On Wed, 05 May 2004 20:36:12 -0600, MikeM wrote:

How do you get the wire from the accelerometer/position pickup into the
cabin during flight?

Depends on the install. Ty-raps, oil door/cowl flap/cowl, "wing
window"/cabin door and duct tape. Put the reflective tape on the prop
face (instead of the back) and shine the strobe through the window.

The newer balancing equipment can log the vibe signature through a
wide frequency range and is useful in these cases also. The older CH
spectrum analyzer could also do a survey of a range of frequencies.

Since the only place that you can put a weight is on the prop spinner
backing plate, can Prop balancing do anything except cancel the "first
order" mass imbalance (usually called a "static" balance)?

Wouldn't you have to have access to the other end of the crankshaft to
add weights there in order to be able to do a true multi dimensional
"dynamic balance"?

I'm not trying to be argumentative; just trying to understand the claims
made by the purveyors of prop balancing services.

If you haven't figured it out by now, I am not selling a darn thing,
nor do I tend to avoid controversy. You bring up some valid points,
I'll do my humble best to address them.

I'm not an engineer, just anonymous guy on Usenet that might know how
to balance props-with trial and error, a little elementary physics,
and some phone calls to powerplant and propeller engineers-and has
possibly balanced quite a few over the years. Standard disclaimer-This
info is worth exactly what you paid for it.

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.

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.

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.

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.

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.

I have wondered that during a ground run, doesn't the buffeting caused
when the prop tip swings within a couple of inches of the ground induce
wierd vibrations in the moving propeller that might not be there if
the prop where swinging in free air? In other words, wouldn't it be
preferrable to record vibration data during an actual flight, and
then postprocess it into a "where and how much weight" solution later?

OK, I have seen what effect (affect?) gusting winds have on dy-bal
jobs, hence the reference to balancing in the hangar. I've often
wondered the same thing. The only in-flight checking I have done has
been on the "weird" ones, and typically did not see a big difference
in the 1st order levels between ground runs and in flight. If I had a
better answer based on personal observation, I would share it.

Again, remember, the average guy just wants it done cheap and in a
hurry, and end up with a turbine-smooth result.

What is a typical charge for this service?

No idea. Had a prop shop next door, so had a relatively high volume of
dy-bal jobs. At that time we were more concerned with taking care of
our customers by offering the additional service (balanced with the
capability to do our own 'planes inexpensively) than making a profit.

When I got out of GA several years ago, we charged $150.00 for a basic
single-engine dy-bal which included 1 hour of shop labor. That would
usually cover a survey run, installing "test" weights, a final run,
and basic permanent weight installation. Any additional labor was
charged at the normal shop rate. A twin was $300.00 and included 2
hours of labor.

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.

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.

Apologize for the length;

TC