Engine Failures- Twins

[uwochris]

Hey guys,

I always thought that the whole purpose of banking towards the operating engine was to offset (i.e. zero) the sideslipping that occured because of the adverse yaw from the failed engine.

However, I recently read something contradictory. It basically stated that banking towards the good engine causes a sideslip, which is desirable as it increases the rudder's ability to offset the adverse yaw. ie) it said that the relative airflow is coming in at an angle, strikes the rudder at a greater angle than it otherwise would, and thus helps it to offset the yaw.

Opinions?

Also, would it be possible to either use rudder or aileron exclusively to offset adverse yaw in single engine ops? ie) no banking + rudder... you only use rudder or aileron. I assume it's possible, but performance would be relatively poor.

Thanks in advance.

Chris.

 

[Ralgha]

Bunk. It's far easier to draw it, but when you have the appropriate bank in, there is no sideslip at all. The airplane is going straight through the air.

In the case of single engine flight in a twin, if you have the wings level and the ball centered, you are in a sideslip, the airplane is not flying straight through the air. Reason: the rudder is producing two things.
1) A moment force that couters the moment force created by the imbalanced thrust.
2) A horizontal force that is not balanced. This can't last, so the airplane sideslips, creating drag to oppose and balance the horizontal force from the rudder.

When you add in the bank, the horizontal lift component from the bank balances the horizontal force from the rudder and eliminates the sideslip.

 

[Timebuilder]

Bingo

Banking sets the thing right for single engine flight.

 

[FN FAL]

Jesus...doesn't anyone tape a piece of yarn on the windshield of the seminole when they are teaching this crap anymore?

 

[A Squared]

Chris,

Ralgha is correct. let me add a little that might make it clearer.

An airplane (symmetrical thrust) which is banked but not turning will be in a side slip, right?

A Twin which is wings level with an engine out will also be in a side slip (see ralgha's explanation)

In a simple sense you could say that you are using the sideslip from banking to counteract the side slip from the engine out, making the airplane go straight.

If you're doing twin training, tape a piece of string to the center of the windshield, like the yaw string on a glider. This will show you the correct amount of bank to keep the aircraft coordinated, when the bank is correct, the string will be aligned with the center of the windshield.

>>>>"Also, would it be possible to either use rudder or aileron exclusively to offset adverse yaw in single engine ops? ie) no banking + rudder... you only use rudder or aileron. I assume it's possible, but performance would be relatively poor."

sure, if you have enough airspeed, you can keep a twin with an engine out from turning with rudder alone. You will lose directional control at a higher airspeed though. Your performance will suffer because you are slipping, hence more drag. Also, as you are slipping with your wings level (maybe that's skidding) the airflow is striking the dead engine side of your vertical stabilizer, which creates yaw force toward your dead engine. So your rudder must overcome both the yaw from the assymetrical thrust *and* the yaw from the uncoordinated flight ... if you are banked so that the airflow is aligned with horizontal axis of the plane, the rudder has less yaw to contend with.

There is some magic combination of aileron and elevator which would keep the airplane from turning with an engine shut down. As airspeed increases, of course the rudder and bank required to keep everything balanced becomes less as the vertical stabilizer becomes more effective at keeping the airplane pointed into the relative wind.

once I was in cruise with an engine feathered and the captain remerked that I should be holding a 5 degree bank away form the feathered engine. The DC-6 when empty is still good for about 180-190 indicated on 3 engines, and a 4 engine airplane on 3 engines isn't nearly as assymmetrical as a twin on one. It might have been an inboard engine too ... anyway, I dutifully banked 5 degrees away from the feathered engine, then I pointed out to him that I was having to hold rudder into the *feathered* engine to keep from turning.. he though about that for a bit than stopped insisting on a 5 degree bank in cruise. The point of all that is that 5 degrees may be a pretty good ballpark figure or the correct bank at low speeds with one at full power, and one windmilling. With the engine feathered, airspeed up and the power reduced on the other, the need for bank is much less. Use a yaw string to get a feel for it.

 

[enigma]

The point of all that is that 5 degrees may be a pretty good ballpark figure or the correct bank at low speeds with one at full power, and one windmilling.

On a slightly different note, the five degree figure is a certification number. It seems that the manufacturers (of small twins) were trying to have the lowest Vmc numbers and were using a lot of bank angle in order to obtain those low numbers. The more bank you use, the lower the Vmc, which seems like a pretty good deal; until you learn that the lower Vmc is derived at the expense of SE performance. You see, more bank lowers Vmc, but also adds climb robbing drag.

In order to keep the numbers relevant, the FAA limits the bank angle that the manufacturer may use in Vmc certification tests to five degrees.

ERAU and the Flight Safety Foundation published a study about 14 years ago that showed the mathematical derivation of the best all around s.e. bank angle for three popular twins. It wasn't near 5 degrees for any of those modern, popular light twins. If memory serves, the number for optimum performance and controllability was never more than 2 degrees.

 

[groundpointsix]

ERAU and the Flight Safety Foundation published a study about 14 years ago that showed the mathematical derivation of the best all around s.e. bank angle for three popular twins. It wasn't near 5 degrees for any of those modern, popular light twins. If memory serves, the number for optimum performance and controllability was never more than 2 degrees.

Do you know where we could find that study? I'd be interested in reading it

 

[Jmmccutc]

Jesus...

oh no not again...look out for super80, he'll be all over this one...

 

[labbats]

I read a similar report where extensive testing was done on Barons. It was found that 2-3 degrees is optimal.

 

[User546]

In a ERAU article I read also, where they compared three different types of twins (Seminole, Duchess, and Barons), it said the optimal bank angle for the Seminole was 2.7 degrees.

 

[Guillotine007]

Do you know where we could find that study? I'd be interested in reading it

pountsix....you know what you are doing. and when is your checkride again?

Klein

 

[ProLine4]

Right on

On a slightly different note, the five degree figure is a certification number. It seems that the manufacturers (of small twins) were trying to have the lowest Vmc numbers and were using a lot of bank angle in order to obtain those low numbers. The more bank you use, the lower the Vmc, which seems like a pretty good deal; until you learn that the lower Vmc is derived at the expense of SE performance. You see, more bank lowers Vmc, but also adds climb robbing drag.

In order to keep the numbers relevant, the FAA limits the bank angle that the manufacturer may use in Vmc certification tests to five degrees.

ERAU and the Flight Safety Foundation published a study about 14 years ago that showed the mathematical derivation of the best all around s.e. bank angle for three popular twins. It wasn't near 5 degrees for any of those modern, popular light twins. If memory serves, the number for optimum performance and controllability was never more than 2 degrees.

Great post enigma. This is very important to point out, as the 12 items of Vmc (especially the "up to 5 deg bank into the operating engine") are for -certification and standardization purposes only- and have absolutely nothing to do with performance or what should be done in a SE situation to allow maximum aircraft performance. It ultimately prevented acft manufacturers from setting a published Vmc that was unrealistically low. especially for higher-powered twins. Had there not been a max bank angle for certification, that would have probably created a horrible situation where aircraft would have been marketed with incredibly low Vmc speeds that were contingent upon pilot input of a large drag-producing and descent-causing bank.

One DE I know makes this topic #1 of all of his ME/MEI checkrides, and you wouldn't believe the number of candidates that talk themselves into a hole because they swear 5 degrees is gospel. Granted this is also a failure on their MEIs part to effectively teach it, but you get the point, its often misunderstood.

I think one of the articles that talked about the optimal bank angle was called "Engine out planning for Light Twins". There's another good FAA document called "Takeoff planning for Light Twins" that the DE always talks about.

 

[enigma]

Re: Right on

Great post enigma. This is very important to point out, as the 12 items of Vmc (especially the "up to 5 deg bank into the operating engine") are for -certification and standardization purposes only- and have absolutely nothing to do with performance or what should be done in a SE situation to allow maximum aircraft performance.

I think one of the articles that talked about the optimal bank angle was called "Engine out planning for Light Twins". There's another good FAA document called "Takeoff planning for Light Twins" that the DE always talks about.

Thanks for the back up. I'm more of an idea man, details just get in the way, so I forget where I read them.

Now I know where to go look for the article instead of having to dig through twentyfive years of saved paperwork.

regards,
enigma

 

[groundpointsix]

pountsix....you know what you are doing. and when is your checkride again?

Klein

April 13th... I don't suppose you want to come back down and give me a refresher on preflighting now that it's warming up?

 

[Guillotine007]

you can come up to cbus and i'll show you on a 58

Klein