Chapter 9: Tracking, Balancing, Ground Running and Storage

Blade tracking

• Tracking determines whether the blade tips rotate in the same plane. It shows only the tip position relative to the other blades, not the blade angle.
• Method: chock the wheels, make sure the magnetos are off and the engine cannot fire, put one blade vertically down, and fix a reference block or pointer close to the tip (within about 1/4 in). Mark the tip position, rotate the propeller by hand and check each blade at the same point.
• A typical tolerance for light-aircraft propellers is ±1/16 in (1.6 mm) between blades; always use the manufacturer's figure.
• Causes of an out-of-track condition: a bent blade, loose or worn blade retention, foreign matter or burrs on the flange or cone seats, under-torqued mounting bolts, or a bent flange/crankshaft. A wooden propeller can sometimes be corrected with shims between hub and flange, as the manufacturer allows.

Checking and adjusting blade angles

• Blade angles are measured at the specified reference station, with a universal propeller protractor (or a digital protractor/inclinometer). The protractor has a ring and a disc, each with a lock, a centre spirit level for the disc and a corner spirit level on the frame, and a vernier scale reading to 0.1°.
• First, establish the reference: put the propeller in position (blade horizontal) and set the protractor to the plane of rotation using a flat surface on the hub or the engine/aircraft reference. Then place the protractor on the blade face at the reference station, level the disc and read the angle.
• Compare each blade's angle with the others and with the specification (e.g. low pitch stop setting). Constant-speed propellers have their low and high pitch stops set in the overhaul shop; ground-adjustable blades are adjusted by loosening the clamps, turning the blades, re-torquing and rechecking.

Propeller vibration and balancing

• Vibration may come from the propeller or the engine. Propeller vibration usually changes with rpm and is felt as a once-per-revolution shake; engine vibration is often more constant. Check blade track, blade angles, spinner, mounting and engine mounts before balancing.
• Static balance — the propeller has no tendency to rotate about its axis when mounted on a frictionless arbor. Checked in the shop on a knife-edge stand (two hardened steel edges, level) or a suspension method. Checks are made with the blades vertical and then horizontal. Corrected by adding/removing weight (e.g. lead wool in balance holes, balance weights on the hub) as the overhaul manual specifies.
• Dynamic balance — balance while rotating, including mass distribution along the axis and aerodynamic (blade angle) differences. Done on the aircraft with a balancer: an accelerometer on the engine measures vibration velocity in inches per second (ips), and an optical or magnetic tachometer pickup gives a phase reference.
• Procedure: run at the specified rpm, record amplitude and phase; fit a trial weight at a known position; re-run; the balancer calculates the correction weight and position. Many manufacturers require ≤ 0.2 ips (lower is better). Weights are attached only at approved positions (usually the spinner bulkhead or hub flange) with approved hardware, and recorded.
• A vibration spectrum survey (frequency analysis) separates propeller, engine and airframe vibration sources.

Propeller engine running

• Safety: run-up area clear, aircraft facing into wind, chocks and brakes, fire cover, clear of people and loose objects. Never stand in the plane of rotation. Hand-propping only by trained personnel with correct procedure.
• Checks after installation or maintenance: static rpm (full throttle against the low pitch stop) within limits; propeller control from high to low rpm — rpm should fall then recover smoothly (exercise the propeller several times to circulate warm oil); governed rpm at the maximum rpm setting; feathering and unfeathering checks (time to feather, rpm drop) where applicable; autofeather, synchroniser and overspeed governor tests; beta and reverse function on turboprops; de-icing ammeter check.
• After the run: check for oil and grease leaks, security of the spinner and safetying, and recheck torque on wooden propellers as required.

Troubleshooting

• Governor inspection and adjustment: check security, drive spline, oil leaks, control arm travel (stop to stop) and the maximum-rpm stop setting. Adjust the high-rpm stop screw per the manual and confirm by ground run (or a flight check where specified).

Propeller storage and preservation

• Moisture and sunlight are the main enemies. The aim is to keep the propeller clean, dry and protected from the atmosphere. Follow the manufacturer's procedures.
• Aircraft storage categories (typical): flyable (fly regularly, e.g. monthly for at least 30 min; grease the propeller more often, e.g. every 6 months); temporary (around 90 days — engine and propeller sealed with plugs and covers); indefinite/long-term (engine oil replaced with preservative oil, circulated through governor, accumulator and hub).
• Metal blades: clean and apply a thin film of corrosion-preventive compound. Keep it off rubber parts such as de-ice boots and seals.
• Wooden propellers: store horizontally so moisture stays evenly distributed (stored vertically, moisture migrates to the lower blade and causes imbalance). Store indoors in a climate-controlled area away from sun, snow and extreme dryness. Protect the metal hub/adapter from corrosion.
• Variable-pitch hubs: fill with preservative oil (or a mix of engine and preservative oil) and seal the openings; this protects internal parts from corrosion.
• Wrap blades in non-hygroscopic material (does not absorb water), use the original shipping container or a sealed bag/container, possibly purged with dry nitrogen and with desiccant. Do not rest propellers on their blade tips; large propellers use racks.
• Governors and accumulators left on the engine are protected by the engine's preservative oil. When removed: fill the passages with preservative oil (types differ for piston and turbine oils), rotate the drive shaft to spread it, fit covers over the mounting face and seal ports with plugs. Relieve the accumulator's pressure for indefinite storage (per the manual).
• Depreservation and return to service: remove covers, plugs, wrapping and desiccant; clean off preservative (approved solvent); inspect for corrosion, damage and expired shelf life of seals; drain preservative oil from hubs where required; re-lubricate; check the air charge; install, torque and safety; check track; carry out ground runs with full functional checks; record everything in the logbook.