Peter – think of your power plant as six independent single cylinder engines joined together. When all six are producing the same power, all is smooth.
When a cylinder starts to produce less power than the others, you will get increased vibration. LOP is very often less smooth than ROP as the %mixture/power curve is steeper LOP. When LOP a small change in %mixture creates a bigger power change than ROP
When you shut down by pulling the mixture you are going will be moments of extreme power imbalance where one cylinder has stopped producing power all together. Combine this source of imbalance with the accurate comments above regarding resonance and you get yourself a shudder
We feather when we put the brakes on, then go through the shutdown checklist, so they are turning fairly slowly when the fuel goes off..
Feathers during shutdown as fuel and feather are same lever.
A PT6 does it as well although not as much as a piston.
Ours don’t, do you feather before you shut down?
Russian tachs are not labelled in actual rpm they are instead normalized to 0-100% engine speed, I believe the engine would be an M14P nine cylinder supercharged radial making either 360 or 400 HP. 65% speed is about 1900 rpm for an M14P and sitting there for 30 seconds before shutdown stirs up a lot of air. The start up procedure is even more Byzantine.
What is “our”? 60% power would, on an aircraft with constant-speed prop, bring the RPM most of the way to redline! In the parking area? No way!!
I tend to think along with achimha: cylinders have different rates of compression leading to different rates of self-ignition.
As for the Rotaxen: yes, mine frightened me much in this respect, until I learned to cut one magneto, count three, then cut the other. For those not in the know: no mixture lever on these engines so there’s only one way of stopping them (apart from shutting the fuel valve and wait for several minutes).
