Myth Busting - Engine Inop Performance

[UndauntedFlyer]

MYTH BUSTING – MULTIENGINE PERFORMANCE WITH AN ENGINE INOP

“THE HIGH DRAG CAUSED BY THE WINGS LEVEL, BALL CENTERED CONFIGURATION CAN REDUCE SINGLE ENGINE CLIMB PERFORMANCE AS MUCH AS 300 FEET PER MINUTE.”

The above statement is simply not true. While it is theoretically correct to say that performance would be enhanced by using some bank and slightly less rudder to correct for an engine that is inoperative, thus eliminating a SLIGHT aerodynamic side slip, actual in-flight tests show no NOTICABLE changes in performance.

Now let’s talk about configuring a twin engine airplane to the much talked about “zero side slip” condition when operating with an engine feathered. This is always a big deal in GA multiengine training manuals and always a subject to be addressed on oral tests for multiengine ratings and multi instructor ratings. This technique is supposed to enhance climb performance when operating with an engine inoperative. Does it? Well the answer is that it does theoretically on paper but in flight the enhancements are impossible to see because they are so slight, if they exist at all. If a person really wants to see enhanced performance they will, but those who give such testimony I believe are affected by a sort of placebo (wishful performance) effect that fools even well intentioned CFI’s. Those people usually see the plane climb 100 to 300 feet per minute better than when configured with the ball centered. The theory goes that with the ball centered and an engine feathered there is a side slip that can be seen with a yaw string taped to the nose ahead of the pilot. And by just letting off on the rudder correction to let the ball slip out just a half a ball width while holding heading with more opposite bank, that that will straighten up the yaw string and all of a sudden the airplane will start climbing better.

I have personally tested this many times, maybe 100 times on typical GA multiengine aircraft. My results of the test are always the same: No noticeable increase in performance. I have even gone out with my yaw string and could find no noticeable displacement in the string when the ball was in the middle or a half ball width displaced as is supposed to improve performance. Time and time again I have tried this and the results are always the same. Of course, when I do this test I actually feather the inoperative engine.

Really though, if you think about it, how could just half a ball width on the inclinometer really have much of an effect on slip stream or on performance? If you use what is well know from your common single engine experiences and think about it, you’ll see that it can’t be true that there is such a bragged about boost. How about this, take any single engine airplane and do the opposite. What I mean is just let the plane go from a ball centered (zero side slip) configuration in the single engine airplane to a half ball width deflection (slight side slip) configuration and does that really make a difference that can be noticed, either on performance or on a yaw string. Certainly not. Oh I’m sure it does make a “DIFFERENCE” but, like the twin, it’s on paper (mostly academic) and not really a noticeable difference. Of course a really huge 2-ball width deflection would increase drag noticeable. But just a half a ball width, give me a break, that’s not enough to notice anything.

It’s interesting to note that the military trained multiengine pilots never hear anything about this GA performance enhancing technique. And no one flying twin engine airline equipment such as the 737, 757, 767 or the 777 ever hear anything about how this technique either.

Actually, this technique is somewhat new to the GA world too. 30-years ago nobody ever heard of it and it was not in any of the FAA books either. Then some how, all of a sudden, this came into print as something new, which it was, and almost everyone embraced it as the gospel, without objectively flight testing it. At that time, to challenge this was blasphemy. It was thought that whoever believed would be safe from danger and would live to be 100-years of age, illness and accident fee. I wanted to believe in it too, but my own flight tests have shown that it’s mostly a performance myth with just a sliver of truth.

So what does all this mean? Just simply this: This much discussed technique of trimming to permit the ball to slip out just a bit when flying with an engine feathered will not hurt anything, but don’t count on it to save your life by producing a noticeable improvement in single engine performance.

Don’t misunderstand me; a person should know all about what is written on this subject in FAA books if they want be successful on their FAA multiengine rating or multiengine CFI tests. And it’s probably NOT a good idea to challenge “zero side slip” for the examiner (unless it’s me), but just keep in mind that this whole thing is mostly an embellishment of a true aerodynamic concept.

Comments/Questions/more myths…..

 

[MauleSkinner]

Have you ever tried it in the clouds at 200 feet AGL, from a ball-centered zero climb? I know a guy who swears it makes a noticable difference there

 

[GogglesPisano]

I always wondered about this myself. In GA the point was hammered home to never fly s/e with the ball centered. Get hired by a jet airline and the training is completely reversed. I figure it's due to the fact that performance isn't as critical in jets given the amount of excess thrust available.

 

[MauleSkinner]

I always wondered about this myself. In GA the point was hammered home to never fly s/e with the ball centered. Get hired by a jet airline and the training is completely reversed. I figure it's due to the fact that performance isn't as critical in jets given the amount of excess thrust available.

Probably more that the performance isn't as critical in jets given the engine-out climb requirements we are required to have, as opposed to not requiring any single-engine performance for a Part 91 light twin.

In other words, you can legally find yourself in the situation I mentioned in my earlier post in a Part 91 light twin. Not so in a jet, unless you have more than simply an engine failure going on.

Fly safe!

David

 

[firstthird]

Your 100+ tests not withstanding, I have my doubts about your assertion. I'm at a hotel and need some reference materials to fully opine but off the top of my head.

1. Light twins are not required to be able to fly on one engine much less climb, thus the old joke about light twins that the second engine allows you to choose the scene of the accident. Whereas transport catagory jets have all kinds of climb gradients they must meet.

2. Jets tend to have spoilers that might deploy if you get too aggressive with the yoke/wing up, degrading climb performance.

3. Light twins have straight wings with a good amount of dihedral whereas jets have swept wings that have nasty tendancies when you start yawing and rolling them around.

4. in military prop training (well, navy prop training) they teach the same thing about wing up and a slightly displace ball. At least I think, it's been 5 years or so and you know that memory is a fickle thing.

I could be wrong about all of the above but when I was instructing in a 4 engine turbo-prop we used to do a demo to show the advantage of the 5 degrees wing up and ball off center, so my experience is different than yours when it comes to 'flight testing' this textbook information.

I would also posit that some low time pilot might inappropriately try to apply some of the 'wisdom' found on this board and get into trouble. As for me, the book answer of wing up and ball out works and since I'm not a test pilot, I doubt I'd be trying wings level, ball centered because someone said it is just as good on a forum.

Anyone else disagree with this 'myth-busting'? We must have some active instructors in light twins that see this scenario all the time.

 

[100LL... Again!]

5 degree myth.

I read a white paper a few years ago that thoroughly debunked the 5 degree myth.

Maximum climb performance was found at 1.5 to around 3 degrees.

The 5 degrees is a certification criteria, not necessarily where best performance is to be obtained. This is important to understand.

 

[MauleSkinner]

The 5 degrees is a certification criteria, not necessarily where best performance is to be obtained. This is important to understand.

True...more bank/slip decreases Vmc. A 5-degree limit is placed on this so that Vmc speeds aren't artificially low. As you said, 1.5-3 degrees yields max performance.

Just as a historical note, I believe they started doing this in P-38's during WWII...the P-38 has counter-rotating props, but they rotate top-outward, instead of top-inward like the Seminoles, Dutchesses, etc. Engine outs were considered a lot more deadly until they started using the bank/slip method.

Fly safe!

David

 

[Tired Soul]

Bank definitely works in the Aztec, can't really say about anything else.
3 or 5 degrees, truly don't know, the attitude indicator is not that accurate.

 

[100LL... Again!]

Bank definitely works in the Aztec, can't really say about anything else.
3 or 5 degrees, truly don't know, the attitude indicator is not that accurate.

Another excellent point.

I tell students that if it don't climb wings level well enough, to get the attitude indicator just slightly dead engine up. That will yield a degree or two.

 

[landlover]

True...more bank/slip decreases Vmc. A 5-degree limit is placed on this so that Vmc speeds aren't artificially low. As you said, 1.5-3 degrees yields max performance.
Fly safe!

David

i have a pet peeve about this myself. Ive come across some people that insist you must bank 5 degrees, no more no less. I always say to them if you needed to bank less or more to live through the flight would you? Like you said 5 degrees is a limitation placed on the test pilot during certifcation.

 

[avbug]

I don't know about this deeply scientific mythbusting effort, but I always demonstrated to students the difference so that they could see for themselves...and there is a difference (not to shoot down your mythbusting effort, here, but it's crap).

As for banking into the good engine being part of a "new" thing wrought on by the FAA...it's in my 60 year old wwii airplane manuals for the airplanes I used to fly...and guess what? It made a big difference.

Lose an outboard engine on a B-24, and without rudder trim, it's about 70 lbs of force on the rudders to hold it. Take out the force with trim, bank into the good engine. Follow the checklist. Clean it up. Then try running it level and ball in the center, vs. not. Dang skippy there's a difference.

 

[skiddriver]

I just pulled out my test cards from my asymmetric flight ride at USNTPS in a U-21. The aircraft was partially instrumented (including a sideslip meter), and we took data to develop static and dynamic Vmc as well other asymmetric flight data.

If the rudder pedals hit the stop before stall/stall warning, we continued to slow the aircraft with 5 degrees of bank into the live engine until we hit a test limit. It gave you a significant difference in Vmc (as we were all taught). Relative climb performance difference wasn't huge, but it was measurable during our tests, which were of course greatly aided by the direct sideslip readout in the cockpit.

 

[UndauntedFlyer]

Let’s talk more about the zero sideslip performance enhancement technique that this thread addresses. First the point is not that it doesn’t exist, the point is that it is so small that it might as well be just an academic point. Just as it would be for a single engine airplane with the ball centered vrs. off by one-half a ball width. As was mentioned in the original posting, how much performance could really be lost in a single engine airplane with a half-ball deflection of the ball in a forward slip? Some for sure, but not really enough to see in the airspeed or rate of climb indicator. So that’s academic too and the degradation in performance would be equal, and neither can be seen.

As was mentioned previously in the original post, I have personally never been able to see any change in performance in a twin engine airplane with an engine feathered and the ball centered vrs. the ball out of center by half a ball width while in the so called zero side slip configuration. I have done this many times in many GA airplanes and have even used a yaw string to see if there is any noticeable deflection with the ball centered or off by half a ball. No change in yaw string deflection is seen. Oh sometimes I hit a slight up draft and the VSI goes up a bit for sure, so I think I have proven the concept correct, but with patience the real result is nothing, no performance enhancement. Time and time again the result is the same. So what can I think when someone reports an increase of 300 – 500 fpm, this must be placebo effect. Maybe there really is a 5 - 25 fpm increase but I can’t read that. And when someone reports that this is in some kind of official book somewhere then that’s fine, after all I have said that it’s in the current FAA Airplane Flying Handbook. But you see, that stuff is all academic. The actual significance and the truth of all this is in real in-flight tests. That’s where the rubber meets the road.

Now if you do this test yourself, you must use real feather because so called zero-thrust settings are all questionable. The only real zero thrust is FEATHER. I say this because, as I said before, here too is where the rubber meets the road.

So in reality does it hurt anything to put in only enough rudder to bring the ball to a half-ball deflection zero side slip configuration? For performance, certainly not, and it’s easier on the leg (or the trim) for a 100 mile diversion to an alternate airport. Plus I suppose the airplane might get an extra mile or two in range. And further, if you lost an engine on takeoff and wanted to take it easy on your leg for the climb out, let off on the rudder a little to half a ball width deflection, using a little bank to correct instead. You certainly won’t hurt performance by this technique, but as for rate of climb enhancement, don’t count on really getting much of anything. Academically yes, less leg muscle yes, but as for performance and safety: It won’t hurt performance and safety is not a concern unless you get too slow and stall cross controlled as it would be with the ball deflected. Remember the wing in a stall always drops opposite the ball and the further the ball is out the harder it drops.

So what is the problem if everybody just believes in this great boost in performance from zero side slip and always just configures to keep the ball out of the center following and engine failure. One thing is mis-identification of a failed engine. I have seen multiengine pilots in training and on check rides try to determine the inoperative engine with the ball out of center. This just doesn’t work for 100% accuracy. To use the “dead foot, dead engine” technique the ball must be centered. Otherwise, as I have see done, confusion causes some pilots to correct for the failed engine with all aileron and the wrong rudder. This can happen, believe me. In this case the dead foot is not the dead engine. If the ball is checked for center position that would show the mistake. So before the student shuts down the wrong engine I ask why the ball isn’t in the center? And the answer is that it’s wrong to have the ball in the center with an engine failure. This is a case of too much cool-aid. I have also seen multiengine students trying to fly single engine instrument approaches in a ball deflected forward slip because of their mis-understanding of the entire reason for the zero side slip concept. They think that having the ball centered with an engine out is never correct. My God, what are they thinking, we are going down hill on the ILS with only partial power on the operating engine. Again, way too much cool-aid here.

Regarding just when all this zero slide slip information started, I really can’t say for sure. If it is in a B-24 manual that’s great because it should be for those guys. I’m sure that if an outboard left (critical) engine was inoperative and particularly if it couldn’t be feathered, the zero side slip would, as I have mentioned, increase the range somewhat. But I can say, as I said in my original post, 30 - 40 years ago, no POH or FAA publication for GA training that I know of explained the zero side slip configuration.

You see, this all came about after a rash of twin training accidents in the 1965- 1975 time period. During that time and before many things were different on multiengine flight training and practical (flight) tests. For example:

• Single engine stalls were trained and tested.
• Fully configured gear and flap down single engine go-around procedures were trained and tested.
• Vmc demonstrations were much more aggressive than they are now. Vmc demonstrations were continued until the pilot was actually losing control.
• There were NO twin engine airplanes with a counter-rotating right engine.

As you might expect there were training accidents. And in particular there were the most training accidents in the newly introduced Twin Comanche. There were also too many accidents in the Beech Barons. Both of these airplanes have higher wing loading than the Apache which was the twin engine training standard at that time. As a result, the single engine stalls and the Vmc demonstrations in these airplanes sometimes got ugly. Because of the higher wing loading the wing would drop hard in the stall and the operating engine would flip the plane inverted into a near inverted spin. Quick and appropriate action most often would save the day, but sometime things just went too far too fast. Way too many training accidents were occurring and finally, the son of a government VIP was killed in a Twin Comanche accident. As a result a somewhat large investigation was ordered by the FAA.

The FAA had a big meeting in Washington and everyone who they though knew anything about multiengine flying was invited to attend. They didn’t know about me then so I wasn’t asked, but I’m told that there were FAA folks, authors, college Phd’s, and manufacturing test pilots from Cessna, Beech and Piper. Everybody wanted to put forth their two-cents worth as to how to stop the accidents. Everyone wanted to make a name for themselves in front of all the FAA big wigs. The conclusions were:

• No more single engine stalls.
• No more Vmc Demonstrations if Vmc is close to Vso except by blocking rudder pedals and then recovering at first indication of a stall, such as the horn.’
• No more single engine go-arounds.
• Vmc was raised on the Twin Comanche with a red paint brush on the airspeed indicator. This was by the power of an FAA AD note.
• Manufactures were encouraged to get a counter-rotating engine for the right side to lower Vmc and eliminate a more critical engine left engine.
• And the “zero side slip” technique was to be taught.

Now I have been told that the college professor, you know the Phd with no multiengine time, is the one that put in his two-cents worth about the zero side slip. He got up in front of everyone with all kinds of charts and graphs to explain the zero side slip technique and then he went on an on until everyone’s eyeballs were spinning. The conclusion was this must all be true, as I agree that it is. My disagreement in this thread is simply with the amount of performance enhancement. They said 300-500 feet per minute and I say no way. Try almost nothing based on my tests only. Some planes may produce more of a performance enhancement but I would be skeptical. There is some, but just as it is in a single engine airplane with the ball centered vrs. off to the side by a half- ball width, it’s just the amount performance change that we are discussing here.

Your comments/questions are welcome.

 

[avbug]

Just one question, really.

Were you born yesterday?

 

[100LL... Again!]

I'm sure it varies a fair bit between different airplane types.

One thing is for certain - the idea that 5 degrees is always the correct amount for max climb is laughable.

 

[UndauntedFlyer]

The thing that I believe is most laughable is the pilot that is flying his final approach single-engine in a twin and trying to hold the ball out of center with rudder and some degree of opposite bank, in other words, in a forward slip.

They think this is the right thing to do for single engine approach safety even though the operating engine is only at about 18"-20" MP and the airplane is holding airspeed and descent rate perfectly.


People like this have been brainwashed with mis-information or as I have previously said, too much cool-aid. It's not their fault though because they are only the product of the instruction they receive and many of the instructors have had way too much cool-aid too.

I do put some blame on the FAA through the PTS for this particular misunderstanding. Under the TASK, "APPROACH AND LANDING WITH AN ENGINE INOPERATIVE," one of the line items or evaluation elements is, "BANKS TOWARD THE OPERATING ENGINE, AS REQUIRED, FOR BEST PERFORMANCE." Of course the misunderstanding here by the student is that best performance isn't necessary unless the power is at maximum full throttle to hold required airspeed and descent profile. So therefore even if the "zero side slip" technique truly did enhance performance, only then with max power set would it be used.


Of course this whole thread disputes the whole zero side slip concept pretty much by saying the performance enhancement are so slight that it's not worth concerning yourself with, at least in most any light twin and its not even a consideration to be discussed in large jets.

 

[nosehair]

this whole thread disputes the whole zero side slip concept pretty much by saying the performance enhancement are so slight that it's not worth concerning yourself with.

...yeah, and what about those people who promote keeping your tires aired up?...yeah, ya might get an extra mile or two gas milage, but..so what?...not what they say about 4 or 5...huh!..(snort)

 

[MauleSkinner]

Of course this whole thread disputes the whole zero side slip concept pretty much by saying the performance enhancement are so slight that it's not worth concerning yourself with.

I beg to differ...if you read my original post

Have you ever tried it in the clouds at 200 feet AGL, from a ball-centered zero climb? I know a guy who swears it makes a noticable difference there

you'd find out about somebody who definitely figures it's worth concerning yourself with.

 

[avbug]

Five degrees is a certification number. It may take more, it may take less. this has always been the case.

Ball out of the center doesn't mean the aircraft is side slipping. Ball out of the center in a bank means the aircraft is banked and traveling straight ahead, with some horizontal component of lift counteracting assymetrical thrust, with a far lower drag penalty than deflecting rudder and aileron to keep the wings level and the ball in the center.

If you don't understand the concept, then you came from a deprived background, or were never fully taught properly, or perhaps were born yesterday.

The amount of bank required may be half a degree, it may be five degrees, it may even be more. Use what is required, when it is required...nothing published should teach otherwise.

 

[minitour]

Ball out of the center doesn't mean the aircraft is side slipping. Ball out of the center in a bank means the aircraft is banked and traveling straight ahead, with some horizontal component of lift counteracting assymetrical thrust, with a far lower drag penalty than deflecting rudder and aileron to keep the wings level and the ball in the center.

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

...wow...

Ball out for sure.

Thanks again.
-mini