Constant-Speed Propeller: Slip Calculation and Fault Attribution
A piston-engined light aeroplane (direct-drive engine, two-blade constant-speed propeller of 1.93 m diameter) is returned to service after a propeller overhaul and blade-angle reset. On the test flight the pilot records 118 kt TAS at 2,400 rpm and 2,000 ft. The propeller manual gives a blade angle of 21 degrees at the 75 percent radius station for this setting, and a cruise slip band of 4 to 7 percent. At the same rpm and altitude, manifold pressure is 1.0 inHg below the flight-manual figure, and the pilot reports the aeroplane 'feels flat'. The airframe has 40 h since its last check, the engine 610 h since overhaul, and no other defects are recorded. The organisation must decide whether to re-rig the propeller or look elsewhere before returning the aircraft to the owner.
1. Calculate the propeller geometric pitch, the theoretical (no-slip) true airspeed, and the percentage slip at the recorded condition.
Read the worked answer
1. Radius at the 75 percent station
r = 0.75 x (1.93 / 2) = 0.724 m
2. Geometric pitch
p = 2 x pi x r x tan(blade angle) p = 2 x 3.1416 x 0.724 x tan 21 deg p = 4.548 x 0.3839 = 1.746 m (about 1.75 m)
3. Theoretical speed
Speed = pitch x revolutions per hour = 1.746 m x 2,400 rev/min x 60 min/h = 251,424 m/h = 251.4 km/h Convert: 251.4 / 1.852 = 135.8 kt
4. Slip
Slip = (theoretical - actual) / theoretical x 100 = (135.8 - 118) / 135.8 x 100 = 13.1 percent
Interpretation: roughly double the top of the 4 to 7 percent band. That is a symptom, not yet a diagnosis.
2. The slip is well outside the band. Using all the recorded evidence, decide whether the correct first action is to re-set the blade angle. Justify the decision and state what should be checked first.
Read the worked answer
Decision: do not touch the blade angle yet.
Reasoning:
- Slip is a dependent quantity. The propeller only sets the no-slip distance per revolution. Actual TAS is set by thrust versus airframe drag, and thrust depends on engine power. Any power shortfall shows up as extra slip even with a perfectly rigged propeller.
- A manifold pressure 1.0 inHg below the book figure at the same rpm and altitude is direct evidence of a power shortfall (roughly 3 to 5 percent of power at that setting). That alone can account for several knots of TAS.
- Re-rigging to a finer blade angle would reduce slip by making the propeller turn faster for the same power, masking the engine defect, and risks an overspeed or an over-coarse condition at another operating point. It would also invalidate the overhaul records.
Check first: induction leaks, air filter restriction, throttle/mixture rigging, ignition timing, magneto and plug condition, fuel flow, compression, and calibration of the manifold pressure gauge. Correct any defect, re-fly, then recompute slip from a fresh stable cruise point.
Only if the engine and airframe are proven good and slip is still outside 4 to 7 percent should the blade angle rigging be re-verified against the propeller manual.
3. The tachometer is later found to read 4 percent high. Recompute slip and state how this changes your conclusion.
Read the worked answer
True engine speed:
n = 2,400 / 1.04 = 2,308 rpm
New theoretical speed:
= 1.746 x 2,308 x 60 = 241,786 m/h = 241.8 km/h = 130.6 kt
New slip:
= (130.6 - 118) / 130.6 x 100 = 9.6 percent
Effect on the conclusion:
- About one third of the apparent slip was a tachometer error, not a propeller or airframe problem. This is exactly why a suspect instrument must be resolved before any rigging change is made.
- 9.6 percent is still above the 7 percent ceiling, but it is now within the range that a 1 inHg manifold-pressure shortfall can explain. The engine remains the primary suspect; blade angle remains secondary.
- Any maintenance limit expressed in rpm (overspeed check, governor check, static rpm band) is now known to have been assessed against a false reading. Re-do those checks with a calibrated tachometer and record the instrument error in the technical log.