Propeller Balancing

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On Dec 14, 7:48 am, "Peter Dohm" wrote:
Reduced capacity for any given disk area.
2) Poor streamlining at speed in airplane propeller applications.
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Dear Peter,

I'm not sure if the 'poor streamlining' was meant to apply to single-
bladed props or to props installed on the clutch-end of the crankshaft
but in the latter case you will find that the tranny flange of the
engine, which is about 13" in diameter, is completely submerged in the
streamline when the prop is fitted with a 12" spinner.

-R.S.Hoover

The comment was meant to apply a single bladed prop; although the "problem"
may not be significant on a relatively slow aircraft.

Peter

On Sun, 14 Dec 2008 10:48:58 -0500, "Peter Dohm"
wrote:



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.

I think that that is because the eccentric thrust and load is not a
reversing load and so is not a fatigue factor on the crankshaft.

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.


it is actually easier and stronger to build a two bladed wooden prop.




OTOH, it does make variable pitch ridiculously simple!

Peter

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

On Sun, 14 Dec 2008 11:45:33 -0800 (PST), "
wrote:


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

a balanced prop is a balanced prop is a balanced prop.
once it is balanced you cant get it any better weightwise.

the remaining factors on a balanced prop are mounting it square to the
shaft and having the aerofoil distributions symmetrical.

what we are describing is *not* a deficient process.
Stealth Pilot

On Dec 15, 1:31 am, Stealth Pilot
wrote:

a balanced prop is a balanced prop is a balanced prop.
once it is balanced you cant get it any better weightwise.

the remaining factors on a balanced prop are mounting it square to the
shaft and having the aerofoil distributions symmetrical.

what we are describing is *not* a deficient process.

I wish it was a simple as that. Most props are built with their blade
mass a little ahead of the hub/root so that centrifugal force will try
to flex them back in line with the hub's plane of rotation. This is to
fight the forward flexing caused by thrust loads. If you lay a prop on
a flat surface, trailing edges down, you'll see lots of clearance
between the TE and the surface on most of them. Turn it over and the
leading edges will be against the table, and on some metal props
they'll lift the hub's front face clear of the table.

So, if the heaviness we want to remove is due to a thicker section of
blade near the hub, or maybe just a denser area of wood, and we add
weight to the opposite tip to counter it, we balance it statically but
not dynamically. The heavier blade tip will flex back more than the
lighter one, putting the prop's tips out of track and screwing up its
dynamic balance. It'll shake. Only the electronic device will find
that.

I'm fortunate to work is an aircraft shop where we have such stuff
available. I couldn't justify owning a $5000 machine myself. Heck, I
could build a small airplane with that $5k.

Dan

Good discussion. Of course, most of it does not apply to Joe Newbie
hacking out his first prop :-)

But it does a good job of explaining what goes in to a well engineered
propeller. What it DOESN'T do is tell the newbie that the efficiency
of his prop will be within a few percentage points of a professionally-
made wooden propeller. Which means Joe Newbie's efforts WILL fly the
plane. And that's the message I'd like to get across.

The propeller's job is to convert torque into thrust. To do that
efficiently the propeller must be perfectly balanced. After mass
balance is dealt with the next most important factor is dynamic
balance, which is beyond the skills of the amateur. But the
homebuilder CAN ensure the propeller is perfectly SMOOTH so that it
presents the least possible amount of drag as it rotates. The amateur
is also capable of ensuring a uniform air foil, especially with a
computer to take care of the lay-out chores... you must still make the
patterns from old beer cans will do for the material... in fact, stiff
cardboard would do well enough.

Wood is a forgiving material. And so is AIR. A propeller that is
nice and smooth, properly balanced and of the required diameter and
pitch, will typically produce within 90% of the thrust produced by a
professionally carved prop.
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If you've taken advantage of YouTube and the archives devoted to
various homebuilts, you will have seen a number of propeller
duplicators in action. You would also have seen that these machines
are quite simple in both concept and construction. If the homebuilder
is building a wooden aircraft it's fair to assume they are well versed
in building from that material. It is a relatively small step between
the spar of a Jodel and a profiler -- or duplicator -- capable of
producing a propeller.

Which raises a very interesting point: If you have a propeller
duplicator you only need to produce ONE BLADE of the prop... and you
may produce it from a variety of materials as well as wood, including
foam-fiberglas composite, cast aluminum, cast zinc and so on.

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The next most common question that arises is 'Why do I need all that
stuff if I'm only making ONE PROPELLER?'

The most correct answer is that you will probably need THREE
propellers to find the one that most closely matched YOUR combination
of engine and air frame. Yeah, I know: It's built EXACTLY according
to the plans. Good for you (seriously). But there are variations
even among commercially built airplanes. With commercially built
engines such as an O-200, driving a commercially built METAL
propeller, these variations are quite small: One Cub was virtually
identical to another. But there ARE variations and it doesn't matter
if your name is Boeing, Douglas or Lockheed ( indeed, there are some
heroic examples which, unbelievable as they might appear, are flying
the world's skies ). If you had to BUY two -- or more -- commercially
built props it can cost you up to $500 to nail down the 'perfect'
prop.

In fact... ( now, you KNEW that was coming, right? ) In fact, a
manually operated profiler ( ie, 'duplicator,' et al ) is exactly
the sort of thing you want to consider as a chapter project. With the
cutting element removed (ie, the portable saw or the ) the frame of
the profiler is flat enough to be hung up for storage, tucked in the
rafters and so on. You must provide the MASTER -- one blade of your
propeller, the practical limits of which are for a prop having a
diameter of about 72" and a depth of about 6". Of the examples you
will find on YouTube and other video sources, the one using a portable
saw as the cutting element is probably the most practical for the
homebuilder, since those using a router have quite an appetite for
cutters.

The 'manually operated' means that YOU are the motive force shoving
the profiler back & forth, advancing an eighth of an inch or so for
each pass -- BOTH back and forth. The tracer/tracker/profiler what-
have-you is a disk of steel or aluminum having EXACTLY the same
diameter as the BLADE of the portable circular saw. The WIDTH of the
tracer must be APPROXIMATELY the same as that of the saw blade --
indeed, it may be considerably WIDER, so long as the advancing edge
(ie, the edge nearest the tip of the pattern) is exactly the same as
that edge of the saw-blade.

The thing you are shoving back & forth whist advancing down the length
of the pattern is a hinged plate or table to which the tracer disk and
the portable saw are attached. This table is attached to a pair of
rods or bars so that it slides back & forth with very little
friction. The weight of the saw is borne by nylon blocks which serve
as bearings. A counter poise or weight
is used as a mass-balance, allowing the tracer to easily follow the
contours of the MASTER PATTERN, which has a very durable finish.

The pattern and the prop-blank are drilled to EXACTLY match, typically
using a master drill guide.

It takes FOUR PASSES through the profiler to produce a near-perfect
propeller. The MASTER PATTERN as well as the blank must be
dismounted, turned over (or reversed) for each new cut, If you are
only doing one propeller, the set-up time makes up an appreciable
amount of the whole, whih is why it makes good sense to produce a
NUMBER of identical props at the same time, Of course, that isn't
practical for our situation but it IS practical for a GROUP of
builders using the same power-plant in the same air frame.

Even when the prop-blank has been run through the band saw (or other
means, such as a saber-saw with a long blade.. [go slow, give it a
chance to cut] ) to remove most of the wood, the tip must be dealt
with manually and the prop will require finish-sanding. But the
process is straight-forward.

-R. S. Hoover

On Mon, 15 Dec 2008 18:35:28 -0800 (PST), "
wrote:



Good discussion. Of course, most of it does not apply to Joe Newbie
hacking out his first prop :-)

But it does a good job of explaining what goes in to a well engineered
propeller. What it DOESN'T do is tell the newbie that the efficiency
of his prop will be within a few percentage points of a professionally-
made wooden propeller. Which means Joe Newbie's efforts WILL fly the
plane. And that's the message I'd like to get across.

snip

The most correct answer is that you will probably need THREE
propellers to find the one that most closely matched YOUR combination
of engine and air frame. Yeah, I know: It's built EXACTLY according
to the plans. Good for you (seriously). But there are variations
even among commercially built airplanes. With commercially built
engines such as an O-200, driving a commercially built METAL
propeller, these variations are quite small: One Cub was virtually
identical to another. But there ARE variations and it doesn't matter
if your name is Boeing, Douglas or Lockheed ( indeed, there are some
heroic examples which, unbelievable as they might appear, are flying
the world's skies ). If you had to BUY two -- or more -- commercially
built props it can cost you up to $500 to nail down the 'perfect'
prop.


snipped so that I can comment on these points.

a well made newbie prop can deliver exactly the same performance as a
professionally carved prop. after all they were newbies once
themselves.

my experience relates to a wooden prop on an O-200 powered tailwind,
but it is still relevant to a vw prop although it turns the opposite
way.

I have been refinishing my prop and balancing it for years so I have
experience with paint layer variations in the shape of the one prop.
Over the polyurethane varnish I use the cheapest aerosol paint can
lacquer that I can find. it dries quickly, is easy to apply smoothly
and .... when it is all chipped and scuffed it wipes of with a rag
soaked in either MEK or Acetone.

tiny variations in the paint surface can have noticeable effects on
cruise speed.
when I inherited the prop as a new owner it was all daggy and glass
resin runs. straight and level cruise was 110knots.
when I cleaned off all the dags and sanded the surfaces smooth the
straight and level cruise was 115knots.
with some other fuselage changes I now cruise at 120knots reliably.

on one repaint I used lots of coats of paint around the leading edge.
about 65 coats on one blade I recall.
the paint didnt last too long before chips were eroding the finish but
while it was pristine I achieved 124knots cruise.

try as I might I cant get the shape again. best I can get usually is
121 knots in pristine condition.

the point here is that making a family of props all subtly varied from
each other can find you the crackerjack best prop for your aircraft.
in australia a prop is typically $3200 so making them yourself can
make the exotic quite affordable.

here is a trick for sizing the prop. make it an inch overlength.
in straight and level flight you should not be able to hit redline
with full throttle.
trim 5mm from each end and rebalance and refinish.
fly it again and the top rpm will be slightly higher.
when the revs come just up to redline rpm with full throttle your prop
is the correct size.

Stealth Pilot

On Dec 16, 4:16 am, Stealth Pilot
wrote:
a well made newbie prop can deliver exactly the same performance as a
professionally carved prop. after all they were newbies once themselves.

I'd be willing to bet that the prop I have--a Colin Walker
unit, uncertified, built by a guy who probably got his start building
one for himself, is MORE efficient than, say, a Sensenich. The builder
of my prop used a more cambered airfoil that pulls better than a
Sensenich, he milled off the leading edge and built it up with hard
urethane and shaped it so that it's a seamless, lightweight abrasion
protection much better than Sensenich's crude riveted-on brass leading
edge that disrupts airflow and can harbor moisture under it, and so
on.

here is a trick for sizing the prop. make it an inch overlength.
in straight and level flight you should not be able to hit redline
with full throttle.
trim 5mm from each end and rebalance and refinish.
fly it again and the top rpm will be slightly higher.
when the revs come just up to redline rpm with full throttle your prop
is the correct size.

I shortened mine from 76" to 72" to get more RPM on takeoff,
and lost performance in all regimes. I wish I could put those tips
back on. It used to cruise at a speed and RPM that indicated zero or
slightly negative slip, believe it or not. Not anymore. Now it slips a
little. Don Downie, an old homebuilder of note, said that long props
would do that.

Dan

Hi

I tryed to replay to this when I was in Thailand but it did not get here.

Cutting 4 inch off is like reduce pitch by 8" when come to power needed.

if diameter is right (depending of what you want) the relative pitch and propeller pitch at 75% r. is about the same, for a standard purpose pitched prop. at the flat botom that is.

A little negative alpha on a CLIMB prop, and little positive alpha on a CRUISE prop.

A speed prop will have maybe 2 degree of positive alpha.

this also depends on the thickness of blade, a thinner/ less camber will need more alpha, and a thicker less alpha. and it also depends on the blade aspect ratio.

You can trade diameter for pitch, in most cases 1 inch diameter for 2 inch pitch.

and it differ 2" in pitch between each of the 4 purpose props, CLIMB, STANDARD, CRUISE and SPEED.

Jan Carlsson
www.jcpropellerdesign.com



here is a trick for sizing the prop. make it an inch overlength.
in straight and level flight you should not be able to hit redline
with full throttle.
trim 5mm from each end and rebalance and refinish.
fly it again and the top rpm will be slightly higher.
when the revs come just up to redline rpm with full throttle your prop
is the correct size.

I shortened mine from 76" to 72" to get more RPM on takeoff,
and lost performance in all regimes. I wish I could put those tips
back on. It used to cruise at a speed and RPM that indicated zero or
slightly negative slip, believe it or not. Not anymore. Now it slips a
little. Don Downie, an old homebuilder of note, said that long props
would do that.

Dan[/quote]