Zero sideslip: Engine failure on multi engine airplane

[derg]

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?

Generally, the autopilot will take care of that and it technically shouldn't be more than three hours. It's not a matter of convenience or hassle, it's a matter of aerodynamics.

We had a guy that would have been able to explain it on a much more technical method than I can from an iPad over a turkey sandwich at a diner, but the "circular logic" crowd shooed him away.

What type of materials are you using to prepare yourself for the MEI?

 

[CaptBill]

I'm not an aeronautical engineer and won't try to explain your yaw string or drag theory. What you want to accomplish when you encounter any sort of catastrophic failure is aircraft stability. When an aircraft loses an engine it will want to go someplace different and behave based on how it is designed. What you as a pilot have to do to survive is manage stability, not efficiency. There are many many factors that will affect how an airplane will perform under a variety of environmental conditions; however, your job as the pilot is to manage it's behavior with whatever that requires. Some single engine procedures require a limited bank angle due to terrain or something that is very unique. Don't get caught thinking there is only one answer for every engine failure. Use a soft touch and allow yourself to hear and feel what the airplane is telling you it needs. Some airplanes need more rudder and less aileron, others just the opposite. The bottom line is "fly the airplane" and return safely to the ground. Science is great to know, but a keen sense of your airplane's personality and a maturity level to do what you need to do vs. what some author wrote in the comfort of his armchair could very well determine if you live or die. Sometimes too much thinking is not good, so I try not to do too much of it as most of you know.

 

[Karnage]

To further what @thevideographer said, read chapter 12 of the Airplane Flying Handbook. It has the image he posted plus a couple others that help to show that centering the ball and maintaining wings level will actually put the nose at an angle to the relative wind.

By leveling the wings and centering the ball, you will be using a lot of rudder and yawing the nose into the operating engine putting the relative wind off the nose onto the side of the failed engine, which would deflect the yaw string towards the operating engine.

Conversely, by leaving the rudder centered and only using ailerons to bank into the good engine you will actually place the relative wind onto the side of the good engine, deflecting the yaw string towards the inoperative engine.

It's actually a combination of aileron and rudder canceling the other's tendency to yaw the plane into or away from the operating engine that will center your yaw string and place the relative wind straight off the nose and give you the best climb performance on one engine in a light twin.

 

[MikeD]

Science is great to know, but a keen sense of your airplane's personality and a maturity level to do what you need to do vs. what some author wrote in the comfort of his armchair could very well determine if you live or die. Sometimes too much thinking is not good, so I try not to do too much of it as most of you know.

Just have to listen to what the machine is telling you, as well as understand the language its speaking.

Just like what I've always said about helicopters: The helicopter does in fact talk to you at all times. But woe be the pilot who 1. Doesn't listen to what the helicopter is telling him (complacency), or 2. Doesn't fully understand the language the helicopter is speaking (inexperience); for the helicopter will interpert both of these failures to listen to it, as a pilot who also has a suicidal death wish same as the helicopter, and who gladly wants to share with the helicopter a double-suicide fate. A fate the helicopter is more than happy to accomodate. And the helicopter is, and has always been, equal opportunity in this regard; for it harbors no hatred, holds no personal grudges, and does not discriminate in any way, shape, or form.

Helicopters have no friends, don't want any friends, and don't care to be your friend.

Understand that fully, and you will be fine as the pilot of them.

Though not as bad or unforgiving (depending), airplanes also talk to you, and various degrees of the above can apply to them too, depending on the circumstances.

 

[CaptBill]

Just have to listen to what the machine is telling you, as well as understand the language its speaking.

Just like what I've always said about helicopters: The helicopter does in fact talk to you at all times. But woe be the pilot who 1. Doesn't listen to what the helicopter is telling him (complacency), or 2. Doesn't fully understand the language the helicopter is speaking (inexperience); for the helicopter will interpert both of these failures to listen to it, as a pilot who also has a suicidal death wish same as the helicopter, and who gladly wants to share with the helicopter a double-suicide fate. A fate the helicopter is more than happy to accomodate. And the helicopter is, and has always been, equal opportunity in this regard; for it harbors no hatred, holds no personal grudges, and does not discriminate in any way, shape, or form.

Helicopters have no friends, don't want any friends, and don't care to be your friend.

Understand that fully, and you will be fine as the pilot of them.

Though not as bad or unforgiving (depending), airplanes also talk to you, and various degrees of the above can apply to them too, depending on the circumstances.

Helicopters hate me so I'm not going to allow one to kill me because they would... Then laugh.

 

[Autothrust Blue]

To further what @thevideographer said, read chapter 12 of the Airplane Flying Handbook. It has the image he posted plus a couple others that help to show that centering the ball and maintaining wings level will actually put the nose at an angle to the relative wind.

By leveling the wings and centering the ball, you will be using a lot of rudder and yawing the nose into the operating engine putting the relative wind off the nose onto the side of the failed engine, which would deflect the yaw string towards the operating engine.

Conversely, by leaving the rudder centered and only using ailerons to bank into the good engine you will actually place the relative wind onto the side of the good engine, deflecting the yaw string towards the inoperative engine.

It's actually a combination of aileron and rudder canceling the other's tendency to yaw the plane into or away from the operating engine that will center your yaw string and place the relative wind straight off the nose and give you the best climb performance on one engine in a light twin.

An even better read is Chapter 39 of Volume One of Barry Schiff's The Proficient Pilot, which explains the relationship between bank angle and minimum controllable airspeed in considerable detail. The less obvious, but more insidious result of flying wings level, ball centered and OEI is a result in lower controllability, as everyone here is pointing out. (The obvious problem is the additional drag of a sideslip adding to an already bad situation.) The rudder will actually be LESS effective if you don't bank toward the operating engine. If you don't have this book, get it; I strongly recommend both volumes. Failing that, the FAA does an okay job of explaining it, but they don't dive in with the level of detail that Schiff does.

Here's a more extreme example: On the Brasilia you HAVE to use the ailerons to keep the airplane under control in an engine failure at V1. It is as much about controllability as it is performance. The effects are very pronounced; the failure of an engine down slow results not just in asymmetric thrust, but a considerable loss of lift on that wing as well, and it's going to try to roll on you as well as yaw towards the inoperative/feathered (we hope) engine. My 'muscle memory' for it is "push [with foot] and turn [with hands]" with both sets of inputs going away from the failed engine.

Operationally, all you really need to know is "bury the rudder and raise the dead," and that failure to bank into the operating engine will result in a less controllable, less-performing airplane. (Too much will do the same thing, too.)

 

[nosehair]

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?

Roberto, have you actually tried to do this? I mean, keep the nose (heading) constant while keeping the yaw string straight while keeping the wings level? Try it and get back to us?

 

[Dugie8]

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.

If your thrust was coming from the center of the aircraft and the yawing motion was simply a "force" that was trying to push the nose one way (say to the left, ie a left engine failure) and you were countering that force with right rudder, a force trying to push the nose to the right, then yes your scenario works.

However, in a normal twin engine airplane, meaning the engines our mounted laterally from the centerline of the aircraft, you have more than just a yawing motion to counteract. Your thrust line is now shifted away from the center of the aircraft and you have to point that thrust line away from the centerline to make the center of thrust down then center of the aircraft. Just like lift vectors in a turn.

 

[Roberto152]

What type of materials are you using to prepare yourself for the MEI?

Derg,

I haven't decided yet if I will get the MEI; my question was prompted by a cursory reading into getting the MEI. While reading about it I came across the banking into the good engine procedure to eliminate uncoordinated flight while flying straight. It claimed a zero bank, ball centered configuration would result in uncoordinated flight. I thought, ok, if this is true, throw out the slip/skid indicator and replace with a yaw string. Now with the yaw string straight must mean coordinated flight (I don't know what the ball would indicate when the yaw string is straight).

I thought perhaps someone on the forum could explain it. Any aeronautical engineers out there?

Nose hair (great call sign!) I haven't tried to do this but if I get access to a twin I'll give it a shot.

Anyone out there care to try? If so please also take note of the ball position during wings level straight yaw string configuration.

I appreciate everyone who has taken time to try to help!

 

[aviatorx43]

You're saying that you can fly with an engine out while maintaining zero bank and keeping the ball in the middle with rudder, and the yaw string will be in the center? I'm pretty sure this isn't possible. The string will be way off center, and it'll only get closer to center when you bank into the good engine and use less rudder.

 

[MercFE]

Now with the yaw string straight must mean coordinated flight (I don't know what the ball would indicate when the yaw string is straight).

I think what you're missing is what was posted in the earlier figure... With wings level and the ball centered, in an engine out scenario, the aircraft is not coordinated. With the thrust off the center line of the aircraft, it would need to have the relative airflow coming from the dead engine side of the aircraft. By turning away from the wind, you can once again center the thrust vector through the center of gravity of the aircraft, and therefore are slipping the aircraft into the wind. All of your installed indicators are saying you're coordinated, but a yaw string would show that the wind is coming across the aircraft from the side.

Turning into the good wing and "slipping" counteracts this force and returns you to truly coordinated flight. As others have stated, it may not be needed at cruise speed, but it doesn't take away the fact that it is in fact happening.

 

[Roberto152]

Regarding the position of the ball, that is NOT what I'm saying. I'm saying fly wings level with the yaw string straight. Forget about the ball. I don't make any claims about the balls's position. With the wings level and the string straight there is no slip or skid (simply because the string is straight). There is no horizontal component of lift (because the wings are level). We were all taught that, unlike a boat, airplanes turn due to the horizontal component of lift (there is none wings level).

Perhaps the aircraft is turning left with the left engine failed, the wings level, and the yaw string straight, but I don't see how that is possible. IF it is turning left it is not doing so via the horizontal component of lift.

Regarding those of you who mention Schiff's book and the other readings mentioned I'm working on getting those. Thank you for the references....

 

[bc2209]

Reading this has me humbled. Three flights into Multi training so far and learning the aerodynamics is fascinating and daunting.

 

[Karnage]

Regarding the position of the ball, that is NOT what I'm saying. I'm saying fly wings level with the yaw string straight. Forget about the ball. I don't make any claims about the balls's position. With the wings level and the string straight there is no slip or skid (simply because the string is straight). There is no horizontal component of lift (because the wings are level). We were all taught that, unlike a boat, airplanes turn due to the horizontal component of lift (there is none wings level).

You won't be able to center the yaw string and fly straight and level. In order to center the yaw string you would need to use less than the required rudder force to maintain a constant track over the ground. By keeping the longitudinal axis of the plane (and the yaw string) aligned parallel to the relative wind you will not be equalizing the drag of the dead engine with the increased yaw of the operating engine. Yes the string will be straight, but you will be in a gradual yaw towards the dead engine and not be able to maintain a straight ground track.

If your goal is the center the yaw string, you need to both bank into the good engine to counter act the loss of lift from the failed engine (accelerated slip stream and all that) and apply rudder to counteract the yaw of the operating engine. It just so happens that when this is done in a plane such as a Seminole the ball gets moved about 1/3 to 1/2 of the way towards the operating engine.

Another way to think of it is that the center position of the ball (which can also be thought of as placing the relative wind directly off the nose which would also center a yaw string) as depicted on the gauge only applies when both engines are operating and producing equal amounts of thrust. When one engine fails, the true center (read position where no slip is incurred to fly a straight ground track and to center the yaw string) gets moved towards the operating engine. This new center is not a fixed location but is instead moves based on the amount of thrust being produced by the good engine and how much more lift is being created on that side of the plane. When an engine fails, the center of the gauge is now no longer the location of zero yaw/slip/skid and that center has moved some amount towards the operating engine.

 

[MikeD]

Regarding the position of the ball, that is NOT what I'm saying. I'm saying fly wings level with the yaw string straight. Forget about the ball. I don't make any claims about the balls's position. With the wings level and the string straight there is no slip or skid (simply because the string is straight)...

Realize too that having the yaw string straight when both engines are operating nicely, versus having the yaw string straight...or as close to straight as possible....when jamming in a boot full of rudder while single engine; are not the same thing in terms of drag, etc, and whether or not you truly are skidding through the air to an extent, or not.

 

[derg]

I think if you draw out the vectors, it'll help understanding.

 

[Roberto152]

Bc2209,

On your next flight can you tape a piece of yarn on the center of the windshield?

If n156499000 (post #34) is correct, flying level with the string straight will cause a boat turn in the direction of the dead engine. In that case the plane can in fact turn with no horizontal component of lift.

Let's see what happens.

 

[Karnage]

You're not turning, in as much as the plane is unable to maintain a straight line track as it's always yawing towards the dead engine requiring aileron and opposite rudder to keep the wings level. While you are able to maintain a heading, your path over the ground will vector into the dead engine. The only way this works out to maintain a straight line ground track is if the local winds are coming from the side of the dead engine and happen to be at the right speed and angle to cancel out the plane's tendency to deflect towards the dead engine. Remember that just because the ball is centered on the gauge (I know you want to ignore the ball but hear me out), the center of the gauge is no longer the location where when the ball is there the plane is in a zero side slip condition. This is why your yaw string will not center. A light twin (like a Seminole) will not be able to maintain a zero side slip condition with one engine failed and keeping the wings level. I know because I tried. For my private multi my instructor and I would use one of the many long straight roads here in Arizona as a reference point. With the left engine failed, even if I maintained heading, I could not stay over the road without banking into the operating engine because wings level alone put the plane into a level slip.

To relate it to your boat analogy (not a good one since boats move in 2 dimension not 3 unless they have a hole in the bottom), think of it as one of those river crossing current correction equations from trig class. Yes the plane (boat) will be pointed straight at the opposite shore when flying wings level when one engine has failed, but the current (in this case the planes's slip condition) will cause the plane to make land at a location downstream and not directly across the river from where it launched. In order to compensate for the current (plane's slip condition when flying wings level with one engine inoperative) in addition to pushing the rudder upstream you have to also turn the boat (bank into the operating engine) so that the bow is pointed slightly upstream as well. Only by both turning the bow upstream and applying rudder to counteract the current will a boat land at a point directly across the river from where it started. It's really a bad analogy because it ignores the whole asymmetrical lift but it works.

It's rather like maintaining level flight in a single engine plane but pushing the rudder hard over and keeping the wings level . The nose will deflect in the same direction as the rudder input and the resulting bank is cancelled out by the opposite aileron input. The plane will veer in the direction of the rudder input putting you into a side slip situation. Yes your heading will remain constant, but the plane will be slipping to the side where the rudder is deflected.

 

[Roberto152]

Ok, so I downloaded the Airplane Flight Manual and reviewed the pertinent parts of chapter 12. There were three scenarios listed. None described wings level, straight yaw string although the first showed a slip with wings level and the ball centered. The relative wind was from left to right (left engine failed). If the pilot reduced right rudder, the ball would move to the right and the yaw string would begin to straighten out. Reduce right rudder until the yaw string is straight.

To throw more gasoline on the fire... If we are banked into the good engine (right) and the yaw string is straight we are not slipping, but there is a horizontal component of lift to the right. In a side slip the aircraft can be banked and fly a constant track which is different from its heading. But in coordinated flight a bank means the horizontal component of lift is turning the aircraft.

Scenario 3 of the AFM chapter 12 shows a shallow right bank with a straight yaw string. Don't we call this situation a shallow right turn?

 

[Karnage]

None described wings level, straight yaw string although the first showed a slip with wings level and the ball centered.

Because wings level with ball centered and a straight yaw string isn't possible in light piston twins without both engines operating. (At least as far as this MEI knows).

Remember that in a plane with wing mounted engines the accelerated slipstream of air coming back from the propeller creates an area of increased lift immediately behind the engine due to the increased airflow. As long as both engines are operating and producing equal amounts of thrust, both wings will be similarly affected. If the left engine stops working you also lose the accelerated slip stream of air coming back from that engine over the wing. Since the right wing with the operating engine still has that accelerated stream of air the right wing will be producing more lift.

Lets take the three scenarios from the Airplane Flying Handbook.

Scenario 1:
If you maintain a wings level attitude and only use the rudder to correct for the failed engine, you will not be able to center the yaw string and maintain level flight. It takes a large amount of rudder (and is very tiring on the legs) to hold the plane so that it stays level. By keeping it level you will end up slipping so that the relative wind comes from the side of the dead engine. This is actually necessary to compensate for the loss of airflow from the left propeller to generate equal amounts of lift on both wings. By placing the left wing forward of the right wing in the side slip you increase airflow over the left wing to generate more lift and decrease flow over the right wing (block the air with the fuselage essentially) and at some point you start to generate equal amounts of lift with both wings and maintain level flight. If you reduce rudder pressure to bring the nose towards the left and center the yaw string you will also be reducing the side slip. Since a centered yaw string means equal amounts of airflow to both wings, you will now be back in a situation where the right wing is producing more lift than the left which will cause you to roll to the left and off center the yaw string again. The yaw string mounted on the nose will only be affected by the relative wind while the wings will be affect by both the relative wind and the accelerated slipstream created by the spinning propeller. Since the left wing doesn't have this accelerated slipstream it will, in level flight with a centered yaw string, be producing less lift than the right wing and therefore the plane will not be able to maintain level flight.

Scenario 2:
By keeping the rudder centered and banking excessively into the operating engine you will be slipping the plane so that the relative wind comes from the side of the operating engine. When one engine has failed the operating engine will cause the plane to yaw into dead engine. Banking alone will overcome the planes tendency to roll towards the dead engine by increasing lift with an increased angle of attack but will not overcome the yawing moment. Essentially the operating engine is taking the place of the rudder in putting the plane into a side slip condition. It's pretty much the same concept as banking to the right then applying large amounts of left rudder to cause a right side slip as you would in a single engine plane. The difference is that instead of using rudder to yaw the plane left you are using the differential thrust created when the left engine fails leaving only thrust coming from the right engine.

Scenario 3: Straight and level (but not wing level) flight.
The left engine has failed and now the left wing is producing less lift than the right because you no longer have the accelerated slipstream of air coming from the left propeller. You compensate for the loss of lift by banking into the operating (right) engine. This increases the angle of attack on the left side to generate more lift while decreasing the angle of attack on the right side and in doing so you create equal amounts of lift on both side of the plane thereby returning you lift to a 100% vertical component of lift. But since you are running at higher power settings on the remaining engine you have an increased yawing motion towards the dead engine. To counteract this you put in rudder pressure towards the operating engine thereby cancelling out the yaw and centering the little string taped to the nose cone.

Somewhere around here I have some crude drawings I use to describe this to students (normally I do this with a large plastic model and lots of hand gestures). After work tonight I will see if I can find them and scan them or use my kindergarten level MS paint skills to come up with something.