Myth Busting - Engine Inop Performance

[MauleSkinner]

Finally an explination that makes sense. Thank you! I just had to "rote learn" it. Now I understand.
-mini

I agree...although I understood it (and taught it), Avbug made it elegantly simple.

Fly safe!

David
P.S. His explanation also indirectly points out that the inclinometer is not a yaw instrument, busting yet another myth!

 

[100LL... Again!]

It is doubtful that more than five degrees would be required for a typical GA aircraft for best single engine climb. More could be required under certain circumstances for controllability, but that is another facet to the issue.

Possibly some specialty aircraft or a particularly old design could require an unusually large amount of bank for best OEI rate.

That said, once controllability is satisfied, the bank will probably be closer to 3 degrees than five for best rate.

 

[avbug]

The certification standard and practical test standard is no more than five degrees, within five degrees, and as required.

14 CFR 23 stipulates:

§ 23.147 Directional and lateral control.
(a) For each multiengine airplane, it must be possible, while holding the wings level within five degrees, to make sudden changes in heading safely in both directions. This ability must be shown at 1.4 VS1 with heading changes up to 15 degrees, except that the heading change at which the rudder force corresponds to the limits specified in § 23.143 need not be exceeded, with the -
(1) Critical engine inoperative and its propeller in the minimum drag position;
(2) Remaining engines at maximum continuous power;
(3) Landing gear -
(i) Retracted; and
(ii) Extended; and
(4) Flaps retracted.

§ 23.161 Trim.
...
(d) In addition, each multiple airplane must maintain longitudinal and directional trim, and the lateral control force must not exceed 5 pounds at the speed used in complying with § 23.67(a), (b)(2), or (c)(3), as appropriate, with -
(1) The critical engine inoperative, and if applicable, its propeller in the minimum drag position;
(2) The remaining engines at maximum continuous power;
(3) The landing gear retracted;
(4) Wing flaps retracted; and
(5) An angle of bank of not more than five degrees.

But note the same standard for transport category aircraft certificated under 14 CFR Part 25:

§ 25.161 Trim.
...
(d) Longitudinal, directional, and lateral trim. The airplane must maintain longitudinal, directional, and lateral trim (and for the lateral trim, the angle of bank may not exceed five degrees) at 1.3 VSR1 during climbing flight with -
(1) The critical engine inoperative;
(2) The remaining engines at maximum continuous power; and
(3) The landing gear and flaps retracted.

FAA-S-8081-14A, Practical Test Standards, Private Pilot, Airplane:

A. TASK: MANEUVERING WITH ONE ENGINE INOPERATIVE (AMEL and AMES)
REFERENCES: FAA-H-8083-3; POH/AFM.
Objective. To determine that the applicant:
4. Establishes and maintains a bank toward the operating engine as required for best performance in straight and level flight.

 

[skiddriver]

The 5-degree limit is to keep the manufacturers from generating an artificially low Vmc number during certification testing (see below).

The drag found in the case where the aircraft is kept wings level is due to the sideforce component from the vertical stab after the rotation has stopped. Once you stop the rotation caused by the failed engine by using rudder (with wings level), the vertical stab is generating a sideforce in the direction of the dead engine that has to be neutralized to maintain straight flight. If left unchecked, it will cause you to follow a centripetal path. Additional rudder toward the live engine is required to counteract this, but this induces a sideslip that exposes the dead engine side of the fuselage to the windstream. This causes a righting moment that tends to once again bring the nose of the aircraft towards the dead engine. With sufficient airspeed across the tail plane and enough rudder, these forces can be balanced, but with significant drag caused by the increased induced drag at the vertical stab, and the increased parasitic drag caused by the dead engine side of the fuselage hanging out in the windstream. Your ball will be centered however.

The most efficient mode of flight is to counteract the rotation caused by the asymmetric engine thrust with the rudder, and then use the very small angle of bank required to counteract the residual side force from the vertical stabilizer (normally in the 3-degree range). You can’t see this condition in the cockpit unless you have a direct reading sideslip indicator. If you did, you would center the sideslip indicator using rudder, and then stop residual rotation from tail plane sideforce using angle of bank. Without the sideslip indicator, you set a small angle of bank and neutralize rotation with the rudder. It’s an approximation, but a close one.

You could continue adding angle of bank to a point where you wash out any rudder application and are counteracting the failed engine caused rotation purely with angle of bank. This generates a very large sideslip, is uncomfortable as all get out, increases stall speed and induced drag simultaneously, and can lead to tail plane stall. It does however have the effect of reducing Vmc to a minimum, up until the point where you hit a stall condition (wing or tail plane) and have the aircraft violently depart controlled flight. That’s why the FAA imposed the 5-degree limit.

And finally, as someone who sat through numerous murder boards for real experimental test flight and has been a working experimental test pilot, I encourage everyone to NOT conduct your own test flying out near (or past) the operating envelope of your aircraft. Spending some time trying to figure out whether 2 ½ or 3 degrees of AOB into the live engine is optimal is one thing. But getting outside of the box is just silly and is only going to end up badly.

 

[skiddriver]

It’s interesting to note that the military trained multiengine pilots never hear anything about this GA performance enhancing technique.

Just the ones who didn't pay attention at flight school, it was in the syllabus at the VT's in Corpus.

 

[UndauntedFlyer]

My position on this is that zero-side-slip is an over rated concept that was sold the FAA by some private pilot with a PhD who was trying to make a name for himself.

I have only seen posts by those who report performance enhancements through placebo effect. Or that say they learned of this in school.

Where does the rubber meet the road?

Do your own tests, for real with one engine feathered. Be patient so as to eliminate the placebo effect. See for yourself. With a yaw string you'll see no change either, ball in center or half way out with opposite bank.

If you're patient and ready for the truth it will be shown un to you.

Large multi engine airliners are performance critical at max GW with an engine failure at V1. Is there a word in the FM about zero side slip for the 777 or the 757 or the 767? The answer is NO. Why? Because it's mostly a myth with a sliver of truth. It’s just not worth mentioning

Don't just believe what is said one way or the other. Prove it to yourself as I have done a hundred or more times.

 

[A Squared]

I have only seen posts by those who report performance enhancements through placebo effect. Or that say they learned of this in school.

Interesting indeed that you casually dismiss everyone who has posted information counter to yours as being misled by "placebo effect" I understand what you are alluding to, that people tend to see what the expect to see or want to see. However,

I have two questions:

How can you be so certain you yourself are not being misled by what you refer to as "placebo effect"?

Are you aware that one of the posters whom you so casually dismiss experienced his "placebo effect" while flying instrumented test aircraft in a Navy Test pilot program?

Personally, I'd rate the Navy test pilot in an instrumented plane as a little more "placebo" resistent than yourself, who appears to have an agenda.

 

[UndauntedFlyer]

The following says basic ground school. I agree with everything here.

The drag found in the case where the aircraft is kept wings level is due to the sideforce component from the vertical stab after the rotation has stopped. Once you stop the rotation caused by the failed engine by using rudder (with wings level), the vertical stab is generating a sideforce in the direction of the dead engine that has to be neutralized to maintain straight flight. If left unchecked, it will cause you to follow a centripetal path. Additional rudder toward the live engine is required to counteract this, but this induces a sideslip that exposes the dead engine side of the fuselage to the windstream. This causes a righting moment that tends to once again bring the nose of the aircraft towards the dead engine. With sufficient airspeed across the tail plane and enough rudder, these forces can be balanced, but with significant drag caused by the increased induced drag at the vertical stab, and the increased parasitic drag caused by the dead engine side of the fuselage hanging out in the windstream. Your ball will be centered however.

The following says it is very slight but says nothing about real performance enhancement. I agree it is slight. Not enought to worry about.

The most efficient mode of flight is to counteract the rotation caused by the asymmetric engine thrust with the rudder, and then use the very small angle of bank required to counteract the residual side force from the vertical stabilizer (normally in the 3-degree range). You can’t see this condition in the cockpit unless you have a direct reading sideslip indicator. If you did, you would center the sideslip indicator using rudder, and then stop residual rotation from tail plane sideforce using angle of bank. Without the sideslip indicator, you set a small angle of bank and neutralize rotation with the rudder. It’s an approximation, but a close one.

The following says that if you you use no rudder and all bank that this can be easily seen and felt. I agree with this too and no one needs scientific instruments for this. Any yaw string will show this at that point.

You could continue adding angle of bank to a point where you wash out any rudder application and are counteracting the failed engine caused rotation purely with angle of bank. This generates a very large sideslip, is uncomfortable as all get out, increases stall speed and induced drag simultaneously, and can lead to tail plane stall. It does however have the effect of reducing Vmc to a minimum, up until the point where you hit a stall condition (wing or tail plane) and have the aircraft violently depart controlled flight. That’s why the FAA imposed the 5-degree limit.

The following says that to do the above example could be dangerous. True in some advanced airplanes. Then the following recommends experimenting with the exact demonstration I recommend. Follow this pilots advice. He is does know what he is saying.

And finally, as someone who sat through numerous murder boards for real experimental test flight and has been a working experimental test pilot, I encourage everyone to NOT conduct your own test flying out near (or past) the operating envelope of your aircraft. Spending some time trying to figure out whether 2 ½ or 3 degrees of AOB into the live engine is optimal is one thing. But getting outside of the box is just silly and is only going to end up badly.

In conclusion, everyone says the same thing: It does makes a difference. My point is that it makes no more difference then the rate of climb difference between flying any airplane with normally operating engines with the ball centered or off just a half a ball width. Its just so little difference that its not enough to worry about and can not be see on the VSI or a yaw string. And all this is only when at max power. Of course the slight side slip could be seen on a highly calibrated scientific instrument.

 

[avbug]

Anybody who knows what they're talking about certainly didn't say it makes no difference. It does.

My position on this is that zero-side-slip is an over rated concept that was sold the FAA by some private pilot with a PhD who was trying to make a name for himself.

Funny. It's in my pre wwii manuals, including a wwii aircraft I flew, and not only was it part of our training syllabus...but in every real-life engine failure I experienced in that airplane, it worked, and made a substantial difference.

Gee...were it not for that placebo effect, I'd probably be dead.

 

[UndauntedFlyer]

Placebo effect is powerful.


Avbug: Thanks for your service in WWII.