Another hornets nest

[hitech]

Golfer:

All 4 engines are off the longitudnal center of the airframe torque is not a factor. I cant see 4 engines trying to roll over an airplane, maybe P-factor or Yaw but not torque. But P-factor is Yaw what am I saying.

Unless I miss read this,It states moving the engs off center line makes torque not a factor.

GScholz: Every action requires equal and oposit reaction.

Obviously there is torque at the prop shaft turning the prop one way . The plane has to have the same torque rolling it the oppist way of the prop.

Now there might be a small amout of torque generated the oppisit way by the gas spining as it exits from the turbin, but I have never looked into it that effect.

HiTech

[GScholz]

Originally posted by hitech
GScholz: Every action requires equal and oposit reaction.

Obviously there is torque at the prop shaft turning the prop one way . The plane has to have the same torque rolling it the oppist way of the prop.

Yes, every action has an equal and opposite reaction. A piston engine is like a rifle firing a piston that turns a crank (thereby generating torque). A jet turbine is like a recoilless rifle blowing an expanding gas out one end to push something in the other direction. The way I picture it, the plane structure is simply "holding" the propeller/fan shaft with a propeller spinning in one end and turbines spinning in the other end, being spun up by an expanding gas blowing through the turbines. The expanding gas leaving the exhaust pipe is the equal and opposite reaction to the turning of the shaft. Of course if you then channel this force through gears that use the plane as leverage it will produce torque on the plane structure.

That's how I understand it.


Another way of creating propulsion without torque is having a small jet engine on each prop/rotor blade.

[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.

[Golfer]

double post

[Golfer]

Originally posted by hitech
Golfer:

 

Unless I miss read this,It states moving the engs off center line makes torque not a factor.

HiTech

Thx for the snip Hitech,

The way I viewed that was it takes away the rolling moment of torque, which it does because it is not on the longitudinal axis anymore.  Think of it this way, your RV on climbout will want to not only yaw to the left, but roll too because the engine produces the torque to make it want to roll left.  In a twin, this isn't really a factor because that torque is moved away from the longitudinal axis and doesn't cut through the Center of Gravity anymore...no more rolling due to torque.

The yawing tendency is still there due to Pfactor and so on, but engine torque will not make the airplane require aileron trim.  Back to your RV on departure, you hold some right rudder to center up the ball and maybe a little right aileron to keep the wings level in a level climb.  If you had a twin RV8 (hmm...maybe I'll start designing, turbodiesels!) with conventional engine installations, you'd have a left yaw tendency but no left roll tendency.

What this picture is supposed to show is the "O"as the CG, the Prop ahead of the fuselage and the logitudinal axis of the airplane.  This is a single:
__^__
. l  I  l
. l  I  l
. l  I  l
. l  I  l
. l O  l


Now, In a twin,  you do not have engines producing torque to make the airplane roll because the torque just doesn't have any way to twist the whole airframe anymore, due to it not being lined up with the CG

Masterpiece of lines and dots:
Dots symbolize thrust line, l's show fuselage and logitudinal axis.


__^__     __^__
.    .   l   l   l   .
.    .   l   l   l   .
.    .   l   l   l   .
.    .   l   l   l   .
.    .   l   O  l  .


Doh!  wonder why these pictures aren't lining up?
-Put in another row of dots all the way at left, disregard them.

[hitech]

GScholz: I agree with you water assement, but somthing is different in the turbin engine. In the water experment the torque or force is transmited to the slowing or changing direction  of the water.  

So lets change this experment slightly. Same setup but this time you are standing on the platform with the bearing and holding the hose. Now there would be a torque transmited from the water exiting the hose.

Where is it transmited to when burrning the fuel? Im scratching my head on this one trying to see where all the forces are going.

And btw turbo props do go have gear boxes between the turbin and the prop.

HiTech

[GScholz]

Originally posted by Golfer
Now, In a twin,  you do not have engines producing torque to make the airplane roll because the torque just doesn't have any way to twist the whole airframe anymore, due to it not being lined up with the CG

Have you ever held a pen or pencil between two fingers (like a cigarette) and wiggled it back and forth (creating a pleasant feeling of torque between your fingers to distract you from a boring lecture)? If you moved the pen off centre were you then unable to wiggle it?

Of course not.

A wing mounted engine will twist the airframe around the engine's crankshaft just like a fuselage mounted engine. Multiple engine will try and bend the structure between them. This will absorb some energy (since the structure will bend somewhat), but it will hardly cancel out the engine torque.

[Golfer]

Have you ever held a pen or pencil between two fingers (like a cigarette) and wiggled it back and forth (creating a pleasant feeling of torque between your fingers to distract you from a boring lecture)? If you moved the pen off centre were you then unable to wiggle it?

Shifting to practical experience:

My conventional twin time includes a majority of time spent in PA-23 series airplanes, though some King Air B100 time.

Since i've been a mere decoration in the cockpit of the B100 and have no formal training, its not being used in the example.

As far as the PA-23 (apache and aztec) go, I don't touch the aileron trim tab.  I use the rudder trim regularly, but that big black handle above my head never gets touched.

In theory, in zero gravity and no other forces acting on the airframe, the engine torque would be able to roll the airplane.  In practical application, it just doesn't happen when airspeed and gravity are factored in.  When on the ground, the weight (gravity) of the airplane keep the engines from torquing left...because they just don't produce enough oomph to torque a 4000lb airplane.  In flight, airspeed which is a stabilizing force, won't let them either.

In space...in theory.  We're not in space.

[hitech]

Golfer: That is incorect, moving the engines off center line or making 2 of them, does not change torque in the roll axis.

Torque is torque no mater what point it is applied to an object.

Your description of flight experience that not much action is required to correct for the torque can be accurate. But it dosn't change the fact that it is not because the engines are out board.

The torque is exactly the same as if they were both on center line.

HiTech

[GScholz]

Originally posted by hitech
GScholz: I agree with you water assement, but somthing is different in the turbin engine. In the water experment the torque or force is transmited to the slowing or changing direction  of the water.  

So lets change this experment slightly. Same setup but this time you are standing on the platform with the bearing and holding the hose. Now there would be a torque transmited from the water exiting the hose.

Where is it transmited to when burrning the fuel? Im scratching my head on this one trying to see where all the forces are going.

In a jet turbine the torque/force is transmitted to the slowing or changing direction of the air moving trough the turbine. Just like with the water hose, when burning fuel the force (expanding gas) is directed aft (giving additional thrust) through the exhaust pipe of the jet engine. Since a jet-turbine is rotating and balanced no other unchecked force is created, except for gyroscopic forces.

Originally posted by hitech
And btw turbo props do go have gear boxes between the turbin and the prop.

If these gears use the aircraft as leverage it naturally will create torque on the aircraft (like on helicopters). I'd have to see how it's set up to get an idea of where the forces would go.

[joeblogs]

Maybe I'm being stupid, but why isn't this just a response to unequal lift across the wings?

I know that if I take an overloaded F6f off a CV and don't get fast enough off the deck, that plane is going to roll left unless I counter.

I always assumed one wing was stalling before the other, especially because it doesn't happen until after you gain some altitude, even if it is just a few feet.

This is less likely if I use rudder trim. Perhaps it's the yaw that determines the wing drop in a partial stall?

-Blogs

[hitech]

GScholz: I assume we aggree that you can not change a linear force to a torque with out there being somthing taking the  same torque in the oposit direction?

For there to be no torque on the airframe it would have to be accounted for in the mass of the spinning air as it exit's the engine.  Now im not postive this isn't the case but if it was wouldn't jets have torque?

HiTech

[Golfer]

Originally posted by joeblogs
Maybe I'm being stupid, but why isn't this just a response to unequal lift across the wings.

I know that if I take an overloaded F6f off a CV and don't get fast enough off the deck, that plane is going to roll left unless I counter.

I always assumed one wing was stalling before the other, especially because it doesn't happen until after you gain some altitude, even if it is just a few feet.

-Blogs

The left roll you will see is due to all the left turning tendencies inherent with a conventional airplane.   A major player in this case is Engine torque.  You're heavy, low, slow with a high angle of attack, engine torque is at its worst here and thats why you need to counter with aileron and rudder.  F4U's have particulary nasty torque and more than a few have become red marks on the back of a carrier.

I'm also not saying that Torque isn't there anymore, it just won't roll the airplane with regards to multiengine airplanes because it doesnt have the leverage to do anything with its torque.

When the engine is moved from the longitudinal axis...i.e. a twin.  This just makes the engine unable to produce a roll movement with torque.  It won't do it.  Torque is still there, but it won't roll the airplane.  Its there in yaw, but not roll.

[hitech]

When the engine is moved from the longitudinal axis...i.e. a twin. This just makes the engine unable to produce a roll movement with torque. It won't do it. Torque is still there, but it won't roll the airplane. Its there in yaw, but not roll.

Gofer this satement is inacurate, Not sure how to convince you the roll torque generated does not change based on where the engine is. It realy is just a physics 101 question.

HiTech

[GScholz]

Originally posted by hitech
GScholz: I assume we aggree that you can not change a linear force to a torque with out there being somthing taking the  same torque in the oposit direction?

For there to be no torque on the airframe it would have to be accounted for in the mass of the spinning air as it exit's the engine.  Now im not postive this isn't the case but if it was wouldn't jets have torque?

HiTech

Yes I think we agree on this one Sir. To my knowledge jet-turbine engines do not produce torque on the engine/airframe.

Edit: Turbine spinning in one direction, air in the other. The forces that create these motions cancel each other out.