Druine D.31 Turbulent
The Turbulent is an ultra-light single seat, low wing monoplane designed by M. Roger Druine in the mid-1950s, the Turbulent is normally fitted with a 30 h.p. Ardem or VW conversion, although engines up to 45 h.p. may be installed.
M. Roger Druine standing before the Turbulent
The fuselage is a rectangular box structure having four longerons, wooden frames, a curved turtle deck and plywood covering. The empennage structure is of wood, the tailplane and fin are plywood covered, the elevator and rudder being fabric covered. The wings are a one piece, two spar structure; the front spar having laminated spruce booms and plywood webs to form a box. A diagonal drag spar is also fitted between the front and rear spars. The ribs are of open lattice type, the aerofoil section being NACA 23012. A built-in fixed slot is located at the leading edge forward of each aileron. The leading edge is plywood covered, forming a D-nose torsion box, while the remainder of the wing is fabric covered. The main undercarriage is of divided type and uses coiled springs for shock absorption. An 8.8 Imp. gallon fuel tank is installed behind the firewall. The airframe being non-aerobatic.
Powered by a 25 h.p. Volkswagen air-cooled car engine fitted with dual ignition and a two-bladed wooden airscrew, the Turbulent runs on 73-octane or premium car petrol, consuming 7.5 litres (1.7 gal) an hour whilst cruising, and has an extreme endurance of three hours.
This is an aircraft designed to be built by amateurs with no more than a reasonable knowledge of carpentry. Complete plans were on sale in France for 5,000 francs, and the materials bought from several firms there. The main difficulty lies in the engine. The Volkswagen is recognized in France but not abroad. (M. Druine intended to fit a 44 h.p. Lutetia vee-four and to fly his Turbulent over to England for a Coventry meeting). So far, 126 sets of plans have been sold all over the world and more than 20 Turbulents have been completed in France. M. Druine calculates that the materials cost about £150, the engine about £90, and that 800 man hours are required for construction.
In 1957 Rollason Aircraft & Engines Ltd was finding that sales of recondi-tioned Tiger Moths were dropping and Norman Jones was looking for another low-cost aircraft for the company to manufacture. He decided that it should produce a licence-built version of the Turbulent and, after introducing some mod-ifications, started to produce the aircraft in 1958. Over 30 were eventually produced by Rollason.
I climbed into the cockpit, fastened the lap-strap, lowered a pair of borrowed goggles and listened while M. Druine bent down and explained the cockpit to me. There were six instruments, from left to right: A.S.I. (km/hr), insensitive altimeter (metres), magnetic compass, rev counter, and V.S.I. (m/ sec). A ball-type slip indicator (below the compass), a magneto switch, throttle, stick and pedals completed the appointments.
I was told to cruise at 2,600 r.p.m. (maximum 2,800) and that was all. The airscrew was swung, and I taxied carefully out. The cockpit is just large enough, the view in all directions excellent and, although the undercarriage is a little hard, directional control is all that one could desire with the engine set at 1,600 r.p.m. The rudder is then effective and the speed over the ground about 4 m.p.h.
At the end of the grass runway I performed some Vital Actions. There was in fact nothing to do but (on this occasion) to turn the aircraft down-wind. I opened the throttle and it accelerated smoothly without any appreciable swing. The tail came up after 40 or 50 yd and after about 200 yd I flew the Turbulent off. I was immediately assailed by a series of sharp bumps and climbed away carefully, avoiding a hangar roof only to hit a violent bump over a road. But the little aeroplane rode the bumps well without any tendency to "fall out of one's hands." An aircraft of this size is naturally greatly affected by turbulence, but the Turbulent is eminently manageable under such conditions. The cockpit was quite warm and only one's shoulders and forehead were exposed to a gentle slipstream. At 2,600 r.p.m. the speed built up to 110 km/hr (60 kt) indicated (120 km/hr, or 66 kt, actual). Stick forces are very light and although depending on the amount of fuel and the weight of the pilot, the trim will vary, the effect is never enough to cause any discomfort. There is a large P.E. on the A.S.I. and 16.5 kt indicated is about 27 kt actual.
I tried a slow run at 35 km/hr (19 kt) indicated with the nose well up and, into wind, almost succeeded in hovering relative to the ground. There was no decrease in control effectiveness and, on opening the throttle, cruising speed was rapidly regained. I next tried to turn the aircraft by taking my hands off the stick and holding one arm out sideways. This resulted in a slight yaw but no change in lateral attitude. One felt very free and cool; this would be the ideal occupation on a hot summer's day.
Next I tried a stall. This is quite remarkable. I cut the power, held the nose a little way above the horizon, and waited. At no more than 28 km/hr (15.3 kt) indicated the Turbulent nodded sharply forward without dropping a wing; immediately the nose fell below the horizon the speed rose to just below 40 kmlhr (22 kt) and, being then unstalled, the nose came up again with the stick still held back. As it passed the horizon it stalled and nodded again-and so on for five or six stalls with the stick held back and without any sign of yaw or wing-drop. 1 tried it into wind and cross-wind with exactly the same results. The Turbulent simply "went downstairs" like a well-behaved child in a strange house. These characteristics are probably due to slotted ailerons and leading-edge slots forward of them, and to a very smooth plywood-covered wing leading edge.
I had to make the approach down-wind over a wood which was causing some vicious bumps. I settled down with a trickle of power at 70 km/hr (38 kt); despite the following wind, the angle of approach was quite steep and I had no difficulty in touching down by the caravan. It required very little round-out and there was not much float, and at no time did one lose a good view ahead over the nose. The landing run was about 150 yd.
Engine: 25 h.p. Volkswagen dual ignition
Engine: VW 1500
Cruise: 78 Kt