F1D "microfilm models", which are about 55 cm (22 inches) in wing span,
are powered by a long thin rubber band driving a large propellor.
They weigh so little (1 gram... yes, one gram!) that they use tiny amounts of energy to fly and so the propellor turns at only about one revolution a second.
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.
Model aircraft which can only be flown in an enclosed space and which are powered by
extensible motors and in which lift is generated by aerodynamic forces acting on surfaces
remaining fixed in flight, except for changes of camber or incidence.
Maximum wingspan of the monoplane model aircraft: 550 mm
Maximum chord of the lifting surfaces: 200 mm
Maximum tail span: 450 mm
Minimum weight without rubber motor: 1,2 g
Maximum weight of the lubricated rubber motor: 0,6 g
The competitor shall be allowed 6 flights of which the best 2 flights will be taken for classification. If the organisers specify rounds for the competition then the competitor is entitled to one official flight in each round. The duration of rounds must be announced in advance.
Only flights of 60 seconds or more will be considered as official. A flight may be terminated by
any physical means within the first 60 seconds. A flight of less than 60 seconds duration will be
considered an attempt and there will be one attempt flight allowed for each of the six official
flights; the attempts will not be accumulative.
A model for starters: de Moustique [860 KB]
. A tutorial (in dutch) [579 KB]
is also available.
Models for experienced indoor builders: the Free Spirit [336 KB] (cat. I), the Kagan [7 KB] (cat. IV).
A lot of plans, tips and articles on IndoorDuration.com