Articles of Interest
Repairing Wood Propellers written by Keith Pickersgill
Some pilots have said that wooden props are generally not repairable, however we have repaired some severely damaged wooden props and used them for many months without problems.
Provided you go about the project properly, it can be done with relative ease without special tools or expertise.
There are two seperate techniques, depending on the type of damage.
Both super-glue, of the Cyano-Acrylate type, and Epoxy can be mixed
with baking-soda, which increases its strength tremendously.
Superglue is used to repair splits along the grain or along the
laminations, due to its great capillary action which causes it to
penetrate deep into the split. One grain (less than one mg) of
baking-soda is crushed and mixed with 2ml of superglue in a CLEAN
glass or stainless steel bowl (table-spoon?) and must be applied
almost immediately. The soda makes it set surprisingly fast, but also
makes it good and strong. Be sure not to use too much glue, which is
no good. You need a microscopically thin layer bonding the two sides.
Once set, you can also use some clear 10 minute epoxy to fill up any
remaining furrows and to add to the structural strength. Mix about 5
grains crushed baking-soda with about 3ml of the active half of the
epoxy, stir thoroughly, then add the other half, the epoxy hardener.
Be warned, 10 minute epoxy acts like 3 minute epoxy when it contains
baking-soda, so work fast. Only mix a bit at a time, and do the job
in stages. Apply a thin layer along the split and rub along the line
with a clean finger-tip, pushing the epoxy into the split. After 2
minutes or so, it will be tacky and you can then wet your fingertip
with a mild soapy solution (I use dishwashing liquid - one drop to
20ml water) and rub the epoxy lightly to flatten any protrusions and
get it to blend in with the wood.
You may need to sand down the joint lightly and repaint. Clear paint
as used on props is usually K2 two-part epoxy paint, try Duco or
Glatex-8. They both come in clear gloss, and clear matt, and thousands
of other colours.
You can either brush it on, or spray paint with a small airbrush. I
use the hobby-style airbrush that normally runs off an aerosol can,
but I use and airline instead, connected to a spare wheel, which I
pump to 5 or 7 Bar (70 - 100 PSI) before connecting the airbrush.
Re-pump the wheel when it gets below 2 Bar.
These paints usually mix 3 parts active ingredient to one part
hardener (activator). Add another one part thinners for brushing on,
or two parts for air-brush spraying.
Now for the other type of repair: That chunk missing from the prop, or
that badly damaged leading edge (struck a stone?) or trailing edge
(dropped the prop?), or even slightly damaged prop-tips.
I recently repaired a prop that an entire silencer, all of 1kg, had
struck repeatedly, as the silencer broke loose and went tumbling
around the inside of the frame as the prop mashed itself against the
silencer. All the lines on the frame were broken, the throttle cable
was ripped out of the carb, much of the wiring was destroyed, and the
sturdy frame was buckled slightly. The heartbroken pilot could not
afford a replacement propeller on top of all his other repairs, so I
undertook to repair what looked like an absolute write-off.
There were a few splits running from the tips about a quarter of the
blades' length toward the hub, running along the laminations and some
along the grain. These were stabilised with super-glue and epoxy as
above.
The tips, thank goodness, were only slightly damaged and I built them
back into shape with Prattley's Steel Puttey, a two part epoxy putty.
The biggest damage was a huge gouge out of the leading edge, about a
third from one tip, and about 6 inches long, 3 inches deep, and as
wide as the deepest (thickest) part of the prop.
This was splintered around the edges with some highly compressed wood
matter in the centre of the impact zone.
I took a Stanley knife and ruthlessly cut all the splintered sections
away, especially anything protruding out from the normal profile. I
then cut away the compressed material inside the gouge, until I had a
neat section missing from the prop, as if it was deliberately cut
away. Then I proceeded to roughen the surfaces inside, where the new
replacement material was to bond to the wood. For this, I simply cut
grooves along the grain, about 2mm deep, and about 1mm apart, running
the full length of the damaged area.
Now the prop had been prepared to start building up again.
In retrospect, it would probably have quicker and easier, to clean the
whole area up with an angle-grinder, and this would have left a nice
rough surface to start building onto.
The replacement material to fill the huge hole, was selected as a
mixture of medium pine sawdust, and Prattley's Steel Putty.
I mixed equal proportions of sawdust with the puttey about 10ml each,
added about 5ml of finely crushed baking-soda, then added the
activating hardener (another 10ml).
Mixed up thouroughly using an old broken bicycle spoke in a small
plastic container, and spooned the mixture into the damaged area.
Using the back of a disposable plastic spoon, I pressed the filler
into shape, gradually building up the form of the leading edge.
It took 5 seperate mixing and filling operations, each comprising of
spooning the filler into the damaged area, then slowly pressing into
the approximate shape of the prop. As it slowly set, one can get the
mixture to mould much like plasticine clay, then make another mixture,
by which time the earlier layer was pretty hard set, and build on top
of this. As long as the earlier layer is not rock-hard and dry, the
new layer will bond very well with the old layer. Ideally it should be
a bit tacky but firm before adding another layer.
To build exactly the correct shape is beyond my artistic skills, but I
made sure that at least there were no depressions where there should
be material. I rather went far overboard and added surplus material,
which I later worked away.
Left overnight to set firmly, in the morning I had what looked like a
mini war-zone, full of craters and wrinkles and fingerprints.
I then took a medium sanding-disk and placed this in a drill-press,
and proceeded to hold the prop up to the spinning sanding disk, and
gradually worked away the bits protruding above the prop, letting the
surrounding wood be my guide. You could do the same by putting the
prop in a vice and using a hand-drill with a sanding disk, or perhaps
even a fine file or surform.
When almost honed to the correct dimensions, I changed-over to a fine
sanding disk and slowly finished off the job.
As a final step, I used a sanding sponge (soft sponge-like block
covered in abrasive material - made by 3M) which bends and conforms to
the shape of the work. This just gave it a nice smooth finish which
blended in very well with the prop's shape.
Now I had a perfectly formed prop, but a huge grey area of filler
material which spoilt its appearance, so I decided to spraypaint the
prop a bright red colour (Signal Red). This entails loosing the beautiful wood colour
and texture, but it would hide the grey blemish.
I checked the balance of the prop and found that the side with the
major repair was only slightly heavier.
I sprayed the prop while hanging from my balancing tool (which I shall
cover in another article if anyone really wants to know the secret to
fine balancing of props).
I just added more paint to the lighter side, until the prop was
balanced.
Once dry, I could not even detect which was the repaired
side, unless I tapped with my fingernail to detect the slightly
different tone of the putty.
Voila! the prop looked spanking new. I put some yellow on the tip (for
safety) and delivered it to a smiling pilot, whom has put many months
flying time on it without any problems. He actually suspects I gave
him a new prop!
The entire repair took about 6 hours work in total, over two days, but it was a worthwile learning experience.
Who says wood props cannot be repaired?
Not long afterwards, someone whom had heard of my repair job brought
me a bucket (pail) full of splinters that he claims had been a prop at
one time. I dug down deep and found the hub, so I gues he was not
joking. Alas, miracles are not mine to perform (perhaps in another
lifetime), so I did not even attempt to tackle this aviation jigsaw
puzzle.
I added it to our growing pile of broken props, now approaching 40 in
various stages of disaster. We plan to hold a bonfire of just broken propellers on Guy-Fawkes night, 4 November.
Everyone invited, just bring an old prop or two. A good evening's BBQ over a few beers, recounting each deceased prop's story.
Address comment to Keith Pickersgill at keith@xplorer.co.za
