Q about Assymetric Ops

[Spelunker]

I am currently studying ATPL Aircraft Systems and came across a question regarding assymetric ops that I cannot figure out:
Q: If the critical engine fails, will that create
a) a long assymetric arm
b) long assymetric thrust.

I understand the whole concept of assymetirc ops, critical engine etc.. but I do not know which of the answers would be regarded as more correct out of the two. I have my thoughts on which one is more correct, but what do you guys think and would you be able to explain.

Thanks,
Spelunker

[Con-Trail]

The critical engine is the engine, which when it fails, generates the biggest yawing moment.

In other words: it has everything to do with the moment arm, not the thrust. (Apart from loosing 50% of it that is)

When you have a twin-prop aircraft with co-rotating propellers, (lets asume that they turn clockwise) the critical engine is the left engine.

Assymetric blade effect
The downgoing blade creates the greatest thrust. With an engine turning clockwise the arm on the right hand engine is greater.

Thus, when the left engine fails, the right engine creates a bigger yawing moment. If the right engine would fail, due to the assymetric blade effect, the yawing moment would not be so great because the moment arm is smaller.

On Jets
There are also critical engines on jets. They are the 2 inboard engines on a 4-engined aircraft: Again the longest arm creates the biggest yawing moment so if 1 inboard engine fails, a big yawing moment would be generated

So I would go for A).

Hope this helps

C-T

[Spelunker]

Thanks Con-Trail for your answer, very helpful. That was the answer I was leaning towards myself, i.e the assymetric arm.

I am having trouble agreeing with you on the Jet assymetric thrust. My book says that:

"in aircraft such as the 747, failure of an outer engine causes greater yaw than loss of an inner engine, as the thrust line is further removed from the aircraft longitudinal axis"

This means that on a 4 engined aircraft, the outer engines must be the critical ones, and failure of an outer engine will cause greater yaw.I believe this is more correct as to what you have said.
What do you think?

Thanks again for your help Con-Trail

Spelunker.

[Con-Trail]

Oop, sorry..yes, you're right. When the outboard engine fails, the arm on the other side is greater. Thus the outboard engines are critical.

I stand corrected:\

C-T

[Keygrip]

....and it keeps being mentioned that, in a cross wind, one of the outboards is "more critical" than the other.

[AerocatS2A]

Presumably the upwind outboard engine yes? (or is there enough fuselage forward of the main gear that it weathercocks away from the wind?)

[Con-Trail]

I don't think weathercocking comes into the equasion. As far as I know weathercocking is turning around the main gear.

In cross wind the tailplane is being pushed aside, causing a yawing moment into the wind.
The crosswind also has an effect on the wings in a way that the angle of attack of the upwind wing is different from the angle of attack of the downwind wing. This will cause a roll effect into the wind.
So if the engine on the windward wing would fail, the yawing and rolling effect would increase. So this will be the critical engine.

[ViciousSquirrel]

What does 'long assymetric thrust' even mean? Sounds to me like a measure of time! In this context it's non-sensical, they're just trying to confuse you.

In a crosswind situation the upwind engine failing would cause the most difficulty when correcting for the crosswind as you are not only trying to counteract the crab induced by the wind, but also the yaw created by the failed engine. This would only matter if you were flying a type where handling dictated that you had to land with some power applied, which doesn't apply to many light twins. Mind you the coordination required as you kick it straight while compensating for the reduction in power can be interesting for multi engine students.

Even so, the left engine (in a standard US twin) would still be the critical engine as determination of the critical engine has nothing to do with crosswind.

[AerocatS2A]

I don't think weathercocking comes into the equasion. As far as I know weathercocking is turning around the main gear.

In cross wind the tailplane is being pushed aside, causing a yawing moment into the wind.

That is weathercocking. The tailplane is being pushed aside causing a yawing moment around the main gear (if the landing gear was not on the ground there would be no yaw.)

ViciousSquirrel, the crosswind situation where you have a critical engine is when in a large aircraft on take-off, not a light twin.

[ViciousSquirrel]

'ViciousSquirrel, the crosswind situation where you have a critical engine is when in a large aircraft on take-off, not a light twin.'

Fair enough, I didn't consider that they might have been refering to larger types :\