Airscrew / Propeller types of Flying Model Aircraft

Most powered model-aircraft, including electric, internal-combustion, and rubber-band powered models, generate thrust by spinning an airscrew. The propeller is the most commonly used device. Propellers generate thrust due to the angle of attack of the blades, which forces air backwards. For every action there is an equal and opposite reaction, thus the plane moves forwards.

Propellers
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 model aircraft, the builder can choose from a wide selection of propellers, to tailor the model's airborne characteristics. A mismatched propeller will compromise the aircraft's airworthiness, and if too heavy, inflict undue mechanical wear on the powerplant. Model 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 methods to transfer rotational-energy from the powerplant to the propellor.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 propellor and powerplant share overlapping regions of best efficiency (measured in RPM.) Direct-drive is by far the most common when using a fuel-powered engine (gas or glow). Some electric motors with high torque and (comparatively) low RPM's can utilize direct-drive as well. These motors are typically outrunners.

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 are rarely used on internal combustion engines, but very commonly on electric motors. This is because most inrunner electric motors spin extremely fast, but have very little torque.

Ducted Fans
Ducted fans are propellers encased in a cylindrical housing or duct, designed to look like and fit in the same sort of space as a model jet engine but at a much lower cost. They are available for both electric and gas engines, although they have only become widely used with the rise of effective electric power for model aircraft. It is possible to equip a model jet aircraft with two or four electric ducted fans for much less than the cost of a single jet or large gas engine, enabling accurate modeling of planes such as military bombers and civilian airliners.

The fan-unit is an assembly of the spinning fan (a propellor with more blades), held inside a shaped-duct. Compared to an open-air propellor, a ducted-fan generates more thrust per crossectional-area. The shaped-duct often limits installation to recessed areas of the fuselage or wings. Ducted fans are popular with scale-models of jet-aircraft, where they mimic the appearance and feel of jet engines, as well as increasing the model's maximum airspeed. But they are also found on non-scale and sport models, and even lightweight 3D-flyers. Like propellors, fan-units are modular components, and most fan-powered aircraft can accommodate a limited selection of different fan-units.

Other
With Ornithopters the reciprocating-motion of the wing structure imitates the flapping-wings of living birds, producing both thrust and lift.

Model aerodynamics
The flight behavior of an aircraft depends on the scale to which it is built. The Reynolds number depends on scale and speed. Drag is generally greater in proportion at low Reynolds number so flying scale models usually require larger than scale propellers.

Mach number depends on speed. Compressibility of the air is important only at speeds close to or over the speed of sound, so the effect of the difference in Mach number between a slow piloted aircraft and a small model is negligible, but models of jets are generally not efficient flyers. In particular, swept wings and pointed noses are used at high Mach number to reduce compressibility drag and tend to increase drag at small Mach number.

Angular momentum also depends on scale. Since torque is proportional to lever arm length while angular inertia is proportional to the square of the lever arm, the smaller the scale the more quickly an aircraft or other vehicle will turn in response to control or other forces. While it may be possible for a pilot to fly an unstable aircraft (such as a Wright Flyer), a radio control scale model of the same aircraft would only be flyable with the center of gravity moved forward, or with avionics. On the other hand, angular inertia, and therefore large scale, generally degrades stability, because it introduces a delay. Static stability, resisting sudden changes in pitch and yaw, is generally required for all models and is usually considered a requirement for piloted aircraft. Dynamic stability is required of all but tactical piloted aircraft.

A contest winning paper glider. Free flight models and flight trainers need to have both static and dynamic stability. Static stability is the resistance to sudden changes in pitch and yaw and is typically provided by the horizontal and vertical tail surfaces, respectively, and by a forward center of gravity. The three dynamic stability modes are phugoid, spiral and Dutch roll. An aircraft with too large horizontal tail on a fuselage that is too short may have a phugoid with increasing climbs and dives. With free flight models, this usually results in a stall or loop at the end of the initial climb. Insufficient dihedral and sweep back will generally lead to increasing spiral turn. Too much dihedral generally causes Dutch roll. However, these all depend on the scale, as well as details of the shape and weight distribution. For example the paper glider shown here is a contest winner when made of a small sheet of paper but will go from side to side in Dutch roll when scaled up even slightly.

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