Zero sideslip: Engine failure on multi engine airplane

R

Roberto152

Active Member
Hello,

I am a single engine CFI considering adding an MEI and was having some trouble believing and understanding the prevailing internet wisdom that I hope some of you MEI's can help with.

The idea is that during an engine failure a zero sideslip for straight flight would occur by banking 2-3 degrees into the good engine and having the ball approximately half way out of the "cage". The articles mention a yaw string would indicate coordinated or uncoordinated flight (I agree about the yaw string). So put a yaw string on the airplane and when the engine fails simply keep the wings level and keep the yaw string straight by using rudder. Now the aircraft is in wings level, coordinated flight. What's the problem with this?

Thanks for any help.
 
You're also increasing your VMC speed unnecessarily, and unknowingly to what degree, by holding wings-level flight.
 
Roberto152 said:
Hello,

I am a single engine CFI considering adding an MEI and was having some trouble believing and understanding the prevailing internet wisdom that I hope some of you MEI's can help with.

The idea is that during an engine failure a zero sideslip for straight flight would occur by banking 2-3 degrees into the good engine and having the ball approximately half way out of the "cage". The articles mention a yaw string would indicate coordinated or uncoordinated flight (I agree about the yaw string). So put a yaw string on the airplane and when the engine fails simply keep the wings level and keep the yaw string straight by using rudder. Now the aircraft is in wings level, coordinated flight. What's the problem with this?

Thanks for any help.
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Do you have Ecklebar's book "Flying High Performance Single Engines and Twins"? There's a long discussion about your topic in Chapter 14, pg 273 if you're using the hardcover.
 
Yes, regarding Vmc, you are correct. It will be higher wings level vs. banked into the good engine 2-3 degrees. To what extent? In the articles I read they ball parked about 3 knots per degree, so Vmc would be about 6-9 knots higher wings level.

The point I find difficult to believe is the attack on the idea of wings level flight after an engine failure. (Assuming we are not near Vmc such as would happen during normal cruise.) As long as the yaw string is straight the aircraft is in coordinated flight.
 
Roberto152 said:
Yes, regarding Vmc, you are correct. It will be higher wings level vs. banked into the good engine 2-3 degrees. To what extent? In the articles I read they ball parked about 3 knots per degree, so Vmc would be about 6-9 knots higher wings level.
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The key is, you don't know. It could be up to 20 knots higher. And single engine time isn't the best time to be playing test pilot, anymore than is already occurring with the emergency at hand.

The point I find difficult to believe is the attack on the idea of wings level flight after an engine failure. (Assuming we are not near Vmc such as would happen during normal cruise.) As long as the yaw string is straight the aircraft is in coordinated flight.
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For cruise thats a different story. Its why many aircraft just place rudder trim a little further into the good engine, as you're not combatting a rolling moment with a higher speed, in the way you are with lower speeds.
 
We even use a touch of bank in large transport category turbojets.
 
Derg said:
We even use a touch of bank in large transport category turbojets.
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Yes, me too, during climb out after a V1 cut when airspeed is slow and Vmc is a factor. And I'm glad I never lost one for real, but in the box I have never flown, and have never seen anyone else fly around dumping fuel while holding bank into the good side. Or being vectored around for a 3 engine approach. It is always wings level once sufficient speed is established (unless of course a turn is desired.)
 
Roberto152 said:
Yes, me too, during climb out after a V1 cut when airspeed is slow and Vmc is a factor. And I'm glad I never lost one for real, but in the box I have never flown, and have never seen anyone else fly around dumping fuel while holding bank into the good side. Or being vectored around for a 3 engine approach. It is always wings level once sufficient speed is established (unless of course a turn is desired.)
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I'm sorry but what? :)
 
Derg said:
I'm sorry but what? :)
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What I'm getting at is the following: I don't doubt you used a touch of bank in the transport categories you fly. I suspect however that those touch of bank incidents were limited to high thrust, low speed scenarios such as after a V1 cut during first segment climb. Perhaps even during second segment climb. In other words I understand and fully agree with using a small amount of bank to keep Vmc in check when thrust is high and airspeed is low.

What makes no sense to me is the notion of having to fly, after an engine failure, in a banked condition to fly straight, coordinated flight! The articles I read suggest that to fly wings level is uncoordinated, and therefore, very inefficient.

If this were the case, when your transport category jet loses an engine half way over the Pacific fuel burn becomes critical and there isn't (probably) the luxury of flying uncoordinated. So if the authors are correct these jets would fly for hours banked toward the good engine(s).

I have never seen anyone during a LOFT environment simulating such a scenario banking the aircraft for straight flight. Have you? And I have never seen anyone flying around banked into the good engine while getting vectored for an approach. I have never seen anyone banking into the good engine while dumping fuel to get below max landing weight. Have you?

I hope my tone did not come across as sarcastic, sometimes it is difficult to know when writing. No disrespect intended...
 
Roberto152 said:
What I'm getting at is the following: I don't doubt you used a touch of bank in the transport categories you fly. I suspect however that those touch of bank incidents were limited to high thrust, low speed scenarios such as after a V1 cut during first segment climb. Perhaps even during second segment climb. In other words I understand and fully agree with using a small amount of bank to keep Vmc in check when thrust is high and airspeed is low.

What makes no sense to me is the notion of having to fly, after an engine failure, in a banked condition to fly straight, coordinated flight! The articles I read suggest that to fly wings level is uncoordinated, and therefore, very inefficient.

If this were the case, when your transport category jet loses an engine half way over the Pacific fuel burn becomes critical and there isn't (probably) the luxury of flying uncoordinated. So if the authors are correct these jets would fly for hours banked toward the good engine(s).

I have never seen anyone during a LOFT environment simulating such a scenario banking the aircraft for straight flight. Have you? And I have never seen anyone flying around banked into the good engine while getting vectored for an approach. I have never seen anyone banking into the good engine while dumping fuel to get below max landing weight. Have you?

I hope my tone did not come across as sarcastic, sometimes it is difficult to know when writing. No disrespect intended...
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As I had mentioned, at higher speed like cruise flight, a bank isn't as necessary....if even necessary....because you're not fighting a rolling moment as you would be in a low speed situation. Hence just trimming off the rudder forces a bit and using that.
 
Nah. Think about the amount of rudder required for zero sideslip.

Lots of drag.

Now if you a little "horizontal component of lift", when you add a touch of aileron, you're actually reducing the amount of drag you're incurring on the airplane.

You'll see that demonstrated during a "Drag Demo" during your MEI training and I don't mean adult men walking around in heels.

It will all be clear. But I absolutely recommend against "Well this doesn't make sense to me so I'm not going to do it" at any point in training.
 
Let us know how this discussion ends with the examiner. Regardless or speed you will be skidding without some bank to counter it. Whether it's a half ball out of center or not may be debatable but the physics is not.
 
I'm confused, how would you keep the airplane flying straight with no bank without being in a sideslip? To fly straight without banking you would have to yaw excessively towards the good engine, meaning the airplane would be traveling sideways. If you don't bank and just add rudder until you're holding your heading it is going to look like this:
 

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Videographer,

I couldn't see the top portion of the picture you attached -- I would have expected more from a videographer!! (Just joking). :). Anyway which direction is the wind coming from in your picture? The reason I ask is because with the yaw string centered the wind is coming straight on the nose of the airplane; this must be the case since it is this same airflow which forces the yaw string straight back. Since this is the case the airplane must be flying straight into the wind. Therefore the aircraft's heading is identical to its track (track through the air mass of course not necessarily its ground track). If the track through the air mass was different than the aircraft's heading the yaw string would not be straight! Coordinated, straight flight.

I have read about the idea of a boat turn. This is the idea that, yes, the wings level, yaw string centered scenario is coordinated but the aircraft is actually turning in the direction of the dead engine. I though about this one for a while because on the surface (boat haha) it sounds believable. But if it is true that the aircraft is turning WITH THE WINGS LEVEL then there must be a force pushing everything in the aircraft toward the outside of the turn. In a left turn with the wings level a marble on a flat surface would appear to roll to the right for example. This same force would act on the yaw string displacing it off of center. But if we keep the yaw string straight AND the wings level there can't be a turning force pushing the yaw string off center.
 
That's why I made this post Derg. I'm trying to get this understanding. So, would you fly your jet transport banked into the good engine for 5 hours over the Pacific? Would that give you an efficient, no slip straight flight path? Or would you fly wings level?
 
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