GScholz more ont turbo props:

[rshubert]

Originally posted by hitech
Can some one describe exatly how torque is transmited to an airframe on a high bypass turbofan?

HiTech

via the hot section of the engine.  The torque produced there is transmitted to the engine mounts, and to the airframe.

Think of it like this:  The hot section pushes power to the cold section via whatever coupling is used, right?  Thus the POWER used by the fan is equal to the POWER generated in the hot section.

Torque of a rotating object is this:

HP * 5250/RPM

IF we hold horsepower constant, then Torque is inversely proportional to RPM.  In other words, as speed goes up, torque goes down.  This is true in any constant-horsepower application of a rotating machine, such as a turbine or electric motor.

Obviously, in a constant HP situation (power) such as the turbofan we're talking about, the torque at the hot section (rotating very fast) is lower than the torque in the fan (rotating much more slowly).  Remember, the power is the SAME.

So, it becomes obvious that while the power transmitted from the fan throught the thrust bearing is the same as the power transmitted through the coupling to the fan, the torque transmitted to the airframe from the hot section is much lower than the torque transmitted by an equal horsepower--but much slower running--piston engine.  The same is true for a turboprop or a turbojet, by the way.  High HP produced with high rotational speed produces less torque than the same HP produced with low rotational speed.

Remember, it's the ENGINE that transfers the torque, not the prop.  Forget all the propellor torque calculations--they only work for engine torque when there is no gear reduction of any kind.  Any other time, you have to think in terms of POWER, and find the engine torque.  That is the torque that is transmitted to the airframe.



shubie

[g00b]

My 1st mis-conception was this.

IF you had a direct-drive turbofan, with no intermediate gearbox between the compressor and the propellor there would be no torque applied to the airframe, with the exception of stator induced torque and mechanical losses in bearings and such.

Correct?

Now you place a gearbox between the compressor and propellor. In order for the gearbox to do it's job it must apply a torque to the airframe to turn high-RPMs and low-torque into high-torque and low-RPMs.

Correct?

I visualize it like this.

Direct Drive: apply a rotational force to the compressor blades, measure at the propellor, should be 100% minus any frictional losses.

10:1 Geared Drive :  apply a rotational force to the compressor blades, measure at the propellor, should have 10 times the torque at 1/10 the RPM. 90% of the torque is going to have to be balanced by the airframe. If you were to spin the compressor while holding the propellor, the plane is going to spin around. i.e. torque.

So... it's really the gearbox that applies the torque to the airframe. There is of course some effect from the stators and compressor blades as well as propwash and various mechanical losses but all those are probably pretty minor compares to whatever mechanical advantage you are getting out of the gearbox.

g00b

[Tails]

By jove, I think he's got it!

[rshubert]

Originally posted by g00b
My 1st mis-conception was this.

IF you had a direct-drive turbofan, with no intermediate gearbox between the compressor and the propellor there would be no torque applied to the airframe, with the exception of stator induced torque and mechanical losses in bearings and such.

Correct?

Now you place a gearbox between the compressor and propellor. In order for the gearbox to do it's job it must apply a torque to the airframe to turn high-RPMs and low-torque into high-torque and low-RPMs.

Correct?

I visualize it like this.

Direct Drive: apply a rotational force to the compressor blades, measure at the propellor, should be 100% minus any frictional losses.

10:1 Geared Drive :  apply a rotational force to the compressor blades, measure at the propellor, should have 10 times the torque at 1/10 the RPM. 90% of the torque is going to have to be balanced by the airframe. If you were to spin the compressor while holding the propellor, the plane is going to spin around. i.e. torque.

So... it's really the gearbox that applies the torque to the airframe. There is of course some effect from the stators and compressor blades as well as propwash and various mechanical losses but all those are probably pretty minor compares to whatever mechanical advantage you are getting out of the gearbox.

g00b

NOOOOOnononononononononononon ono!

There is ALWAYS torque applied to the airframe.  ALWAYS.  It is equal to the torque produced by the engine.  NOT THE PROPELLOR or fan section or whatever.  See my post above that explains it.

Take a plane, such as a P-47.  Put a 2000 HP P&W radial on it.  Turn said engine at 2700 rpm, and go 400 mph.  Can we agree that the torque is 2000*5250/2700, or 3889 ft.lbs?

Same plane, same weight, same aerodynamics.  Replace the engine with an Allison turboprop engine, hot section RPM 16000 rpm, producing 2000 HP, and the plane is going the same speed.

Now, the torque is 2000*5250/16000, or 657 ft.lbs of torque.

The propellor torque (assuming we used the same prop and the same prop RPM) is the same in both cases.  Remember, the prime mover--not the power absorbing device--is what transmits torque to the mount.  The propellor is beating on the AIR.  The engine is beating on the PLANE.

[g00b]

hrmmm, I don't see how a direct drive turbofan could produce torque. Nor do I see how the same propellor driven at the same speed could apply a different amount of torque to the airframe no matter what the power source is.

HiTech said it right from the beginning, it just didn't sink in right away.

g00b

[Tails]

When you slice away all the fancy names, a high bypassing fan is really nothing more than a ducted turboprop with a LOT more blades on the 'propeller'. Direct drive or no, if a turbine engine is turning something, including itself, it is going to make torque, with one exception:

Multiple compressor/turbine sections that contra-rotate. To brake it down into something a pilot can understand, if your N1 (low-pressure compressor /w associated turbine disc) turns one way, and the N2 (high pressure compressor, /w associated turbine disc) turns in the other, the toque forces counter and the engine transfers no torque to the air frame.

This is assuming a perfect world of course. Now I just need to dig into my books and see which engines actually do this, I know there's a few....

[rshubert]

Originally posted by g00b
hrmmm, I don't see how a direct drive turbofan could produce torque. Nor do I see how the same propellor driven at the same speed could apply a different amount of torque to the airframe no matter what the power source is.

HiTech said it right from the beginning, it just didn't sink in right away.

g00b

Remember, it's not the PROP that transmits torque, it's the ENGINE.  Big difference, man.    That's a very important concept, here.  Goob, you are hung up on one issue.  You think "torque" is power.  It isn't.  Power is power, torque is force.  They are different.

Another issue I see here is a lack of a gut level understanding of Newton's Laws.  I'm sure you studied them in school, everybody does, but applying them is sometimes not so obvious.  Remember this:  For every action, there is an equal and opposite reaction.  If you believe and understand that law, the rest becomes apparent.  Newton's Laws are immutable constants--that's why they aren't called "Newton's suggestions" or Newton's recommendations".

Don't feel bad about this lecture, and don't feel like I am flaming you or anybody else--remember that this stuff is not obvious, and can be difficult to grasp, since it can't be tasted, felt, or smelled.  I suggest you follow the link below, and get a good handle on the basic physics involved here.  Then, you will see the light!

http://science.howstuffworks.com/fpte.htm

[rshubert]

Originally posted by Tails
When you slice away all the fancy names, a high bypassing fan is really nothing more than a ducted turboprop with a LOT more blades on the 'propeller'. Direct drive or no, if a turbine engine is turning something, including itself, it is going to make torque, with one exception:

Multiple compressor/turbine sections that contra-rotate. To brake it down into something a pilot can understand, if your N1 (low-pressure compressor /w associated turbine disc) turns one way, and the N2 (high pressure compressor, /w associated turbine disc) turns in the other, the toque forces counter and the engine transfers no torque to the air frame.

This is assuming a perfect world of course. Now I just need to dig into my books and see which engines actually do this, I know there's a few....

You're partially right.  In the case above, the engine itself absorbs the torque loads.  I'll bet there are some serious thrust bearings on those compressor sections.

[Tilt]

Turbo fan or piston engine there is somewhere a spinning shaft that is geared up/down to a rotor.................

Given the same rotor and the same footlbs at its tip then the opposing force applied to the mounting system is the same.

[GScholz]

Will this never end?

1. Propellers do not create torque, crankshafts and pistons do.

2. An engine does not need to apply torque on the airframe to produce thrust ... rocket engines don't.

3. If the engine produces thrust by balancing torque forces, no torque is applied to the airframe (which is what I believed turboprops did ... now I don't know what to believe).

[Holden McGroin]

Torque is an element of spinning power. Rockets do not spin, hence no torque.  Turbines spin, hence torque is present in the turbine.

In a turbojet, the turbine powers the compressor and all the compressor gas goes thru the turbine.  The torque of the compressor can balance the torque of the turbine, so the net torque to the airframe can be near zero. (Though spinning the same way, the turbine torque is the opposite direction of compressor torque because the turbine is removing power from the airstream and the compressor is adding power to the airstream)

The power is from expanding combustion gasses and the turbine takes a bit of that power and spins the compressor to keep the brayton combustion cycle giong.

In a turboprop, the power of the engine spins a prop which imparts power to the airstream.  That requires that there be rpm and torque at the prop, and that torque must be counteracted in the engine mount.

A turbo fan works much the same way as a turboprop, the horsepower of the bypass airstream is imparted by a spinning rotor disk;  hence rpm and torque.

Turbine and compressors work with spinning blade disks (rotors) and static blade disks (stators) which could be analogus to pistons and cylinders.  The moving piston (rotor) reacts with the static cylinder (stator) to react with expanding or compressing gas to create torque and rotational speed.

[GScholz]

Originally posted by Holden McGroin
In a turbojet, the turbine powers the compressor and all the compressor gas goes thru the turbine.  The torque of the compressor can balance the torque of the turbine, so the net torque to the airframe can be near zero. (Though spinning the same way, the turbine torque is the opposite direction of compressor torque because the turbine is removing power from the airstream and the compressor is adding power to the airstream)

In a turboprop, the power of the engine spins a prop which imparts power to the airstream.  That requires that there be rpm and torque at the prop, and that torque must be counteracted in the engine mount.

How does the propeller transfer the torque to the engine mount if there is no mechanical connection?

Originally posted by GScholz
I thought up a simple home experiment for those fortunate enough to not live in the arctic. Take a shaft (wood pole etc.) and attach a bearing to it with a handle on it ... so that you can hold the shaft while turning it.

You should now have a handle with a bearing on it with a shaft running through it.

On one end of the shaft you mount a small propeller. On the other end you mount a small fan/turbine.

Now take your garden hose and mount it on the handle/bearing so that the nozzle points in the direction of the fan/turbine (actually you should use at least two nozzles to ensure a balanced application of force on the turbine, but what the heck). You now have a very primitive jet turbine powered by tap water rather than expanding air.

Turn on the water and hold this construction by the handle.

The water will deflect off the turbine blades and make the turbine rotate in the opposite direction. Since the turbine is mounted on the shaft it will turn the shaft, and since the propeller is mounted on the other end the propeller with turn also.

You now have a turning propeller creating power, but not torque, since your hand is not mechanically connected to any moving parts at all ... except you are holding around the shaft. The only torque I can see this will create is from friction in the bearing.

See? Turning prop, forward propulsion, no imparted torque on the structure. The torque is countered (and created) by the deflection of the water.

[MiloMorai]

Originally posted by GScholz
How does the propeller transfer the torque to the engine mount if there is no mechanical connection?

What are those tubes from the gear case attached to?

[Holden McGroin]

How does it work w/o connection? same way a cylinder head pushes a piston down with force but with no mechanical connection. Expanding gasses shove against a static cylinder head, and a moving piston.

The airfoils which are the stators in the turbine have a fluid dynamic differential pressure associated with the hot gas flowing past them.

One side it high pressure due to Newtonian ram effect, the other side is low pressure due to Bernolli's equation.

This differential pressure on the stator ring is connected to the turbine casing. The casing then has a force (torque) that needs to be countered in the aircraft structure.

In your water turbine, you will put into the engine a certian amount of thrust, associated with the impulse of the tap water nozzle against the turbine.  You will get the same amount of thrust minus the mechanical inefficiency of the prop, turbine, and bearings.  You still need to absorb the impulse of the water jet in the structure of the boat, so your action has a reaction.  

If the turbine you use is a pelton wheel which would be appropriate, the nozzles would be pushing 90 degrees to the shaft at the pelton wheels radius, and the force of the water impulse at a radius is torque.

[niklas]

Hitech is correct.

The sum of all forces must be zero, the sum of all moments must be zero. Always. Even in acceleration where you have to deal with virtual forces by the "heavy" mass (F = m*a)

So when there´s an imbalance of moments ( prop´s moment being higher due to reduction gearbox compared to the turbine shaft moment), where´s the difference gone that makes the sum zero? It´s the imbalance on the gear wheel, axis distance in the gear box comes into the game and so on. At the end it will result in an moment to the frame.

niklas