R44 200ftAGL engine out Autorotation video

[John R81]

It takes time to observe the loss of power in the engine because this is an indirect measurement which requires time for the lower power to have an effect on what you are measuring.

Each cylinder has 2 spark plugs, each of which is fed from a single separate mag. Working correctly, each initiates a flame front in the fuel / air mixture within the cylinder which travel outwards from the spark and eventually (in fractions of a second) meet to provide full combustion. Before the test the engine manifold pressure is set to provide power sufficient to maintain the rotor at a steady 75% ROTOR RPM. That level of power is required because of the resistance at zero blade pitch (air on the rotors causing drag and friction in the mechanical systems).

Switching to one functional mag at the same manifold pressure (same volume of fuel / air in the cylinder) means only a single spark and flame front within the piston, and that is all that you have changed. The burn from a single flame front now has to cover the whole of the cylinder volume and hence the fuel / air mixture is slower to achieve full combustion. This is less efficient, the burn having to be at the correct point in the cycle of the engine (timing) for peak efficiency, and less power is now produced by each cylinder even though the manifold pressure has not changed. Resistance in the system is initially unchanged (ROTOR RPM is 75% when you start the test) but so less power will then be less than needed to sustain the ROTOR RPM at that same speed and it will slow to a point where the resistance forces balance the new (lower) power output.

The drop in engine performance is instantaneous but what you are measuring is not. You are watching the ROTOR RPM, and it takes finite time for resistance to slow the blades to the new equilibrium. You notice the loss of power straight away - not as smooth and RRPM begins to drop. With a "good single mag" the new ROTOR RPM steady state will be achieved well within the test time.

A weak (failing) single mag will provide a weaker spark than intended and provide a smaller ignition patch, or bad timing on that mag will provide a mis-timed spark; in either case these additional issues will result in further power drop than expected. The ROTOR RPM then falls further to achieve the new equilibrium between resistance and power. The bigger the power drop, the longer it takes for the ROTOR RPM to decay to the new equilibrium - there is quite some momentum in the system, after all. Hence the duration of the test is what Robinson have determined is enough to to check that the mag is within serviceable range, i.e. long enough for the ROTOR RPM to drop out of range and be spotted if it is not within serviceable range..

I was told that the difference in drops between L and R mag is due to the position of the two spark plugs in the cylinder, which results in different flame progression in the cylinder - though I can't vouch for that.