"Kich the Ball" reversed in some AC?

[A320PI]

Hi,

I am studying aerospace engineering and also hold a commercial pilots license. Recently, a Prof tried to convince me that older racers with the cockpit far behind the CG would have a reversed indication on the inclinometer. This would mean stepping away from the ball to coordinate the turn. I can not really believe it since it is the resultant of the weight- and the centrifugal force vector that causes the ball to displace. Therefore it should no depend on the position of the instrument relative to the CG. Can anyone confirm this or convince me otherwise?
Thank you folks!

[pacrat]

I suspect your Prof is considering aircraft of the DH Comet and Gee Bee racer era. If so the reversal of the 'stand on the ball' theory has nothing to do with cockpit C of G relationship but is more likely a function of the old 'Turn and Slip' indicators of the day. They had no ball and displayed slip/skid by means of a needle at the top of the instrument. I offer my letter to the editor of an Aviation News publication which was recently published and discusses aspects of your question.

It was a delight to read your article on the Auster in the last issue. When you wrote of the "difficulties pilots experienced flying the aircraft in balance" you are spot on to say "the rudder authority was robust compared to ailerons that were flawed in that they suffered a good measure of adverse yaw". Add to this the old turn and slip gauge that displayed slip and skid at the top with a needle, rather than the under mounted ball of the later "Bat and Ball" that the next generation of pilots trained on. Modern aircraft types bank into a turn with aileron and unchecked will develop a slip into the turn. Very early aircraft had little or no fin so it was the case that rudder was the primary tool for turning, then bank was added - the slip that resulted from the bank would thus balance the initial skid from rudder application - thereby streamlining the fuselage with airflow and improving comfort. Instructors of old talked of 'leading the turn with rudder' and follow with aileron. Rudder alone produces a skid and not the more familiar slip from unbalanced bank however the secondary effect of yaw was very strong on many of these types, Auster included. To guide the 'Bat and Ball' trainees we speak of standing on the ball. This of course translates to standing on the top needle in the Auster with older instrumentation. Having a top mounted indicator to guide what to do with your feet below has it's own concerns in regard to Human Factors and ergonomics of cockpit design. A rudder more powerful than ailerons and confusing instrumentation set the seen for the mishandling of this delightful aircraft. With and understanding of the issues and adapting your handling to suit the problem is easily overcome.

The problem is flying in balance. Balance of what? - Slip and skid.
This can equally be considered the balance between applied rudder and aileron. If the instrument says I need more Left rudder it is also saying I need less Left aileron, or perhaps even some Right aileron. Considering that the rudder easily overpowers aileron and given the sensitivity of the slip indicator trainees with a focus on rudder are doomed to hours of over-controlling before they can hope to demonstrate true mastery of the Auster. From the teachings of old on these types, we lead with rudder and match bank to the turn by considering the needle as the joystick. If the needle is out right apply pressure to the left and vice versa. The aim is needle always centred but as aileron is not as powerful as the rudder we reduce over-controlling. Rudder for turning and aileron for trimming the turn.

This thinking also returns an element of ergonomic logic to having the needle above the turn rate indicator. It's always worked pretty well for me in my many happy hours in Austers.

The other point to consider from your Prof's argument is of course that with the standard glass tube and ball slip indicators of the later era of aircraft - applying a boot full of rudder let's say to the Right, would move the tail Left as the nose went Right. If mounted aft of the C of G the inertia of the ball would show it moving right as the tube moves left, being fixed to aircraft structure aft of the C of G.

This would be a transient condition only. As I see it in a steady turn an element of slip or skid would still be correctly displayed as would be the rudder requirement to correct it. This misleading transient indication could be a threat to pilots chasing the ball and potentially set up a 'Pilot Induced Oscillation' in yaw. Perhaps this was a factor in some of the fatal Gee Bee
racer accidents, given the types reputedly poor directional stability to start with.

[gusty_tailwind]

My apologies for resurrecting a relatively old thread, but this topic is one I've been wondering about for some time. My question is:

What force is it that causes the ball in the inclinometer which is part of the Turn Coordinator (or Turn & Slip indicator) to move up one side or the other of the instrument?

I've been trying to imagine the simplest case in which the ball would move, in order to deduce the forces at work. This case seems to be the application of rudder in straight & level, unaccelerated flight, where the turn which would result in a plane having positive dihedral is countered by the application of opposite aileron, establishing a skid. In this case, the inclinometer, mounted forward of the CG, shows that rudder opposite to whatever had been applied needs to be applied. So, for example, if right rudder was applied, the ball would move left, indicating that left rudder is needed in order to get coordinated again.

What is it that causes the ball to move, and are those forces reversed when the inclinometer is mounted aft of the CG?

[Plastic Bonsai]

The slip ball deflection is caused by the non-gravitational forces acting on the aircraft in the lateral/athwartship/y-axis. In air on a conventional fixed wing aircraft this is principally caused by the aerodynamic forces generated on the airframe by lateral incident flow generally termed "sideslip". If the incident flow is coming from the starboard side then the lateral aerodynamic forces act to port and cause the slipball to deflect to the starboard and vice-versa.

The location of the slip ball relative to the aircraft cg does not affect the slip ball steady state response.

[rotorfossil]

I think that just to confuse the issue further, old aircraft before the "turn and slip" needles presentation with the slip at the top, had a inclinometer with a bubble in a curved tube where the ends of the tube curved downwards at each end. Trying to remember not to "step on the bubble" but the reverse was seriously difficult in an Albatros DVa replica. The comment about leading with the rudder was very true. The rudders on aircraft of that vintage were quite powerful due to the small fins but as someone once said of the Sopwith Camel with its huge ailerons, the primary effect of ailerons was adverse yaw and only secondarily of roll. The bubble, which was not well damped could go from one end of the tube to the other in a flash. It was easier to do it by the feel of the wind on the side of your head!

[italia458]

Gusty tailwind...

Inertia.