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

Hello Folks,
I asking for responses from any who have converted from a 2-blade to 3 blade
prop on their plane. I not interested in any performance claims but just
specifically the issue of cockpit vibration.
The prop mfg's and STC rep's are aware of this issue but express their
bewilderment to me. I've been looking for a common thread but haven't
found it yet. . There may be some engineer in a backroom somewhere who
knows the answer, but I haven't talked with him yet.
I don't have a better answer than anybody else.
I have been polling other owners groups and will later post what results I
can come up with.

It goes across the popular airframe models. Simply some are happy, others
are not. Even after a dynamic prop balance, some owners are not l not
happy with the different vibration, and different noise.
At the present, the conventional wisdom for those contemplating conversion,
is to not let go of the 2-blade until you are sure and make the deal to be
able to return the 3-blade for a refund if you choose.

It seems maybe that 6 Cyl Continentals which do not have a crankshaft 5th
order dampner or counterweight do not like 3 blades. i.e. Cessna 182 with
O-470-L or a Beech V35 with IO520-BA. Every Cessna produced with a
3-blade option, has an engine with a 4th,5th and 6th counterweight. Others
engines didn't, like the O-470L which has a 4th order and two 6th orders.

It could be that the models with scimitar blade design are the chief
culprit.
Some owners of Pipers and Mooneys with 4 cylinder Lycomings have asked me
about the vibration. All of them were scimitar blades.

Another factor is whether the conversion was done in conjunction with a
fresh engine overhaul versus midtime. Some speculate that at midtime, the
crank counterweights may have already worn in, taken a set, to the old prop
vibration.

I'd appreciate those who wish to participate if you let me know what
airframe and engine you have and what prop you went to, scimitar blade or
not. And whether the conversion was done fresh or at midtime and whether
you were happy or not with the cockpit vibration and noise.
Also useful would be a short note on how you percieved the vibration.

You can email me directly if you wish.


Thank you very much

Kent Felkins
Tulsa Oklahoma

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"

In article xCzDc.117287$eu.53283@attbi_s02, Jay Honeck
wrote:

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

Doesn't a two-blade weigh less?

Isn't a two-blade more efficient?

On Sun, 27 Jun 2004 13:05:33 GMT, "Jay Honeck"
wrote:

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!


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. Many
thought it was insignificant, but many thought it was worrisome. A
couple returned the new prop because they really didn't like it. I
took the approach that additional vibration no matter how minor is
un-needed. It could do nothing to the aircraft, or it could do
something, either way a small plane is noisy enough and has enough
vibration that I didn't want to add anymore, and my luck would be that
I'd have the same violent tail vibration one guy reported. No Thanks.

Other things I didn't like about the 3 blades were the fact that they
are a new "plastic" (carbon composite) type of material. No one knows
for sure what UV does to them. We may see some interesting stories
over the next 10 years if they don't hold up real well. Again its a
debate. Some people say composites are fine, some people say they
will become problematic. Until a 10 or 20 year study shows real
documentation it's all a matter of opinion. Also no ADs have been
issued for these yet (although many blades were scrapped before use
due to a bad production process). I like knowing that my prop
probably won't have any real expensive ADs issued against it. Since
its been in use for so long we generally know the limitations. (I
know once again a subjective opinion). With the composites I just
think a big surprise might be around the corner.

The last thing is that the 3 blades really kill your glide
performance. Almost everyone agreed on this one. My 235 already
leaves a little to be desired at best glide speed, why would anyone
want to shorten it even more? I know you can use the prop control to
overcome some of that, but what if you need to go-around for some
reason, you've got one more (critical) thing to do. The performance
gains on the other side didn't seem significant enough to me to
balance this out. Very slightly better climb performance an no one
reported better cruise.

When all was said and done I spent about 5Gs to do my governor, buy a
blade and have everything put back together. It would have been about
10 for the 3-blade. When I looked at the extra money, and some of the
downsides/unknowns. I kept the extra money in my pocket.

Hope this helps, I kind of streamed my thought process.

z

P.S. Glad you got some momentum behind your airport.

EDR wrote:

Doesn't a two-blade weigh less?

If tghe blade material is the same, the 2-blade prop weighs less.

Isn't a two-blade more efficient?

Yes.

George Patterson
None of us is as dumb as all of us.

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"

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"

Jay Honeck wrote:

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?

I thought the same, but it doesn't appear to be the case, at least not
with 4 cylinder engines. I've read that the 3 blade props seem more
compatible with 6 bangers. Supposedly, this is related to power pulses
of the engine, but I still don't see how that is any better with two
long blades rather than three shorter ones. I'd think the longer blades
would flex even more with the power pulses making more vibration, but
this apparently isn't the case. I'm not familiar enough with the
physics of engines, props and vibration to know what is at work here.

Our flying club just replaced a two-blade prop on our 67 Arrow with a
three-blade. We aren't happy with it at all. We haven't had it
dynamically balanced yet and plan to do that this year after the engine
is replaced. We were told that the balance is a composite of the prop
and the engine so we were advised not to balance it now since our engine
is near TBO and will be replaced this coming winter.

The Arrow vibrates a LOT more with the three-blade prop, especially at
RPMs less than about 2200. After someone mentioned a placard against
operation in certain RPM ranges, I checked the tach more closely. It
has no markings on the tach, but there is a small placard near the tach
that says to avoid a certain RPM range (I think it was 1500 - 2200) at
certain levels of MP. I need to write it down next time I visit the
airplane as I don't remember the details now.

As others have mentioned, the power-off glide distance is dramatically
reduced. This is useful if you are flying a fast approach to mix with
the big boys at the larger airports. When you chop the power on short
final, it is like dumping speed brakes. It is a real hazard for
emergency landings. The first one I tried while getting checked out in
this airplane ended up about a mile short of the field I'd selected. I
was amazed at the sink rate as compared to the Skylane I owned
previously. I don't think the Arrow has even an 8:1 glide ratio now,
compared to probably 12:1 or so for the Skylane. I thought it was just
the Hershey bar wing, but my instructor said the Arrow was much better
prior to the prop swap.

Bottom line, none of the club members who'd flown the airplane with the
two-blade prop would make the switch again now that they've flown the
three-blade. I never had the chance to fly the Arrow with the two-blade
prop, but even in a absolute sense, I don't like the three-blade. It's
only advantages a 1. it was cheaper than a new two-blade (this
surprised me also), and 2. it looks cool on the ramp. Other than that,
it is all negative: more weight, more vibration, more drag power-off,
and slightly less cruise speed. Supposedly, a three-blade prop will
give better takeoff and climb performance in exchange for the loss in
cruise, but none of the club members say that this has been the case
with the Arrow. Takeoff and climb are about the same and cruise is
about two knots slower. Could be that 180 HP just isn't enough to gain
the takeoff and climb benefits.


Matt

I thought the same, but it doesn't appear to be the case, at least not
with 4 cylinder engines. I've read that the 3 blade props seem more
compatible with 6 bangers. Supposedly, this is related to power pulses

I guess that makes sense.

It would seem logical that a third power-producing blade would be superior.

Otherwise why would any planes have more than two blades?
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jay Honeck wrote:

: It would seem logical that a third power-producing blade would be superior.

: Otherwise why would any planes have more than two blades?

At some point having 2 blades to absorb the engine power leads to blades
that are so long the tips go supersonic. This is bad, for many reasons
including the noise. (Republic Aviation actually made an airplane that had
intentionally supersonic blades: google "thunderscreech".) When you reach
this point the only solution is to use more blades. Hey, the ATR-72 has 6
blades on each prop!

In regards to your O-540, many O-540 airplanes have 3-bladers from the
factory. Commander 114, Piper 6/300, Navajo, etc. I don't know if they
use different crankshaft vibration dampers or not. My (solid crank) O-360A4A
does not have vibration dampers, perhaps the Arrow's does not either,
leading to the extra vibration with a 3-blader.

There is an excellent article in "Torque Meter" about the design of the
P&W R2800 crankshaft that describes the difficulties in crankshaft design
and handling torsional vibrations, as well as descriptions of how the vib.
dampers work. The article discusses the interplay of prop, engine, and
reduction gearbox. The article is on-line, but I don't have the link handy.
--
Aaron Coolidge