What does a Porsche 911 and a F-16 fighter jet have in common ?
The F-16 was the first production fighter aircraft intentionally designed to be slightly aerodynamically unstable, also known as “relaxed static stability“, to improve maneuverability.
Most aircraft are designed with positive static stability, which induces aircraft to return to straight and level flight attitude if the pilot releases the controls. This reduces maneuverability as the aircraft must overcome its inherent stability in order to maneuver. Aircraft with negative stability are designed to deviate from controlled flight and thus be more maneuverable.
However, to make it flyable by the average human — and avoid the need for constant inputs by the pilot, the F-16 has a fly-by-wire flight control system . The flight control computer accepts pilot inputs and manipulates the control surfaces in such a way as to produce the desired result without inducing control loss.
So what about the 911 ?
Not intentionally, but 911’s have always been inherently unstable, the huge engine mass at the very back-end making for a potentially lethal pendulum. Over the years, and 7 iterations, Porsche has tried to deflect physics through mechanical and electronic expedients, and has mostly succeeded. However, the basic inbalance is there, and still can be used advantageously by a skilled driver.
Like in an F-16, not much input / provocation is needed to make it change direction (ok, nowadays you have to switch off the nannies). That’s what always made them such an effective tool on road / track / gravel. But, for a hands on demonstration of “Relaxed Stability” check the next video by the great Chris Harris.