Toecutter and John, thanks for the informative responses. I am
interested in the methods. I have a few more ??

wrote:

snip

As you've indicated, the best way (I'm aware of) to do a vibe survey
is to mount a velocimeter as close to the front of the engine as
practical, and another on the accessory case.

Are the accelerometers two axis (X-Y) or single axis?

If single axis, are they mounted so as to be sensitive left-right or
up-down?

Do all balancers use the second accelerometer on the accessory case?

Do the engine mounts allow more up-down, or left-right shake if
the engine-prop are out of balance?


In a perfect world, with a simple 1st order front end "prop" shake,
the rear of the engine will be driven in the opposite direction as the
front (when the front is pulled "up" by the vibe, the rear is being
pushed "down"). Reducing a simple mechanical imbalance by adding
weight to the prop hub/backplate will reduce the amount of vibration
measured at both ends of the engine.

A mechanical imbalance on the rear of the engine will have the same
affect (effect?) on the front. In most cases, reducing the the
measured 1st order vibe at the front will also reduce it at the rear.
In other cases, adding additional weight to take the vibration cycle
"beyond" 0.0 IPS, actually creating vibration 180 degrees out of phase
from the original imbalance (at a reduced level) will reduce the
measured vibration at the rear of the engine by a larger amount. My
thoughts are that this would tend to reduce the 1st order vibes that
are felt from the driver's seat.

Is the criteria for "best subjective" balance some combination at what
the front and rear accelerometer see?

In other words, if all you had to work with was the front accelerometer,
and you balanced to minimise its amplitude at the fundamental shaft
rotation, could you create a situation that the pilot may interpret as
making things worse?

The problem (common to most aspects of GA maintenance) is the average
owner wants the job performed yesterday with $$ spent being the
primary consideration.

The quickest way to do that is to reduce the vibe level at the front
(single velocimeter installed) to below .1 IPS. It is also the most
practical/economical method.

Is this "good enough" in most cases?

A spectrum analyzer comes in handy when diagnosing "strange"
vibrations, or when a massive reduction in 1st order vibes doesn't
seem to make a big difference in the cockpit. The most common cause of
this that I have seen is 1/2 order vibration. The frequency is lower,
it tends to shake the panel (and your backside) a lot harder. We used
an old low-tech box, but it was simple to dial in 1/2 the rpm, and
measure the 1/2 order vibe.

The only "mechanical" explanation I can imagine for 1/2 RPM vibration
modes is that there is a weak cylinder, which in a 4 cycle engine only
fires every other shaft rotation?

Again, as you've indicated, IMHO this is nothing that can be remedied
by adding weights to the hub/backplate. It just allows you to "see"
that there are higher levels of other vibration frequencies present,
and confirms that the backside is giving the brain good data. I really
think it is impossible to change the levels of these vibrations at non
1/1 frequencies by hanging weights on the front.

Is any of this making sense? Forgive me if it is not, some of this
stuff is hard for me to describe face-to-face, let alone with fingers
and a keyboard.

Makes perfect sense. I am an engineer and know enough about electronics,
instrumentation, mechanics, & digital signal processing to be totally
dangerous.

snip

At that time, due to a relatively well-defined FAA ruling (that was
several years old, just hadn't trickled down to the field) if no
approved maintenance manual procedure for dy-bal/weight attachment was
present, a 337 was required. Not a real big deal to fill out a 337,
but it takes time. And at that time, GA "approved" dy-bal procedures
where pretty much non-existent. Am not sure if that has changed in
recent years.

Is the 337 necessary to do the ground testing, or only testing during
flight, or to drill a hole and put a bolt into the spinner backing
plate?


Our equipment was basic, and experience with how much weight needed to
be added where was a necessity. I had a knack for it, and usually
didn't have to do a lot of weight swapping and additional survey runs.
The primary advantage to the newer digital equipment is that it uses a
optical "tach" pickup, and tells you exactly how much weight to add
and where. A secondary advantage is the ability to do spectrum
analysis. The disadvantage is that with anything other than the
"perfect world" scenario, you're right back to trial-and-error and
experience.

I have run into some balancer equipment salesmen at airshows who make
all sort of claims about "you dont have to know anything to use our
equipment" which automatically set off my "bull**** detector".

Appreciate your experience and willingness to share it


MikeM