Originally Posted by
BDAttitude
Switching an ignition circuit off leads to a step in power produced by the engine, not a step in engine speed. So no freewheeling.
On a prop with low inertia and no constant speed governor active, the new power level will lead to a nearly instantaneous drop in speed due to low masses and high aerodynamic resistance.
I strongly suspect that I will take a little longer for the rotational masses in a R44 to settle for the new power level.
Freewheeling does occur. The sprag clutch leaves the engine free to decelerate to the extent that switching a mag off will make it decelerate without any inertial effects from the transmission or rotor system. Also, this test is performed at 75% RPM which is, intentionally of course or you could not perform the test, below the 80% point at which the governor takes over, so that is not a factor either. The only rotational masses the engine has to contend with during this test are those of the engine itself. Thus your assertions are not entirely correct. However, it is true that, because of the sprag clutch, there are no frictional or drag forces loading the engine from the transmission or rotor system, so that does have some affect on the deceleration of the engine.
I was asking how long that takes and if this magneto check is practical therefore.
A tired mag will approach the 7% in 2 second limit, so not long in that case. I brand new mag can take 5 or even 10 seconds to show even a small drop, say 2%. One must be careful and patient to detect a bad P lead. I suspect most do it by ear, but I like to visualize a discrete drop in RPM. In any case the check is eminently practical.