Tuesday, May 05, 2015

A history of porpoising

Skirted ground-effect Formula 1 cars of the late 1970s and early 1980s were occasionally afflicted by a type of instability referred to as 'porpoising'. Many cars suffered, but the phenomenon is nicely described by Peter Wright in relation to the development of the Lotus T80:

"The car was so sensitive that, above a certain critical speed, it became aerodynamically unstable in pitch. One test day at Silverstone, Mario Andretti coined the term 'porpoising' to describe the phenomenon when he observed daylight under the front wheels while at speed on the straight.

"Since 1977 I had been working with David Williams, Head of the Flight Instrumentation Department at the Cranfield College of Aeronautics. He had designed and built a digital data system for use on the T78 when it had become apparent that it would be absolutely essential to gather data from the chassis in order to progress with the development of ground effect. When the T80 porpoising started, I discussed the phenomenon with him, and he offered to model it and validate the results with the data we had. He established that it was an aero-elasticity problem, akin to flutter in an aircraft wing. The changing aerodynamic loads, as the car bounced and pitched, excited the pitch and heave modes of the sprung mass on its springs and tires." (Formula 1 Technology, p36 and p308.)

However, pace Wright, the same phenomenon had already been identified and named at least as early as the 1940s, albeit in the field of seaplane hydrodynamics; specifically, during the take-off and landing of such craft. A Wartime Report issued by NACA in June 1943, begins:

"Porpoising is a self-sustaining oscillatory motion in the vertical longitudinal plane...Observations of porpoising show that there are two principal oscillatory motions (1) a vertical oscillation of the center of gravity and (2) an angular oscillation about the center of gravity. These two motions are seen to have the same period but to differ in phase." (Some systematic model experiments on the porpoising characteristics of flying-boat hulls, Kenneth S.M. Davidson and F.W.S. Locke Jr, p7).

The British were also heavily involved in the early study of porpoising, an Aeronautical Research Council report in 1954 defining the phenomenon as follows:

"Porpoising, basically, consists of a combination of oscillations in pitch and heave. It includes both stable and unstable oscillations, a stable oscillation being one which damps out. (A review of porpoising instability of seaplanes, A.G.Smith and H.G.White, p5).

All of which is an important reminder that ground-effect was of crucial importance to hydroplanes long before Formula 1 happened upon the phenomenon.