Propeller Balancing

Is your airplane fitted with pneumatic wheels? The odds are, it is.
That means you probably have tools in your kit that allow you to
remove the wheel, remove the tire & tube from the rim, to re-install
them, inflate the wheel and so forth. Indeed, if you're a serious
airman you give your the same degree of attention you devote to the
engine and air-frame, which means you have the tools and fixtures
needed to maintain your wheels.

All of that also holds true for wooden propellers. Not only do you
pay careful attention to its finish, protecting it with a waterproof
sleeve when it must be parked out in the weather, you have the means
of periodically refinishing and balancing your propeller.

With regard to balancing, go here...

.....http://www.youtube.com/watch?v=uw95sC3keQs

.....and watch the video a time or two. The video shows balancing the
prop by installing a relatively massive amount of weight very near the
center of rotation. That is, he's using a very short radius.
Another common method is to use an additional coat of paint or varnish
near the TIP of the LIGHT blade. That is, using a relatively small
weight having a relatively LARGE radius. Both methods work equally
well but the small-mass/large-radius is better suited for the amateur,
in my opinion, since it calls for fewer tools and parts (ie, drill
bit, drill motor, lead slug of suitable size, wooden plugs and so
forth.).

Right now you don't have a prop to balance but I want to make you
aware of the things you WILL need, not just when making a propeller
but when MAINTAINING your propeller in the years to come.

The most important of those things is the BALANCE STAND and the
BALANCE BAR.

The Balance Stand is literally anything that can hold a pair of knife-
edges at PRECISELY the same height. Here in the States it is common
to use joiner blades as the knife edges and a section of precision-
ground oil-hardening steel for the balance bar.

There is no requirement to spin the propeller round & round on it's
balance bar; you only need to see which blade is heavier, which means
the Balance Stand may be a bench-top device, such as a pair of
accurately cut boards that have been grooved to accept the router
blades. The 'truth' of the jig is determined by placing a straight-
edge across the pair of knife edges and using a precision protractor
or level to ensure the blades are level. Nowadays, if you have the
money, you would probably use a digital protractor, most of which are
accurate to a tenth of a degree. Or you can use the old fashion
method which consists of a carpenter's framing square, a bit of thread
and a plumb-bob.

The point that needs to be made here is that your propeller balancing
tools are something you will use periodically, not just when you carve
a propeller. Just as you keep a set of tools specific to your landing
gear tires and wheels, so too must you keep a kit of tools specific to
balancing your prop.

-R.S.Hoover

wrote in message
...

Is your airplane fitted with pneumatic wheels? The odds are, it is.
That means you probably have tools in your kit that allow you to
remove the wheel, remove the tire & tube from the rim, to re-install
them, inflate the wheel and so forth. Indeed, if you're a serious
airman you give your the same degree of attention you devote to the
engine and air-frame, which means you have the tools and fixtures
needed to maintain your wheels.

All of that also holds true for wooden propellers. Not only do you
pay careful attention to its finish, protecting it with a waterproof
sleeve when it must be parked out in the weather, you have the means
of periodically refinishing and balancing your propeller.

With regard to balancing, go here...

....http://www.youtube.com/watch?v=uw95sC3keQs

....and watch the video a time or two. The video shows balancing the
prop by installing a relatively massive amount of weight very near the
center of rotation. That is, he's using a very short radius.
Another common method is to use an additional coat of paint or varnish
near the TIP of the LIGHT blade. That is, using a relatively small
weight having a relatively LARGE radius. Both methods work equally
well but the small-mass/large-radius is better suited for the amateur,
in my opinion, since it calls for fewer tools and parts (ie, drill
bit, drill motor, lead slug of suitable size, wooden plugs and so
forth.).

Right now you don't have a prop to balance but I want to make you
aware of the things you WILL need, not just when making a propeller
but when MAINTAINING your propeller in the years to come.

The most important of those things is the BALANCE STAND and the
BALANCE BAR.

The Balance Stand is literally anything that can hold a pair of knife-
edges at PRECISELY the same height. Here in the States it is common
to use joiner blades as the knife edges and a section of precision-
ground oil-hardening steel for the balance bar.

There is no requirement to spin the propeller round & round on it's
balance bar; you only need to see which blade is heavier, which means
the Balance Stand may be a bench-top device, such as a pair of
accurately cut boards that have been grooved to accept the router
blades. The 'truth' of the jig is determined by placing a straight-
edge across the pair of knife edges and using a precision protractor
or level to ensure the blades are level. Nowadays, if you have the
money, you would probably use a digital protractor, most of which are
accurate to a tenth of a degree. Or you can use the old fashion
method which consists of a carpenter's framing square, a bit of thread
and a plumb-bob.

The point that needs to be made here is that your propeller balancing
tools are something you will use periodically, not just when you carve
a propeller. Just as you keep a set of tools specific to your landing
gear tires and wheels, so too must you keep a kit of tools specific to
balancing your prop.

-R.S.Hoover

Mr. Hoover. I watched the youtube and noticed that he balanced with the
lead slug and then added some more weight with the wood plug and then didn't
check his balance again. I've electronically balanced my helicopter tail
rotor, which BTW turns at 2700 rpm and got it down real close and then added
my cotter pin to the bolt washer system I was using for the added weight.
In my case I was also in close to the rotor center. The weight of the cotter
pin was quickly noted by the electronic balancer. I agree with you I would
rather use an additional coat of paint on the tip.

Stu Fields

On Fri, 12 Dec 2008 12:41:33 -0800 (PST), "
wrote:


blades. The 'truth' of the jig is determined by placing a straight-
edge across the pair of knife edges and using a precision protractor
or level to ensure the blades are level. Nowadays, if you have the
money, you would probably use a digital protractor, most of which are
accurate to a tenth of a degree. Or you can use the old fashion
method which consists of a carpenter's framing square, a bit of thread
and a plumb-bob.

The point that needs to be made here is that your propeller balancing
tools are something you will use periodically, not just when you carve
a propeller. Just as you keep a set of tools specific to your landing
gear tires and wheels, so too must you keep a kit of tools specific to
balancing your prop.

-R.S.Hoover


what I'm about to tell you is illegal in the country of origin but I
do it once or twice a year because to not do it is stupidly dangerous.

honestly you dont need planer blades.
I use two 6 or 8 inch wide hardwood plank sections that have been
dressed all round in a commercial planer.
the pivot of my balancer is a polished half inch diameter rod that was
originally a part of a photocopier.

I have machined from half inch 6061 aloooominum two tapered conical
sections that achieve a jamb fit in each side of the propeller boss
hole and are a slop free slide fit on the shaft.

The principal requirement in machining these conical sections is that
all machining is done at once without taking the workpiece out of the
lathe, that way the entire component cant help but be precisely
concentric.

in use I sit the two boards on the kitchen table after giving them a
dust off. I use a torpedo bubble level to check that the tops are
level. The rod is fed through the middle of the prop and the two
conical sections are fed over the rod and jambed into the prop's
central hole.

I sit the assembly on top of the boards, close all doors and windows
to prevent any slight breezes, and wait for the heavy blade to
descend. it is sensitive enough that a quarter of a typical 'post it'
note gummed on the end of one prop blade will make that blade descend.

I add paint or varnish from a rattle can to achieve balance. allow it
to dry a few hours then recheck.

using this technique I achieve a vibration free engine installation
for months on end.

btw there is no point in using different length screws on the spinner
and expecting the thing to remain balanced.

if you have a wooden prop and you maintain it you will have something
that can outlast the airframe.

Stealth Pilot

Dear Stealth (and you other guys, too)

Thank you for telling us how you do it. I wish more people would
share, because the truth is, you don't need a lot of stuff to detect
an imbalance in a two-bladed prop.

What's needed is some means of reducing the contact area between the
prop and its support so that a relatively small amount of imbalance
( I use cigarette papers ) will be enough to cause the heavy limb or
blade to drop. Indeed, a prop may be balanced using a support built
entirely of wood. Other successful balancers I have seen used string
( !! actually, a fairly strong twine ), safety wire, water pipe and so
on.

The pipe affair was rather interesting. Until I saw it, I'd failed to
appreciate that a round section contacting another at right angles was
dealing with a relatively small area of contact -- small enough to
provide the sensitivity needed for the fellow's propeller.

The method described in my original post (above) was aimed mostly
toward the Chugger People, in that I assumed a table saw was
available. The planer blades are commonly available from Harbor
Freight, a retailer here in the States. Three-quarter inch dia bar or
tube is commonly available from a variety of sources... but they could
always use a smaller rod & cones, etc.

What really needs to be conveyed, in my opinion, is the mechanical
principles that we are working with. Once a person understands the
How behind the Why, the world becomes literally filled with 'propeller
balancers.' Or even Piston balancers... connecting rod balancers...
and so on. I see the Hat Trick as getting the message across without
losing the person's interest... adding a dose of humor to the physics
if need be.

Oddly enough, there are those who feel the only proper way to teach a
subject is to stick to the fundmentals. No humor allowed. These
people insist that the humor dilutes the message, and since 'these
people' are often professional educators, they win by default.

-R.S.Hoover

I use a balancing stand made from shelving angle. The angle is
about 1.5" x 1.5" and has slots that allow you to bolt together just
about anything. I ran a file across the critical edges, draw-file
fashion, to level and smooth them. It works fine.

I found, with the wooden prop off my Jodel, that it was out of
balance *chordwise*, not spanwise. The maker had drilled the center
hole off by about 3/32" and the bolt hole circle is radiused from
that, so the whole prop was/is orbiting the crank axis just a little.
My shelf-angle stand is tall enough that I can support a prop
vertically as well as horizontally to catch such imbalances.

Of course, we now have a Chadwick-Helmuth electronic balancer that
does the analysis with the engine running. Washers and small hardware
are attached at the specified place on the spinner backplate. Makes a
big difference. Saves instruments and radios.

Dan

On Sat, 13 Dec 2008 15:38:06 -0800 (PST),
wrote:

I use a balancing stand made from shelving angle. The angle is
about 1.5" x 1.5" and has slots that allow you to bolt together just
about anything. I ran a file across the critical edges, draw-file
fashion, to level and smooth them. It works fine.

I found, with the wooden prop off my Jodel, that it was out of
balance *chordwise*, not spanwise. The maker had drilled the center
hole off by about 3/32" and the bolt hole circle is radiused from
that, so the whole prop was/is orbiting the crank axis just a little.
My shelf-angle stand is tall enough that I can support a prop
vertically as well as horizontally to catch such imbalances.

Of course, we now have a Chadwick-Helmuth electronic balancer that
does the analysis with the engine running. Washers and small hardware
are attached at the specified place on the spinner backplate. Makes a
big difference. Saves instruments and radios.

Dan


dan you've mentioned a *very* important point. when making a prop, cut
the blank to outline shape then drill the hub holes.
the hub holes and the back face of the prop then become the datums to
work from. that way you get the prop evenly disposed about the hub
hole and boss.
if you dont have a prop that is symetrical about the centre you have a
clock mount for the wall.

the chadwick-helmuth 'little finger extended while drinking tea'
balancer may have achieved static and dynamic balance for you but what
of the asymetric thrust??
it would be an interesting prop to watch you fly behind. (while
standing on the ground) must make some interesting noises.

for veedubber; the critical thing to find out with any balancer that
is home made is whether a very light weight added to a tip moves the
prop. I was amazed that mine moved with quarter of a post it note.
indeed experimentation has shown that balancing a prop in a room full
of still air is far more important than knife edges.

Stealth Pilot

"Stealth Pilot" wrote in message
...
On Sat, 13 Dec 2008 15:38:06 -0800 (PST),
wrote:

I use a balancing stand made from shelving angle. The angle is
about 1.5" x 1.5" and has slots that allow you to bolt together just
about anything. I ran a file across the critical edges, draw-file
fashion, to level and smooth them. It works fine.

I found, with the wooden prop off my Jodel, that it was out of
balance *chordwise*, not spanwise. The maker had drilled the center
hole off by about 3/32" and the bolt hole circle is radiused from
that, so the whole prop was/is orbiting the crank axis just a little.
My shelf-angle stand is tall enough that I can support a prop
vertically as well as horizontally to catch such imbalances.

Of course, we now have a Chadwick-Helmuth electronic balancer that
does the analysis with the engine running. Washers and small hardware
are attached at the specified place on the spinner backplate. Makes a
big difference. Saves instruments and radios.

Dan


dan you've mentioned a *very* important point. when making a prop, cut
the blank to outline shape then drill the hub holes.
the hub holes and the back face of the prop then become the datums to
work from. that way you get the prop evenly disposed about the hub
hole and boss.
if you dont have a prop that is symetrical about the centre you have a
clock mount for the wall.

the chadwick-helmuth 'little finger extended while drinking tea'
balancer may have achieved static and dynamic balance for you but what
of the asymetric thrust??
it would be an interesting prop to watch you fly behind. (while
standing on the ground) must make some interesting noises.

for veedubber; the critical thing to find out with any balancer that
is home made is whether a very light weight added to a tip moves the
prop. I was amazed that mine moved with quarter of a post it note.
indeed experimentation has shown that balancing a prop in a room full
of still air is far more important than knife edges.

Stealth Pilot

I agree with you regarding the proper proceedure, and also believe that a
multi-blade propeller that is not concentric may contain additional hidden
flaws--such as voids--and likely does. Such a blatant lapse in workmanship
and inspection should clearly be viewed as a "warning shot across the bow!"

However, I also suspect that far too much can be made of the eccentric
thrust problem.

Over the decades, there have been a number of experiments with single blade
propellers--primarily on helicopters--with an opposing counterweight. While
my intuitive reaction was to question the probable bending force applied to
the crank shaft--or drive shaft--I have never actually heard of that being a
problem.

Therefore, I can only presume that the lack of popularity is due to other
factors, such as:
1) Reduced capacity for any given disk area.
2) Poor streamlining at speed in airplane propeller applications.
3) Strange appearance.

OTOH, it does make variable pitch ridiculously simple!

Peter

BTW, I still can't quite accept the idea either.

On Dec 14, 4:18 am, Stealth Pilot
wrote:

the chadwick-helmuth 'little finger extended while drinking tea'
balancer may have achieved static and dynamic balance for you but what
of the asymetric thrust??
it would be an interesting prop to watch you fly behind. (while
standing on the ground) must make some interesting noises.

It's not noticeable. The pitch is symmetrical, and the
chordwise offset, being as small as it is, just causes the airflow
across the blades to have a very small difference in chordwise angle.

Dan

On Dec 14, 3:18*am, Stealth Pilot
wrote:

for veedubber; the critical thing to find out with any balancer that
is home made is whether a very light weight added to a tip moves the
prop. I was amazed that mine moved with quarter of a post it note.
indeed experimentation has shown that balancing a prop in a room full
of still air is far more important than knife edges.
------------------------------------------------------------------------------------------------------------------------------------------

On my 'back bench' (ie, the bench facing the lathe) I have a
machinist's steel tool box. It contains the high precision bits &
pieces every machinist accumulates over the years, such as my ten-k
mikes (ie, accurate to .0001"), broaches, reamers and so forth. In
the top right-hand drawer is a packet of Zig-Zag cigaret papers.
These are used as a gauge when making certain set-ups. But they also
serve as my 'test mass.' When a prop has been finished, I put a
cigarette paper on one of the blades to ensure the prop not only
drops, but on that side. The blade is then leveled and the test mass
is placed on the opposite limb. Some argue that I'm merely testing
the drag of my pivot and while pivot-drag is surely a factor, the
amount of TIME the blade takes to deflect by a certain amount, such as
down to the top of the bench, tells me if the prop is properly
balanced. That is, so long as each limb takes the same amount of time
to deflect for the same amount of distance, I know the balance is
equal on both sides of the prop.

Occasionally I get some guy in the shop who wants to argue that the
modern-day, all-electronic balancing is superior to my clumsy efforts,
especially with regard to TIME, in that the electronic balancers give
you an answer RIGHT NOW... and the answer is accurate to millionths of
a gram, or whatever.

Cost of such equipment is rarely mentioned :-)

This is another case of a newbie falling prey to some hi-tek
huckster. While it would be nice to have a balancer that gave such
precise results ( ie, four zeros preceding the significant digit ), in
the real world that degree of precision only applies to turbines ---
devices spinning at tens of thousands of revolutions per minute. If
you happen to have ACCESS to such a machine, you are lucky, but there
is simply no need for that degree of precision when you're dealing
with two-bladed props for Volkswagen engines.

After balancing the prop with the shiny-side out, I like to flip it
over and check the balance again. 'Shiny-side' = the prop is
finished with a good grade of VARNISH. After the varnish has cured,
the side of the prop facing the pilot [tractor installation assumed]
is given a light sprayed-on coat of FLAT BLACK paint, so as not to
reflect the sun into the cockpit.

-R.S.Hoover