Propeller IMBALANCING

To All:

Recent posts have covered several aspects of propeller balancing,
largely for the purpose of convincing those who have never carved a
prop to give it a try. Posts have dealt on the benefits of using as
many laminations as PRACTICAL, so as to ensure adequate stiffness in
the finished prop. Maple laminae a quarter of an inch thick is the
goal to shoot for but few amateurs are lucky enough to have access to
such material. In fact, most amateurs are stuck with using whatever
is available from the local Borg -- typically birch in 3/4" thickness.

In making up the blank, recent messages have stressed the importance
of applying glue to both surfaces in quantity sufficient to result in
a massive squeeze-out. This offers visible evidence the particular
joint will have enough glue to eliminate any possibility of a 'dry' or
glue-deficient joint.

The next step will be explaining how the propeller's pattern is laid-
out upon the blank, including however many stations the builder
chooses to use. As with the number of laminae, when it comes to
stations, more is better. Thanks to the universal availability of
computers and the low cost of a homebuilder's time, the the use of ten
or more stations is encouraged and the next series of messages will
address this issue. The ubiquitous nature of computers makes it
practical to give the amateur builder nothing more than the equations
used to produce the airfoils of each station, allowing the computer to
do the required laying-out of the airfoils. In a similar vein, a
pattern will be provided for the laying-out of the propeller's
center. This pattern is to be glued to the blank, making it difficult
to err when it comes time to drill-out the hub recess, center hole and
bolt holes, after which the propeller's profile may be cut out as well
as the initial angles at each station.

The result of the above is a rough blank that requires only smoothing
to produce a usable propeller. The airfoil patterns printed out
during an earlier step are used to ensure each station has the proper
curvature.

However, I do not believe we have covered a particular balancing
problem in sufficient detail. That problem occurs when the profile of
the blank is cut-out inaccurately, leaving one blade longer than
another. The amount of the error is typically small and typically a
percentage of the average station width. That is, the more stations
you have, the better your chances of making a SMALL error.

The reason this is worthy of mention has to do with the performance of
the finished propeller, in that the shorter blade, while equal to the
longer in MASS and therefore is found to be in BALANCE, will NOT be
equal to the longer blade in terms of THRUST. This is because the
outer-most third of the propeller blade produces more than 50% of the
blade's total thrust. The upshot is that your beautifully finished
propeller will produce less thrust than it should, and the thrust it
does produce will be asymmetric leading to a host of problems the cure
of which calls for reducing the length of the longer blade so as to
match the other, altering the profile so as to match the other, and
then rebalancing.

Bottom Line: Be careful to keep the blade-length EQUAL throughout the
carving & finishing.

-R.S.Hoover

wrote in message
...
To All:

Recent posts have covered several aspects of propeller balancing,
largely for the purpose of convincing those who have never carved a
prop to give it a try. Posts have dealt on the benefits of using as
many laminations as PRACTICAL, so as to ensure adequate stiffness in
the finished prop. Maple laminae a quarter of an inch thick is the
goal to shoot for but few amateurs are lucky enough to have access to
such material. In fact, most amateurs are stuck with using whatever
is available from the local Borg -- typically birch in 3/4" thickness.

Ask around. Likely you know _someone_ with a thickness planer that would be
willing to run a couple dozen feet of lumber for you. Not all of them may
get you down to 1/4 inch, but you can get down below 1/2...

(I never knew how handy one could be until I got one)

Hang in there dewd.

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

Balancing a propeller to be, start best when the the lamells is planed and is going to be glued, you check it for heavy ends by balancing it on a rod at the center line mark, then turn them so the heavy ends alter direction, the one with no heavy ends to see, can be best used in those lamels that goes from tip to tip.

Jan Carlsson
www.jcpropellerdesign.com

Originally Posted by

To All:

Recent posts have covered several aspects of propeller balancing,
largely for the purpose of convincing those who have never carved a
prop to give it a try. Posts have dealt on the benefits of using as
many laminations as PRACTICAL, so as to ensure adequate stiffness in
the finished prop. Maple laminae a quarter of an inch thick is the
goal to shoot for but few amateurs are lucky enough to have access to
such material. In fact, most amateurs are stuck with using whatever
is available from the local Borg -- typically birch in 3/4" thickness.

In making up the blank, recent messages have stressed the importance
of applying glue to both surfaces in quantity sufficient to result in
a massive squeeze-out. This offers visible evidence the particular
joint will have enough glue to eliminate any possibility of a 'dry' or
glue-deficient joint.

The next step will be explaining how the propeller's pattern is laid-
out upon the blank, including however many stations the builder
chooses to use. As with the number of laminae, when it comes to
stations, more is better. Thanks to the universal availability of
computers and the low cost of a homebuilder's time, the the use of ten
or more stations is encouraged and the next series of messages will
address this issue. The ubiquitous nature of computers makes it
practical to give the amateur builder nothing more than the equations
used to produce the airfoils of each station, allowing the computer to
do the required laying-out of the airfoils. In a similar vein, a
pattern will be provided for the laying-out of the propeller's
center. This pattern is to be glued to the blank, making it difficult
to err when it comes time to drill-out the hub recess, center hole and
bolt holes, after which the propeller's profile may be cut out as well
as the initial angles at each station.

The result of the above is a rough blank that requires only smoothing
to produce a usable propeller. The airfoil patterns printed out
during an earlier step are used to ensure each station has the proper
curvature.

However, I do not believe we have covered a particular balancing
problem in sufficient detail. That problem occurs when the profile of
the blank is cut-out inaccurately, leaving one blade longer than
another. The amount of the error is typically small and typically a
percentage of the average station width. That is, the more stations
you have, the better your chances of making a SMALL error.

The reason this is worthy of mention has to do with the performance of
the finished propeller, in that the shorter blade, while equal to the
longer in MASS and therefore is found to be in BALANCE, will NOT be
equal to the longer blade in terms of THRUST. This is because the
outer-most third of the propeller blade produces more than 50% of the
blade's total thrust. The upshot is that your beautifully finished
propeller will produce less thrust than it should, and the thrust it
does produce will be asymmetric leading to a host of problems the cure
of which calls for reducing the length of the longer blade so as to
match the other, altering the profile so as to match the other, and
then rebalancing.

Bottom Line: Be careful to keep the blade-length EQUAL throughout the
carving & finishing.

-R.S.Hoover

Dear Jan and the Group,

Thank you for mentioning this point. It is something you do
immediately after marking the MEASURED center of the board... and
which I completely forgot to mention. I use a large screw-driver as
my 'balance beam,' and a pencil mark on the EDGE of the board to
indicate the HEAVY end.

There is also another thing you must consider when the laminae are
THICK, as in 3/4" you must also take the grain into account, flipping
the boards so that you get face-to-face or back-to-back. What you
DON'T want is face-to-back on any of you boards. Of course, if you
can plane them down to about 3/8" the 'warp factor' is much less.

-R.S.Hoover


On Dec 15, 12:33*am, Jan Carlsson Jan.Carlsson.
wrote:
Balancing a propeller to be, start best when the the lamells is planed
and is going to be glued,

On Mon, 15 Dec 2008 08:33:59 +0000, Jan Carlsson
wrote:


Balancing a propeller to be, start best when the the lamells is planed
and is going to be glued, you check it for heavy ends by balancing it
on a rod at the center line mark, then turn them so the heavy ends
alter direction, the one with no heavy ends to see, can be best used in
those lamels that goes from tip to tip.

Jan Carlsson
www.jcpropellerdesign.com


Jan
good discussions of the dark art arent they.
have you made a prop so good that you kept it and made the customer
another?
Stealth Pilot

Did you have to ask that sort of question? :-)

hmm, almost, I made one for my airplane, and keept it when I sold it (with original prop on) Mine was 10 MPH faster and climb better (without overspeeding)

Jan Carlsson
www.jcpropellerdesign.com

Originally Posted by Stealth Pilot[_2_]

On Mon, 15 Dec 2008 08:33:59 +0000, Jan Carlsson
wrote:



Jan
good discussions of the dark art arent they.
have you made a prop so good that you kept it and made the customer
another?
Stealth Pilot

On Dec 14, 9:14*pm, "Capt. Geoffrey Thorpe" The Sea Hawk @See My
Sig.com wrote:
wrote in message

... To All:

...*Maple laminae a quarter of an inch thick is the
goal to shoot for but few amateurs are lucky enough to have access to
such material. *In fact, most amateurs are stuck with using whatever
is available from the local Borg -- typically birch in 3/4" thickness.

I've never seen solid birch lumber in a Borg, nor can I recall
a Borg that did not carry solid maple. But what they carry will
depend a lot on what grows locally. Doug Fir is scarce out here too.
Poplar is as cheap as pine.'

Regardless, select straight-grained wood. Avoid any wood with a
wavy or quilted surface or wild grain.

Near any big city you will find one or more hardwood lumber dealers
who will carry or can order any common North American hardwood --
for a price.


Ask around. Likely you know _someone_ with a thickness planer that would be
willing to run a couple dozen feet of lumber for you. Not all of them may
get you down to 1/4 inch, but you can get down below 1/2...

(I never knew how handy one could be until I got one)


With any planer I have ever used you can get down to 1/8" by
fastening the workpiece to a thicker board using double sticky
(not foam backed) carpet tape. The trick is getting it apart after
planing. Use a LOT less tape than you think you'll need.
Lacquer thinner will soften the glue on the tape if you can't
get it apart any other way.

That works well for planing short pieces too.

Resawing is the practice of cutting a thick board into two
or more thinner boards, as opposed to ripping which makes
narrower boards from wider ones.

With a good bandsaw and a little practice you can resaw 3/4"
lumber to get two pieces each near 5/16 and then plane it
down from there, rather than planing all the way down from
3/4" to 1/4" or so wasting most of the wood.

You can also resaw on a table saw if the board isn't too
wide, here's how:

You raise the blade up until it will cut about one half inch shy
of the middle of the board and make one pass against the
rip fence. Then flip the board so that the same side is against
the fence and cut from the other side. Then finish with a
hand saw. Leaving an inch or so in the middle to cut by hand
prevents the kerf from pinching the blade.

Resawn Borg lumber will often cup due to residual stresses
left over from rapid kiln drying. If it does, try repeatedly
moistening and heating the the concave side until it is
the convex side and then let it dry for several days.

You don't want residual stresses in your propeller. If you
eschew resawing and plane all the way down from full
thickness, plane equal amounts from each side to
minimize cupping.

You can also get more (and probably better) advice
from rec.woowdorking, er, rec.woodworking.

--

FF

On Dec 18, 11:03*am, Fred the Red Shirt
wrote:

Resawn Borg lumber will often cup due to residual stresses
left over from rapid kiln drying. *If it does, try repeatedly
moistening and heating the the concave side until it is
the convex side and then let it dry for several days.

You don't want residual stresses in your propeller. *If you
eschew resawing and plane all the way down from full
thickness, plane equal amounts from each side to
minimize cupping.

You can also get more (and probably better) advice
from rec.woowdorking, er, rec.woodworking.

FF
------------------------------------------------------------------------------------------------------

Good advice. Thank you very much.

-R.S.Hoover