This issue first appears in UE 4.9, but I have marked it as UE 4.10, as the issue is still present, and that is the engine version I am currently using.
The issue is that for a given impulse applied to a physics simulating object (not on an object’s center of mass, to produce change in angular velocity), the same impulse induces a much larger change in angular velocity for engine versions 4.9 and 4.10 than it does under exactly the same conditions in UE 4.8 (i.e. the moment of inertia is smaller for an identical object in 4.9 and 4.10).
I have produced a minimal example:
This video shows an impulse of (0, 60000, 0) applied to the physics-simulating object at location (0, 0, 0), slightly below the objects center of mass, in UE 4.8:
This is exactly the same conditions, and the same impulse in UE 4.10:
As you can see, the object is rotating much faster in 4.10 than it is in 4.8, but it’s linear velocity is the same.
I logged the values of the linear and angular velocity after the impulse for both engine versions:
in UE 4.8:
Linear Velocity: X=0.000 Y=399.967 Z=0.000
Angular Velocity: X=399.995 Y=-0.001 Z=2.000
in UE 4.10:
Linear Velocity: X=0.000 Y=399.967 Z=0.000
Angular Velocity: X=696.273 Y=-0.001 Z=3.481
The speed of rotation from this same impulse in 4.9 and 4.10 seems unreasonably high to me, also I can’t imagine any reason to deliberately cause this change with the engine update, so I assume this is a bug.
Otherwise, could someone tell me if there is a way to manually alter a physics object’s moment of inertia?, as my game uses ragdoll physics integrally, and having just updated to UE 4.10 from UE 4.8, all the ragdolls are spinning erratically at the slightlest provocation.