Propeller

This page covers airplane and multirotor propellers. For collective/cyclic pitch helicopter propellers, see helicopter rotor.

Terminology

Propellers are commonly rated in AxB terminology, where A is the diameter in inches, and B is an indication of the pitch. B indicates angle by stating how many inches the prop would move forward in one rotation if cutting through some ideal cohesive solid, like butter - as if it were a perfect 'screw'. In reality, an airstream slips off the blades and so this is a vast overestimate.

Directionality

Fixed-pitch props are highly directional. They are designed to 'scoop' their way through the air with concave surfaces, rather than having it slide off the convex surfaces when mounted backwards. One *can* use a prop as a tractor or (reversed) as a pusher in a one-prop configuration with a brushless motor, by switching out the wires to reverse the direction of rotation. For twin-prop planes (optionally) and multirotors (non-optionally), however, the roll/yaw torque induced by having the props haft out on a moment arm is considerable, and it is desirable to make the props rotate in opposite directions while pointed in the same direction. These situations make it impossible to use the same model of prop for both motors: special prop pairs of CW and CCW-oriented blades are created and sold for the purpose, although they are somewhat harder to find than normal props.

Typical Variants

Fixed Prop

By far the most common choice in fixed-wing planes is two blades with a fixed pitch, because they're easy to balance and relatively efficient. As one goes up to 3 and 4 blades, efficiency drops because the blades rotate fast enough to have turbulent airflow interactions between each other; This is similar to tandem-wing parasitic drag.

Folding Prop

Ubiquitous on powered gliders, these props mount to a central hub with a pivoting shaft, so that when they don't have the centripetal force of the powered rotation, they can fold neatly out of the way. Care must be taken to ensure that mechanically, they can only fold outwards, as folding inwards would cause an unbalanced startup which likely breaks something as throttle is added. In many prop configurations, these are also safer than fixed propellers when coming near the ground unpowered, or even be necessary for landing in some landing gear-less designs.

EDF

An electric ducted fan is a compact, shrouded propeller with long-chord blades, designed to operate at high RPM. They are ~half as efficient as normal unshrouded props, and do not have an inherent power:weight advantage, but are commonly used to symbolize jet turbine engines safely in scale designs, so EDFs are often applied when someone wants to go fast. They may be considered somewhat safer to operate, both in the hand and in the landing zone, due to the shroud blocking obstructions at the dangerous tip edge. Typical sizes are 40mm diameter on a small plane to dual 90mm diameter on a large plane. Other than their differences during propelled flight, there may be slight aerodynamic differences during gliding compared to non-folding props.

Exotic Variants

Single-Bladed

There is some curiosity around 1-bladed propellers which mitigate their asymmetry with a squat counterweight, and could theoretically be more efficient than 2-bladed props. This would be fine with a flat prop, but the physics dictate that the increased drag on the airfoil of a useful blade scales up faster than the counterweight's centripetal force with increased RPMs, unbalancing the blade. It could be optimized to work well at a particular RPM and airspeed, but would be unbalanced for others, unless some way of dynamically rebalancing the counterweight could be achieved. This remains experimental in nature, with several helicopter prototypes

Coaxial

Some airplane props have been made with coaxially rotating shafts and concentric nose props. As with a coaxial helicopter (but with a much simpler mechanism), these correct for most induced roll, and increase thrust somewhat for the same footprint, at the expense of a considerable efficiency hit. Used historically in some full-scale designs, like tailsitters, for the roll characteristics. Coaxial shafts are still somewhat undesirable mechanically from a wear and tea perspective, but the main problem is in aerodynamic efficiency problems from airflow interactions between the blades; This is similar to biplane parasitic drag.

Variable Pitch

Collective-pitch props do exist for fixed-wing, but the less efficient untwisted shape necessary to operate them hurts efficiency significantly. They're generally confined to 3D planes as a gimmick, because they can hover on their nose with a lot of difficulty.

Accessories

• A prop saver is a rubber band and bracket designed to protect the motor when it strikes an obstacle
• A spinner is an end cap designed to hide the motor shaft and provide a slightly smoothed airflow

Links

• Beginners Guide to Motor and Prop Selection - RCGroups
• Understanding Propeller Size - RC Airplane World (refers to gas engine sizes)