Asymmetric Balance

[Afterglow]

Hi just curious about this...

On the type I fly E190 when climbing away OEI in order to have a neutral aileron input the trapezoid (balance ball) needs to be not entirely central, i.e. lacking a bit of corrective rudder, why would this be? Is the aircraft 'slipping' in this case?

I assume this effect applies to all swept wing jet aircraft?

[rudderrudderrat]

Hi Afterglow,

Affirm.

We have a similar system on Airbus. When the Beta target is centred, then we have minimum drag due control deflections - but some side slip, so end up with a constant bank into the live engine(s) to maintain heading. The effect is due to the asymmetry of the thrust line being balanced by the sideways vector of the displaced rudder. The sideways drift causes one wing to generate more lift than the other. (swept wings)

The slip target is a different function and colour - with zero slip the wings are level and heading is constant.

[Afterglow]

Hi RRRat,

Thanks for your reply, just getting my head round this now!! So I see what your saying about the thrust line hence the slip, but I just want to clarify my understanding...

Is the Beta target the airbus 'slip indicator' or a min drag indicator?

Not quite sure I understand why with bank into the live engine you'd have min control drag, as with neutral aileron and thus a slight lack of rudder you'd have less control deflection into the airstream, however would I be then correct in assuming you'd have a constant heading but not a constant track due to the slip? I suspect I haven't got this!

Thanks again,

A

[aviatorhi]

My jet experience is exclusive to Boeings, but should apply to all swept wing jets (even though terminology might not match)

Ailerons are used to control wing roll. In this case of an engine failure they'll be the first control surface you use to stop any wing roll. On a swept wing jet, the rudder is (typically) the least used flight control in regular operations, so many pilots (especially pilots new to jets) are unfamiliar with the relationship between rudder and ailerons on those aircraft. Any asymmetry around the vertical axis will cause a rolling moment, since life is increasing on the forward moving wing and decreasing on the rearward moving wing. However, with an engine failed you have two choices, flying with the wheel slightly off or flying with the ball slightly out. This is for a few reasons, (1) on many types, the aileron-flight spoiler mixing is directly related to the angle of control wheel displacement, hence a neutral control wheel is desired; (2) neutral control wheel is equivalent to minimum drag, in an engine out climb (particularly after an MTOW takeoff) drag is not your friend; (3) full lateral control would be unavailable to the pilot if the aircraft is not flown with a neutral control wheel.

There is also a controllability factor to consider... if excessive left rudder is used, the aircraft yaws left causing the advancing right wing leading edge to be ‘longer’ (moving more directly into the relative wind) while the retreating left wing leading edge becomes ‘shorter’ (moving less directly into the relative wind). This excessive left yaw induces a left roll. If the pilot overcompensates with right right aileron, additional right roll is induced, and the cycle continues.

[john_tullamarine]

There is a variety of threads on this topic in Tech Log .. one you might start with is here (http://www.pprune.org/tech-log/110180-oei-why-does-balance-ball-show-slip.html).

In general,

(a) for a specific Type, the OEM TP folk have spent a lot of time looking at OEI operations and the appropriate Type techniques for the pilots are specified in the AFM, FCOM, etc.

(b) for generic thinking ..


First, we have the dynamic bits where hands get sweaty for a little while ..


(i) failure creates an undesired yaw .. the closer to Vmc the more dramatic the dynamics will be. Keep in mind that things happen very, and increasingly, rapidly in the last few knots coming back to Vmc.

(ii) yaw is controlled with rudder (roll is controlled with aileron). The more dynamic the situation, the more rapidly the pilot has to do whatever may be required.


then we settle down into some sort of quasi-static continued flight situation ..


(iii) IF we end up wings level, then there will be a residual out of balance force in that the rudder causes a lateral force and acceleration toward the dead side. This results in a sideways velocity with gives a small sideslip angle and, generally, reduced climb or (increased descent) rate. There will be some minimum IAS which can be achieved in this configuration.

(iv) if we are clever, we can bank a little into the live side and generate an opposite direction slip to balance the rudder-induced slip. To keep everyone honest and to keep the aircraft away from too much slip toward the live side, the Standards impose a maximum bank into the live side of 5 degrees. Unless the AFM says different, it is reasonable to presume that controlled flight at Vmc will require this bank angle. We also find that, with a bit of bank, we don't need as much rudder as the slip into the live side also generates a stabilising yawing moment .. ergo, we can slow down a bit by using a bit more rudder and achieve a lower speed. The AFM Vmca speed usually will be for the 5 degree bank into the live side configuration.

(v) while we are back near Vmc (which we really should avoid like the plague) we are interested in control and staying blue side up - not performance.

(vi) once we can get to a speed margin somewhat above Vmc, the control problems are reduced greatly and we can concentrate on performance .. however abysmal that may be in the real world.

(vii) generally we will get best climb capability somewhere near zero slip (think about it .. how do you increase your ROD on final once you are configured to full flap, gear down, and throttles closed ?). Generally this will occur for a bank angle into the live side of around 2-3 degrees.

(viii) I don't know what your thoughts are but, for me, flying 2-3 degrees wing down is hard work (and inappropriate for some AH systems due to re-erection considerations).

Considering that

(a) climb penalties tend to be similar for wings level and 5 degrees

(b) climb improvement around 2-3 degrees is not incredibly dramatic

(c) jets sometimes have spoiler considerations with which to contend

we generally end up flying wings level for the post failure continued flight scenario .. ie after the dynamic things have been sorted out.