paragliding training center
Dust, dirt, sand, and small rocks sucked through the propeller can often ruin it. When you start and run the engine keep it away from the ground as much as possible. Avoid accelerating until it is on your shoulder. Note that placing the motor on your back is less dangerous than sitting it on the ground. Be careful to counter prop-thrust by securely grasping the engine frame and leaning against it. Flying at a beach can quickly damage a wood or carbon fiber propeller. If a pilot must have fun at the beach, he should launch into the wind (!) and apply as little power as possible until well away from the sand. Otherwise, the propeller will be sand blasted. Note: It is advisable to attach a good air filter if flying near sand as it is particularly bad for the internals of any engine.
Resist the temptation to use a paramotor to propel a bicycle or a skate board. The bicycle wheels kick up dust and rocks which can reach the propeller and be sucked through it, damaging the propeller. Moreover, engine power is significant and you could have an unpleasant surprise when it's time to stop. (This advice is from Miniplane because – surprise – guys have used a paramotor in lieu of a scooter.)
Do not fly with the propeller damaged or unbalanced. The blades are subjected to centrifugal forces of close to 2,000 G's. A difference of only 1 gram will cause intense vibrations of 2 kg or more at normal speeds and is sufficient to separate the frame welds or even the tubes. The studs that attach the redrive to the engine casing have been known to shear under sufficient load. These SS studs should be replaced with tempered steel studs for added strength. Note: Propeller strikes to the frame or the ground are more than sufficient to bend the hub. A bent hub will cause severe vibration and possible damage to other parts of the engine no matter how well the propeller is balanced. Always check the face runout of the hub if you have a prop strike or the propeller vibrates no matter how carefully you have balanced it.
The propeller must not have any cracks and should be inspected often. The paint must be intact over the surface of the propeller otherwise leaking oil can penetrate into the fibers of the wood making subsequent repairs impossible.
When parked in a vehicle, it is advisable to keep a wood propeller in a horizontal position to prevent moisture near the vehicle floor from warping the blades. A perfectly centered propeller stored vertically can become quickly unbalanced in a hot car. High humidity can cause slight deformations in a new propeller. This can be compensated during the assembly by increasing the torque on the appropriate mounting bolt(s).
Here are some tips on attaching the propeller, whether carbon fiber or wood. Make certain that the hub is not bent or damaged.
Minor dings and chips to the propeller can be repaired in minutes by using cyanoacrylate glue (Super Glue) and ordinary baking soda (sodium bicarbonate) as a filler. However, a product is available called Q-Bond which works the same as Super Glue but includes a superior filler than baking soda.
Larger dings and chips may be be repaired with JB Weld 5 minute epoxy.
Major damage to a propeller is much more difficult to fix. If the structural integrity is weakened the the propeller can come apart at full speed. It is not only dangerous to any bystanders but also will most likely damage the hub. Sometimes, the broken-off propeller can go straight up through the wing, making a nice, neat hole in it.
Repaired surfaces can immediately be painted with automotive nitro-type paint (it dries quickly and resists gasoline and oil). Do not use cheap spray paint e.g. Rustoleum as it will not render a flat finish, is difficult to wet-sand, and does not adhere well. If nitro paint is not available polyurethane paint can be used e.g. Valspar.
If you do not refinish the damaged prop (wood or carbon fiber) immediately, the waste oil from the exhaust can be absorbed by the damaged area and then it will be difficult or impossible to remove it. Paint will not stick to surfaces that have any oil residue on them.
You will need a balancer like this one, which is sold by Blue Sky PPG for about $15. For about twice the cost, California Power Systems has the Steve String Balancer which contains a bubble level and is superior to the one sold by Blue Sky. The propeller must be balanced both in the long dimension (tip to tip) and the short dimension (leading edge to trailing edge). Start with balancing it tip to tip.
Minor out-of-balance can be corrected by applying thicker layers of paint on the lighter side. Note that you want to spray enough paint to make the lighter side/end heavier for the moment. When the paint dries, it will weigh less. If it is badly out of balance, holes can be drilled on the heavy side/end and then sealed with epoxy or Q-Bond, lightening the heavier side/end.
Balancing the short dimension of the propeller (leading edge – trailing edge)
Balancing the long dimension of the propeller (tip to tip)
Photos courtesy of Fresh Breeze
Remember that what seems like an out of balance propeller may be a damaged hub. There a number of outfits that can fix badly damaged props – Google "propeller repair".