Unusual Instability - Inertia Coupling
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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 ?
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 ?
The Jaguar is another aircraft that has a reasonably well documented inertial autorotation mode. It is a mode that exists at low angles of attack whereby the principal inertial axis is depressed below the flightpath vector. Essentially, autorotation occurs in this situation when the rolling inertia exceeds the aerodynamic roll damping and thus the roll continues with no lateral control applied. Note that this is a completely different mode to aerodynamic autorotation which occurs at a high angle of attack and which may result in a spin.
Multiple rolls can trigger inertial autorotation as the roll rate increase progressively when the inertial axis diverges from the flightpath vector, thus increasing the roll inertia. However, a far more role relateable trigger in the Jaguar is entering a roll at less than +1g, or simultaneously pushing and rolling such as when rolling out of a hard turn or trying to reverse the turn direction as quickly as possible without climbing. The minimum normal acceleration for entering a rapid roll in a Jaguar is +0.5g with no external stores (and then only for a maximum bank angle change of 180 deg), and +1g with external stores. It is worth considering next time you watch the RAF Jaguar display that a vertical roll is actually a 0g rapid roll i.e not a permitted manoeuvre in a Jaguar!
It is possible to see the onset of inertial autorotation in the Jaguar even during 360 deg rolls entered at +1g. As the roll progresses, the indicated normal acceleration reduces to around 0. At high speeds of 450 KIAS and greater (low AoA) this occurs after only a 90 deg bank change. However, at lower speeds (higher AoA), the bank change before this is observed increases until at 250 KIAS (minimum speed for rapid rolling) it does not occur at all.
The recovery from inertial autorotation in the Jaguar is to smoothly pull the stick rearwards until the motion stops (whilst maintaing neutral lateral and directional control inputs). This agrees with the theory that to stop the motion you have to reduce the angle between the inertial axis and the flightpath vector and so should, theoretically, work for any aircraft so long as its longitudinal controls are still effective. A very interesting topic!
Multiple rolls can trigger inertial autorotation as the roll rate increase progressively when the inertial axis diverges from the flightpath vector, thus increasing the roll inertia. However, a far more role relateable trigger in the Jaguar is entering a roll at less than +1g, or simultaneously pushing and rolling such as when rolling out of a hard turn or trying to reverse the turn direction as quickly as possible without climbing. The minimum normal acceleration for entering a rapid roll in a Jaguar is +0.5g with no external stores (and then only for a maximum bank angle change of 180 deg), and +1g with external stores. It is worth considering next time you watch the RAF Jaguar display that a vertical roll is actually a 0g rapid roll i.e not a permitted manoeuvre in a Jaguar!
It is possible to see the onset of inertial autorotation in the Jaguar even during 360 deg rolls entered at +1g. As the roll progresses, the indicated normal acceleration reduces to around 0. At high speeds of 450 KIAS and greater (low AoA) this occurs after only a 90 deg bank change. However, at lower speeds (higher AoA), the bank change before this is observed increases until at 250 KIAS (minimum speed for rapid rolling) it does not occur at all.
The recovery from inertial autorotation in the Jaguar is to smoothly pull the stick rearwards until the motion stops (whilst maintaing neutral lateral and directional control inputs). This agrees with the theory that to stop the motion you have to reduce the angle between the inertial axis and the flightpath vector and so should, theoretically, work for any aircraft so long as its longitudinal controls are still effective. A very interesting topic!
