Steve,

When I installed an MT 3-blade electric constant speed prop, I dynamically balanced the flywheel end of the engine with the damper plate installed but without the reduction drive installed.  This was accomplished by changing the weight of the damper plate attachment washers in two adjacent positions.  The result was 0.018"/sec at 4000 RPM.  This could not be improved further due to the angular location of the imbalance changing from one run up to the next.  My guess is that this was a result of about 0.020" of radial clearance between the corners of the square damper plate and the flex plate.  The damper plate may shift until there is no radial clearance at some location and this angular location is not consistent.

After installing the reduction drive and prop, dynamically balancing the system with the sensor on the reduction drive as close to the prop as possible resulted in a one per prop revolution signal of 0.035"/sec.  There was a three per prop revolution signal on the ground that disappeared in flight.

In flight at 2100 prop RPM (~6000 engine RPM) and above, the system seemed to be vibration free.  At prop RPM less than 2100 and greater than 1800, there was a definite vibration felt in the control stick but not noticeable when placing a hand on the instrument panel.  If the plane was oriented such that the reflection of the outer prop disk was visible, the arc of the blade tips was very clear when there was no vibration felt in the control stick.  This arc reflection became less distinct when the stick vibration was present.  My guess is that one or more prop blades was fluttering slightly fore and aft or all three blades were not tracking as closely to each other when the vibration occurred.

After placing the prop controller in manual mode in flight thus allowing no subsequent pitch change and then landing, it was found that the pitch of one of the blades as measured at the tip was different from the other two by one degree while the other two were consistent within less than 0.2 degrees.

Repositioning the pitch change blocks within the prop hub allowed the pitch of all three blades to be consistent within less than 0.2 degrees at the blade tips.  In flight, the RPM dependent vibration in the control stick was essentially eliminated.

There is about one degree of slack in the pitch change mechanism for each of the blades.  When in flight in constant speed mode, a slight pitch change causes that control stick vibration to momentarily return and then disappear. One of the blade's seals causes more resistance to pitch change rotation than the other two.  It appears that the aerodynamic twist in addition to the blade counterweight generated torque is insufficient to allow that blade to follow its pitch change block as rapidly as the other two.  This doesn't seem to me to be a safety of flight problem and the issue can be avoided by operating in manual mode if it is too annoying.  That behavior may also be resolved upon prop overhaul.  The overhaul interval is specified at 100 hr which seems unreasonably short to me, possibly determined by lawyers as opposed to engineers.

This may or may not have any bearing on what you are seeing.

Steve Boese

> On Dec 6, 2020, at 6:08 PM, Stephen Izett stephen.izett@gmail.com <flyrotary@lancaironline.net> wrote:
>
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>
> Thanks Dave
>
> That makes sense, particularly for those of us with the damper already.
>
> I suppose I’m also questioning our own vibration and noise from our fwf. I’d be interested to know what others have found.
> Our prop is an electric adjustable.
> We balanced each blade pretty precisely and then dynamic balanced it at 6000rpm, and according to the equipment we used we got it was right down in the noise at <0.05 inches/sec.
> It did require quite a bit of weight to get it balanced and Ive always wondered why that was. ~8 X 3/4 inch washers and the nuts and bolts that hold them to the spinner backplate at a radius of about 5 inches.
> I’ve pondered:
>        1. The Engines Rear counterbalance being wrong. But I thought it was on the back of the Eccentric Shaft when I got the engine originally, and then I purchased and bolted on the flex plate.
>        2. The Damper itself, but all the bolts, washers and nuts appear symmetrical.
>        3. The Prop Hub and Spinner - all appears symmetrical.
>
> I wish I had checked the balance dynamically without the blades and spinner to isolate what is causing the need for the weights.
>
> We can certainly hear/feel the difference in flight when we adjust the rpm’s between say 6K and 7K rpm.
> Its really smooth at 7K but rougher at 6K.  Though I’d prefer to run her at around 6K.
> There doesn’t seem to be a significant change in the fuel flow/IAS from using 6K vs 7K. I suppose the equivalent of driving in 3rd or 4th gear.
> I’d just prefer to lower the wear in case that is significant!
>
> I suppose it's this vibration difference that makes me want to get rid of the damper if others have found its not necessary.
>
> Cheers
>
> Steve