Critical Engine

[Honiley]

A few of us winding ourselves up over this one...

A twin engined aircraft has both props which rotate to the left as viewed from the flight deck thus making this aircraft have a critical engine - but which engine and why?

Thank you,
Confused Of Honiley

[Sky Wave]

The Right Hand Engine will be the critical engine assuming that they rotate counter-clockwise when viewed from behind. (Left engine if clockwise)

Why? The down going blade creates more thrust than the up coming blade (Needs a picture to explain why). The Left Hand Engines down going blade is furthest away from the aircraft with the right hand engines down going blade being relatively close to the fusalage. That means that if an engine is failed the left hand engine causes more of a yawing moment than the right hand engine because the highest amount of thrust is out on the wing and a long way from the fusalage. The greatest amount of thrust on the Right Hand engine is close to the fusalage and the center line of the plane. All this means that losing the right engine is worse than losing the left engine and therefore the right engine is the critical one.

Most engines rotate clockwise so left engine is usually the critical engine.

Much Easier to explain with pictures, perhaps run a search.

Try this link.

http://www.fergworld.com/cfi/pdf/Multi-engine_Instructor_Quick_Reference.doc

[blueplume]

When the aircraft is level both blades provide equal thrust.
The increased thrust of the downgoing blade compared to the upgoing is a phenomenon that occurs when the a/c is in a climb..

The definition of "critical engine" can be found in the literature, loosely defined (by me) as being the engine the loss of which will provide the pilot with the greatest amount of difficulty in controlling the a/c under the most adverse circumstances i.e. high temp., high alt., high weight, a/c loaded to CofG limits.

When level the yaw and therefore roll tendency exists because of the greater distance from the longitudinal axis of, in this case, the downgoing blade of the left engine.

[Sky Wave]

the engine the loss of which will provide the pilot with the greatest amount of difficulty in controlling the a/c under the most adverse circumstances i.e. high temp., high alt., high weight, a/c loaded to CofG limits.

The most adverse conditions are Low Temperature, Low Altitude, rearward C of G and high power on the remaining engine. Weight has no effect.

[blueplume]

Erm, yes, I see what you mean. Low altitude and low temp. means high density and high thrust available etc.. I stand corrected.

[Astra driver]

Of course, one might consider either engine "critical"!

Especially with a big mountain in front!

Actually, SW weight does have effect as far as control is concerned;
Increased weight means increased lift required, which means greater lift avaialble for the side slip used to control the yaw.
Of course a greater weight might also mean you are still going down hill, swings and roundabouts I guess.