GScholz more ont turbo props:

[hitech]

Golfer:
Study this drawing:




It realy is showing how to convert torques to forces and forces back to torque. And why the torque about the CG is just the sum of all torques.


HiTech

[Straiga]

Hitech did you get all that paper work I sent you. If you didnt get it I will fax it to you. Then we can have a better discussion about torgue not being a factor and Im sending more.

The XP-38 actually had conventionally-handed counter-rotating engines, but it was found that the slipstream over the centre-wing section disturbed the airflow and lead to elevator buffeting, so from the YP-38 aircraft onwards the direction was reversed.

HoHun where is this info your talking about.

Straiga

[Straiga]

Lets look at hitechs picture a second. Put the landing gear down when the airplane is sitting on the ground and if you started the right engine and went to full power torque is present and the right wing that mounted to the fuselage will tip down oposit torque do we all agree at this point. Now when this wing moves down it will tend to raise the left landing gear of the ground.

Now if we just started the left engine only the left wing that is mounted to the fuselage will move up. This will tend to lift the right landing gear of the ground.

With a multi-engine with both props turning in the same direction each wing will roll in the same direction, But when you mount a fuselage to both wings this rotation is stopped. This roll moment is then translated into a yaw moment.

If the right engine is running and release the brakes the plane will turn to the left. If the left engine is running only it will have a harder time trying to turn to the left. But either way its a yawing moment. So why doesnt a airplane roll over on its back when sitting on the ground holding the brakes with both engines running a full power. It doesnt does it.

There are other forces at work, the length of the wing, engine horse power, how big of a rudder and vert stab. How big of a horizontal stab and elevators, gross wieght of the airplane, airspeed, desinty altitude where the engines are mounted to the wing, and also how far they are from the CG and what ind of leverage they produce. All of this comes together to make airplane and will determine how it will fly.

Ok you would think in a single engine airplane that using aileron to counter torque would be the same for a multi-engine but its not. Especially when you have an engine failure, using a lot of aileron only pronounces the problem (MAKES IT WORSE) so what about this. When you have an engine out, the wing with the good engine has prop wash crossing that wing, while the other is just getting relative wind. If the plane slows where the left wing stalls the right wing with the good engine because of assmetric lift will roll the plane to the left. So by adding more aileron to counter this roll it will stall the wing faster. But not from torque roll effects. Just from an unequal lift between the two wings. One wing stalls the other does not so what happens next a spin. Whats a spin a yawing moment about an axis. This is what happens. Torque if any has little to do with this.

Straiga

[jigsaw]

Something that I don't get about HT's drawing that one of ya'll might be able to explain;

What are the -10 and -12 representing?
If you're measuring the arm, shouldn't that still be a positive number?

[HoHun]

Hi Straiga,

>HoHun where is this info your talking about.

It's from "P-38 Lightning in action" by Larry Davis.

Regards,

Henning (HoHun)

[HoHun]

Hi Straiga,

>Put the landing gear down when the airplane is sitting on the ground and if you started the right engine and went to full power torque is present and the right wing that mounted to the fuselage will tip down oposit torque do we all agree at this point.

Hm, actually I'd say revving up the right engine on the ground will try to force the left wing to go down, loading up the left landing gear leg.

>Now if we just started the left engine only the left wing that is mounted to the fuselage will move up. This will tend to lift the right landing gear of the ground.

Revving up the left engine will give the same result as revving up the right engine.

>With a multi-engine with both props turning in the same direction each wing will roll in the same direction, But when you mount a fuselage to both wings this rotation is stopped. This roll moment is then translated into a yaw moment.

Hm, I disagree on the fuselage bit. A flying wing would behave just the same with regard to torque.

>If the right engine is running and release the brakes the plane will turn to the left. If the left engine is running only it will have a harder time trying to turn to the left. But either way its a yawing moment.

But that's the result of thrust, not of torque.

>So why doesnt a airplane roll over on its back when sitting on the ground holding the brakes with both engines running a full power. It doesnt does it.

In Hitech's picture 2, it will try to roll over onto the back. If you add landing gear legs (say at 5 ft out from the centreline), you'd find that the left landing gear gets loaded up with an extra 20 lb(f) with both engines revving at full power. (Puny engines Hitech gave us! ;-)

>But not from torque roll effects. Just from an unequal lift between the two wings.

Ah, good point - I hadn't thought about that :-) But that's with one engine out - with both engines running there is a torque-induced roll tendency in a twin just as for a single, or even for a twin when one engine is out (though aileron obviously is not the way to compensate for it :-)

Regards,

Henning (HoHun)

[HoHun]

Hi Jigsaw,

>What are the -10 and -12 representing?
>If you're measuring the arm, shouldn't that still be a positive number?

That's just a convenient way of representing the direction of the arm. Obviously, pushing down the left wing or pushing down the right wing will have the opposite effect, so an arm to the left of our reference point given a negative length to represent that.

Regards,

Henning (HoHun)

[Straiga]

Ah, good point - I hadn't thought about that :-) But that's with one engine out - with both engines running there is a torque-induced roll tendency in a twin just as for a single, or even for a twin when one engine is out (though aileron obviously is not the way to compensate for it :-)

Hmm so would you say rudder is the primary flight control used to countering this effect?

The other points I still disagree with you though, But we will get you to see the light.

When you fly any single and multi-engine in AHII with auto pilot on you see aileron trim set to the right but no rudder trim is used at all. This is a adverse aileron yaw situation why is rudder not used to counter this yaw. Also If you let a plane takeoff in auto pilot and go eat and come back a half hour later the plane has made a 90 degree left turn in course heading. I believe this is also due to adverse aileron yaw with no rudder. I would think that the plane would track straight out from takeoff with a no wind condition if rudder was used to control direction. Remember that anytime aileron is used so must rudder.

Im still insist that in flying multies I have not used aileron to counter any torque or have experienced any torque roll tendincies.

Straiga

[hitech]

Havn't recieved the images yet. Sent you our fax number.

And to brush up on your physics:

Scanned from Tipler Physics (colage physics book): Pages 322 & 323

Key piece is in the description of Figure 12-25.

The couple exerts the same torque FD about ANY point in space.

 




Hitech

[Straiga]

Hitech I re e-mailed you that paper work and sent it to others they got it did you?

Straiga

[HoHun]

Hi Straiga,

>Hmm so would you say rudder is the primary flight control used to countering this effect?

Yes, certainly. Ailerons have unwelcome side effects, as discussed for the single-engine engine.

>The other points I still disagree with you though, But we will get you to see the light.

Thanks for the confidence :-) This thread has already been quite instructive for me because it's difficult for a non-pilot to see the three-dimensional chain of reactions around the different axes of an aircraft as clearly as a pilot does.

Maybe I should emphasize here that the torque I'm expecting from a multi-engine aircraft doesn't really mean that the pilot is struggling with large roll moments, but rather that torque is one of several forces that are in balance when the aircraft in smoothly flown while the pilot only perceives the necessity for a yaw.

Torque is not so much noticable in its direct, but in its indirect effects. That's why I consider the yaw pilot-induced, though a pilot might see it as inevitable result of the torque - which of course is true since the other option would be to let the plane roll out of control and crash.

>Im still insist that in flying multies I have not used aileron to counter any torque or have experienced any torque roll tendincies.

Hm, maybe the P-factor effects requires rudder input anyhow so that you automatically counter the torque roll in the same action? I figure smooth flying might make the torque virtually unnoticable because it's only one factor in an equilibrium.

Regards,

Henning (HoHun)

[Straiga]

HoHun, Logical thinking your getting there. Most twins in flight use very little rudder trim in stable unexcelerated flight. Aileron trim just sits there collecting dust. I have sent hitech a lot of paper work. I hope he gets it, its some good reading, and I will send more when he post what I have sent him.

Maybe I should emphasize here that the torque I'm expecting from a multi-engine aircraft doesn't really mean that the pilot is struggling with large roll moments, but rather that torque is one of several forces that are in balance when the aircraft in smoothly flown while the pilot only perceives the necessity for a yaw.

You would be supprized how little trim is needed in a multi-engine they are that stable until you lose a engine and thats when the fun starts. But even when this happens prop slipstream and P-factor far outway what torque does to the airframe. I have never felt any roll moment tendencies do to torque in any multi.

Torque is not so much noticable in its direct, but in its indirect effects. That's why I consider the yaw pilot-induced, though a pilot might see it as inevitable result of the torque - which of course is true since the other option would be to let the plane roll out of control and crash.

The only roll situation would be because of a single engine failure and the lost of lift on one wing while the prop wash is present across the other wing so the plane rolls due to assimetric wing lift. Any plane will roll even a glider (NO ENGINE, NO TORQUE) with this unequal lift happening. This is the first step to a spin.

Hm, maybe the P-factor effects requires rudder input anyhow so that you automatically counter the torque roll in the same action? I figure smooth flying might make the torque virtually unnoticable because it's only one factor in an equilibrium.

Ok Hitechs thinking is that a single engine airplane will roll left due to torque, and that a multi-engine has twice the amount of torque so it will roll twice as much. I havent seen this at all if anything Id say its less torque than even in a single engine airplane, effecting the airframe in a multi.

I will let the blood pressure rise on some people, and will be back to continue.

Later Straiga

HoHun I will get you out of that dark room yet!

[hitech]

Ok Hitechs thinking is that a single engine airplane will roll left due to torque, and that a multi-engine has twice the amount of torque so it will roll twice as much. I havent seen this at all if anything Id say its less torque than even in a single engine airplane, effecting the airframe in a multi.

HiTech thinks no such thing. We are talking torque created in the roll axis, we are not talking how much the plane roles. How much a plane rolles is effected by many other forces that counter the  torque created by the engine.

These forces are primarly .
1. Slip stream over the wing. AOA higher on one side of the prop then the other.
2. Incindence greater on one wing than other.
3. One wing having more area then other.
4. Ailron riging.
5. Flap rigging.

But what I do know is that 2 engines the same size create twice as much torque  about the CG as 1 enging does no mater where you place them.


HiTech

[HoHun]

Hi Straiga,

>I have never felt any roll moment tendencies do to torque in any multi.

Hm, that might be because the rotational inertia grows quicker than the torque.

The P-51D weighing 6356 lbs empty according to NACA TN No. 1629 has a rotational inertia of 4486 slug ft^2 in the roll axis.

If we make us a Twin Mustang from two of these by joining them with a distance of 15 ft between the two fuselages, that will give as an aircraft with a rotational inertia of a stunning 53000 slug ft^2 (2883 kg empty, 4.572 m space, 6198 kgm^2 Single Mustang, 72661 kgm^2 Twin Mustang).

This increase of rotational inertia by a factor of almost 12 easily outweighs the increase in torque by a factor of 2.

(In normal twins, you don't actually move the entire mass out of the centre line but only the engines, but these usually a bit farther than on the P-51, in relation to the propeller diameter.)

This calculation shows us that twins with off-centreline engines should be much less susceptible to roll accelerations than singles. Accordingly, despite the increased torque, the torque would be much less noticable and probably completely buried by P-factor and slipstream effects.
 
>Ok Hitechs thinking is that a single engine airplane will roll left due to torque, and that a multi-engine has twice the amount of torque so it will roll twice as much. I havent seen this at all if anything Id say its less torque than even in a single engine airplane, effecting the airframe in a multi.

Maybe my Twin Mustang example has contributed to resolve this apparent contradiction. I still think the pilot mostly notices torque changes and not so much the constant torque, so the high roll inertia of a multi really seems to be a reasonable explanation for your experience.

>HoHun I will get you out of that dark room yet!

I can already see the tunnel at the end of the light ;-)

Regards,

Henning (HoHun)

[Straiga]

HoHun read the post that says straiga heres your multi-engine pictures.

Straiga