4.5 Propellers and Rotors

The Propeller Is a Rotating Aerofoil

A propeller or rotor blade is an aerofoil that happens to travel in a circle instead of a straight line. Everything you know about lift, angle of attack and drag applies to it — the only complication is that a rotating blade meets the air at a different speed at every point along its span.

The three angles

Schedule 4 topic 7 names three angles that candidates routinely conflate.

• Blade angle (also called geometric pitch angle) — the angle between the blade's chord line and the plane of rotation. It is a fixed property of the blade at any given station, set by its manufacture.
• Helix angle — the angle of the blade's actual path through the air, which is the resultant of its rotational motion and the aircraft's forward (or descent) motion. It changes with airspeed and rpm.
• Angle of attack of the blade — the difference between blade angle and helix angle. This is what actually determines the thrust the blade section produces.

Pitch is the related distance measure: the theoretical distance the propeller would advance in one complete revolution if it moved through the air like a screw through a solid. A propeller marked 9×4.5 has a 9-inch diameter and a 4.5-inch pitch. The gap between that theoretical advance and the real advance is slip, and slip is where the air gets accelerated and thrust gets made.

Blade twist

The tip of a rotor blade travels much faster than the root because it covers a much larger circle in the same time. On a 15-inch propeller the tip travels roughly six times the speed of a point 1.25 inches from the hub. If the blade angle were constant along the span, the tip would meet the air at a tiny angle of attack while the root would be stalled.

The fix is blade twist (also called washout): the blade angle is largest at the root and decreases toward the tip, so that the angle of attack stays roughly constant along the span. Some blades add taper — narrowing toward the tip — to reduce tip losses and structural mass. Both features are why a proper propeller cannot be substituted with a flat paddle of the same diameter.

Thrust and Torque

Thrust is the useful force produced along the rotational axis. Torque is the resistance to rotation the blade generates, and by Newton's third law it produces an equal and opposite reaction on the airframe.

On a single-rotor helicopter that reaction is what a tail rotor exists to counter. On a multirotor it is countered by pairing: half the rotors turn clockwise and half counter-clockwise, so in a balanced hover the torques cancel. Deliberately unbalancing them is how a multirotor yaws — speed up the clockwise pair relative to the counter-clockwise pair and the airframe rotates in the opposite direction to the now-dominant torque.

The practical consequence is that rotor direction matters. Fitting a clockwise propeller to a counter-clockwise motor produces a blade that is aerodynamically backwards: it generates a fraction of the intended thrust, the torque balance is destroyed, and the aircraft will typically flip on take-off. Propellers are marked (often CW/CCW, or by hub colour) and the correct assignment is part of the pre-flight.

Choosing Diameter and Pitch

Propeller selection is a trade, and Schedule 4 links it directly to the electrical unit (REES topic 3) through current draw.

A large-diameter, low-pitch propeller moves a large mass of air slowly — which is the efficient way to make thrust when the aircraft is not going anywhere. That is why heavy-lift and long-endurance multirotors run big, gentle propellers at modest rpm. A small, high-pitch propeller moves a small mass of air quickly, which suits an aircraft that is already travelling fast, and wastes energy in a hover.

Two practical rules follow:

• Over-propping an airframe — fitting a propeller larger or coarser than the motor and ESC were sized for — raises current draw beyond the components' rating. The symptoms are hot motors, hot ESCs, voltage sag and, eventually, an ESC failure in flight. Always match the propeller to the manufacturer's specification.
• Balance and condition matter more than they look. An unbalanced or chipped propeller injects vibration into the airframe, which contaminates the IMU accelerometer readings and can produce hover drift, oscillation, or "toilet-bowling" in position hold. Inspect propellers before every flight for nicks, cracks and delamination, and retire damaged ones rather than filing them smooth.

Fixed-Wing Propellers: Tractor and Pusher

A fixed-wing RPA carries its propeller in one of two positions:

• Tractor — propeller ahead of the wing, pulling the aircraft. The propeller wash flows over the wing and tail, which improves control response at low airspeed but adds drag and noise over the airframe.
• Pusher — propeller behind the wing, pushing the aircraft. The wing meets clean, undisturbed air, which suits sensors mounted in the nose and keeps the propeller clear of a nose-mounted camera. The propeller is also more exposed on a belly landing.

Some larger fixed-wing and VTOL RPA use variable-pitch propellers, which change blade angle in flight so the blade can hold an efficient angle of attack across a wide speed range. Small multirotors almost universally use fixed-pitch propellers and change thrust by changing rpm alone — which is why multirotor thrust response is limited by how fast the motors can spin up and slow down, and why a heavier propeller (higher rotational inertia) makes an aircraft feel less crisp.