By looking at the FFT results of intensity (G) vs. frequency (hz) and using the calculation V(in/sec) = 61.5 x G / hz, the 1X prop vibration was reduced from 0.21 to 0.04 in/sec or by a factor of about 5. This is not evident when looking just at the raw
data.
Subjectively, before balancing, a vibration could be felt mostly in the seat back and in the fore-aft direction of the control stick. It was not objectionable but I was constantly aware that it was there. After balancing, the vibration was felt only in
the range of about 5200 engine rpm (1800 prop) to 6000 engine rpm (2100 prop). Outside of this range the vibration was not felt. It is possible that this resulted from the beat seen in the raw data which has a frequency of about 5 hz causing an airframe
resonance. It has been reported that the human body is quite sensitive to pressure vibrations in the range of 5 to 10 hz. Unfortunately, rpm within that range would be desirable in cruise flight.
It was found later that one of the three prop blades was about one degree in pitch different from the other two. This should not contribute to a prop weight imbalance but could contribute to the airframe vibration signature. It was possible to readjust
the parts within the prop hub to get all three blades consistent in pitch to within 0.1 degree. After this adjustment, the vibration in the 5200-6000 engine rpm range was gone most of the time. There is about 1 degree of lost motion when the prop pitch adjusting
mechanism changes direction. In other words, one can overcome the seal friction of a particular blade and change the pitch angle by about 1 degree back and forth by twisting the blade by hand near the root.
In flight, there is an aerodynamic twisting force on each blade in addition to that from a counterweight. Apparently, this total torque is insufficient to immediately overcome the seal friction of one of the blades and keep that blade pitch pin block
against the fine pitch side of the pitch change mechanism. The result is a vibration that persists for a few seconds after a slight decrease in commanded pitch. This happens in level flight in constant speed mode when an up or downdraft is encountered. The
momentary vibration can also be induced in manual mode by decreasing the pitch. Thus the "most of the time" comment above.
Photos of the pitch change mechanism inside the prop hub may be seen in the following link which was also posted previously. The photos show the triangular fore plate installed or removed. The black plastic blocks engage a pin installed off-centered
in each blade root and are captured with a small amount of clearance when the fore plate is installed. When the center jack screw is turned, the plates move fore or aft, thus rotating the blade and changing the blade pitch.
In my aircraft at least, there are many things affecting the vibration characteristics and everything seems to affect everything else.
Steve Boese
I see that you cut the vibration amplitude by something on the order of 50%. How does it feel? I mean, did it go from "It's like flying a 1970's cheap motel massage bed" to "I can't tell if the engine is running", or was
it like "I almost can't feel it" to "I'm not sure if I can feel it"?
Granted, this is a totally subjective measurement.