Critical engine with counter-rotating propellers

FullDeflection

FullDeflection

Well-Known Member
A brief search yielded no relevant topics...but this seems to be an interesting topic. One that I'd like to learn more about.

Do aircraft utilizing counter-rotating propellers have a critical engine? If said aircraft has 4 engines, does it have 2 critical engines, with each having an equally negative effect on engine-out performance? Those 2 engines of course being the two furthest away from the fuselage.

To me, it would seem that the thrust vector from each engine would be straight back from the propeller hub, and thus would have the same effect on engine-out performance.



An-70

1751556.jpg



Tu-95MS 'Bear-H'

1740230.jpg
 
or none depending on how you look at it. If you understand the concept what does it matter what it's labeled?
 
I thought if the plane has counter rotating props, it does not have critical engine. :dunno:
 
You're asking about contra-rotating props, not counter-rotating.
icon14.gif


Most coaxial rotor helicopters do not have a tail rotor (or other anti-torque device), so I guess there is not much torque produced, which would result in symmetric complications from a loss of power on either side.
 
If said engine is on fire, it becomes the critical engine. Or the one with the pumps, yada yada.
 
7700 said:
a twin-engine airplane with counter-rotating propellers has two critical engines
Click to expand...

Pessimists say there are 2, optimists say there are none.

I'm an optimist.

Regarding a plane with contra rotating props... I'd suspect there would still be a measurable amount of force experienced from the mass of the engine spinning along. The props however would neutralize each other.

Are there an mechanics who have ever worked on a contra rotating system? I'd imagine they are a mtc nightmare!
 
Autothrust Blue said:
In the Semi-Nole, the entire aircraft is critical. Duchess too.
Click to expand...

(Most of this is aimed at the OP, first sentence is for you Auto)

Both engines have the same effect, neither is a critical engine by the typical definition. The critical engine is the one where the down going blade will be closest to the fuselage. This means the other engine, rotating the same way, would have its down going blade farther from the fuselage. From weight and balance we know this is a greater arm and with a greater arm means a greater moment (or greater rotational force applied).

Typically your left engine is the critical engine since the props turn right when we are sitting in the aircraft. So the right engines down going blade is farther from the fuselage than the left engines. Therefore, if the left engine (critical) quits, the rotational forces applied from the right engine will be greater than they would be for the left because the right engine has a greater arm.

Remember: Arm x Force = Moment

Here's a simple diagram:

Crit%20eng%20and%20Vmc.gif


Sorry if anything is inaccurate. I am not an MEI and am not 100 percent certain on the presented material, beyond knowing it was taught to me. But we know how 100 percent accurate student memories are, right... ;)
 
shdw said:
(Most of this is aimed at the OP, first sentence is for you Auto)

Both engines have the same effect, neither is a critical engine by the typical definition. The critical engine is the one where the down going blade will be closest to the fuselage. This means the other engine, rotating the same way, would have its down going blade farther from the fuselage. From weight and balance we know this is a greater arm and with a greater arm means a greater moment (or greater rotational force applied).

[unfunny but largely accurate and insightful technical stuff truncated]
Click to expand...

I forgot my massive 'sarcasm/otherwise not to be taken seriously' tags. :laff:
 
I always looked at critical engine as the engine who loss woujld cause the greatest decrease in performance and capabilities. With counter rotating props or contra rotating I would assume a failure of either engine would cause an equal loss of performance. In that case I would fall back on the capabilities part of my above definition. in that case is there a non redundant system component driven by one engine alone? If so that engine would be critical. But then what if one engine drives the electrical and one the vacuum...which system is more critical?
 
Codyjp said:
Pessimists say there are 2, optimists say there are none.

I'm an optimist.

Regarding a plane with contra rotating props... I'd suspect there would still be a measurable amount of force experienced from the mass of the engine spinning along. The props however would neutralize each other.

Are there an mechanics who have ever worked on a contra rotating system? I'd imagine they are a mtc nightmare!
Click to expand...

I would also imagine huge amounts of drag - even with feathered props thats a lot of metal out there with contra rotating props.
 
Here is the definition from 14 CFR 1.1:

Critical engine means the engine whose failure would most adversely affect the performance or handling qualities of an aircraft.


Very basic definition, with lots of interpretations.

In the standard Twin Comanche PA-30, the left engine is critical. The c/r Twin Comanche PA-39 is commonly referred to as having no critical engine.

The engines on the Lockheed P-38 Lightning are c/r, but opposite of the traditional c/r twins; the outboard blades are descending. This was done to increase performance, but resulted in a difficult handling aircraft if an engine was lost on takeoff and very marginal performance on one engine if it was heavy. The Lightning is commonly referred to as having two critical engines.

With in-line twins, if there is a performance loss that is greater with a specific engine, then there is a critical engine. Though rarely would you hear anyone refer to these aircraft as having a critical engine. This also could apply to the c/r twins.
 
stratoduck said:
The Lightning is commonly referred to as having two critical engines.
Click to expand...

Which is a nonsensical statement. You can't have two engines which "most adversely" affect directional control. The definition means that one engine is worse than another to lose; when they're both the same, labeling them both as critical has no meaning.

With in-line twins, if there is a performance loss that is greater with a specific engine, then there is a critical engine. Though rarely would you hear anyone refer to these aircraft as having a critical engine. This also could apply to the c/r twins.
Click to expand...
"Critical" with respect to performance is a different paragraph in Part 23 and pertains to climb gradients. The critical engine with respect to performance might be different from critical with respect to control. Most of time when we're talking about critical engines, we're talking about Vmc, which isn't concerned with performance.
 
tgrayson said:
Which is a nonsensical statement. You can't have two engines which "most adversely" affect directional control. The definition means that one engine is worse than another to lose; when they're both the same, labeling them both as critical has no meaning.
Click to expand...
I wish I had the brain power and brass ones to say that to my examiner during my MEI ride, just like that.
I just nodded.
 
Douglas said:
I wish I had the brain power and brass ones to say that to my examiner during my MEI ride, just like that.
I just nodded.
Click to expand...

I picture George Carlin adding what tgray said, word for word, into his skit on expressions we say...

[YT]hHhYLJMi7CE[/YT]
 
You must log in or register to reply here.
Back
Top