Repair and post-repair checks

Example 1 How much force does a small chip create?

A chipped blade tip loses mass on one side, equivalent to an unbalance of 9.5 g·mm; after rebalancing, 2.0 g·mm remains. The propeller and hub weigh 60 g and turn at 5,000 rpm in hover, where each motor carries about 7.85 N of lift (from UAT 302).

import math

RPM, M_ROTOR, HOVER_THRUST = 5000, 60, 7.85     # rpm, g, N
omega = 2 * math.pi * RPM / 60
for label, u_gmm in (("chipped", 9.5), ("after balancing", 2.0)):
    force = u_gmm * 1e-6 * omega ** 2           # g·mm -> kg·m
    e_um = u_gmm / M_ROTOR * 1000               # µm
    print(f"{label:<16} U {u_gmm} g·mm: F {force:.2f} N ({force / HOVER_THRUST:.0%} of hover thrust), "
          f"e {e_um:.0f} um, e*Omega {e_um / 1000 * omega:.1f} mm/s")

chipped          U 9.5 g·mm: F 2.60 N (33% of hover thrust), e 158 um, e*Omega 82.9 mm/s
after balancing  U 2.0 g·mm: F 0.55 N (7% of hover thrust), e 33 um, e*Omega 17.5 mm/s

A chip too small to notice creates a rotating force of about a third of that motor’s lift. It rotates 83 times a second, speeding up fatigue in mounts, screws and sensors. Rebalancing cuts the force about five times, but a chipped or cracked propeller should be replaced as the manufacturer directs, not rebalanced and reused, because damaged material may break in flight.

Example 2 Acceptance criteria after replacing a motor and propeller

Averages from 2 minutes of hover, compared with UAV 3’s baseline (simulated values). The organisation’s criteria: vibration below 30 m/s² per the ArduPilot documentation, each motor output within 5% of the mean, and hover current within 5% of the baseline.

baseline = {"vibe_max": 14.0, "hover_current": 21.0}
after = {"vibe_max": 16.5, "hover_current": 22.6, "rcou": [1512, 1498, 1571, 1505]}   # m/s², A, µs

checks = []
checks.append(("vibration < 30 m/s²", after["vibe_max"] < 30))
mean_out = sum(after["rcou"]) / len(after["rcou"])
spread = max(abs(o - mean_out) / mean_out for o in after["rcou"])
checks.append((f"motor outputs within 5% of mean (max {spread:.1%})", spread <= 0.05))
dcur = (after["hover_current"] - baseline["hover_current"]) / baseline["hover_current"]
checks.append((f"hover current within 5% of baseline ({dcur:+.1%})", abs(dcur) <= 0.05))
for text, ok in checks:
    print(f"{'PASS' if ok else 'FAIL'}  {text}")
print("return to service" if all(ok for _, ok in checks) else "not released: investigate")

PASS  vibration < 30 m/s²
PASS  motor outputs within 5% of mean (max 3.3%)
FAIL  hover current within 5% of baseline (+7.6%)
not released: investigate

Vibration passes, but hover current is above the baseline beyond the limit, even though motor 3’s output is only about 3% above the others. The new propeller set may not match the original model, or the replacement motor may have a different KV; this must be checked before return to service. Looking only at vibration would release a drone with shorter endurance without anyone noticing.