What was the tactical purpose of the unusual flight control system that allowed the Northrop YA-9 to generate sideforce without banking?

Question

What was the tactical purpose of the unusual flight control system that allowed the Northrop YA-9 to generate sideforce without banking? (And, how was the system controlled by the pilot?)

(Image is public domain USAF photo, via Wikipedia)

According to Wikipedia, on the Northrop YA-9, a prototype ground attack aircraft that was a competitor with the A-10,

Split ailerons were fitted that could be used as airbrakes. When these airbrakes were operated asymmetrically in conjunction with the aircraft's rudder, sideways control forces could be applied (and the aircraft moved sideways) without yawing or banking, easing weapon aiming.

It's clear how something like this might work-- if the aircraft can be prevented (via the yaw torque generated by the asymmetrical drag created by deploying the split ailerons, on the wing opposite from the deflected rudder) from yawing out of alignment with the instantaneous direction of the flight path, then the rudder can be used in a "backwards" sense (left rudder for a right turn) to generate sideforce and curve the flight path to the left or right, creating some change in the aircraft's heading, without banking. Note that there would be no sideways airflow over the aircraft-- a "yaw string", if present, would stay centered, while a slip-skid ball would not.^1,2

Is it a great tactical advantage to be be able to change the aircraft's heading this way rather than by banking? Does this advantage pertain to air-to-ground gunnery in general, or only to operations at very low level, e.g. near treetop height?

Footnotes:

1. Note that if this conjectural description above is in fact accurate, the quoted Wikipedia article is somewhat in error-- while the aerodynamic sideforce component is not generated by yawing the nose to one side of the direction of the flight path, as it is in a conventional slip, there is a net horizontal force component, so the flight path will curve, which (due to the aircraft's "directional stability") will involve a change in heading, so the yaw rate is not zero, and the maneuver cannot be describe as being carried out "without yawing". It's hard to imagine how the described set of flight controls could possibly be used to truly push the aircraft "sideways" through the air-- accomplishing a change in flight path with no change in heading, which would result in sideslipping flight with all its attendant aerodynamic consequences-- while also keeping the wings level.

2. Note also that if the objective truly is simply to allow the aircraft to move sideways through the air without changing heading, a conventional wing-down "sideslip" accomplishes that. (And a "sideslip" is generally not described as a useful maneuver for gunnery purposes!) Even setting aside the question of whether or not such a maneuver would really be useful, it's hard to how the described set of control surfaces could really accomplish what the Wikipedia article contends, allowing the aircraft to sideslip (changing the direction of flight path, without changing the aircraft heading) without banking at all. The problem isn't the balance of yaw torques, but rather the fact that something must counteract the aerodynamic sideforce generated by the airflow striking the side of the fuselage. If we can somehow increase the rudder deflection required to produce a given slip angle, we might eventually reach a point where the sideforce from the deflected rudder is strong enough to fully cancel the sideforce from the air striking the side of the fuselage. Again, this would involve opening the split ailerons on the side opposite the deflected rudder. But an aircraft designed to be maneuvered in this way should have a very large rudder, located only a minimal distance aft of the CG, and should have minimal fuselage side area. The fixed vertical fin should be minimal in size-- an all-moving vertical fin with no fixed portion at all would be ideal. Such a system could perhaps be implemented on a swept-wing flying-wing aircraft shaped something like the XB-35 or YB-49, but would seem entirely unsuited to the YA-9, with its large fuselage and large fixed vertical fin.

Answer 1

This way of controlling the flight path without banking or yawing allows for more direct control of alignment with a ground target: if you need to move sideways to line up on a given target from a specific direction, with this system you have direct proportional control of sideways velocity (and thus integral control of the sideways position): giving control input for x seconds will displace you y meters, after which you continue flying straight on target again.

In contrast, with banking and yawing you have integral control of the sideways velocity (and thus double integral control of sideways position): giving a control input for x seconds will initiate a y degree turn, after which you will be displacing at z m/s towards the position you want to achieve. To stop the movement and point at the target again, you need to bank/yaw in the opposite direction.

Now you can imagine how useful this proportional, direct control would be for lining up, especially for small corrections. Here's some games to show how controlling position becomes more difficult with each additional integrator layer: https://github.com/partomatl/control-game or https://clixplatform.tiss.edu/phet/en/simulation/legacy/maze-game.html)

Answer 2

Is it a great tactical advantage to be be able to change the aircraft's heading this way rather than by banking? Does this advantage pertain to air-to-ground gunnery in general, or only to operations at very low level, e.g. near treetop height?

No, and no. The proof: The A-10 also has split ailerons, and the control logic in question here could have been incorporated into it's design also, however it was not.

Had such maneuverability offered great tactical advantages it surely would have been adopted. The Fly off between YA-9 and YA-10 (later to become the A-10) proved otherwise (from "Fairchild Republic A-10 Thunderbolt II: The 'Warthog' Ground Attack Aircraft", Peter C. Smith):

Their findings were that there was no significant difference in weapons delivery accuracy between the two prototypes.

When asked which aircraft felt right to fly, most of the test pilots opted for the YA-10. This was a vital factor due to the similarity of the delivery accuracy test results.

The Conclusions and Recommendations section specifically mentions the Side Force Control feature of YA-9, but even despite this recognition, the YA-9 lost the bid. During flight testing it did not deliver significantly different results from the traditional controls of the YA-10

Aiming with a fixed aircraft gun is not all that complex: target acquisition is just basic flying, and fine adjustments up and down is handled with aft and fore stick movement, left and right with pedals + ailerons (to keep wings level). Pedal-aileron interaction is very "grass roots" flying, and there is little sense in developing more or less complex hydromechanical systems to do special tricks. If you consider the typical shooting distance of a mile, give or take some, a very slight movement of pedals will move the impact point several meters, and very quickly.