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Hi,
I would like to know how the propeller torque is controled in a multi-engine prop plane?
Do the engines rotate in different directions? ( I don't think so)
Regards
Torque is controlled the same way in a multi engined airplane as in a single, with the rudder. However in some prop planes the engines do counter rotate, but I believe the majority do not.
The majority do not counter rotate, but in some training aircraft, as with the piper seminole, they do counter rotate. Torque in a multi is a completely different animal than torque in a single, as with a multi with standard props, the left engine is known as the "critical engine" because its failure would cause a very noticeable left turning tendancy (the right engine has its own left turning tendancy + induced yaw due to the failed left engine not producing any thrust), whereas if the right engine failed there would be a very diminished left turning tendancy (Left turning tendancy - moment of the fuselage - induced yaw toward the right).
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Didn't your CFI ever yell right rudder, right rudder! at you?
Of course, but what on Earth does "torque" have to do with that?
Ok, indirectly, very indirectly, it does.
There are two reasons for the (in "normal" rotating props) left yaw effect of the propeller: The p-factor and the incidence of the rotating slipstream on the rudder.
P-factor: When flying at an AOA, the descending blade will have a higher airspeed and AOA than the ascending one, because of how the forward speed combines with the tangential speed of the rotating blade. This effect cannot be the responsible of the left turning tendency during the T/O run in a tricycle geared plane for two reasons: It would not be present at the beginning of the T/O run because there is no forward speed to combine with the tangential speed, and the airplane runs with no/minimum AOA during the T/O run.
Rotating slipstream: Most of the air that passes through the prop disc is pushed straight back. However, the smaller fraction that becomes involved in the boundary layer of the blades is also pushed tangentially due to the friction with the blades. This fraction of air, when the backwards movement is combined with the tangential movement, forms the "rotating slipstream". That rotating slipstream will hit from below on the left wing and from above in the right wing, from below in the left stabilizer and from above in the right stabilizer (except in some T tails), and from the left in most fins (not it most singles with twin fins and twins with a single fin). Since the air on the fin comes from the left, the fin is pushed to the right, and the airplane yaws to the left.
Now, if the blades push the air tangentially in one direction, the air pushes the blades tangentially in the opposite direction (third law of motion), and that's the blades' drag. The rotating slipstream is the result of the blades' drag. And the torque is what keeps the prop turning despite that rotating drag. In that sense, the rotating slipstream is related with the torque.
The direct effect of torque is to try to roll the airplane. If the airplane applies a clockwise torque to the prop, then the prop applies a counterclockwise torque to the airplane (again, third law of motion), what means a left roll. That means that torque is arrested with ailerons, not with rudder.
The effect is usually small, partly because it is small in itself and partly because the same rotating slipstream, indirectly related with torque, tries to bank roll the plane in the opposite direction (remember: "That rotating slipstream will hit from below on the left wing and from above in the right wing"). However, it has caused accidents in WWII warplanes flown by inexperienced (for the type) pilots (there were a couple of those accidents in the last years), where the airplane rolls inverted immediately after lift off or after initiating a go-around and crashes.
But basically, the torque is a moment in the longitudinal axis and the rudder makes a moment in the directional axis, and those are perpendicular axes. So torque by itself cannot be arrested with rudder.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Gabriel,
[Flyboy mode] You could have simply stated:
Torque causes the aircraft to roll.
"P" factor cause the aircraft to yaw.
If any one requested an explanation, go into detail[/Flyboy mode]
Don
Standard practice for managers around the world: Ready - Fire - Aim! DAMN! Missed again!
Gabriel,
[Flyboy mode] You could have simply stated:
Torque causes the aircraft to roll.
"P" factor cause the aircraft to yaw.
If any one requested an explanation, go into detail[/Flyboy mode]
So if I say "say again?" it's too short but then an explanation is too long?
Maybe. But I thought that a "say again" would be enough at least for the professionals. But after two other senior members insisted with the original (and wrong) proposition, I thought that giving just "titles" would not be enough.
Even then, I started that last post with:
Of course, but what on Earth does "torque" have to do with that?
There are two reasons for the (in "normal" rotating props) left yaw effect of the propeller: The p-factor and the incidence of the rotating slipstream on the rudder.
This is more or less the same that you proposed. That who understood that part could move on to another thread. Who didn't could look down for the detailed explanation.
All that, plus, I wouldn't be me if I didn't give a "scientific" explanation.
For example: The P-factor you mention is almost a myth. It has no means of being the responsible of the left yaw in the take-off run and it is not the main responsible of the left yaw in the initial climb. The slipstream rotation you don't mention is. As I explained in the previous post.
Unfortunately, a lot of sources blame the P-factor and forget the slipstream rotation. Including "official" training material like the AIM and the Argentinian "Private Pilot Handbook". But they are wrong. If you asked me for a source, I'll point you here:
It's the most correct non-scientific, pilot level, training material I've ever seen. Very well written, very well explained, and very correct!
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Of course, but what on Earth does "torque" have to do with that?
Ok, indirectly, very indirectly, it does.
There are two reasons for ...But basically, the torque is a moment in the longitudinal axis and the rudder makes a moment in the directional axis, and those are perpendicular axes. So torque by itself cannot be arrested with rudder.
Thanks,
I was wondering how a displacement along longitudinal axis maybe corrected by Rudder.
Then Rudder is used to correct slipstream displacemnets and should the Trims in aelorans used to correct propeller torque itself?
And does Propeller rotational torque cause any yaw movemnet? Somebody told me something that was not sure about...
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
Are you talking about torque from the turning prop, or torque as a measure of engine output?
It's almost the same (unless the prop is gearboxed).
Nearly all of the torque measured as engine output (on the shaft) will be used to turn the prop, with a small fraction used to turn the alternator, fuel pump, etc.
--- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
--- Defend what you say with arguments, not by imposing your credentials ---
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