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White Diamond

An advanced microlight design concept

Part 2 - The design described in detail

Article by Andrew Gibbs

The design solution
Like any design, the final solution arrived at was a compromise between many factors. The whole airframe is designed so that when disassembled it can be carried on a trailer of road-legal width to save on hangarage costs.

White Diamond general arrangement (GA) drawing. This scale drawing gives dimensions and shows the location of all the main parts of the design.

Low induced drag (the drag created by generating lift) is achieved by using a high aspect ratio wing combined with taper, giving a favourable span-wise lift distribution. The joined wing tips are designed to further reduce induced drag by eliminating tip vortices as far as practically possible, the theory being: no wing tips - no wing tip vortices. Safe stall characteristics should be achieved by using a slightly thicker section in percentage terms towards the wing tip, combined with washout.

The wings are of composite sandwich construction, where a strong, thin skin of fibreglass is combined with a very light foam or honeycomb core, whose main function is to stiffen and space the load bearing skins apart. The wing tips are similarly constructed, with the addition of anti-shear ribs. Incidentally, the principle of sandwich construction is nicely illustrated by a well-refrigerated Crunchie bar - the structure is light, stiff and strong!

The wings are split into 4 light and easily manageable panels for easy assembly and disassembly. The upper and lower panels are nearly identical which helps lower production costs.

The fan duct is also of composite construction, being formed from inner and outer shells. The outer shell incorporates integrally moulded upper wing roots. The duct is first fitted to the fuselage using steel tubes which serve both to align the whole assembly accurately and later help maintain its shape in use. Load bearing glass fibre vanes are bonded in place afterwards, completing the assembly. The boom is a hollow filament wound composite structure of comparatively thick wall to carry flight loads and the forces transmitted by the fin during taxiing.

Tailplane and boom
The fin and rudder are deliberately the only parts of the airframe to project into the accelerated airstream from the fan. This was desirable to help provide directional control while taxiing and taking off in crosswind conditions. The horizontal tailplane is detachable and is of entirely conventional steel tube construction. The non-detachable fin projects mainly downwards and houses the tailwheel. The lower fin is of sturdy composite construction since it must take taxiing and landing loads.

Choice of a ducted fan system
Research revealed that microlights were often operated from private airstrips. These present a significantly different operating environment than licensed airfiields. From a safety perspective, I had two main concerns; firstly, loose surface debris is commonplace, and represented a danger to conventional propellers. Secondly, I discovered that since private airfields were unregulated, they permitted anyone to be present near aircraft and consequently were associated with a much greater risk of a member of the public walking into a rotating propeller.

A ducted fan propulsion system offered a solution to these problems, especially as it could be located behind the lower wing. Other good reasons for choosing a fan system are that it is quiet in terms of noise footprint and efficient at the speeds expected - according to the highly regarded reference work 'The Design of the Aeroplane' a fan is quieter and more efficient than a propeller up to about 110kt, after which the drag of the duct itself becomes limiting. Although the calculated maximum speed of White Diamond on 60hp would be around 130kt, its cruise speed would be about 100kt, which would use a fan to best advantage.

Engine selection
I wanted to avoid the use of the then common Rotax 2-stroke engine, both because my research indicated that pilots seemed to experience a lot of engine failures using these, and because of it's environmentally poor total-loss lubrication system. Instead, I chose the twin cylinder 60hp 4-stroke Emdair CF077A. This offered better fuel economy than a 2-stroke, meaning increased range and savings in oil costs, although these advantages were partly offset by the 4-stroke's inherent weight penalty. Alternatively, almost any engine of 45-80hp could be substituted, particularly as the engine is located very close to the CG.

Central to the instrumentation concept is an on-board computer. Inputs to the computer would include the fuel level, flap setting, angle of bank and manually entered inputs such as pilot and baggage weights. These inputs allow the computer to accurately estimate the 1G stall speed, which varies with weight. This value would be displayed as a floating point indicator on the expanded airspeed scale. The computer would also display the flight time elapsed and flight time remaining based on current fuel consumption. Additionally, engine and carburettor temperatures would be displayed, and a warning given if carburettor icing became likely or if engine overheating was imminent. Even today carburettor icing is a significant problem for general aviation aircraft and it seemed to me then that a simple temperature gauge would offer a worthwhile safety benefit.

Controls
A side stick situated on the right hand side of the cockpit operates conventional elevator and aileron controls via cables which run through the duct vanes. The vanes are very strong and are reinforced to maintain structural integrity of the airframe and protect the control runs in the very unlikely event of the fan bursting. The throttle and flap controls are located on the left side of the cockpit. The cockpit is comfortable, being roomy, quiet, fully enclosed and fitted with a padded seat. For summer use an alternative half-length canopy could be fitted for the pleasures of open-cockpit flying. Visibility is excellent during taxi, level flight and turns due to the large canopy and the virtual absence of any airframe forward of the pilot.

Since all of the heavy masses (pilot, fuel and engine) are located around the CG, the machine should be reasonably manoeuvrable in all three axes. The maximum take-off weight is 290kg with an 85kg pilot, 10kg of baggage and a very generous 50 litres of fuel aboard, enough for 5 hours to dry tanks.

A white paint finish was chosen, as the reflective properties of white are necessary to prevent the composite structure from overheating in strong sunlight. Looking at the finished design, the choice of name became very easy!

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