Unusual Instability - Inertia Coupling
Inertia Coupling, a form of instability, only becomes prominent with aircraft having high roll rates and when successive rolling occurs. The result is an undesirable increasing pitching as rolling continues. The cause usually arises from the mass distribution of an aircraft whereby mass fore and aft of the cg is flung centrifugally away from the axis of roll. It may occur with an aerobatic piston engined aircraft having a heavy tractor engine in front preventing the performance of a clean rolling motion and causing the roll axis to wave around in a circular or eliptical motion.
As an ageing TP I have not heard of any flight or wind tunnel testing of this instability despite there having been some attributable losses of military aircraft. Nor have I heard of any pilot being practised enough to use flying controls to dampen or reduce the effect. Perhaps we will hear of some aerobatic pilots who have this ability or who allow the instability to develop as a deliberate spectacle. Fly by wire computer controlled flying controls would have an inherent capability to control the instability but because a sequence of rolls is not a useful manoeuvre, except as a spectacle, inertial coupling probably gets scant attention in design and acceptance..
A military aircraft having a high susceptability was/is the French Mirage III delta. Two experiences with the effects took me too close for comfort.
The first was my investigation into the crash of a Mirage III which had the pilot practicing for a flying display. He performed a high speed low level run over his base ending with a series of rolls. At the end of the third roll considerable pitching was observed. Continuation of the roll resulted in the nose pitching down uncontrollably into the ground. Yet another case to support the absolute need to practice all risky manoeuvres at a safe altitude.
The second involved a Mirage III test aircraft investigating engine surge boundaries at high alpha and high altitude. The engine surged and caused the aircraft to enter a spin. Full pro spin aileron was used for spin recovery. The pilot did not recognise spin recovery and held on full aileron causing rapid rolling. The violent pitching associated with the rolling led him to believe that he was still spinning. He ejected going through 10,000 ft at 850 Kts CAS and suffered considerable trauma to both legs from flayling. The Mirage made a very deep hole.
john_tullamarine describes observing a Mirage III pitching down dangerously close to the ground after a succession of rolls during a display.
The extent of the problem with Inertia Coupling is little known so any PPRuNers having had experience should reveal all for the benefit of those it might bite some day.
Someone must know the susceptability of the present generation of high performance aircraft. What about the Migs and Sukhois ?