wnewbury

Honker Biplane by Thornburg. Overpowered by the 19, but that's what the throttle is for. I like the PAW a lot, except for the smell.

Jennifer Curtis

I like the PAWs a lot too, but its the vibration
rather than the smell that bugs me. Mine haven't
been as well balanced as other brands.

On an undersized plane, with less mass to
dampen the vibration (regardless of lower
throttle) the elevator servo seems to wear
out too quickly. It would be the rudder on
one with a horizontally mounted engine,
though just as fatal on a 3 channel plane.

Jenny

AMB

Jennifer You may want to try unbalancing the prop and mounting on the shaft in a position that counteracts the engine vibration this is an old stunt that has been uses over many years Hope this helps --Martin

Jennifer Curtis

AMB,

I tried an unbalanced prop, but with unbalance on the
blade so far from the crankshaft, it was very RPM
sensitive, with the lowest vibration point at one
particular speed. This would work on a control
line plane but not on RC.

I ended up drilling a hole in the prop hub
(on the prop, not on the engine prop drive)
and sticking some lead in it. I also ground as
much extra metal as I could from the pin
side of the crankshaft. Together this was
an improvement, but not as good as a
properly balanced engine.

Jenny

Recycled Flyer

Hi Jen,
is your engine a full circle crank web one?

Many of the PAW DS's were and can be significantly improved by grinding the cheeks off them or if you want to really address this PAW offers a piston lightening service as well.

Beyond that the vibration could simply be the harsher rapid onset ignition combined with a model really designed for a smoother running glow engine.
What size engine are you using and what size engine bearers? Most PAWS I wouldn't go under 1/2" square hardwood that run back to the wings high point at least or a radial mounted model with extra attention from F1 back to the same high point.

Also try some ignition improver, this will lower the compression needed and should give a slightly smoother run.

Cheers.

Jennifer Curtis

I haven't tried lightening the piston, but
have already considered and implemented
everything else you mentioned.

The last crash and the additional weight
in incurred did more to dampen the
vibration than anything except the lead
in the prop.

I plan to leave it the way it is now until
the engine or the plane is used up.

My other PAW engines will only be
side mounted in 4 channel planes,
so it will be the rudder servo that fails
and I will have a decent chance of
maintaining control.


Jenny

ps: I just remembered that PAW sent
a replacement steel carb when the
original aluminum one wore out from
excessive vibration. I'm sure they are
aware that vibration is a real issue for their
engines.

AMB

Jenny Just a thought are your servos "shock mounted" ie using rubber grommets to lessen the vibration to them?? good luck with all the endevers martin

wnewbury

Mine doesn't have excessive vibration. Guess I got a good one. Long ago (in RCM?) someone said you should mount the prop straight up and down when the engine is at top dead center. That this would cure vibration problems. I couldn't tell any difference.

Jennifer Curtis

AMB

Of course the servos are shock mounted with
rubber grommets. The vibration is entering
through the servo arm.

The up and down vibration of the engine causes
an up and down vibration in the elevator, which is
translated to a continuous push pull vibration in
the pushrod. This causes premature wear in the
servo gear teeth and bearings.

The elevator is made of the lightest possible balsa.
It is mounted with pin type hinges for freedom of
movement. Easy hinges could possible reduce
the elevator's motion, but would be a difficult
modification with the wood already full of epoxy.

WNewbury

You would think that a balanced propeller could
be mounted in any direction. In theory it is true,
but a prop that is statically balanced, may not be
so well balanced when spinning. If there is a little
extra weight near the hub on one blade and a little
extra near the tip on the other it could look perfect
on the dubro balancer. At 10,000 rpm the heavier
tip would have more momentum and try to pull
the engine off center.

It will do this regardless of which direction it is
mounted. It will, however, add its own vibration to
that of the engine if mounted straight up at TDC.
It will subtract from the engine if mounted straight
down at TDC. Since the prop appear balanced,
you need to try both directions to find which one
is better. some props are so good you can't tell
a difference.

The propellers I use are pretty good in this respect.
I usually mount the propeller so that when the
engine stops, the prop is in a near horizontal
position (so it doesn't break in deadstick landings).

The PAW engines NEVER quit in the air, so this
is not an issue. On these engines, I mount the
prop in a direction to make it easy to flip start.
Then I mark where I want to add the lead weight.
After I weight the prop, I mount it in the same
orientation.

Jenny

AMB

Jenny just an idea will it work?? good question mount a full servo arm on a cross piece in the plane hook elevator servo rod to one side, and an other from the servo to the other side of the servo of course you will have to change servo
direction setup via the transmitter servo settings but it might take the vibration load off the servo since most would be to the arm between the elevator horn and intermediate servo arm ???? martin

with two shorter rods maybe less whip/vibration just a thought do not know if this will solve the issue

Recycled Flyer

What model of PAW are we talking about here?
The thread is about the 19 so running on that assumption are you suffering from a bent crankshaft?
Loose engine bolts?

Pics may help solve this one.

Jennifer Curtis

My plane is powered by a .19, possibly the same as WNewbury's,
though changes have been made over the years. There is nothing
actually wrong with the engine. It is just a shaker.

The pushrod idea won't work. The push pull force will still be
there. That is how the elevator works. If the secondary arm
was very flexible, it would absorb the vibration. It would
also allow the elevator to flop up and down and encourage
flutter.

The only thing that would work is mass balancing the
elevator. That is too much engineering for such a small plane.

This problem is not one to fix, but one to live with, kind of
like being short.

jenny

AMB

Jennie an other thought on your next plane check out Davis Model products ISO-MOUNT it bolts thru the fire wall and should help reduce the vibration stated 20-60 size engine but may fit the 19 martin

Jennifer Curtis

I've used the iso-mounts before. On larger engines they
work ok. On smaller engines the improvement is not much.

I found that to get the most effectiveness, they needed to
be lightly tightened, but then there was trouble with the
rubber wearing through, or the nut breaking free from
the rubber.

The small ones seem to help most on 40 to 60 size
two-stroke engines. I didn't try the large ones.
My biggest engine is a 91 surpass. It ate through
the rubber on the small mounts pretty fast.

I used to have a source for even smaller ones,
but I don't remember where. After several
failures I gave up on them.

Jenny

steve111

I wonder if the issue is something other than an inherent vibration problem with the PAW 19. I've used a number of these extensively over the years, and have never had an issue with vibration, even when using 3/8" square bearers of modest length. What size prop are you using? Perhaps the engine would be happier in a different rev range.

Jennifer Curtis

Perhaps the vibration is inherent only to the engines
I have. The .15 that was on the same plane
(before the .19) was also a shaker, just not as bad.

The .19 is 23 years old, and the .15 is 25 years old.
Hopefully some design updates have happened.
since then


Jenny