UndauntedFlyer
Ease the nose down
- Joined
- Feb 26, 2006
- Posts
- 1,062
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…..
“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…..
