot]

Another crazy thought,

Mr. Lappos said the S76 engines produce from the exhaust about 100 pounds of thrust a HIGH SPEED,

but the thrust the engine is producing to give 100 pounds of thrust at HIGH SPEED, I imagine could produce a lot more thrust in a hover, a LOT more.

I'm not really sure how to explain this, when I learned to fly jets (business jets) It was explained to me that suppose you were standing on the brakes of a Lear on a runway, then went to TAKEOFF power setting of say 97% N1, the engine would be producing a "jet" of air that had a velocity of say 50 ft/sec, since the air surrounding the engine exhaust was near zero the, the difference between the 50 ft/sec "jet" of air and the surrounding relatively still air is what gave you thrust.

Then as you let go of the brakes and started to accelerate down the runway, with THE SAME POWER SETTING and N1 speed, what would happen when the aircraft reached an equivalent speed to 50 ft/sec?

If the velocity of the "jet" of air coming out of the turbofan would equal that of the surrounding air, you would theoretically stop having thrust and thus accelerating.

To solve this I know as the aircraft accelerates, more air is pushed inside the engine, more air goes thru the engine and thus the thrust increases, without moving the power setting, so in reality when the aircraft is already accelerated to 50 ft/sec, the engine is now producing a "jet" of air much higher than 50 ft/sec to keep producing thrust, say now 100 ft/sec, BUT THE INTERESTING PART IS THAT IT'S WITH THE SAME 97% N1 POWER SETTING!

So now a turbofan engine that was producing a STATIC 4,000 lbs. of thrust is now producing twice that because of the all the air that is being rammed into the inlet at high speed, a turbofan's engine inlet has the benefit of being exposed to the relative wind, and its easy to understand how this happens, but a helicopter with its particle separators, or ducting is not subject to same, or is it? Think of the Bell 212/412 inlets that actually have to suck air away from the particle separator duct?

So my point is that if an S76 produces 100 pounds of thrust at HIGH SPEED, with a power setting of say 90% N1, without the benefit of much ram air (remember particle separators, and indirect inlet) increasing considerably the amount of air going try the engine, and thus exhaust. Then the amount of thrust produced at a hover with the same 90% N1 power setting would be a lot more than 100 pounds according to my uneducated guess, remember you are a hover and the air surrounding the engine outlet is almost perpendicular to the exhaust due to downwash.

I'm sorry if my explanation isn't good but English is a second language to me and its sometimes had to explain myself, feel free to tell me I am crazy for thinking this.

Ian, Hilife,

Thanks so much for the reply, I will need some time to digest that information.

[maxtork]

Blender Pilot

Remember "high speed" for that S76 is a bit different then the high speed of a jet. Vne is only about 150kts (Ha only...I'm used to flying helicopters where Vne is about 100kts lol). Also the inlets on the S76 are not that bad...they are fairly direct and there is no particle seperators.

Max

[helicopter-redeye]

ive never seen one of these on a navy gazelle which one have you seen

Five from memory, all flown in cammo by the Royal Marines (which is part of the Navy?) So it was prob. a AH1 rather than an HT2 or 3.


Thanks for clarifying what drives the fan. I've never seen a diagram that shows the linkage but I assumed it was engine powered rather than an electric motor ...

[md 600 driver]

i think from memory the marine ones are a ht4 or ah4

a link to notar explained
http://www.kulikovair.com/Notar2.htm



steve

[helicopter-redeye]

You know, when you look at the diagram, you can see how a chap can get confused. It looks like an jet outlet pointing down the tail cone.

;)

[Hilife]

BlenderPilot

When you next see your business jet instructor, kick him in the Christmas Crackers if you would.

It is a common misconception that thrust in a jet engine is caused by the reaction of the exhaust gas stream on the ambient air, it is not. If this were true, then how do rockets propel themselves in space where there is no atmosphere?

There are a multitude of thrust forces acting within the engine at every stage, for arguments sake let’s stick to the exhaust/nozzle section and subsonic air flow.

The best basic example I have seen to explain thrust is the classic party balloon.

Imagine if you will blowing up a balloon and tying a knot around the teet so no air escapes. The higher pressure air within the balloon trying to escape is acting with equal force on the entire inner surface of the balloon and therefore opposing forces cancel each other out and the balloon remains stationary.

Untie the knot and let go of the balloon and it shoots off in a direction OPPOSITE the opening at the teet. NOT because the escaping air is pushing on the ambient air outside but because the forces exerted on the inner wall OPPOSITE the now open teet are greater than at the exit hole where there is not longer a surface for the pressure to act upon. (Newton’s third law of motion)

A jet propulsion engine works on exactly the same principle. The jet engine expels high velocity higher mass air rearwards thereby exerting a force not on the ambient air but on the area OPPOSITE the open exhaust outlet, this being the rear of the turbine.

The corporate jet you learnt to fly in would have embodied a pitot style inlet and by moving forward through the air you create a ram effect increasing inlet air pressure further still. The higher inlet pressure corresponds to higher pressure air through the entire engine cycle therefore engines can benefit from this (think supercharging). But I think I am correct in saying that without this inlet ram effect, thrust actually decreases with increased forward speed so this benefit merely maintains thrust levels and would not double them as your instructor suggests, but I may be wrong here.

Regarding S-76 thrust I’m really rattling my brain here. Firstly, the A and C Series have the inlet at the front in the inlet plenum; the B has its inlet at the rear and does not have a particle separator as such but bypass doors which are usually extended when hovering close to the ground. I would hazard a guess that neither design has any noticeable inlet ram effect even at high forward speeds.

You have referred to an N1 (NG Gas Generator speed) of 90% for both scenarios – hover and forward flight. Assuming that there were no gas losses in the compressor side (bleed valves closed and cabin services off), the volume of air passing through the engine would be the same, therefore I would assume that exhaust gas thrust would be the same.

However, if both scenarios resulted in different Tq settings, then it is fair to suggest that NG/Nf and ITT readings would differ and therefore exhaust gas thrust would vary accordingly.

As a result of my thoughts above, I too may well deserve a kick in the Christmas Crackers, but lets hope not, it is Sunday after all.

[BigMike]

I think the off-set on the NOTAR exhaust was more to do with the airflow around the tail-boom pushing the hot ehaust down on to the ground (fire hazard), during early testing with the standard 500 setup.

BM

[Reflex]

Regarding the 76.

The intake and particle separator on the B model directs the air into the intake with the EAPS off and effectively slightly pressurises the intake area.

As I understand it, the intake area of the A & C models becomes pressurised above 40 - 50kts.

[SilsoeSid]

I hope this tidies up any loose ends;

http://www.geocities.com/pprunessilsoesid/notar.jpg

Using the natural characteristics of helicopter aerodynamics, the NOTAR® anti-torque system provides safe, quiet, responsive, FOD-resistant directional control. The enclosed variable- pitch composite blade fan produces a low pressure, high volume of ambient air to pressurize the composite tailboom. The air is expelled through two slots which run the length of the tailboom on the starboard (right) side, causing a boundary-layer control called the “Coanda Effect.” The result is that the tailboom becomes a “wing,” flying in the downwash of the rotor system, producing up to 70 percent of the anti-torque required in a hover.
The balance of the directional control is accomplished by a rotating thruster.
In forward flight, the vertical stabilizers provide the majority of the anti-torque, however directional control remains a function of the jet thruster.

Source (http://www.mdhelicopters.com/rotorcraft/models/MD900_Technical.htm)

http://www.geocities.com/pprunessilsoesid/AirlinersNetPhotoID533238.jpg


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