in reply to:

- Kent, Do you by chance know why the prop blades should be in the same
plane as the crank throws? Your description (1 & 7 oclock) is at odds
with that).

In 4 cylinder engines, I think I can see a basis for a 45 degree
offset to prevent second harmonic engine yaw from coupling into the
airframe as first harmonic due to the different swinging inertia of a
two blade prop.

I'm not an engineer, I have not done a modal analysis and do not know how
the 2nd harmonic will act.
45 degrees? The position is 30 degrees after TDC or BDC which ever crank
throw you are watching.
I have no idea why Lycoming does not put a bushing exactly at TDC. Though I
do recall seeing one there in the lightweight flange models.
Perhaps a Commanche. Or maybe the crank shop installed the bushing in the
wrong hole after regrind!

A prop installed on the 90 degree bushing can display a very acceptable
dynamic balance reading on the front end of the engine. However the engine
and prop assembly as a whole can be roughly moving in a dynamic couple.
This couple will be apparent if a second vibration sensor is used on the
aft of the engine. A differential of 0.30 - 0.40 Inches per second
is not unusual.

The pros do not always get it right the first time either. Some examples:
The first 300 of the Mooney 201's have a service bulletin that moves the
index bushings in the crank flange. Changes the IO-360 A1xx to a
IO-360-A3xx. A Lycoming service bltn say about the same for some Aviat
Huskys. Grumman AA-5A service manual is different from the AA-5B. 5a is
before TDC, 5b is after.

Perhaps the timing of the combustion impulse in consideration with the prop
vibe survey is a factor. Faint rumour is that it will be difficult to
certify a metal prop on a 4 cyl Lyc with electronic ignition. Did Van's
aircraft get Hartzell to bless their's?

And finally in reply to


I'd like to believe the balance people would have caught that sort of
thing rather than hang compensating weights on the spinner.

Been around aviation long?

Besides, heck, if a flying club was smart enough to know they were chasing
1st order vibres, they should have been savy to suspect prop index.
And another besides...... a lot of balance technicians are barely
acquainted with theory and don't need more than minimum experience.
Their box tells them how to do it!

Kent Felkins