Centennial of Flight 2003: Australian Aeromodellers Tribute

What exactly are Free Flight Model Aircraft?

Free Flight Model Aircraft come in a bewildering array of sizes and shapes depending upon their intended purpose. The two basic types are Indoor Free Flight and Outdoor Free Flight

Indoor Free Flight

Indoor models are flown inside in places ranging from living rooms up to giant airship hangars with the most common places being sports halls (ie the local gymnasium). These aircraft are super light and fly very slowly, some as slow as walking pace! The pinnacle of these types are the F1D Microfilm class.

Microfilm is an extremely thin and light cellulose film which is made by pouring a special lacquer solution onto a tank of water. This solution spreads out on the surface of the water and hardens into a film just as if it were painted onto a wall. Then the builder lifts the film off the water with a wooden frame and allows it to dry before using it to cover the wings and tail of his model. This film is so thin it refracts light just like a soap bubble and shows the most beautiful colours of the rainbow.

Class F1D

F1D Microfilm models, which are about 65 cm (25 inches) in wing span, fly via a long thin rubber band driving a large propellor; they weigh so little (1 gramme... yes, one gramme!) that they use tiny amounts of energy to fly and so the propellor turns at only about one revolution a second and they move so slowly that even small air disturbances can upset their flight (such as a person walking past while they are in the air).

In addition, because they need such a small amount of energy to fly, on one fully wound rubber motor they can stay in the air for up to 50 minutes at a time in a large airship hangar. Even in a small gymnasium, flights of 25 minutes are not uncommon.

Other Indoor Free Flight models include F1L (smaller less complex version of the F1D), Indoor Handlaunch Gliders (you throw them up and they glide to the ground) and Peanut Scale, which are small scale copies of real aircraft.

Outdoor Free Flight

Outdoor models have an even wider array of types ranging from the International Competition Classes to Vintage model aircraft to Scale copies of real aircraft. Most Free Flight (FF for short) competitions are based around the idea of flying several rounds in which you need to achieve a target time or maximum (max for short). For example, class F1A is flown to seven rounds. In each round the Max is normally 180 seconds. If more than one person achieves a perfect score of 7 x 180 seconds, then additional flyoff rounds are flown with the Max increasing by 120 seconds each time. Unlike the first seven rounds which everyone can complete, these flyoff rounds are knockout rounds; that is if you drop any time you are out.

The three main international or FAI classes (the FAI is the World governing body for all aviation sports) are F1A Glider, F1B Wakefield and F1C Power. These classes represent the pinnacle of FF development, both technically and in competitive terms. Technically, the modern FAI FF model is a far cry from the 'stick and tissue' model which many people think of; they are generally highly developed aerodynamically and constructed with the latest aerospace materials such as carbon fibre and Kevlar.

Class F1A

Class F1A is for gliders which are towed to their initial starting altitude on a line of maximum length 50 metres. They are latched to this line and remain on tow until the competitor decides that they are in good air (ie a thermal, which is rising air). During this time the competitor has total mobility and can tow the model anywhere on the field that they choose until they are happy with the air they have found. At this time the competitor rapidly increases the speed of the model on tow (usually with a short sprint). This increases the tension in the towline up to perhaps 10kg and this tension causes the latch on the model to release. At this point the competitor lets their end of the line go and the model zooms upwards with it's very high airspeed and settles generally 5 to 15 metres higher than the towline height. At the moment the towline leaves the model the This high speed launch technique is highly refined and gives a definite performance advantage.

Once released, the F1A model spends the rest of the flight as a pure glider, soaring bird-like in big circles, riding the air currents. In order to avoid losing the model in a thermal, at the time the competitor desires the model to return to the ground an onboard timing device flips the tailplane up to a special angle which super-stalls the wing and causes the model to descend to the ground with the wing acting as a kind of parachute. This process is known as dethermalising the model.

Class F1B

Class F1B, also known as Wakefield (after the Wakefield Cup presented to the World Champion), is the oldest competition class in model aviation. In fact it is the oldest existing Open International Sporting Competition in the world, having been first presented in 1928.

Wakefield is for aircraft powered by 'extensible motors'; ie rubber motors. Current rules allow a 190 gramme airframe to carry a maximum of 40 grammes of rubber, which is normally made of many strands of thin rubber strip about 30 cm in length. This short, thick skein of rubber is wound with a special geared winder to perhaps 450 turns which drives the propeller for about 40 seconds after the model is launched. The aim is to get the model as high as possible during the motor run so that the flight duration potential can be as high as possible. At the end of the motor run, the propeller senses that the rubber has run down and locks it's rotation and the blades automatically fold against the side of the model to reduce drag during the gliding phase.

Modern Wakefields have many refinements compared to their predecessors back in 1928; most have a delayed propeller start, so that the competitor can throw them with the propeller locked and gain several metres extra height before the prop starts; many have complex variable pitch propellers which automatically adjust the blade angle to compensate for the fact that the rubber motor power reduces during the motor run.

Just as for Class F1A, F1B's are flown to seven rounds, the first round target is 210 seconds and all others are to 180 seconds. Flyoffs are run to the same rules as for F1A.

Class F1C

Class F1C is for models powered by internal combustion engines. The engines most commonly used are highly developed miniature 2.5 cc engines. These tiny engines drive a carbon fibre propeller at speeds around 30,000 rpm and produce more than 1 brake horsepower in output. They are allowed to run for only 7 seconds in order to pull the aircraft as high as possible. At the top of the climb the best aircraft can hit speeds of over 150kph.

An onboard timing system shuts the engine down at a little less than 7 seconds, applies a brake to stop the propeller and applies a number of predetermined angular changes to the tailplane and rudder in order to transition the model from a vertical climb position to a horizontal gliding attitude without losing any altitude.

These models are the race cars of the FF world, climbing super fast to great altitude but then transitioning into the magical slow, elegant glide which epitomises Free Flight.

F1C is also flown over 7 rounds, the first being to a Max of 240 seconds and the rest being for 180 seconds. Flyoffs are the same as for F1A and F1B.

Free Flight Competition is more than just having the best aircraft potential. It is a complex tactical and involved activity which demands a wider range of skills than any other sport. Competitors must not only be able to create and tune a high performance aircraft but they need to understand micro-meteorology in order to determine when a thermal is present for the model to ride. The best aircraft in the world will not achieve a Max in a downdraft.

The competitor must also understand what his opponents are capable of and choose his moment to fly accordingly.

The most impressive Free Flight aircraft seem to fly as if they have a pilot on board; rather than just flying in large even circles they bob and turn, sometimes this way, sometimes that, as if they know where the core of the thermal is and what they should be doing. Many times aircraft launched in very bad downdrafts have been seen to fly in a straight line until they contact a rising thermal where they turn again, going up instead of down! This behaviour is not accidental; it is the result of good design principles and careful adjustments although sometimes it seems mysterious.

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