David-aviator
Well Known Member
This is in the FWIW category, just a thought or two on getting a decent prop balance job. What follows has to do with the MT electric system but may be applicable to any moveable blade system.
Typically in the past, for me, it was done by simply pushing the throttle up to a predetermined rpm to get the initial vibration number. With the MT prop this is always with blades in the "green light" fine pitch mode. There is little appreciable load on the prop, you can take it all the way up to the fine pitch limit and the blades are as flat as can be. Yes there is some thrust, but it is minimal. That's why it takes so much rpm to get this thing moving on the ramp.
It would seem if we want a more realistic vibration number to start the process, the blades ought to be off the fine pitch stop to more closely approximate what is going on in flight. If it were practical, the entire process would be better if done in flight.
Anyway, yesterday I suggested we get the blades off the fine pitch stop to see what would happen. This is accomplished by setting the controller to 1700 in auto and pushing the throttle up and letting the system do the rest. On the first run, manifold pressure about 22 inches, you could hear and feel the blades come off the fine pitch stop and settle in at 1700 rpm. The airplane was shaking a bit and the blades were flexing under the load. The guys at the vibration meter could see it happening. The dot on the scope was jumping all over the place for a moment and then settled down in one quadrant. He then had me go up to 2200 and same thing happened. With these initial numbers, he then calculated where the weights would go and how much. We repeated the process 3 times, installing temporary weights, removed the spinner, installed permanent weights under the aft plate flange with the recessed screw heads between the spinner and plate, ran it again, made a minor adjustment and it was done. The process took about 2 hours but we got very satisfactory results.
The difference in vibration between flying in and flying out was quite apparent. The operation was somewhat smooth before, but it is much more smooth yet some more. It is not unlike flying behind a turbine.
Typically in the past, for me, it was done by simply pushing the throttle up to a predetermined rpm to get the initial vibration number. With the MT prop this is always with blades in the "green light" fine pitch mode. There is little appreciable load on the prop, you can take it all the way up to the fine pitch limit and the blades are as flat as can be. Yes there is some thrust, but it is minimal. That's why it takes so much rpm to get this thing moving on the ramp.
It would seem if we want a more realistic vibration number to start the process, the blades ought to be off the fine pitch stop to more closely approximate what is going on in flight. If it were practical, the entire process would be better if done in flight.
Anyway, yesterday I suggested we get the blades off the fine pitch stop to see what would happen. This is accomplished by setting the controller to 1700 in auto and pushing the throttle up and letting the system do the rest. On the first run, manifold pressure about 22 inches, you could hear and feel the blades come off the fine pitch stop and settle in at 1700 rpm. The airplane was shaking a bit and the blades were flexing under the load. The guys at the vibration meter could see it happening. The dot on the scope was jumping all over the place for a moment and then settled down in one quadrant. He then had me go up to 2200 and same thing happened. With these initial numbers, he then calculated where the weights would go and how much. We repeated the process 3 times, installing temporary weights, removed the spinner, installed permanent weights under the aft plate flange with the recessed screw heads between the spinner and plate, ran it again, made a minor adjustment and it was done. The process took about 2 hours but we got very satisfactory results.
The difference in vibration between flying in and flying out was quite apparent. The operation was somewhat smooth before, but it is much more smooth yet some more. It is not unlike flying behind a turbine.
