Mx: Propeller Problems

Start with the simple and cheap. Here are some that qualify.

• Repair cracks. Under air loads they can cause an aerodynamic imbalance besides sapping power.
• Static balance.
• Make sure the bolts are tightened evenly. If the prop is askew widthwise on its mount (making one blade have a higher angle of attack than the other), an aerodynamic imbalance can cause vibration.

Problem

Possible solution or link.

1. Prop bolts are loose.

2. Center bolt that holds prop hub against drive spline is lose.

3. Something is preventing the prop hub resting on its backing. The prop should mate flat against the back plate. If it doesn’t then it will be rocking on the spline which will eventually wear.

4. Reduction drive problem with mount or bearings.

1. Leading edge is rough.

2. A crack or other deformity is causing drag.

.

1. Static imbalance. One blade is heavier than the other.

 – For carbon fiber props, if they have been submerged or exposed to moisture, the interior foam may hold some water, making them off balance. Balance and let dry.

2. Aerodynamic imbalance. This is where one blade is producing more thrust than the other. Causes may be:

 – Prop is on laterally crooked such that one blade has a higher angle of attack than the other.

 – One blade has is larger than the other.

 – One blade has been sanded down so that it has a higher angle of attack than the other.

 – One blade is warping under air loads. This is more probably with weaker woods or thin (flat) blades.

3. Prop is cracked. If you are unsure about fixing props it is far better to send it someone who is. Separating blade parts could be lethal! Among many methods to repair such prop, (as long as the crack does not go through the Hub) is to epoxy the damaged area, filling void’s with urethane filler (about the same density as wood) and then cover the whole prop in Fiberglass, Unless it is just a small crack then cover it and opposite blade with glass to balance. Suggestion from John Fetz.

1. The center bolt on some machines comes loose easily, enough to fall off. It happens when you throttle off.

Good Locktite is a start but the real cure is safety wiring the bolt. You’ll need a special bolt that is long enough to extend past the prop face. Below is a picture of one built by Damien of TN. He welded the extension and drilled through. Now he can safety wire the bolt around one of his other prop bolts.

This was built for a Snap 100 which, for some reason, has a problem with the prop falling off when the center bolt comes loose. Thanks to Damien who sent me one of these. He welded a piece onto the bolt so that it could be safety wired to a prop bolt.

2. The bolts holding the prop to its hub are not equally tight and a vibration can set up causing prop failure at the hub. Prevention is simply insuring that the bolts are uniformly tight and checking them frequently.