Dynamic prop balance

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

wrote:

I have checked the vibe level in-flight on numerous occasions, on
aircraft that acted "strange" after a standard dy-bal.

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

ty wraps and 200 MPH tape.

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)?

You are correct on most engines you do not have access to the rear of
the crankshaft.

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"?

Yes

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

It would seem to me (and forgive me, it's been a few years) that on a
constant-speed propeller that achieving cruise rpm is quite do-able.
I'm thinking that we useta set the rpm approx. 100 rpm over the
desired "cruise" setting using the throttle, and retarded it using the
prop control.

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?

This would give a 2 X per rev vibration. Mass imbalance gives a 1 X
per rev vibration.

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?

Much more trouble and the chance of removing some paint when removing
the tape holding the wires exists.

A proper dy-bal job performed by a knowledgeable technician is a
decent value. We did all the company aircraft mainly because we owned
the box...

What is a typical charge for this service?

Most A&P's that work on helicopters have the balancer needed.



Thanks,

MikeM


What you are trying to do is balance to 0.1 IPS or less. To do this
requires corrections in the range of a few gram-inches.
This is the weight in grams multiplied by the distance said weight is
from the center of rotation.
All props have some slop in the mounting to the crankshaft. Lets say
you have a prop that weighs 22 Lb or about 10 kilograms. If it has a
0.001 inch slop between the prop and the crankshaft bolts then
0.001*10,000 grams is 10 gram inches. The correction weight would be
2 grams at 5 inches or 10 grams at 1 inch.

To get an acceptable vibration level I used to balance a tail rotor of
a H269 helicopter to within one tenth of a gram. It does not take
many bug splatters to equal this amount!

John