The Rotation-Only Breakthrough
The rotation-only concept for variable sweep was pioneered by Dr. Barnes Wallis at Vickers-Armstrongs, Weybridge, around 1954. Starting in 1959, brilliant work by a NASA Langley Laboratory team, including Dr. Wallis, made variably swept wings a practical design option. Team members William J. Alford, Jr., Edward C. Polhamus, and Wallis found a practical way to eliminate the translation, or fore-and-aft motion of the wing inboard ends, drastically simplifying the variable-sweep rotation/translation mechanism to rotation alone.
The clue was to pivot the wings well out from the airplane centerline and to bring the wing trailing edges when fully swept parallel and close to the horizontal tail leading edges. In the Alford-Polhamus-Wallis design, the wing pivots are on the outboard ends of a glove, a diamond-shaped, highly swept inboard fixed-wing section. Wing spanwise loads are carried primarily on the outboard or unswept panels when the wings are in the forward position. The wing’s spanwise load shifts relatively inboard, to the glove, when the wings are in the aft position. This relative load shift is exactly what one wants in order to minimize movement of the total wing aerodynamic center when the wings go through its sweeping routine (Loftin, 1985). Alford and Polhamus jointly hold the U. S. patent on this design.
An additional benefit of the Alford-Polhamus-Wallis arrangement arises from downwash changes with wing sweep. Bringing the wing trailing edge close to the horizontal tail drastically increases the downwash rate of change with angle of attack, reducing the tail’s stabilizing effect. That is, the tail’s increasing up-load with increasing angles of attack is reduced. In effect, the wing acts as a huge turning vane, aligning with itself the airflow into the horizontal tail. Reduced stability from the horizontal tail is just what is needed when the wing is swept back by rotation alone.
Another way of thinking of the Alford-Polhamus-Wallis arrangement is to consider the horizontal tail as an extension of the wing when the latter is fully swept back. Surface area at the rear of a lifting surface carries a smaller airload than does the same amount of area as an independent lifting surface. The lower airloads on the horizontal tail result in reduced static longitudinal stability, again just what is needed.