Balance a propeller by resting its hub shaft on two level knife edges and checking that it stays put with the blades horizontal and again with them vertical, removing wood or sealing coat from the heavy blade until it does, then confirm the blade tips pass through the same point (tracking) and repeat after every layer of finish. An unbalanced propeller makes a vibration that repeats once per turn and fatigues the engine mount and airframe. Static balance is necessary but not the whole job, and a finished aircraft is judged by how it runs.
Part of Aerodynamics and Flight
Removing the heavy side of a propeller before it vibrates the engine and airframe apart.
What Is Balanced, And Why It Matters More Than In A Machine On The Floor
A propeller out of balance by a small mass at its tip produces a rotating force that grows with the square of speed. On an engine fixed to a heavy bed that is a nuisance; on a light mount at the front of an airframe it fatigues welds, cracks fittings and loosens bolts (the carrier’s own list of consequences). The corpus’s Static Balancing article gives the knife-edge method and its sensitivity limit, and Dynamic Balancing the trial-weight method for a rotor running at speed.
What You Need
- Two straight, hard, level bars (knife edges) set on a firm stand, checked level with a spirit level or water level (Levels and Plumb Bobs).
- A close-fitting shaft or bolt through the hub bore.
- A wind-free room and patience.
Method: Static Balance
Step 1: Put the propeller on the edges with the blades horizontal. If it rolls, the lower blade is heavy.
Step 2: Correct the heavy blade by removing a little wood from the back face away from the leading edge, or add weight to the light blade. Do not cut the leading edge or thin the root.
Step 3: Repeat until it stays in any position. Rotate in 90° steps.
Step 4: Turn the propeller so the blades are vertical. If the blades balance horizontally but not vertically, the hub bore or the blade is not symmetrical about the centre; the propeller is not good, and weight added here hides a fault.
Step 5: Check tracking. Mount on the engine shaft and turn slowly past a fixed pointer. Both tips should pass the same point within the accuracy of the pointer.
Step 6: Re-balance after sealing and finishing. Each coat of varnish, epoxy and the metal tip changes the weight of each blade.
Dynamic Balance Is Measured On The Engine
After the static work, run the engine at a series of speeds on the ground and feel for vibration, as the carrier says. If it shakes, the trial-weight method of Dynamic Balancing finds the correction using vibration readings, but a weight added to a propeller hub or spinner must be fixed so it cannot come off at speed. Also note that a worn engine, a loose mount or an unbalanced crankshaft vibrates too (Engine Mounting).
Common Mistakes
- Balancing before finishing — the coats upset it.
- Balancing on bent knife edges — the result is false.
- Gluing lead onto a rotating blade — it can separate at speed.
- Ignoring moisture — the wood gains and loses weight with the seasons; recheck.
Safety
Spinning mass and flung weights
A propeller on a stand can spin when tapped, and one on the engine is lethal; tie the aircraft down, stay out of the plane of rotation, and keep hands away from the propeller until the engine is dead, the ignition is off and the propeller is not free to turn.
What’s Next
- Learn next: Engine Mounting, where the remaining vibration goes
- Combine with: Ground Testing, the engine run-up