Most powered model-aircraft, including electric, internal-combustion, and rubber-band powered models, generate thrust by spinning an airscrew. The propeller is a type of airscrew and is by far the most commonly used device. The blades of the rotating propeller push against the atmosphere, and by Newton's Third Law, the air's reactionary force pushes the aircraft.
As in full-size planes, the propeller's dimensions and placement (along the fuselage or wings) are factored into the design. In general, a large diameter and low-pitch offers greater thrust at low airspeed, while a small diameter and higher-pitch sacrifices thrust for a higher maximum-airspeed. In RC aircraft, the builder can choose from a wide selection of propellers, to tailor the plane's airborne characteristics. A mismatched propeller will compromise the aircraft's airworthiness, and if too heavy, inflict undue mechanical wear on the powerplant. Scale RC aircraft propellers are usually specified as diameter × pitch, given in inches. For example, a 5x3 propeller has a diameter of 5 inches, and a pitch of 3 inches. The pitch is the distance that the propeller would advance if turned through one revolution in a solid medium. Additional parameters are the number of blades (2 and 3 are the most common).
There are two different methods to transfer rotational-energy from the powerplant to the propeller.
- With the direct-drive method, the propeller is attached directly on the engine's spinning crankshaft (or motor-rotor.) This arrangement is optimum when the propeller and powerplant share overlapping regions of best efficiency (measured in RPM.)
- With the reduction method, the crankshaft drives a simple transmission, which is usually a simple gearbox containing a pinion and spur gear. The transmission decreases the output RPM by the gear ratio (thereby also increasing output torque by approximately the same ratio). Reduction-drive is common on larger aircraft and aircraft with disproportionately large propellers. On such powerplant arrangements, the transmission serves to match the powerplant's and propeller's optimum operating RPM. Geared propellers were once used very commonly on electric motors, but with the advent of brushless motors are rarely seen.
In some designs of aircraft, the propeller is replaced by a ducted fan unit. In jet-powered or ducted fan scale aircraft, the engine is a single-piece assembly with no user-changeable parts. The turbine-wheel spins at extremely high speed (>150,000 RPM), limiting most adjustments to the original factory.
Ornithopters are aircraft that may have a similar physical shape to a plane, but do not use airscrews at all. In ornithopters, the reciprocating-motion of the wing structure imitates the flapping-wings of living birds, producing both thrust and lift.
Differences in propellers can drastically change flight characteristics, even if nothing else on the craft changes. Changes in pitch or length can adjust thrust but will also adjust the amount of strain on the powerplant. A prop with more blades creates more thrust if the prop turns at the same speed.
Some tips for propeller selection:
- If a plane's cowl is low to the ground during takeoff or landing (short or no landing gear), a shorter prop with more blades can replace a longer prop and provide equal thrust.
- The more air the prop moves, the more thrust is provided. A prop with a steeper pitch and/or wider blade will displace more air at the same speed. The more air displaced in each rotation, the more responsive the craft is in regard to its relative weight. 3D flyers require more air displacement in order to support the weight of the craft. A narrow prop with high pitch that may be used for racing can displace a lot of air at high RPMs but may not create enough displacement per rotation to be used for a higher drag bi-plane of the same wingspan.
- If a prop is too long and/or too heavy, it can create a gyro effect which will counteract yaw and pitch and make the craft less responsive. A little gyro effect can be beneficial for stability, so this needs to be determined on a craft by craft basis.