Inertia Coupling

[Shaft109]

I have heard this phrase a few times but can't find a definitive explanation on it; could someone enlighten me please?

[Milt]

Simply stated

If you continue to roll an aircraft which will occur roughly around its centre of mass, that mass out towards the fore and aft extremeties will be describing a circle and may generate high inertial forces tending to enlarge the circles. Without some damping the circles may enlarge dramatically and lead the pilot to believe he has entered a spin.

Low level high speed rapid rolls, particularly in a delta will probably lead to unintentional ground impact on about the third roll.

[MarkMcC]

Simply stated
If you continue to roll an aircraft which will occur roughly around its centre of mass, that mass out towards the fore and aft extremeties will be describing a circle and may generate high inertial forces tending to enlarge the circles. Without some damping the circles may enlarge dramatically and lead the pilot to believe he has entered a spin.
Low level high speed rapid rolls, particularly in a delta will probably lead to unintentional ground impact on about the third roll.

Nicely put. And amply demonstrated on the Mirage 2000...

[Teadriver]

I'm no expert, but here's my view, for what it's worth.
When the aircraft's pitching, rolling or yawing, the mass of the aircraft - disposed about the CG - and the rate of rotation in each axis produce inertias. Any additional motion applied at right angles to that plane of rotation results in precession forces being applied , displaced by 90 degrees around the plane of rotation (remember the old school stuff?). Most aspects of aircraft handling involve a balance between aerodynamic and inertial forces, where the aerodynamic forces the pilot generates through control inputs, or those resisting him through natural aircraft stability, are also aided or resisted by inertial forces.
If a typical military jet, with most of the mass in the fuselage, is rolled at a high rate the inertia in pitch tends to cause angle of attack to increase- it's like a dumbell - which may be sufficient to over-ride the aircraft's natural stability that is trying to keep it down. At the same time, the rotation caused by the pitch-up coupled with the rotation in roll to induce an increase in sideslip - which is sometimes why structural failure of the fin can result from prolonged rapid rolling.
Another area where this is very obvious is in spinning, where some aerodynamic forces are pro-spin, some anti spin, and the same with the inertial forces. You can have hours of endless entertainment twiddling with saucers trying to work out which rotation in which axis produces pro- or anti-spin inertial forces! For example - and it's a generalisation - the additional roll rate caused by applying in-spin aileron usually couples with the nose-up pitch rate to produce anti-spin yaw, which is why in-spin aileron is included in the spin recovery controls for some aircraft (again - generalisation - usually military types with high fuselage masses). And next time you do a spin, notice how much residual roll you get as the aircraft recovers - that's usually the inertial effects dissipating as angle of attack and sideslip reduce.
It initially surprised me how powerful the inertial effects are considering the relatively low rotation rates you get, particularly in pitch and yaw, ut there we are. And the most reliable way to get the Hunter into an inverted spin was by using an inertial coupled entry, although in all the spns I did I never did work put what went on during the entry, other than it involved some spectacular pitch tumbling...

[Milt]

Must be a few stories out there from pilots being caught out and put at risk from the effects of inertial coupling.

Tony Svensson TP was investigating high altitude engine surging in a Mirage IIIO delta. Following an engine surge at low IAS he entered a spin. Full in spin roll control was applied to recover. The spin recovery was not recognised masked by the continuing roll then mixed with inertial cross coupling. Now disoriented by the wild gyrations of the aircraft he unfortunately continued full roll input which further aggravated the conditions and departed the aircraft going through 10,000 ft at 850 Kts IAS as shown by the flight test instrumentation. He recovered from horrific injuries caused by the high speed ejection.

Where are you now Tony?