Free flight (model aircraft)
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The essence of free flight model aircraft is that the model's flight is not controlled by the flyer once the model has been released. These models must be aerodynamically stable. If their flight is disturbed by a gust of wind or rising air current, they must be able to return automatically to a normal flight path. This stability is achieved by a combination of the configuration and proper trim of the model.
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[edit] Types
Free flight models may be broadly divided into four categories:
- gliders (tow-line and hand-launched)
- rubber-powered (pure duration, and scale with duration)
- power (Co2,gasoline-powered, or electric)
- indoor (pure duration, and scale with duration)
When flown competitively, the usual aim is maximum flight duration. In the case of models flown outdoors, the modeller attempts to launch the model into rising column of air, a "thermal". These outdoor free flight models tend to be designed for two very different flying modes: climbing rapidly under power or tow, and gliding slowly while circling with minimum fall rate. Much of the challenge in designing and flying these models is to maintain aerodynamic stability in both modes and to make a smooth transition between them.
Models flown indoors do not depend on rising air currents, but they must be designed for maximum flight efficiency, because of the limited energy stored in the rubber or electric power source.
Within each category, there are different classes. Typically, there is an FAI world-championship class, a so-called mini class, an open class, and possibly any number of national or unofficial classes, for which regional or national competitions may be held.
[edit] Gliders
Gliders have no onboard motive power. The only energy inputs are the launch, and any rising air encountered during the flight. The FAI glider class is F1A, also known as A/2 or Nordic glider. The model must have a maximum area (wing plus tail) of less than 34 dm2, and a minimum weight of 410 g. Launch is by hand tow, using a cable of less than 50 m length. The mini glider class is A/1 (F1H). A/1 gliders must have less than 18 dm2 total area, and weigh at least 220 g. Open glider contests are rarely flown, and most competitors in such contests use F1A gliders. Other glider classes include magnet-steered gliders - essentially a free flight slope soaring class, and hand-launched glider (usually abbreviated HLG). HLGs are small models which are launched from level ground simply by being thrown hard. This is one of the more athletic of the free flight disciplines.
[edit] Rubber-powered
Rubber-powered models are powered by the stored energy of a twisted elastic material. These range from the simple rubber-band powered toys available in many toy stores, up to the open rubber class, examples of which often use 200 g of rubber in their "motor". The FAI rubber class is F1B, also known as Wakefield. Charles Dennis Rushing has written on the history of the Wakefield Cup. F1B models may have a maximum of 30 g of rubber motor, and the empty weight of the airframe must be at least 200 g. The maximum total area of the model must be less than 19 dm2. The mini rubber class is Coupe d'Hiver (also known as F1G). "Coupe" models have no area restrictions. The maximum weight of rubber allowed is 10 g, and the minimum empty weight of the airframe is 70g. (need Coupe history here - 100g vs 80g, min cross-section etc). Open rubber is a popular event, featuring large models with enormous amounts of rubber crammed into them. Open models often have 50% of their flying weight composed of rubber. P-30 is a common beginner's event. A P-30 must use an unmodified commercially available plastic propellor. P-30 has a maximum wingspan of 30 inches, and uses 10 g of rubber. The empty airframe must weigh at least 40 g.
[edit] Power
Power models are those with an onboard power source which is not a rubber motor. Frequently this is an internal combustion engine. However, the category also includes compressed gas motors and electric power. The FAI power class is F1C. F1C models are equipped with an internal combustion engine of up to 2.5cc and need to weight at least 300 g per 1cc (i.e. minimum weight of a 2.5cc equipped model is 750 g). These engines are usually custom made for optimal power output and often yield 1 HP at more than 30,000 RPM.
[edit] Indoors
Indoor models are, as the name suggests, models designed to fly indoors. These models are typically very light weight. The international FAI indoor class is F1D. It requires models to have a minimum weight of 1.2 grams and a maximum wing span of 55 cm. They are powered by .6 grams of rubber. Such models require a large space, such as a sports hall, aircraft or dirigible hangar. There is even a salt mine in Romania which has several times hosted the FAI world F1D championship. Times approach forty minutes in the highest sites that are currently accessible.
Although most indoor aircraft are rubber-powered, both gliders and power aircraft (usually compressed gas or electric) are also flown indoors. Some events concentrate on scale or semi-scale replicas of man-carrying aircraft. Others feature unusual flight configurations, such as ornithopters, helicopters or autogiros.
