Oblique-winged aircraft (which have one long wing that pivots around a central point, so that the wing is straight at low speeds but strongly-swept - one side forwards, the other side back - at high speeds), while theoretically more efficient than conventional aircraft, have a number of issues which have prevented them from seeing any significant use; one of these problems are their various unusual cross-coupling modes, including a significant tendency towards pitch-roll coupling (where pitching up or down makes the aircraft roll to one side or the other, and vice versa) and a need to hold a significant bank angle just to maintain coordinated flight and keep from slipping sideways (even with no thrust asymmetry), all of which become increasingly severe at high sweep angles.
For example (emphasis added):
McMurtry found the handling qualities were very close to the simulator, but that the aircraft’s asymmetry resulted in unusual trim requirements, asymmetric stall and inertial coupling. [...] For instance, the AD-1 required about 10° of bank in order to trim the aircraft with no sideslip at 60° wing sweep.
As hoped, the low-cost, low-speed, low-technology AD-1 successfully demonstrated the concept of a manned oblique wing aircraft, sweeping the wing to a maximum of 60°. The aircraft experienced cross coupling between pitching moment and aileron deflection. This pitch-roll coupling and the aeroelastic effects contributed to unpleasant handling qualities at sweep angles above 45°. [...] [A Summary Of A Half-Century of Oblique Wing Research, pages 20-21.]
The proposed wing pivot was also to be canted so that at 0° sweep, the wing was canted to 0°, but at 65° sweep it would be canted to 10°. The wing incidence would also have been increased (a standard feature of the F-8) with higher sweep angles. These features were expected to reduce aerodynamic cross-coupling, such as the necessity to bank the whole aircraft to trim sideslip, as found in the AD-1 flight testing. [...] _[A Summary Of A Half-Century of Oblique Wing Research, page 23.]_
Flight tests of AV1 and AV2 proved the design to be controllable in both pitch and roll. Noble states, “The elevon mixing is standard symmetric and the aircraft responds equally to left and right inputs. There is some slight roll coupling associated with up and down elevator. I believe this can be eliminated with fine tuning of the elevon mixing.” _[A Summary Of A Half-Century of Oblique Wing Research, pages 32-33.]_
Why do oblique wings come with these strange cross-couplings?