Thermal Bug (oz2979)

About this Plan

Thermal Bug. Free flight rubber model. This is a modern redrawn plan in PDFvector format.

Quote: "Thermal Bug, By Brian Lewis

The first Thermal Bug was designed in the early part of 1948; it weighed 2-1/4 oz and had a 2-1/2 min motor run. Since then the design has been developed through seven models, and a Wakefield version is being built for the 1953 season.

Mk.2 won first place in the 1950 Southern Cross Exhibition and went on to win two club contests, and win second place in the Southern Counties rally. Mk.3 was lost on its first flight and spent two weeks in a potato field, which wrote it off. Mk.4 was flown in the Northern Heights gala, but after climbing to 100 feet the motor peg slipped out, prop and rubber shot out of the nose, and the rest of the model sank for 30 secs! Model No. 5 won a Southern Cross contest first day out. Flown in the 1951 Bill White trophy it was lost on its second flight, but placed 13th; another flight of 2-1/2 min would have placed it first.

The sixth model was flown in the Farrow Shield and returned top time in the SE Area. Later it was flown in an Icarian/Luton/ St. Albans triangle match at Eaton Bray and placed third.

Trimming the model is straightforward enough. Just move the wing fore or aft to get the glide, then trim the bamboo runners to project 1/4 inch either side of leading and trailing edges. This will make sure the wing position is the same every time the model is assembled. Try not to use any downthrust, but use a little sidethrust if nessessary. A tight right hand turn is best. The Thermal Bug has plenty of spiral stability so do not be afraid of overdoing the turn, provided the model is launched to the left of the wind it will get away safely every time.

Though the first Thermal Bugs were very light, due to influence of indoor flying, the later jobs are made more robust using quite hard balsa. This has paid of in consistency of performance, less repair work and fewer warps.

This type of job makes an ideal contest model. It costs about 3s all up to make and can be built in a week of evenings. Trimming is usually completed in half a dozen flights. By the way, final trimming is done by bending the prop hinge to alter the pitch and obtain the best climb: motor run ratio. This can be altered to suit conditions - fine pitch for gusty weather, coarse pitch for still air."

Quote: "The Thermal Bug is a rubber-powered contest model of 28" wingspan. It was built from plans that I scaled up from an article in the 1952 Aeromodeller Annual. A beautiful floater, it goes up and never seems to want to come down. All up weight is 4oz (113g)".

Supplementary file notes

Article (one page with schematic drawing) from 1952.

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