Survey - 3 blade prop conversion- Cockpit vibration, happy or not

I did a bunch of "research" (asking as many 235 owners as I could
find) about this two years ago when one of my blades failed
inspection. Almost all of them reported some sort of vibration.

I've heard this before, but I don't understand why an extra blade would
inherently add vibration?

If anything, shouldn't an extra blade reduce vibration?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Hello Folks,

Theoretically, a 3 blade whould be smoother, quieter and more efficient,
but it simply isn't like that in the real world.
I became aware of common dissatifaction of cockpit vibration in the time
following AD 98-18-02, which mandates 500 hour disassembly of the Hartzell
BHC series.
Many owners chose to buy a 3-blade conversion instead. Surprisingly, the
3-blade was several thousand dollars cheaper than an approved 2-blade.

A couple dozen owners of Commanche, Mooneys and other makes contacted me
for dynamic prop balancing. Many wtill were not happy.
At the time it seemed most were conversions with scimitar blades.
I presumed it was because of 3 blades installed on a 4 cylinder Lycoming.

A 1956 C180 owner sent his back, and he later informed me that the prop mfg
admitted there was a problem sometimes withthe 3-blade on the installations.
They suggested it was due to counterweights.
Other C182 owners were not happy either.

Later I became involved with the issue on Beech V35's. Some of these are
McCauley conversions with scimitar blades. Debonairs with th O-470's
generally regret 3-blades. Even STC holders like Bery D'Shannon have heard
complaints.

Where I'm lacking is that I did not fly many of the planes myself, nor can
the owners actually define the vibration complaint. One response was
"...the panel is smooth now but the vibration moved down to tthe rudder
pedals..."

I wonder if their tail controls needed rigged, or maybe it was because the
scimitar blades have different tip vortices affecting the tail surfaces.
Aircraft empennages are lightweight and suffer much from vibration, not only
propwash but also mechanical excitation being transfered from the
powerplant. Other day, I helped push a C414 into the hangar with a
powertug that had a 5hp Briggs & Stratton. The horizontals were humming
from the little engine thump.

A related issue perhaps, is Vans Aircraft tried to get Hartzell to bless the
a 2 blade installation on a 4-cyl Lyc with electronic ignition. Hartzell
refused.
.. Whether it was liability, or the expense of redoing vibration surveys, or
if they actually saw something critical in a a vibration survey I do not
know.
Electronic ignition varys the engine timing and also may have a slightly
different spark characteristic which influence the cylinder pressures, thus
the power pulse affects the torsion which affects how props vibrates.
I get a gut feeling that certified approval of electronic ignitions on 4
cylinders may be long in coming.

Like a tuning fork, all props have one or more resonance points. The tips
flutter fore and aft, plus the blade is stressed by a bending moment in the
rotation and anti-rotation axis. Prop mfg's survey the vibration patterns
of a particular engine-prop combination. This is why one sees tachometer
yellow arcs and red arcs at on some aircraft and also why engines have
different counterweight configuration.

Take care
Kent Felkins

"Jay Honeck" wrote in message news:JeDDc.101103$2i5.20125@attbi_s52...
I did a bunch of "research" (asking as many 235 owners as I could
find) about this two years ago when one of my blades failed
inspection. Almost all of them reported some sort of vibration.

I've heard this before, but I don't understand why an extra blade would
inherently add vibration?

If anything, shouldn't an extra blade reduce vibration?

It should, but the science of vibration studies is complicated.
I know very little about it, but old mechanics who battle three-blade
vibration will sometimes tell you that removing the prop and rotating
it 180 degrees on the crank will sometimes fix it. The 185 (IO-520)
has been known to throw the alternator belt with three-blades, and
rotating the prop usually stops it. Don't ask me to explain it, but
there must be some sound reason behind it.

Dan

Generally a three blade will seem smoother on a six cylinder engine. Same
for boats, three blade for 6cyl, four blade for 8cyl. This is for
noise/vibration only performance may be different.

Mike
MU-2

"Jay Honeck" wrote in message
news:xCzDc.117287$eu.53283@attbi_s02...
Interesting work, Kent. Thanks for doing it.

Any word on the O-540s? A 3-blade is in our future, if (when) our 2-blade
needs to be over-hauled.

But only if it doesn't change the vibration for the worse!
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

I installed a 3-blade on my IO360C1C Arrow because old prop was out of
spec and cheaper than a new 2-blade. I think there is less noise,
it's nice to be able to ignore the 2100-2350 RPM avoidance placard for
the old 2-blade, and climb performance is definitely better.

I had substantial vibration initially, but after 180 rotation and
dynamic balance it is now very smooth - no complaint.

Cruise speed is hard to judge because of lack of controlled
conditions, but I can still get 175 mph TAS out the old 69 Arrow. It
certainly isn't any faster, and I can't prove that it's slower.

And, as others have said, it does look pretty sexy. Shorter blades
also pick up less FOD.