Chris decided in early May 2001 to build a giant scale model of one of the classic aircraft, the J3 Cub. There are quite a few 33% scale models in the LMA, but Chris had a Transit van to fill so he decided on a 50% model.
Chris Gale with his 50% J3 Cub.
Specification of the model is:
Wing Area: 45 sq. ft.
Length: 11ft 5in
Engine: 3W 150 Power Master Boxer Twin
Propeller: 32″x10″ carbon fibre.
The model outline was based on the 25% scale plan by Balsa USA. The structure was changed to meet the needs of the larger model. A conventional balsa and ply construction has been used. Ribs were made from 1/8″ liteply. Spruce was used for the spars and longerons. The main spar is 1 1/8″ x 1/2″ and there are 1/2″ x 1/2″ sub spars.
The wheels are ones typically used on stack trucks. The undercarriage was designed and manufactured by Mike Jackson.
The electrical equipment includes two HiTech 805 servos for each aileron. This was over engineered, but Chris wanted some redundancy included in case of a servo failure. Each half of the elevator uses a HiTech 700 servo; one also being used on the rudder. Two Futaba PPM receivers are used each powered independently. Failsafe operation was provided by an SM Services system. Four, 6V, 4AH gells cells were used to provide the power. These weigh 7lb and are placed beneath the ‘pilot’.
The model was covered in Diatex 1000 which is a material used on full-size microlites and gliders. After covering, two coats of 50% thinned dope was applied followed by cellulose grey primer to provide a good and opaque base for the sprayed synthetic yellow.
You can see that Chris’ model uses a Super Cub type cowl. Chris said that many full-size owners used this cowl to help protect the engine from the elements; especially if the aircraft was stored outside.
The model was completed in December 2001 and test flown in March 2002. Chris reports some problems with the first flight caused by the large wooden propeller originally used. This was loading the engine too much resulting in a short flight with an enforced landing. This was cured using a carbon fibre propeller. The PPM receivers were also picking up some interference. Chris cured this by quadrupling the length of the aerials. The model is now a very stable and graceful flyer. Just perfect for calm summer evenings!
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