I read on the US Socata site that some people found differences in vibration according to the propeller angle.
The 3 blade props are notorious for vibration, relative to 2 blade props, but it isn’t clear whether this is due to worse QA at Hartzell on the 3 blade version.
Does this make any sense?
Look at the TCDS for the propeller, in “Note 9”. If there is a specified orientation for the prop on the crankshaft, it’ll say there. If not, it shouldn’t matter. Some props are doweled so misinstallation is not possible.
Otherwise, a dynamic balance, with the spinner installed.
Most metal props on high-power pistons are indexed. Given torque pulses and crankshaft torsional elasticity, blade position is important for system excitation (or rather, its absence).
Dynamic balance is great indeed since no matter how well a prop is balanced at the shop, it will typically vibrate in the order of 1ips after install, but always under 0.15ips after dynamic balance. Problem is you need to redo it every 200hrs or so…or more often if you like to cut grass with your prop…
The ramifications of the higher vibration are plentifold, but @Peter I thought your prop was two-bladed?
I don’t understand how the orientation could possibly matter since the moment of inertia is the same regardless of angular orientation.
Mine is 3B. It hasn’t been balanced since it was last overhauled, a few years ago, and I haven’t done it because I’ve not had the opportunity to get it done easily. Also I found that balancing didn’t make any difference to the vibration actually measured on the airframe.
Peter wrote:
I don’t understand how the orientation could possibly matter since the moment of inertia is the same regardless of angular orientation.
According to physics (which I consider to be relatively accurate 
) it doesn’t matter one single bit if the propeller has 3 blades or more. For 3 blades or more the gyroscopic forces become symmetric. For two blades or or less, meaning for two blades in all practical considerations, this is not the case. A two blade propeller will have a vibrational mode whenever the axis of rotation is changing. The formulas for this is rather complex, so I don’t remember them. The axis of rotation can change in pitch and yaw, and any combination, and the vibrations will change accordingly. If the aircraft has lots of pitch changes, you would mount the propeller so the piston strokes do not coincide with the gyroscopic induced vibrations.
For a 3 blade or more none of this matter one bit. I would therefore assume it is best to have a set position for the prop, because this will make repeated balancing easier ?
I would therefore assume it is best to have a set position for the prop, because this will make repeated balancing easier ?
I definitely agree that if a prop has been dynamically balanced, and you need to remove it for some reason, then it needs to go back in the same orientation, because the weights which have been added are partly or wholly attached to the backplate. Sometimes they get attached to the prop flange but IME that’s unusual (for balancing outside the factory or the prop shop).
The axis of rotation can change in pitch and yaw, and any combination, and the vibrations will change accordingly. If the aircraft has lots of pitch changes
OK; I get that. But that’s a very subtle 2nd order effect.
Peter wrote:
But that’s a very subtle 2nd order effect.
Far from it. It’s a first order effect, but removed altogether for 3 or more blades (does not exist at all due to gyroscopic symmetry). With three or more blades the moment of inertia is the same in any direction of pitch/yaw. With two blades it goes from literally zero to max two times for each rotation. It has nothing to due with aerodynamics, it’s 100% mechanical, and exists at full force in vacuum. The issue is that it only experiences while the axis of rotation is changing in yaw or pitch. A two bladed metal prop (heavy prop) can break havoc on the shaft due to this, if it’s not taken into account. A metal two bladed prop is the worst prop imaginable.
However, practical experience does not confirm the above. 2B props tend to run much smoother than 3B props.
In cruise, and on the ground when doing prop balancing, there is no significant pitch activity, hence my “2nd order” comment.
If you look at the link I posted above and ask yourself the obvious question, you may conclude that most if not all airframe vibration is not coming from the engine (through the engine mounts) but is coming from the oscillations in the slipstream.
Peter wrote:
However, practical experience does not confirm the above. 2B props tend to run much smoother than 3B props.
That’s an interesting comment because I’ve always had the opposite impression, that three blade props had slightly lower performance but vibrated slightly less. That is the popular perception anyway Link Link but whatever difference may exist fundamentally can easily be overshadowed by detail design, manufacturing and maintenance issues.
If I run into my CS prop designer/manufacturer friend later today I’ll ask him for his view.
