A propeller is a pair of twisted rotating wings whose blade angle is set so each section meets the air at a useful angle of attack, and its diameter, pitch and speed decide how much thrust it makes at the aircraft’s speed. For the carrier’s 170-190 cm propeller at 2,200-2,800 RPM the blade tips travel at about 196-279 m/s, and a 120-150 cm pitch gives a pitch speed of 158-252 km/h, the flight speed at which that propeller makes no thrust. Efficiency and the best pitch for your aircraft are measured values that this corpus cannot supply.
Part of Aerodynamics and Flight
How a propeller turns shaft power into thrust, and the geometry and speeds that constrain its design.
Each Blade Section Is A Small Wing
At a radius r from the hub the blade moves at 2π × r × (rev/s) in its own plane and the aircraft moves forward. The air meets each section at the angle whose tangent is flight speed ÷ rotational speed, so the blade must be twisted: steeper near the hub, flatter near the tip, so the angle of attack stays useful along the length. Each section is a wing, and the same ideas of angle of attack and stall apply (Airfoil Geometry, Stall Dynamics). The thrust is the reaction to the air the propeller accelerates backward, which is the same momentum account as a wing’s lift (Bernoulli’s Principle).
Worked Example: Tip Speed And Pitch Speed
Inputs from the carrier: diameter 170-190 cm, speed 2,200-2,800 RPM, pitch 120-150 cm.
- Tip speed = π × D × (RPM ÷ 60). Smallest: π × 1.7 × 36.67 = 195.8 m/s. Largest: π × 1.9 × 46.67 = 278.6 m/s; rounded, about 196 m/s and 279 m/s.
- The speed of sound in air at ordinary temperature is about 340 m/s, so the tips run at roughly 0.58-0.82 of it, before the aircraft’s forward speed is added. Propeller efficiency falls and noise rises well before the speed of sound, so the high end of the carrier’s range is not a comfortable operating point.
- Pitch speed = pitch × RPM ÷ 60. Smallest: 1.2 × 36.67 = 44.0 m/s = 158 km/h. Largest: 1.5 × 46.67 = 70.0 m/s = 252 km/h.
Pitch speed is the speed at which the propeller would advance as in a solid with no slip, and so produce no thrust; a fixed-pitch propeller works usefully at a flight speed some way below it. The right ratio comes from propeller data or from test, not from this article.
Diameter, Pitch And Compromise
- Larger diameter moves more air more slowly, which is more efficient, but is limited by ground clearance, tip speed and weight.
- Lower pitch gives better thrust at take-off and climb; higher pitch gives faster cruise (Aerodynamics and Flight). A fixed-pitch propeller is a compromise.
- Two blades are the simplest to make and balance.
Direct Drive Needs An Engine That Suits The Propeller
Many car engines reach their power at a higher speed than the propeller can use at this diameter; the engine must be geared down, which adds weight, vibration and a failure point. If the engine runs at a speed that puts the tips near the speed of sound, reduce the diameter or the speed rather than trusting the estimate.
Testing
Measure the static thrust with a spring balance with the aircraft tethered to a strong anchor (Improvised Weighing; Ground Testing). It tells you the thrust at zero speed, not at flight speed.
Common Mistakes
- Copying a pitch figure without the engine’s speed, which changes everything.
- Over-speeding a wooden propeller beyond its rated speed.
- Ignoring the nicks and cracks that a leading-edge stone strike leaves.
Safety
A propeller that fails takes the engine with it
A blade that breaks or a hub that loosens at full speed throws an unbalanced mass that can tear the engine off its mount. Inspect the propeller before every flight, run it up only with people out of its plane, and do not exceed the speed it was balanced and tested for.
What’s Next
- Learn next: Carving Propellers, making the blade
- Combine with: Propeller Balance, making it run true