Critical engine

[unreal]

Okay, I have a bit of a situation. A friend of mine is going through his MEI training at the moment, and I was trying to help him study. My question: "Name the four things that make an engine critical." He named three: p-factor, torque, accelerated slipstream, but I contend that he's missing one more: spiraling slipstream.

I know I've seen the reference to "spiraling slipstream" as a factor that makes an engine critical, but like a big idiot, I can't find that reference. Any help, or am I just insane?

 

[KeroseneSnorter]

Kersheners (sp?) books have it in there.

I thought it was in the Faa's test prep as well, at least it was way back when. I know I had to spit it out to the DE when I took the Multi and MEI rides.

 

[User998]

You are correct. According to Paul Craig's "Multiengine Flying" (arguably the bible for initial multi students), on pages 21-24, it lists and details the four items which make an engine critical.

Below are the notes directly from my MEI Lesson Plans folder:

-Four Things That Make An Engine Critical

P-Factor
·Descending blade produces more thrust that the ascending blade.
·Descending blade on right side will have longer thrust arm, then the descending arm on the left engine.
·The yaw produces by the loss of the left engine will be greater that yaw produced by the loss of the right engine, making the left engine more critical.

Spiraling Slipstream
·Left engine’s slipstream hits horizontal stabilizer, counteracting the yaw caused by the loss of the right engine.
·Right engine’s slipstream too far out to hit stabilizer, and is unable to counteract the yaw caused by a left engine failure.

Accelerated Slipstream
·P-Factor causes longer arm to the centerline of thrust of the right engine, that then left. The centerline of lift is also farther out on the wing, resulting in a greater rolling tendency with the loss of the left engine.
·Rudder is more effective with the left engine running. (Slipstream)

Torque
·For every action, there is an equal and opposite reaction.
·Since props turn clockwise, the aircraft tries to roll counter-clockwise.
·If you lost the right engine, the aircraft will yaw right but roll to the left, thereby canceling each other.
·If we lose the left engine, the aircraft will yaw left and roll left , into the dead engine.

 

[gkrangers]

Well think about it...the loss of the spiraling slipstream from the critical engine will hurt control...

PATS.

CMEL ride this Saturday....

 

[unreal]

Cool, thanks. His school doesn't teach it that way, so my word alone isn't going to sway him (and really, it shouldn't). I'll check out that book, thanks.

 

[FL420]

Spiraling Slipstream
·Left engine’s slipstream hits horizontal stabilizer, counteracting the yaw caused by the loss of the right engine.
·Right engine’s slipstream too far out to hit stabilizer, and is unable to counteract the yaw caused by a left engine failure.

Horizontal or vertical stabilizer?

Accelerated Slipstream
·P-Factor causes longer arm to the centerline of thrust of the right engine, that then left. The centerline of lift is also farther out on the wing, resulting in a greater rolling tendency with the loss of the left engine.
·Rudder is more effective with the left engine running. (Slipstream)

What is this centerline of lift?

 

[MauleSkinner]

I always thought the one that quit was the critical one...it definitely has more negative impact on performance than the one that's still running

Fly safe!

David

 

[Holding Short]

Horizontal or vertical stabilizer?

Well... if you sit there long enough without doing something about that failure, it will be vertical.

HS

 

[kevdog]

I always thought the one that quit was the critical one...it definitely has more negative impact on performance than the one that's still running

Fly safe!

David

I always thought the one still turning was critical, since if it fails, you are SOL.

 

[UndauntedFlyer]

Personally, I think that except for P-factor and maybe torque reaction to a degree, the rest is a bunch of crap and of very little significance.

The only thing an applicant should be responsible for on an FAA test on this subject is what is in the POH or the Airplane Flying Handbook (AFH). Paul Craig is not listed as a reference source for any task in the PTS for anything.

If the examiner wants that info too, he is overstepping the line of fairness.

 

[minitour]

Personally, I think that except for P-factor and maybe torque reaction to a degree, the rest is a bunch of crap and of very little significance.

I'll be honest, I don't buy spiraling slipstream in a counter-rotating prop twin...I just don't see it...but that's me.

I do firmly believe in accelerated slipstream, and P-Factor....torque is there IMHO, but not as much.

I agree with you...the applicant should be responsible only for that information that is required or referenced in the PTS. With that said, I think the applicant should have more knowledge than that...but the examiner shouldn't be "grading" him or her on it. If they wanted it tested, it would be in the PTS.

-mini

 

[UndauntedFlyer]

I think the applicant should have more knowledge than that (what's required in the PTS) ...but the examiner shouldn't be "grading" him or her on it. If they wanted it tested, it would be in the PTS.

-mini

The problem here is that if the examiner brings up things that are not in the PTS or in the referency sources for the task, then that examiner may be making the applicant nervous. This causes the applicart to question everything about his/her training and pretty soon the applicant can not even answer such simple questions as: "What is your birthday?"

 

[minitour]

The problem here is that if the examiner brings up things that are not in the PTS or in the referency sources for the task, then that examiner may be making the applicant nervous.

I agree.

The problem is we've got way too many CFI's teaching the PTS rather than teaching all of the important and unimportant information that a pilot may or may not need, but should have just in case.

Then we've got examiners pushing their views on subjects vs. what is in the referenced sources for a certain TASK in the PTS which can make you nervous too. My CSEL ride was like that...I wasn't allowed to use rudder pedals from "positive rate - gear up" to "final check, gear down". Now, nowhere in the PTS or referenced sources did I see that lazy eights were done without rudder, or eights-on-pylons, etc...but the examiner wanted to see it done like that. What's up with that? By the time my power-off 180 came around, I had the seat cushion pulled halfway up my azz-krak with nerves.

What we need is the opposite, the CFI's that teach WAY more than is necessary and require the student to perform better than +/- 100' for altitude +/- 10-degrees for heading, etc. and examiners that want to see you perform safely within the tolerances of the PTS.

IMHO

-mini

 

[gkrangers]

On my CMEL ride he wanted to hear PATS.

Also asked when a multi has a Vmc of 0...

Hey, will the CSEL addon have any ground? Or just the PTS maneuvers?

 

[minitour]

Also asked when a multi has a Vmc of 0...

This I have to hear....when is it?

I don't think I've seen singles with a Vmc of 0 let alone a twin....

(you are using "Vmc" in the "minimum controllable speed" sense, right? As it...right before loss of control (twin) or stall (single/twin)?)

 

[gkrangers]

This I have to hear....when is it?

I don't think I've seen singles with a Vmc of 0 let alone a twin....

(you are using "Vmc" in the "minimum controllable speed" sense, right? As it...right before loss of control (twin) or stall (single/twin)?)

Centerline thrust......

Thats what he asked ("0"), but maybe he meant no Vmc speed.

 

[avbug]

I always found the best way to explain a critical engine is to explain VMC; the definitions are intertwined, and showing the effects at their most critical time (approaching control loss) emphasises the part each factor plays in making it a critical engine.

I agree too, that in most light twins, the loss of either engine is critical, regardless of the nuances and semantics taught. The loss of the remaining engine isn't nearly as critical as what you do with it while it's still operating. Once it's gone, life gets simple again.

While CG isn't intrinsic to explaining why which engine is the critical engine, explaining Vmc and relating the issue of CG to Vmc will also help illustrate the nature of the critical engine. Torque, "p factor," and other elements are far less important than assymetrical thrust differences...which is the heart of the matter. The bottom line is that in some airplanes, the loss of one engine may be a bigger bother than another.

One way you can also explain it, which works well for a student, is to use a model of a four engine airplane, and explain the differences at each wing station, or for each engine, during a failure. The inboard engines may require half rudder, for example, and less aileron deflection, resulting in higher climb performance, less drag, etc. The loss of the outboard engine may require full rudder, much higher drag, and a complete loss of climb performance...you can establish any performance you want for the model airplane, but it's the illustration that counts in allowing the student to see the difference from one engine to another.

Another handy way to explain it is to put the airplane on a stick, running up through the vertical axis, or the yaw axis of the airplane. Get a model of an A-26 or something with large props to ilustrate the point. Make small arrows and big arrows out of laminated cardboard, with some velcro or sticky tape on the back. With the student looking down on the model, place the biggest arrow pointing back from the descending side of the prop arc, toward the trailing edge, showing where the greatest thrust is...for each propeller. Show which one is farthest from the long axis of the airplane, and which will have the greatest effect or arm. Have the student push the model from each side, and see for himself or herself which has the greatest effect.

It can all be explained verbally, but just like a picture is worth a thousand words, little examples such as that leave an indellible impression in the students mind. If you can explain it to the student, you can explain it to the examiner...and the examiner will be happy. He's not there to hit you with "gotchas," he's there to see it you can teach the basics. Don't attempt to recite what you read in the book; attempt to really teach it, and the examiner/inspector will be happy.

The most effective illustration I've seen in the classroom involved a rotorwing session in which the instructor had me hold a bicycle wheel by the axles, with the axles vertically, as he spun the wheel. He was demonstrating gyroscopic precession, and that simple demo established it in my mind so much better than him telling me. Same for Vmc. Find ways to teach it so the student will remember it...models work really well for that, and you can apply and show all the principles you want on that model. Make it a discussion.

Spiraling slipstream, in my opinion, has little effect on a non-centerline-thrust multi engine airplane. What effect it does have is negligible in comparison to the other elements.