Jet Damping and Inertial Effects

While he was at the Douglas Company plant in El Segundo, California, Hans C. Vetter described a damping effect to be expected from jet air intakes and exhausts. He argued that the air in a jet duct travels in a radial path with respect to the center of gravity when the airplane performs rotational oscillations. Pressure forces on the structure result which are in the direction to resist angular velocities, adding “Coriolis” damping to the aerodynamic damping moments provided by the wing and tail surfaces. Jet damping moments depend on the distances from the center of gravity to the jet intake and exits and on other dimensions (Vetter, 1953).

Artificial yaw and pitch damping, used on almost all modern jet aircraft, tends to swamp out jet damping effects. Furthermore, jet damping is most significant at low airspeeds and high thrust levels, normally encountered only at low altitudes. But the airframe’s natural damping is best at low altitudes. Still, careful designers include jet damping in their calcu­lations. Vetter’s theory implies that rocket-powered aircraft also have Coriolis jet damping, but of course only for the rocket’s exhaust.

The angular momentum of propellers and the rotating parts of jet engines create inertial reaction moments when an airplane pitches or yaws. This is of interest in the analysis of inertial coupling (Chapter 8) and spins (Chapter 9).