Sorrell SN7S Hiperbipe (oz8083)

About this Plan

Sorrell SN7S Hiperbipe. Radio control scale model. For .20 power. Scale is 1/8.

Quote: "IT IS SELDOM that a full size cabin biplane is designed with a full aerobatic capability as part of its specification but, the Sorrell 'Hiperbipe' (High PERformance BIPlane) fulfils this requirement. Aircraft designed to perform flick manoeuvres, vertical rolls, lomecevaks and other advanced aerobatics, tend to be utilitarian, with the overall comfort of the pilot being a secondary consideration. Not so with the 'Hiperbipe'. The interior of the cabin is extremely well appointed with more than generous seating space for the pilot and his passenger, afforded by the extra wide fuselage. With a touring speed of 150mph, a stall speed below 50mph and ample power from the 180hp Lycoming engine for short take-offs, this biplane really does combine the virtues of a high speed tourer and a sporting aerobatic aeroplane.

To achieve the high standard of specification, set by himself, the designer, Hobie Sorrell, introduced a number of features that, if not exactly unique, are not to be found on the average club aircraft. The fuselage was designed as a lifting body, hence the constant width, aerodynamically shaped, fuselage.

Negative stagger wing style, used so successfully on the Beech series of biplanes, assists in the upwards vision and the straight leading edge of the top wing gives a good reference for aerobatic manoeuvres. All four wings are fitted with full length 'flaperons' ie ailerons that are capable of being drooped to give flap effect, whilst retaining their use as ailerons. To incorporate so many unusual features, in such a unique design, would be outstanding in any circumstances, when one considers that this is also a genuine 'home build' aircraft the achievement become all the more amazing.

There is no doubt that many 'Hiperbipes' will be constructed in the near future and this should offer the modejer more scope with regard to colour schemes. For modellers requiring additional information on this delightful little biplane - it only has a wing span of 22 ft 10 in - plan packs are available from the Aeromodeller Plans Service, PO Box 35, Bridge Street, Hemel Hempstead, Herts price 35p plus 10p postage. Quote Plan Pack No 2990.

I must admit that, despite the obvious attractions of the 'Hiperbipe', the aircraft was not high on my list of priorities for modelling. The list was, as always, quite extensive and it was only through the continuous prompting of my son, Andrew, that I set to and started drawing. As there had already been .40 and .60 powered models of the aircraft designed I opted for a smaller, one eighth scale, design suitable for the popular .20 engines. The hotter versions of this range of engines should give the model a sprightly performance in keeping with the prototype. No problems were encountered during the drawing up stage, but it was fairly obvious that the fuel tank and radio equipment would have to be positioned well forward to maintain the correct balance point. All up weight and wing loading - the same difference - is important with small models.

Ironically, the smaller a scale model becomes, the slower it must fly to give a scale impression. Conversely, the smaller the scale of the model, the harder it is to achieve the low wing loading necessary for good slow flight characteristics. The weight of the radio control equipment remains as a constant and the weight of the power unit is not proportional to its capacity. We must, therefore, keep a close eye on the construction of the model - selecting balsa wood densities with care - and the finishes. It is very tempting to go overboard with the finishes on a model of the Hiperbipe type, but restraint is necessary. The prototype model was covered with heat shrink white film and trimmed with 'Fablon' turquoise self-adhesive plastic. Total weight less fuel, was 3lb 5oz.

Wing construction features a built-up, fully sheeted, structure and, because of the narrow chord, no main spar is included. Ailerons are only fitted on the top wing to simplify the control installation and the flaperon feature of the original aircraft is not reproduced. It would be possible to use foam wings but, if you decide to use this method, keep them as light as possible by using balsawood skinning. Wing interplane struts are housed into semi-circular cut-outs in wing ribs and require no other fixing, they will 'knock out' in the event of a hard landing. Windows to the cabin area are not glazed but painted black, or covered with black plastic. Glazing the windscreen and cabin windows may be feasible, as the fuselage sides are reinforced with .8mm plywood, but, you are on your own if you decide to do this - it will need some modifications to former F3 and the top wing fixing.

A glass fibre cowl was used on the original model, formed from a balsa plug and female glass fibre mould. For modellers not wishing to produce their own glass fibre, or balsawood and plywood, cowl a stout vacuum formed plastic cowl can be supplied for a cost of 90p plus 35p post and packing. Available from DB Models, 3 East Street, lrchester, Northants NN9 7BG.

With the small ground clearance, when wheel spats are fitted, the model is more suitable for operation from tarmac or paved strips, unless hand launches are attempted. For the average club flying field conditions, it would be wise to dispense with the spats (no doubt some of the full size aircraft will do likewise), but this does detract from the sleek appearance of the model.

Ideally, the spats should be formed from glass fibre and a method of producing these, using silicone rubber moulds will be described in part two of the constructional article. Before you commence construction of the model, you should work out the radio equipment installation and the engine..."

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