Fixed-pitch & constant-speed propellers
| Fixed-pitch | Constant-speed | |
|---|---|---|
| Blade angle | One only | Governor varies it to hold the RPM selected; moved by engine oil at engine pressure (TP 1102) |
| RPM | Directly related to power: rises and falls with throttle | Held by governor; propeller pitch lever sets RPM |
| Throttle | Changes RPM | Changes MP, not RPM (once RPM set) |
| Efficient | At one airspeed/RPM combination only | Matches RPM to power needed (TP 1102) |
| Overspeed/overstress by pilot | Cannot normally | Pilot has that control and risk (TP 1102) |
Fixed-pitch propeller:
- Read RPM on tachometer (TP 1102); shows RPM, not horsepower.
- Higher density altitude: same RPM, LESS power (thinner air). Holding indicated RPM in climb takes more throttle (PHAK ch.7).
- Initial trainer prop: compromise. Balances take-off performance and cruise speed (PHAK ch.7).
| Fixed-pitch type (PHAK ch.7) | Pitch | Drag | Take-off/climb | Cruise |
|---|---|---|---|---|
| Climb propeller | Lower | Less | Higher RPM, more power | Less performance |
| Cruise propeller | Higher | More | Less power | Better efficiency |
Constant-speed propeller:
- Most engines: max horsepower near max RPM, most economical cruise power well below it. Constant-speed prop matches RPM to power needed (TP 1102).
- Governor adjusts automatically through flight (PHAK ch.5):
| Phase | Blade angle | RPM | Result |
|---|---|---|---|
| Take-off | Small | High | Maximum thrust |
| Speed builds | Bigger | Lower | Less fuel consumption and engine wear |
| Pitch lever (TP 1102) | Blade angle | RPM | MP |
|---|---|---|---|
| Fully forward | Smallest (fine pitch) | High | Drops |
| Pulled back | Greatest (coarse) | Low | Rises |
- Raising MP with the pitch lever alone is undesirable.
| Control | Sets |
|---|---|
| Propeller pitch lever | RPM |
| Throttle | Power |
Manifold pressure (MP):
- Inches of mercury (inHg), read on manifold pressure gauge, mainly fitted to constant-speed aircraft (TP 1102).
- Engine stopped: gauge reads ambient pressure (29.92 inHg). Running: below ambient.
- Engine failure: MP rises back to the ambient value for that altitude (PHAK ch.7).
Exact RPM and MP settings: POH/AFM.
Trap: expecting the throttle to change RPM on a constant-speed prop. Governor holds RPM; throttle moves MP. And RPM alone does not show power on a fixed-pitch prop: higher density altitude, same RPM, less power.
References
- TP 1102
- PHAK ch.5, ch.7 (FAA-H-8083-25)
- POH/AFM