V1 cut Zero Sidelsip bank angle

[C-12Bubba]

Howdy;

Ya know how you use 1/2-3/4 ball and 3-5 degrees bank toward the good engine in a turboprop? What's the deal with jets? Why do you center the ball and maintain wings level (as with a 737)? Aerodynamically, does this give you best climb rate and zero sideslip?

Most of my reference books don't go into enough detail to explain the above questions...

Thanks for input!!!

 

[enigma]

Here's what my MD80 FCOM says: Keep the ball centered with constant rudder, wings level with little or no aileron and constant pitch attitude.

I'd guess that a slight bank into the good engine might accomplish a minimal increase in performance, but in the Maddog, any aileron over five degrees automatically gets the roll control spoilers up which is most certainly undesirable when single engine.

For a restricted rudder ( the rudder is restricted at speeds above 200kts in order to protect the structure and it is possible for the restricting mechanism to fail to the restricted setting), the FCOM allows that it may take up to 8 degrees of bank into the good engine to maintain constant heading at V2 or 135kts (minimum speed with restricted rudder), whichever is higher.

I'll guess that this all means that McBoeing has calculated the zero slip angle of bank to be 8 degrees, but that the extra bank improvement is negated by the deployed roll control spoiler so they suggest a wings level attitude unless the airplane also has a failed rudder limiter which precludes using rudder.

I know this, if I had a dead engine and a restricted rudder, I'd probably also have the good engine on fire and a heart attack happening in the cabin. I can only hope that this would be in the sim. If I was real, I don't think that I would attempt a single engine go around. We'd just land on the grass, taxiway, or whatever was in front of the pointy end.

edit: maybe I'll try it in the sim next recurrent

enigma

 

[JetPilot_Mike]

For a restricted rudder ( the rudder is restricted at speeds above 200kts in order to protect the structure and it is possible for the restricting mechanism to fail to the restricted setting
enigma

Now that is a mouthfull!!

 

[enigma]

Now that is a mouthfull!!

That's me. King of the run-on sentence. That one may or may not be a run-on, but many are

cheers,
enigma

 

[F16TJ]

C-12 Bubba...

Mostly has to do with swept wings that most jets have, vice the relative straight wings on props/turboprops. A characteristic on swept wing airplanes is higher roll rates when engine out. This is caused by the asymmetric thrust yawing the aircraft. As the aircraft yaws, the upwind wing has increased lift due to a better relative wind (the downwind wing has less lift). The relative difference in lift on the two wings causes the excessive roll rates, which is more pronounced when turning away from the good engine. The effective use of rudder (coordinated flight, the ball centered) keeps the lift differential between the wings equal, as they both "see" the same relative wind.

Life was so much easier in the Viper...motor workie, keep flying; motor no workie, step over the side!

Out...

 

[J Dawg]

In any prop, the wing with the operating engine is creating more lift than the one with the dead engine. This is because of the induced lift of the operating engine's slipstream. To counteract the roll caused by the imbalance of lift, a bank angle causing enough horizontal component of lift into the good engine is required. This angle is published as 5 degrees by the FAA. My aerodynamics classes at college taught 1-2 degrees was a more accurate zero sideslip angle.

As far as the 1/2 - 3/4 displaced ball thing - with a bank angle now established, the ball is simply lying at the lowest point in it's track (gravity)

In turbojets, there is no induced lift by propellers, and both wings create the same lift when single engine. Therefore, when single engine, keep the wings level, and enough rudder to keep the ball centered (the ball is still lying in the lowest point in it's track, which is the centered position when the wings are level).

 

[FatesPawn]

To counteract the roll caused by the imbalance of lift, a bank angle causing enough horizontal component of lift into the good engine is required. This angle is published as 5 degrees by the FAA. My aerodynamics classes at college taught 1-2 degrees was a more accurate zero sideslip angle.

Buzz.

Countering roll purely done by deflecting the ailerons... NOT by necessarily banking.

In ANY multi-engine, engine-out condition (including centerline thrust multiengine airplanes with counter-rotating propellers) a slip develops because of THRUST ASYMMETRY.

The "Horizontal component of lift" (banking into the good engine) is used to counter the resulting sideslip that ALWAYS develops in asymmetric thrust conditions. This sideslip is a result of the rudder countering the yawing tendency. The Force vectors added up (Rudder + the moment created by asymetric thrust) results in sideways motion through the air even with the wings level.

Banking is used to counter the sideslip, NOT the ensuing rolling tendency.

Jet aircraft would benefit from this as well, though many times (as discussed above) this results in poping a roll spoiler depending on aircraft type and killing more lift than is generated by preventing uncoordinated flight.

my $.02

 

[TIS]

So far, I see only bits and pieces of correctness here

There are two things at work in a jet but before I get to that let me first talk a bit about the 5° bank into the good engine thing.

The reason you would do this is to help with rudder effectiveness and to reduce drag. It comes from the Vmca certification rules in either Part 23 or Part 25. It does not necessarily have anything to do with single engine performance, but rather with single engine loss of control. I have flown plenty of turboprops in which it is not at all necessary to bank 5° into the good engine to achieve satisfactory or even optimal performance in an engine-out situation.

Having said that, there may well be some performance advantages to banking into the operative engine in the event of an engine failure. Given that a Part 25 certified airplane must climb to certain specifications after the loss of an engine, the act of banking 5° into the operative engine might indeed be required to achieve the minimum, required performance. In other words, if the AFM says to do it then you should do it because it is a tested and accepted pilot technique applicable to the situation. Otherwise I would argue that you needn't gratuitously offer 5° of bank in any single-engine situation just because that's what you did in a Duchess or a Seminole.

Now, in a swept wing jet things are a bit different. The two things I alluded to above are important to understand if you are, or are about to start, flying one. First, the sweep of the wing plays a role that is best described graphically but I shall atempt to describe it verbally.

When an engine fails that airplane yaws, right? Let's just say that you're observing from the top of a 737 when the right engine fails. The airplane yaws to the right, correct? Where do the wings go? The wing with the failed engine on it moves aft or, retreats, while the wing with the operating engine moves forward or, advances. The airplane however, continues to fly more or less straight ahead. It might roll without corrective action, but it flies straight ahead.

Now let's take a look at that advanced wing. Get out your CFI model if you have to in order to understand this. The advanced wing now has air flowing over it in a way that was not intended when the wing was designed. Effectively the chord of the wing is now longer and hence the amount of lift generated by it is greater. Now, you might say that the same thing is true about the other wing but the situation is a bit different in that the advanced wing receives uninterrupted ariflow while the retreated wing does not. The end result is that the advanced wing produces more lift and induces more roll in this crabbed orientation than if the airplane were held straight with the rudder (ball in the middle).

Putting the ball in the middle puts the airplane back in a straight-ahead flying condition that puts both wings where they should be in terms of airflow and lift balance. The airplane will actually fly a bit better if you keep the plane straight with the rudder than if you don't.

Now, moving on to the other issue that you probably have to consider (definitely in any Boeing), we need to talk about spoliers. If the airplane in question has roll assist spoilers (737, Gulfstreams, etc.) you will engender their operation automatically by using the aileron controls. You won't be able to prevent it. Roll spoliers assist the ailerons by destroying lift on the wing that is directed downward when the ailerons are deflected. If the boards are up, they're taking away from the overall lift that the airplane is producing.

Allowing the ball to remain un-centered will mean that partial roll control must be maintained by using the ailerons. This will, at some point, possibly from the moment you add aileron, raise some or all of the spoilers on one wing, destroying some of the lift and increasing the total drag on the plane. That's hardly a recipe for better performance in a single-engine situation.

Applying enough rudder to straighten out the longitudinal axis of the airplane solves this problem by eliminating the rolling moment with the rudder instead of the ailerons and roll spoilers. Your wing stays cleaner and so do your shorts!

The bottom line is that 5° of bank is basically inappropriate unless the AFM or POH says that's what you should do in response to an engine failure, despite whatever you may have learned growing up.

TIS

 

[J Dawg]

TIS - you make soom very good points


I'd like to add some practical scenarios. Lets say after departure in a light twin, an engine fails. the airspeed is well above VMC at this point, but climb performance is lacking, with an obstacle approaching. The absolute best way to get the most climb performance would be to level the wings (100 verticle lift component), and to let the airplane yaw off course (rudder centered), assuming there are no higher obstacles to the side. Resume normal SE procedures when obstacle clear.

However, if airspeed is a problem, and VMC is approaching, the thing taught to students 99% of the time is to reduce power on the operating engine and lower the nose. Another consideration (never taught) is to increase bank angle into the operating engine. As TIS mentioned, the greater the bank angle, the more effective the rudder becomes. A 20 degree bank angle will reduce VMC dramatically.

Also, the published airspeeds are at Max t/o weight at sea level. If single engine and unable to maintain a climb at VYSE, pitch up just a little to loose a few knots. Chances are you're (hopefully) above sea level and less than max t/o weight when the engine fails.