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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.

 

First flown in 1951, the initial French D.3 version, was powered by a 25 hp modified Volkswagen engine.

 

 

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.

 

  

 

 

Experience
Turbulent F-PGYQ, which belonged to the designer, was kept in the Hurel-Dubois experimental hangar at the Villacoublay subsidiary airfield.

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.

 

 

Some early standard Rollason-built D.31 were powered with 30 hp Ardem or Porsche engines. The Rollason D.31A developed version featured an improved main spar.

 

The first British built Turbulent, G-APBZ, made its first flight at Croyden on January 1, 1958. 

 

Gallery

 

 

Engine: 25 h.p. Volkswagen dual ignition
Prop: two-bladed wooden
Fuel: 73-octane or premium car petrol
Wing span: 6 m 55 cm (21ft 6in)
Length: 5 m 25 cm (17ft 3in).
Consumption cruising: 7.5 litres (1.7 gal) an hour
Max endurance: 3 hours

Engine VW 45hp.
Gross weight: 620 lbs
Empty wt: 395lbs.
Fuel cap: 8.75gal.
Wingspan: 21’7”.
Length: 17’6”.
Wing area: 77.5 sq.ft.
Vmax 109mph.
Cruise 100 mph.
Climb rate: 450 fpm.
Ceiling 9000 ft
Range 250 sm.
Take off run: 500’.
Landing roll: 450’.
Seats: 1.

Engine: 1600 VW.
Cruise: 80 mph.
Climb: 500 fpm.
Range: 300 sm.
Span: 21 ft 7in.
Length: 16 ft 6 in.
Empty wt: 380 lbs.
Max wt: 620 lbs.

Engine: Ardem 4002, 30.7 h.p.
Span: 20’ 2”.
Length: 17’ 4”.
Wing Area: 80.7 sq. ft.
Empty Weight: 410 lb.
Loaded Weight: 660 lb.
Wing Loading: 8.2 lb/sq. ft.
Max. Speed: 95 mph.
Cruise Speed: 75 mph.
Stall Speed: 45 mph.
Initial Climb: 450 fpm.
Range: 340 miles.