Editing Model aircraft (section)

==Model aerodynamics==
[[File:International contest paper glider.svg|thumb|A contest-winning paper glider]]
{{See also|Flight dynamics (aircraft)|l1=Flight dynamics}}

The flight behavior of an aircraft depends on the scale to which it is built, the density of the air and the speed of flight.

At subsonic speeds the relationship between these is expressed by the [[Reynolds number]]. Where two models at different scales are flown with the same Reynolds number, the airflow is similar. Where the Reynolds numbers differ, as for example a small-scale model flying at lower speed than the full-size craft, the airflow characteristics can differ significantly. This can make an exact scale model unflyable, and the model has to be modified in some way. For example, at low Reynolds numbers, a flying scale model usually requires a larger-than-scale propeller.

Maneuverability depends on scale, with stability also becoming more important. Control [[torque]] is proportional to lever arm length while angular inertia is proportional to the square of the lever arm, so the smaller the scale the more quickly an aircraft or other vehicle turns in response to control inputs or outside forces.

One consequence of this is that models in general require additional [[longitudinal static stability|longitudinal]] and [[directional stability]], resisting sudden changes in pitch and yaw. While it may be possible for a pilot to respond quickly enough to control an unstable aircraft, a radio control scale model of the same aircraft would be flyable only with design adjustments such as increased tail surfaces and wing dihedral for stability, or with [[avionics]] providing artificial stability. Free flight models need to have both static and dynamic stability. Static stability is the resistance to sudden changes in pitch and yaw already described, and is typically provided by the horizontal and vertical tail surfaces respectively, and by a forward center of gravity. Dynamic stability is the ability to return to straight and level flight without any control input. The three dynamic instability modes are pitch ([[phugoid]]) oscillation, spiral and [[Dutch roll]]. An aircraft with too large a horizontal tail on a fuselage that is too short may have a phugoid instability 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 (aeronautics)|dihedral]] or sweep back generally leads to increasing spiral turn. Too much dihedral or sweepback generally causes Dutch roll. 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 goes from side to side in Dutch roll when scaled up even slightly.