Wood is the most commonly used material for PPG propellers.
It has many benefits: it is light, not very expensive and in case of a strike it will disintegrate faster thus causing less damage than a composite one. However, I have seen wooden props that after disintegration, the splinters damaged lines and even canopies.
Before we start, it is well worth mentioning that there is a very good article on the Technical Info page at www.poweredparaglider.com written by Keith Pickersgill a few years ago. It explains how to repair propellers.
Prop strikes are very common in the PPG world. Most of us have damaged propellers because of different reasons.
They mostly occur on take off or landing. Some machines with flimsy propeller cages are very prone to prop strikes during a forward power assisted inflation. Other causes are brake toggles that are let go too early after lift off, or even foreign objects that get in the prop during the flight. I have seen cameras and even batteries sucked into props.
A damaged wooden propeller can be easily repaired if the damage is not to extensive.
There are different types of propeller damages:
1) Damage beyond repairs Remedy: discard propeller
2) Chipped propellers with missing wood Remedy: cut and glue in missing part making sure the wood is identical with the one of the propeller
3) Dents and nicks caused by small particles and object which can be easily fixed. Remedy: Use the same methods as described in point #4 (below)
4) Cracks that sometimes can even reach half or more of the length of the blade. Remedy: Follow the method shown below.
In this presentation, we will repair a propeller that had multiple cracks over 1/2 of its length.
As seen in the pictures below, the process is very easy and will make the repair strong enough to allow a safe flight.
However, we must stress that the person who repairs the propeller, will be fully responsible for his safety. It is beyond my control to actually see how the repair was made.If in doubt, better replace the propeller.
The most common tools we need are the following: a Dremel machine with different cutters, a spatula, EPOXY glue (regular and the jell type, a razor blade, a sharp knife, a piece of plastic and...faith. I do not recommend using Crazy Glue with baking soda on wood. From my research, I learned that the difference in rigidity between wood and the cyanoacrylate (Crazy Glue) will not ensure a good bond that would withstand vibrations!
If the propeller has multiple cracks, we should try to use as little glue as possible and try to get the glue as far in the crack (towards the hub) as we can ( this will make balancing easier)
If the crack is very long and ends at the tip of the propeller (which is very common) we should try to insert a wedge into the crack to open it up as much as possible. Using a jell type fast setting two part EPOXY we should try to push the glue all the way through the crack. We will find a spot where we can not push more glue because the crack is too small for the spatula or even a blade to go through. This portion will have to be cut open in another step.
Once we applied the glue, we squeeze the propeller blade and let the excess of glue flow out. This is why the jell type Epoxy will be very handy here. Now we can clamp and squeeze the broken parts together. After setting, the excess of epoxy can be cut off with a blade or a sharp knife.
After this operation, we have the crack fixed from the tip of the propeller as far towards the hub as we could reach with the glue. This spot on the crack should be well marked in order to see where to start cutting in the groove, as described further.
Now the rest of the crack has to be repaired.
Note: any crack should be cut open even a little further than it developed. This will ensure that the crack is stopped.
The picture below, shows such a crack which was glued with the above method (right side of the picture) The primary glued area presents only a hairline.
The picture also shows how to further open the crack in order to be able to apply the glue:
Sometimes, we can not cut all the way through the crack. In this case, I use a small drill and drill all the way through the propeller blade, making sure the drill follows the angle of the crack.
This will ensure that the whole surface of the crack is glued and will not open again.
First, I use a shallow layer of a regular epoxy glue which having lower viscosity, can flow much better into the crack. Once this primary layer is set, I use a Jell type epoxy to complete the operation.
Note: the second layer of glue should be poured sparingly not to be too high. I deliberately used more glue in the picture to emphasize this.
If we use too much glue, even with my "plastic foil" method presented below, we will have a lot to sand! We use enough glue to have it level and just a little higher than the two lips of the crack.
Now, we will use a "glue leveling" device. This is a regular sheet of plastic that we cover the glue with.
We start from one end and apply the foil pushing it down with a finger. This will cause the glue to dissipate quite evenly and if the operation is done with care, we may not even need to sand.
The glue will be feathered and smooth. I prefer not to sand but to scrape the glue after the plastic is removed. Using a wetted razor blade when the glue is half way set, I can achieve a very smooth finish.
Using sandpaper we will remove some of the original varnish thus exposing the wood to moisture. This will involve more work and a lot of varnishing. If the Epoxy is carefully scraped but left over the bare wood,
it will act as a water barrier and no varnishing is needed.
When I presented the first part of the repairing process, I mentioned that (counting from the propeller tip) we should spread the crack open and pour epoxy as far as we can and then squeeze the broken parts together
to eliminate the excess of glue. This is very important because especially close to the tips, the more glue we use, the more counterweight will have to be added on the opposite tip.
The second part of the operation involved cutting a groove into the crack in order to apply glue to all the surface (which could not be forced open) thus reinforcing the crack.
But this operation added a lot of weight since epoxy has a higher density than wood. Using only varnish to balance the propeller may not work well in this case.
This is why, I apply counterweight in the form of lead or soldering wire in a predrilled hole close to the hub. The Lead is imbedded with epoxy.
The propeller shown in the picture has a 22 cal. Lead bullet, sunk close to the hub in order to balance it. I also added some more weight in the form of soldering wire as seen in the picture below.
I usually use more Lead and then, with the Dremel, I can balance the prop removing some of it.
This is the balancer I use. It is an improved Paratour device.However, there are other types available as the one offered by Beresh&Hirsh propellers.
It has a bubble and the propeller can have a longitudinal and transversal balancing. After smoothening the repair and balancing, the propeller should be waxed.
I use an automotive or marine wax.Before the first flight, the propeller should be run a few times at full power and checked for cracks. I have used the above method for years and it never happened that the repair gave in.
Please note that an unbalanced propeller may be very dangerous. The high amplitude vibrations, sometimes not even felt by the pilot, will be detrimental to all parts of the paramotor and the propeller itself.
It is recommended that wooden propellers have checked the balance once in a while and should be kept in a HORIZONTAL position so they absorb moisture evenly thus keep the balance.
Wooden propellers with any damaged varnish surface should be revarnished to prevent them from absorbing moisture.