Jet Engines and the Forces They Create - Confirmed?

[buster]

last year i did a jet engine workshop n one of the instructor there told us that both the engines on an aircraft(ofcourse an aircraft with an engine on each wing or maybe more on each) are run in same direction(i mean same sense of rotation) n this induces a transeverse force of the aircraft, making it to, also, go perpendicular to the fuselage...
i want to confirm it, is it correct?(i don't have any source proving this, just some talks with the instructor over there)
for the transeverse motion, we thought that the engines be revved up in different sense to cancel out the force..
but first, is this thing even true?
n if it is, why is it so?

 

[Danger]

You'll have to alter your wording in order for it to make sense. If I understand it correctly, you're asking about the result of engine torque on the flightpath.
Multi-engine prop aeroplanes normally have contra-rotating engines.
As for turbines, I must assume that the torque is more or less irrelevant. That's Fred's department, though, not mine. (Also RainmanAero, Russ, Integral, and a few others with much more flight experience than me.)

 

[jambaugh]

Buster, it is likely true that jet engines of a particular design all turn the same direction as it is easier to assembly-line produce all engines the same.

I'm also not clear on your wording. You speak of "transverse force" but I think you mean torque=twisting force...but yes in general the engines will produce a torque on the aircraft, both due to "revving the engines" and possibly due to action on the airflow.

Keep in mind the turbine blades of a jet will impart a rotational component to the air flow which via Newton's equal and opposite law will result in a reaction torque. But there are stationary vanes in the engine opposing this rotation of the airflow converting it to linear reward thrust (and receiving a compensating torque). The less rotation in the exhaust the more efficient the jet thrust. Most jets and turboprops I wager essentially eliminate this rotational component to the air flow and so produce no torque during steady operation.
But if there is any exhaust rotation then there is a corresponding torque on the craft.

Prop plane engines will, like helicopters, induce a torque due to their action on the airflow. Some dual prop designs will use counter-rotating engines to prevent this and others simply use trim to counter it. Conventional helicopters with their long props require the tail rotor to counter this torque but you'll note some two prop designs use counter rotating lift props to cancel the torques out.

As far as revving torque, jet engines don't change rpm's very quickly and during steady flight not at all so this torque is not of much consequence.
Remember the turbine blades are not that massive so their moment of inertia is small compared to the whole plane. The torques due to changes of engine speed will be much less than those due to each wing considered separately. I would guess also that passengers repositioning themselves within the jet will have more of an effect.

Note that planes are built to be stable in flight which means shifting weight slightly or addition of a little torque will be countered by the reaction of increased lift on one wing and decreased lift on the other as the craft shifts slightly.

(Compare this with roll stability in boats where buoyancy increases on one side to compensate for shifting weight.)

But the short answer to your question is Yes, if the turbines of the jet rotate counter-clockwise then revving the engine will cause the plane to try to rotate clockwise in reaction to the rev.

Now from your post I didn't sense that you were referring to gyroscopic precession but that is another effect that will be present when a jet engine is turned.

 

[DaveC426913]

But the short answer to your question is Yes, if the turbines of the jet rotate counter-clockwise then revving the engine will cause the plane to try to rotate clockwise in reaction to the rev.

And one of the reasons why you could never hope to beat a Sopwith Camel in http://en.wikipedia.org/wiki/Sopwith_Camel#Engine_variants".

 

[FredGarvin]

First off. Yes it is true that we do not make engines that rotate in different directions for alternate side applications. The extra cost in tooling and required components would be prohibitive.

As far as the transverse force you mention, I can't really say I have heard any airframer mention it. Of course there is the reaction torque required to restrain the engine. The other issue with forces is the gyroscopic effects of the rotating group. Just like a gyro, the engines will resist a change in the orientation of its axis.

 

[buster]

so the reason why they all rotate in same direction is the cost management?

initially we did thought of the gyroscopic effect but i havnt studied it yet, so cudnt come with a solution taking gyroscopic effect