One extremely important check which must be done on the blades before any dynamic balance is performed is to verify the blade angles. I'll explain in detail how this can be done on the aircraft shortly.
I recently had my prop dynamically balance on my YAK 52. Before this was done,
all the prop hub weights were on one side. After dynamically balancing the prop,
we had to add more weight to the same side of the hub. The vibration was reduced
dramatically though. We balanced the prop while running at 70%. Now here's the
catch.
On my airplane there has always been a resonant vibration between about 46-48%
and the high 50% area. I was told this is relatively common and is usually
caused by mismatched blade angles and don't trust the "tic" marks on the blades
or counterweights to set the proper blade angle. So dynamically balancing the
prop without first accurately setting the blade angle on both blades only masks
the root of the problem. My blades had a 2 degree difference between the blade
angles. This is where the problem stemmed from.
The correct setting for the blade angle is 14.5 degrees, 8" in from the tip of
the prop blade. Some of the props have a vertical line (if the blade is
horizontal of course) marked on them. Usually they are the white ones. This line
is typically the 8 inch line and is used to set the blade angle. You will need a
smart level which measures angles electronically and has a digital readout in
tenths of a degree.
Place the prop in a horizontal position. Next remove the hub spinner by removing
the cotter pin and nut. Then remove the piston from the center of the hub being
careful not to spill the oil into the hub unless you want oil spray all over the
place for the first couple of hours after the next start (remember, we have a
dry hub). Using the counterweights, you can now move the blades back and forth
quite freely. Now you have a machined surface (the front of the prop hub) to use
as a reference point. Place the smart level on the front of the hub and measure
the angle. This is the airplanes angle of incidence. Make note of the angle. Now
go to the prop blade on the left side as you face the airplane. Measure 8" in
from the tip in two places approximately 1" on either side of the approximate
center of the tip and make a vertical line across the blade. Be sure the blade
is up against it's mechanical stop in the flat position. Next measure the blade
angle using the smart level at this line CENTERING the smart level over the
line. Make note of the blade angle. On mine, this is what I saw on the first
blade. Again, my blades had a 2 degree difference between them. NOT acceptable.
Angle of incidence - 2.7 degrees
Blade angle - 79.3 degrees
Doing the math, 90 degrees minus 79.3 degrees = 10.7 degrees PLUS 2.7 degrees (A of I) = 13.4 degrees blade angle
Now rotate the prop 180 degrees and take the same measurements on the other
blade. Don't forget to measure the A of I again after rotating the prop. The
reference point is obviously very important and by rotating the prop or moving
the airplane, you could change the A of I. So be sure to measure it each time
for each blade.
To make the blade adjustment, at the rear of the counterweight is a 17 or 19 mm
(can't remember which size) nut with a cotter pin thru it and the bolt. Loosen
the nut so there is no clamping tension on the blade shank. Now gently rotate
the left blade to achieve the 14.5 degree blade angle. Check your readings a
couple of times and don't forget to set the calibration on the level beforehand
for each blade. When the angle equals approx. 14.5 degrees, tighten the
counterweight clamp. DO NOT reinstall a cotter pin at this time because you will
want to recheck all your settings before you do put the new cotter pin in place.
Do the same for the other blade.
After making the adjustment on both blades, recheck the settings again by
rotating the prop and measuring the angle of incidence each time before
measuring the blade angle. Reinstall the piston without any oil in it (because
you WILL spill oil into the hub when you reinstall it), piston lock plate and
screw, spinner, nut and cotter pin. Don't forget to safety wire the piston
locking plate with the screw that you had to remove to be able to remove the
piston in the first place.
When this is finished, then you can dynamically balance the blades. But be sure
to start with no weights on the hub though.
Hope this helps.
Dennis Savarese
YAK 52 N152JB CLW