Direct-Thrust Moments in Pitch

Static longitudinal stability for a powered airplane is defined as the tendency to return to a trimmed angle of attack and airspeed following a disturbance, with throttle fixed. Having the airplane’s center of gravity below the line of action of the thrust vector, or the thrust line, provides a stabilizing pitching moment increment under these conditions, whether the thrust comes from propellers or jets.

With fixed throttle, thrust remains more or less constant as airspeed falls below the trim value, actually increasing somewhat in the case of propellers. On the other hand, the aerodynamic forces on the rest of the airframe necessarily decrease at lower airspeeds. The net result is that the diving, or nose-down pitching moment, caused by the thrust line passing above the center of gravity increases relative to the other forces, producing the desired restoring nose-down effect. For single-engine airplanes, tilting the entire engine and propeller assembly nose down by a few degrees raises the thrust line relative to the center of gravity.

The powerful (1,900 horsepower) Curtiss SB2C Helldiver came into production in 1942 and soon became a valuable addition to the U. S. Navy’s offensive capability. Along with the Helldiver’s speed and bomb-carrying capacity came an unenviable reputation for poor longitudinal stability and handling difficulties at the low speeds used in carrier approaches. Propeller down tilt seemed to be a natural fix for the SB2C.

The propeller down tilt idea was tested on two generic wind tunnel models at the NACA Ames Aeronautical Laboratory; one model was quite similar to the SB2C (Goett and Delany, 1944). The SB2C-like model had a forward, or unstable, shift in the neutral point (center of gravity for neutral stability) of about 10 percent of the mean aerodynamic chord in a 2,100-horsepower climb. This longitudinal stability loss was cut in half with 5 degrees of propeller downward tilt.

The exigencies of wartime prevented the application of down tilt to the SB2C – poor stability was just another hazard that Navy pilots had to contend with in those days. Many of the youngsters who flew SB2Cs from carriers had graduated into the type direct from training aircraft of the North American SNJ Texan class. The SB2C was the first really high-powered airplane they encountered.

These relatively inexperienced pilots apparently thought poor longitudinal stability, man­ifested by difficulty in establishing a fixed trim airspeed during carrier approaches, was what one should expect on a big, fast airplane. In discussing possible SB2C propeller tilt with Navy fleet pilots, Bureau of Aeronautics engineers were met with “Leave it alone! It flies just fine!” Although the SB2C missed out on propeller down tilt, this feature was used later to increase stability for three single-engine high-powered propeller airplanes. The Douglas AD Skyraider and the Grumman F6F Hellcat and F8F Bearcat all had nose-down tilted engines.