Gym Dandy (oz14520)

About this Plan

Gym Dandy. Radio control indoor biplane model, for electric power with geared coreless WES Technic 5-24 motor.

Direct submission to Outerzone.

Update 25/4/2023: Added article, thanks to RFJ.

Quote: "A truly practical, fully functional radio controlled, electric-powered indoor flier. There's nothing marginal about the performance of this one! Gym Dandy, by Dave Ridgway.

The ready availability of 'micro' radio equipment at an affordable price has made the flying of radio controlled models indoors a practical proposition. However, the stumbling block for such electrically powered models has still been the heavy (relatively) batteries needed to cope with the high current demands of many small motors.

This has been addressed by the use of a highly efficient 'coreless' type electric motor, fitted with a 6.25:1 ratio gearbox to reduce the current demands by a factor of at least three! Obviously, this allows the use of smaller cells, leading to a reduction in overal weight. But also, the low current demand means that the smaller cells on the market which can thus be used and which are not designed for high current demands, have an easier life.

What is not so obvious is that this motor/gearbox set-up requires more cells (6-7), so where's the advantage? Surely, you may ask, this means more weight! Well, yes, but it also means that you can use a BEC speed controller and eliminate the separate receiver battery (and switch). So the net result is a saving of about 50% on the battery weight.

The model was designed around certain parameters - wing loading, overall size and weight being the main criteria. Although it is generally considered that the lower wing on a biplane is quite inefficient, would such be the case at the very slow speeds at which an indoor model would fly? The results of experiments with a parasol monoplane layout tend to indicate that the extra wing is well worth having because in monplane configuration, flying speed was considerably increased.

A wing loading of less than 2 oz/sq ft was targeted, making for an all-up weight of 5 oz. The Mark 1 prototype came out at 4.2 oz (with 7x 5mAh cells) and 4.8 oz with 6x 120mAh cells. To get down to these weights, is not difficult, but you MUST use selected wood. The model's structure has been so designed to enable the lightest density wood to be used and except where mentioned otherwise, MICRO X indoor grade balsa was used exclusively on the originals, but it is quite possible to select from stock wood.

Construction: As most of the construction is conventional and quite traditional, I will only cover relevant aspects of the building process that require special attention.

Wings: The 'U' shaped leading edge forms the main strength of the wing, so it is important that this component is executed soundly. A piece of stock 1/4 in leading edge section is used as a former around which is wrapped 1/32 sheet to form a 'U' channel section. Achieve the wrap-around by soaking a suitable sized piece of 1/32 sheet in water for about 1/2 hr. Then, place on the former and wrap with 1-2 in bandage. Dry thoroughly, remove from former and cut to length - it is easier to make four shorter lengths than two long ones!

The ribs are best made by the slicing method - cut a length of 1/16 sheet to exact length of ribs, using a 1/16 ply template. Cut top surface - measure leading and trailing edge depths and cut bottom surface. Spar slots are made with a small file, whilst holding together a block of the ribs, five at a time (one panel). Shape the trailing edge before construction and pack up at the front over the plan for camber. I glued the ribs into the leading edge first, then pinned these over the plan and built the wing in normal manner over the plan..."

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