Critical Engine??????

ipilot

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Hey guys!!!
I was just wondering if a multi engine jet aircraft has a critical engine. If there is a critical engine, what factors make it critical? Will the effect be more pronounce in a B747 with enignes having thrust line away from the CG and less pronounced in a learjet having engine closer. Is the jet aircraft more easier to control rather than a prop driven with an engine faliure? Thanks in advance to all.......


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Draginass

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There are no "critical" engines on jet aircraft. On four engine aircraft the outboard engines have a longer moment arm, thereby causing more yaw with a failure (depending on thrust output) than an inboard engine. Difficulty in controlling an outboard engine failure on a 4 engine aircraft like a 747, is not too hard except at low speed. At low speeds (say below 50-70k), the rudder is not effective, and an outboard engine failure will result in a significant centerline deviation unless an abort is initiated right away.

On a two engine jet like a 757, the engines are more inboard (less moment arm), but each is producing HALF the total thrust, as opposed to 1/4 on a 4E aircraft. Engine failures on a 757 are more challenging than a 747, in terms of both control and performance.

In general, 4E aircraft do not have as much performance as 2E aircraft when all engines are operating, but have more performance during single engine failure.
 
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avbug

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On some multi engine airplanes, every engine is critical. Some airplanes, such as twin engine airplanes using contra-rotating props (outward), place both engines in the critical category, with respect to the conventional use of the term. Most light twins will lose up to 80% of their available performance as a result of one of two engines failing. In such a case, one may safetly assume that both engines are critical (especially at high density altitudes and weights).

Remember that an engine may be critical in other than the conventional sense; some aircraft use particular systems only on certain engines, and the loss of those systems under certain circumstances may prove critical. The loss of that engine becomes a bigger issue than other engines.

There are multi engine jet aircraft which do have a critical engine, but I can't think of a good example right now. (One that leaps to mind, but isn't a good example, is the P2V-5/7/SP2H. It's not a good example because we didn't use the jets all the time, and when not operating, the #1 recip was critical. Both jets were also "critical" because they were prone to fire, or failure on a regular basis). Anybody?
 
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SentryIP

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Short Answer - No

ipilot,

There's no critical engine in jet aircraft since jet engines do not have a (pronounced) P-factor. Take a Cessna 310 where both engines rotate clockwise (as seen from the cockpit). The left engine is the critical engine because it's failure would more adversely affect handling and/or performance characteristics. On a Beech Duchess, with counter-rotatng engines, loss of either engine will have the same affect (left engine rotates clockwise and right engine rotates counterclockwise) and P-factor is the same. On jet engines, there's no P-factor. As far as the distance of the engines from the aircraft centerline, yes, it does make a big difference. It's a simple statics problem where the moment of a force, M, is equal to the force (engine thrust), T, times the perpendicular distance, d (M=T x d). Aeronautical engineers use this distance and thrust to determine the size of the vertical stabilizer and rudder and come up with a Vmca (minimum speed for directional control-air) and Vmcg (minimum speed for directional control-ground) for the flight manual. Hope this helps. Fy safe.
 

SentryIP

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Critical Engine Definition

avbug,

From your post, I don't think you understand the definition of "critical engine." A critical engine is the engine whose loss will most adversely affect the handling and/or performance characteristics of the airplane. Also, I'm not certain what you mean by light twin airplane with both engines counterrotating outward. Do you mean the left engine rotating counterclockwise and the right engine rotating clockwise? That would increase the P-factor and I don' know of a reason anyone would design an aircraft like that. Keep you speed up.
 
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ipilot

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Thanks to draginass, avbug and SentryIP for the insights. what I could understand is that there is no critical engine on a jet aircraft but the engines which are farther from the CG will be equally critical so the outward engines on a 747 or any engine on a 757 will be equally critical.
So will there be a Vmc or Vmcg speed for a jet engine? As both engines r critical( equally adverse performance), is there an airspeed at which the rudders will be effective and u should maintain at or above that, may be V1 or above on take off???
Thanks again.....
 

avbug

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Sentry IP,

Yes, I understand the definition. At this stage in my career, it would be a little late to be trying to figure out the basics. Reread the post, and you'll note that I clearly stated that reference was made to engines which are critical in the conventional sense, and those which are critical for other reasons.

Critical refers not only to the aerodynamic assymetrical thrust situation which would most adversely affect the handling qualities of the airplane, but "the engine whose failure would most adversely affect the performance or handling qualities of the aircraft." See FAR 1.1

Additionally, in aircraft which are not affected aerodynamically by the loss of one engine over the other, the loss of one engine may present other issues such as loss of hydraulic, vacum, electrical, or other concerns. In such cases, loss of that engine may effect the operation of the aircraft more than the loss of any other powerplant, and that engine may be considered critical.

Critical is conventionally taken to mean an engine affected by assymetrical thrust caused by the descending propeller, however, there are clearly other reasons that an engine may be critical. I made reference to the P-2V/SP2H, in which failure of either jet created a more critical thrust situation, in terms of total thrust, and in terms of assymetrical thrust, than either recip.

Simply because you are not familiar with contra-rotating propellers with the descending blade on the outside, doesn't mean they don't exist, or haven't been designed. Certain of the Aerostar series had this feature, as did the P-38 lightening. Both engines were critical. If I'm not mistaken, I believe the F7F also utilized this arrangement. I may have confused you previously by referring to them as "counterrotating", but you understood the intent. The descending blade is on the outside of the arc, with the left engine turning counterclockwise as viewed from the rear, and the right engine turning clockwise.

In the non-conventional sense, both engines on most light twins are critical, in that the loss of either engine results in very undesirable performance (eg, the inabilty to maintain altitude, or serious erosion of climb performance). The reference in this case is clearly outside the conventional definition, and I so stated in the previous post. It is more tongue in cheek than a specific reference to the critical engine.

In this sense, even counterrotating props such as on a beech duchess become critical at high density altitudes and gross weights; the airplane is seriously impaired with the loss of either engine, and both engines are critical to operation of the aircraft.

Years ago I spoke with an A-10 IP who was extolling the virtues of the A-10. He stated that it was his idea of a fighter aircraft (spare me the role-lecture). He stated that it was fast (relatively speaking), maneuverable, single seat, and single engine. I commented that the airplane did indeed appear to have two engines. No, he replied. It had one. It just had a half engine on either side. Humor aside, the same thing may be said for most light twins, except that rather than a half-engine on either side, when one fails, the other resorts to a 1/5 engine, yielding only 20% of the necessary performance.
 

SentryIP

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avbug,

Thanks for your reply. I'll take your word for it that you understand the definition since we both quoted the same reference. I read your first post again and it seems you're saying that any engine is a critical engine. In that sense, the critical engine is the engine that's still operating, not the one you lost :D

As far as airplanes with contra-rotating propellers or counter-rotating propellers, if losing either engine has the same affect, then which is the critical engine? Semantics, I guess. If there's an advantage in having contra-rotating propellers over counter-rotating propelers, please let me know. It's the same gain/loss provided the rudder and stabilizer are larger on the contra-rotating airplane to compensate for the larger moment. A larger stabilizer and rudder will result in more drag.

If there's a multi-engine airplane out there that will cause you to lose total systems such as hydraulics, electrics, etc, with one engine inoperative, I'd like to know about it so I never fly it. Fly safe.
 

avbug

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It's probably me. I have a tendency to digress toward vagueness, or to lack being concise (concision? Is there such a word? Conciseness). At any rate, I believe we are talking about the same thing.
 
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