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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