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Most pilots over-rev their engines while flying because propellors "unload" in the air as the aircraft gains speed.
ENGINE CHOICE: Before selecting a prop, one should first determine if the engine is capable of the expected performance of the aircraft. Refer to Tip - Power to Weight.
OPTIMUM RPM: Whatever the engine, RPM is critical to propellor selection. The goal is not to achieve maximum RPM on the ground, BUT to achieve optimum RPM in the air. Therefore it is important to KNOW the RPM at which the engine creates maximum horsepower. Don't guess! The information can usually be found in the operator's manual the manufacturer provides with new engines.
As props unload by some 20% in flight, it is recommended that the full throttle RPM of the engine on the ground (static RPM) approximate 80% of the manufacturer's designed maximum horsepower rpm or the engine could over-rev at full throttle in flight - even level flight.
PROP PITCH REQUIRED TO ACHIEVE A DESIRED SPEED : ( / means divide)
SPEED (Mph) / 60 (Min/hr) x 5280 (Ft/mile) x 12 (In/ft) / FULL RPM (unloaded) = PITCH (In)
SPEED RESULTING FROM A PARTICULAR PROP PITCH : ( / means divide)
PITCH (In) x FULL RPM(unloaded) / 12 (In/ft) / 5280 (Ft/mile) x 60 (Min/hr) = SPEED (Mph)
SIZING PROPS: Having determined the speed / pitch requirement of the prop, it becomes necessary to size the prop diameter up or down to achieve the correct static RPM (80% maximum).
NOTE: Larger diameter, lower pitch props appear to work best and the slower speeds greatly reduce the stress on servos and airframes. Lower prop RPM also greatly reduces prop / engine noise.
PROP LOAD COMPARISON: Based on the premise that propellor load is simply the amount of air moved in a single revolution (LOAD = RADIUS x RADIUS x 3.14 x PITCH), one is able to choose a comparably loaded prop of different diameter / pitch.
Large diameter - Low pitch (more thrust / less speed)
Small diameter - High pitch (less thrust / more speed)
Note: Comparison is only valid between similar brands / lines of propellers.
- for maximum thrust use stiff, wide bladed props
- cutting down larger diameter props adds tip area
- 60% of blade area should be clear of the cowling
PROP TIP SPEED should NEVER EXCEED 640 MPH: (/ means divide)
TIP SPEED (mph) = 2 x 3.14 x Prop Radius (inches) x Max RPM x 60 / 63,360
PROP ARC VISIBILITY
- Propellor tips painted white / yellow increase the visibility of the complete propellor arc