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I'm currently working on my PPL, and whilst trying to understand fully how the turn coordinator works, I got down a rabbit hole that makes me question my understanding, and that I can't dig myself out of, namely:

  • Why, in a multi-engine airplane with one engine inoperable, is the ball no longer centred when the aircraft is flown coordinated?

There are multiple resources (such as this Wikipedia article), which reference that fact, but I couldn't find anything that actually explains why that is. I don't quite understand why the ball would suddenly sense aerodynamic forces differently when an engine fails. If the centripetal force is equal to the centrifugal force, it will be equal whether the engine is out or not, and the ball has no knowledge of its state (or even what an engine is).

I would very much appreciate a complete, from the ground up explanation that doesn't assume any knowledge of multi-engine operations. I have a basic understanding of aerodynamic forces involved in single-engine powered aircraft flight, including the concept of coordinated flight, as well as the typical steam gauge instruments, including a turn indicator/coordinator.

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    $\begingroup$ With asymmetrical engine thrust, there is no such a thing as level coordinated flight. You will have either sideslip, or a mismatch between turn and bank, or more likely both. $\endgroup$
    – PcMan
    Jan 8 at 6:06
  • $\begingroup$ Some answers have gone rather deep into this-- I'll try to find links. $\endgroup$ Jan 8 at 10:12
  • $\begingroup$ @JamieC -- I'm sure that there's going to be an accusation of gratuitous over-editing here, and I'm not sure what's the most appropriate place to respond-- note that the last few edits to my answer were made within a few hours of an edit to the question itself that completely changed the meaning of the title, and also bumped the question up to the top of the page again. Some of these last few edits were in direct response to this change in the question. Sorry, that's just the way my head works, when I'm thinking actively about a topic, additional points keep coming to mind $\endgroup$ Jan 10 at 13:30
  • $\begingroup$ @JamieC -- (or is the point simply that the closing paragraph should be deleted?) $\endgroup$ Jan 10 at 13:32
  • $\begingroup$ What is this "ball" you speak of? $\endgroup$
    – rob74
    Jan 10 at 14:59

2 Answers 2

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  1. Asymmetric thrust means now you have more thrust on one side than the other. A yawing moment results.
  2. You want to balance this yawing moment in order to still fly straight. This means the airplane needs to produce an opposing yawing moment somewhere else. The most suitable part to do so is the vertical tail surface, which will do this by producing a side force by means of rudder deflection.
  3. With that side force, the airplane would accelerate sideways, so you need a compensating side force which balances that of the vertical tail. This can be achieved with a slight bank angle, so your wing creates this compensating side force.

Now you fly at a sideslip and a nonzero bank angle, but still on a straight line. This makes the ball move away from the center position.

Note that a similar effect makes Sikorsky-type helicopters hang sideways in hover: In order to compensate for the side force created by the tail rotor to counter rotor torque, the rotor disk needs to be tilted sideways in order to create this countering side force. Otherways the hovering helicopter would drift sideways. From the tail rotor lever arm and the amount of tilt you can directly read how efficiently the rotor converts torque into lift.

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  • $\begingroup$ Fascinating that computer controls may eliminate the "snaking" tendency which forced early (subsonic) jet designers to use a large tail. We may get that Horton flying wing yet in commercial aircraft. $\endgroup$ Jan 8 at 20:30
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    $\begingroup$ So much more efficient to put the trimming, control and stabilizing surfaces at the end of a long moment arm using a tube. Then you can make the tube really big to put stuff in. Packaging the volume lengthwise in a tube fuselage means way less frontal area than the same volume put cross-ways inside a wing, and without all the stability and control issues. There's a reason that 99.9% of airplanes are NOT flying wings beyond special niche applications, and it's not really control issues; it's the packaging issue of having all the interior volume oriented spanwise. $\endgroup$
    – John K
    Jan 8 at 22:38
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    $\begingroup$ @JohnK Exactly, and this is even more so once the tube containing that stuff is pressurized. Adding a tail saves surface area and structure overall. $\endgroup$ Jan 8 at 22:51
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    $\begingroup$ @PeterKämpf i cant fix this because its only a 1 character edit, but I think you mean 'oRder' in relation to a Sikorsky. $\endgroup$ Jan 9 at 1:46
  • $\begingroup$ @RowanHawkins Thank you for spotting this! Fixed. $\endgroup$ Jan 9 at 17:27
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(Note: the first two paragraphs of this answer were aimed at the original title of this question, which was "Why does the ball no longer point straight down in a multi-engine airplane with inoperable engine?")

First let's address a misconception that some readers may take from your title. In linear, constant-heading flight in any aircraft, "coordinated" or otherwise, the slip-skid ball always "points" straight down in relation to the earth. If you have a conception that in linear, constant-heading flight in a multi-engine aircraft with a failed engine, the ball is not pointing "straight down" toward the earth, then you have a misconception-- regardless of whether or not we are applying the optimal rudder input and bank angle to keep drag to a minimum.

But "straight down" is not the same as "centered", if the aircraft is banked. More to the point, regardless of bank angle, and regardless of whether the intended flight path is a straight line or a turn, it turns out to be not optimal to keep the slip-skid ball exactly centered, when dealing with a failed engine on a multi-engine aircraft.

Now moving on to your actual intended question as I understand it-- I'm assuming that the real essence of your question is "why is not optimal to keep the slip-skid ball centered, in the case of engine failure in a multi-engine aircraft?" Or to put it another way, in this case, "why is the ball not centered in 'coordinated' flight?"

You need to start by asking yourself what is the meaning of "coordinated flight". We could define "coordinated" in several different ways, one of which would be that "the ball is centered". But if by "coordinated" we mean that the the fuselage is pointing straight into the relative wind, not yawed to one side-- i.e. if we mean that a tuft of yarn ("yaw string") mounted in the middle of the windscreen would be centered, not blowing to one side-- then we don't always want the ball to be fully centered, especially in the case of a failed engine on a multi-engine aircraft.

What's going on here is that when the fuselage is fully streamlined to the relative wind and not yawed to expose either side of the fuselage to the airflow-- the optimum situation for minimizing drag-- then the ball cannot be fully centered, because the rudder itself, which is strongly deflected to compensate for the failed engine, is generating a significant aerodynamic sideforce, and therefore a significant acceleration, toward the failed engine. Compensating for this sideforce by banking slightly toward the good engine stops the flight path from curving, i.e. stops the aircraft from turning, so that the aircraft's heading remains constant, but doesn't affect the position of the ball, so the ball continues to be off-center.

If we applied enough rudder pressure to fully center the ball, this would mean that we were exposing the side of the fuselage (the side nearest the failed engine) to the airflow, thus creating an aerodynamic sideforce that exactly cancels the sideforce from the deflected rudder. (And in this case we could hold heading with the wings level-- there would be no turning tendency.) This is not optimal--drag is greater than it needs to be.

Note that in both cases the torque from the deflected rudder is cancelling the torque from the thrust asymmetry. In the latter case, the slightly greater rudder deflection is actually creating a little extra torque, that is used to keep the fuselage and vertical fin pointing slightly sideways in relation to the relative wind and airflow-- again, this is not optimal.

In theory this happens any time we deflect the rudder for any reason. If we want to keep the fuselage streamlined to the airflow, and the rudder is deflected at all, the ball should not be fully centered. Rather, we should apply slightly less rudder than would be needed to fully center the ball. Otherwise we are exposing the side of the fuselage, the side opposite the deflected rudder, to the airflow. But normally this effect is trivial enough that we don't worry about it. Not so when the rudder is strongly deflected to counteract the torque from a failed engine.

We could imagine some other system of yaw control-- such as the "clamshell" split ailerons on the B-2 Spirit-- that would create plenty of yaw torque, but no significant aerodynamic sideforce. In this case, if there is a thrust asymmetry, to fully align the centerline of the aircraft with the direction of the flight path, relative wind, and airflow, the ball should simply be centered, not left deflected to one side.1

Maybe it would help to explain it this way: one way of looking at things is to say that you should use the rudder as needed to hold the heading constant, and then, while continuing to hold the heading constant, bank toward the good engine enough to move the ball slightly off-center (one-half diameter is often recommended). But another way of looking at things is to say that you should apply rudder as needed to bring the ball nearly but not fully to center (again one-half diameter off center is often recommended), so that the fuselage is fully streamlined to the airflow, and then, while keeping the ball in that position, bank as needed toward the good engine to stop any turning tendency and hold the heading constant. In theory, you'll end up in the same place with either method. Granted, in the real world, there are reasons why it's very important to immediately counteract any yaw toward the failed engine, so the first method-- prioritizing holding the heading constant-- may be better. But thinking through how the second method would work, may give you a better understanding of what is really going on in terms of aerodynamic forces and torques.

PS-- I guess I'm starting from an assumption here that you understand the difference between sideforce and torque. I'm also starting from an assumption that you understand what the slip-skid ball (inclinometer ball) really indicates. The slip-skid ball responds to the net sum of all lateral (sideways) accelerations acting on an aircraft, excluding the lateral acceleration component due to gravity. And this is using "lateral" or "sideways" in relation to the aircraft's own reference frame-- that's why banking doesn't affect the position of the ball in and of itself, nor does it generate any aerodynamic sideforce. (Obviously it's a different story if we are also changing some other variables, for example if we are manipulating the rudder as needed to force the flight path to remain in a straight line as we vary the bank angle, in which case we'll be demonstrating a sideslip.) For (much) more, see this related ASE answer: What does the balance ball actually indicate?

See also this related ASE answer: Why should you not turn in the direction of an inoperative engine?

And see also this related answer: How can using split throttles help when landing twins in crosswinds?

Closing note: note that this answer and at least one other posted answer are basically in agreement, except that this answer uses the word "sideforce" specifically in relation to the aircraft's own reference frame. By this convention, banking creates a horizontal (sideways?) force that keeps the flight path from curving (keeps the aircraft from turning), but does not actually create a "sideforce" in the aircraft's own reference frame, hence the bank angle does not directly affect the deflection of the slip-skid ball. Also, the present answer, while possibly more verbose than absolutely necessary, is intended to encompass situations where the aircraft is not simply travelling in a straight line.

Footnotes:

  1. But on the other hand, minimizing sideslip (centering the yaw string) is not that important, in terms of minimizing drag, in an all-wing aircraft like the B-2, with minimal cross-section as seen in side view. Consider this quote from the 35th Wright Memorial Lecture by Jack Northop on the development of flying-wing aircraft: "for very long-range aircraft there is a valuable compensating advantage in being able to fly under conditions of asymmetrical power without appreciable increase in drag." In such a case the pilot should simply make whatever control inputs most efficiently balance the aircraft in roll, with minimum drag penalty, and so long as the aircraft has efficient ailerons, minimizing sideslip (centering the yaw string) will be less important than in a more conventional aircraft.
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  • $\begingroup$ You're right, I should've said "centred", calling it "straight down" is inaccurate and confusing wrt the substance of my question. I'll fix it. $\endgroup$
    – mathrick
    Jan 9 at 19:51
  • $\begingroup$ @mathrick -- Sounds good -- $\endgroup$ Jan 9 at 20:43
  • $\begingroup$ Note that this answer and Peter Kampf's answer are basically in agreement, except that I'm using the word "sideforce" specifically in relation to the aircraft's own reference frame. Banking creates a horizontal (sideways?) force that keeps the flight path from curving (keeps the aircraft from turning), but does not actually create a "sideforce" in the aircraft's own reference frame, hence the bank angle does not directly affect the deflection of the slip-skid ball. $\endgroup$ Jan 9 at 21:21
  • $\begingroup$ Also it appears that this answer, while obviously more verbose, might be better suited to understanding situations where the a/c is not simply travelling in a straight line. $\endgroup$ Jan 9 at 21:21

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