Wed Jan 10, 2018 2:36 pm
AIUI, there is NO automatic Advance- Retard an aircraft magnetos.
There is no need, as the engine spends most of it's life in a very narrow speed-band, so a fixed timing is perfectly easy to optimise and simplifies things. Unfortunately, the advance needed for 2500 RPM, is far too early for the slow rise of the piston when using a starter or hand-propping....thus, the ignited mixture can expand and produce power before the piston/crank has cone "over the top"....Piston is stopped and reverses direction before reaching the TDC position (The classic "kick-back" )
The Magneto is really 2 items in one,- a generator and a transformer (ignition-coil) Timing the "break" ensures that the primary coil produces it's optimum output at the right time. and the points, driven by the same
revolving armature, break that current at exactly the right time . this sudden interruption in the current causes the induction of a voltage into the secondary winding....so you get a spike if output, which is taken to the spark- plug.
Instead of a direct coupling to the engine, an Impulse- drive is arranged to drive a "clock-spring" and the magneto is furnished with a centrifugal catch which , when it reaches it's stop, stops the magneto revolving.....continued rotation of the engine winds up the spring, which has it's other end fixed to the magneto-side of the coupling....continued turning then brings the "trip" round to release the catch.....the spring unwinds, "****" the magneto round smartly and producing a nice fat spark.....the delay in timing is a product of the number of degrees rotation the spring winds up until it's released . When rotational speed id high enough, the centrifugal catch parks and the spring is sufficiently tensioned that it holds the mechanism firmly against it's stop in the "unwound - run" position.
From this, you will see that too-fast a cranking- speed will disable the impulse- coupling.
Said coupling not only retards the timing when it operates, but because the magneto gets a fast "break" it makes a strong spark.
A simple device that is a very ingenious solution to 2 problems.
!- sufficient rotational speed to generate adequate output.
2- a later spark to ensure the power of the ignited mixture is produced after Top Dead Centre.
Hope this is helpful.