Flight Test

It is occasionally slightly odd how some snippets of information arrive at your own doorstep. As an example, take this case when searching for a little bit of background information on the Turbi. Patrick Caruth, always generous to a fault, gives of his time to fly his BN Freelance as a photoship for Neil to ‘air’ his camera for our team effort flight tests. He lent me a book quite some time ago and having ‘refound’, while looking for another, settled to read about Britten Norman Ltd., The Last Grand Adventure in British Aviation? by Derek Kay.

The only three-engined aircraft I had on my licence was the Trislander, an Islander with an extended fuselage and a third engine attached to its fin. A novel solution indeed. So I settled down to learn more about the company’s projects and maybe to glean a little more on the Trislander.

Very soon into the second chapter there was a rather surprising revelation. In 1956 Desmond Norman, one half of the partnership, had arranged with both the Popular Flying Association, and with Associated Rediffusion Television programme This Week to film the complete construction of a Druine Turbi! There was no other information, but it sent me down a rabbit hole… or two. It was perhaps G-AOTK, or a very appropriately registered G-APFA, as both commenced their builds in 1956, the year of the broadcast. Project numbers PFA 230 and 232. ‘Oscar Kilo had a 65hp Czechoslovakian Walter Mikron 3, an engine whose four inverted inline cylinders blend with the fuselage lines of the Turbi, whereas Foxtrot Papa had the arguably more mechanically balanced four cylinder horizontally opposed Continental A65-3 with its familiar, but rather pronounced, cylinder cooling ‘eyebrows’ that protrude, catching the slipstream.

Design origins

Roger Druine was born in France and his surname, if you remove the final letter ‘e’, indicates his ancestors were boiler makers, true engineers. Starting from age 16, he produced many designs and various subtypes, but he is noted for three principle aircraft designs. His most popular design, the single-seat Turbulent is constructed of wood with a VW engine. The first one I saw up close was Nigel Lemon’s, who incidentally started building it when still at school aged 14. A subtype with a strengthened spar gained a full Certificate of Airworthiness here in the UK. Nigel must have been in his mid-twenties when I was fortunate to meet him, and later test flew his Turb’ from the active rural Hertfordshire farm strip at Benington. It was delightful, nimble, light to handle and of course, well constructed.

Above The Turbi has functional, but handsome, lines, flattered by it’s elegant colour scheme.

Below The ‘Working End’ displaying the contrasting tones of the wooden prop and the engine cooling eyebrows.

The two-seater bigger brother, Druine D5, was an expanded tandem two-seater version, and again the fuselage was constructed of a wooden rectangular box, having four longerons as wooden frames, and a plywood covered turtle deck, again the tail, fin and tailplane were strengthened by a ply covering. The low cantilever wing is a one piece structure with two spars. All control surfaces are fabric covered, having a fixed main undercarriage and tailskid. Although much later, it was again at Benington I first saw a Turbi from afar whilst joining their circuit.

The third aircraft of Roger Druine’s was the Condor D 60 series (61, 62, 62A, 62B and 62C, each with a change of engine and increased power). It was manufactured both in France and in Britain by Rollason at Croydon, and with its closure, then at Redhill. This too was the then home of the late Tiger Club who among many activities had a formation display team of four Turbulents. As an aside, Prince Philip also enjoyed flying this single-seater.

Rupert’s Turbi

The Turbi we tested carries serial PFA 229, registered just ahead of the Rediffusion televised construction and was undertaken by Dr Frank Roach, a Glaswegian GP and John Raye a cabinet maker, who’s very skills must have been a great advantage in an all-wooden project. Rupert Hibberd, ‘Bravo Oscar’s owner for the last 30 years, has, from its build, a host of documents and supporting letters from Harold Best-Devereux, who, at the time was one of the ‘top brass’ of the PFA and well connected with the EAA. The Turbi caught Rupert’s attention at the 1977 Sywell Rally. He thought she fitted the bill as ‘a delightful looking aircraft’, and ‘a simple, easy to look after machine’.

That long wingspan was reminiscent of a Klemm 25/ BA Swallow, the only thing not to his taste was the blue, red and yellow colour scheme. Twelve years later he had the good fortune to acquire it, and has owned and operated her for 30 seasons since.

That vintage similarity is not difficult to appreciate and its large wing area of 145.3 square feet provides a very light wing loading of 7.5lb per square foot. This is just a smidgeon under the Tiger Moths 7.6, and being so lightly loaded, theoretically the Turbi has a slightly better turning circle.

Rupert had built time on Austers – he owned two –whose characteristics certainly gave a good appreciation of deft rudderwork to tame its wayward, adverse yaw. A less attention-demanding Jodel with a C-90 followed, then a DR 1050 Ambassadeur with a Continental 0-200 and a DR1051M with a 105hp Potez 4E.

The very name ‘Turbi’ derives from Monsieur Druine’s earlier single-seater the Turbulent, which of course appears partially in the D5s naming, naturally enough with its success a two-seater version was called for, thereby getting the two place the friendly suffix of ‘bi’ added to the shortened Turbulent, giving us Turbi.

The aircraft in Rupert’s own words is not a true two-seater. Yes, it has two cockpits, but full fuel and full cockpits can invariably place it out of limits, one item has to be reduced. However,as an aircraft for a summer evening, he says it’s hard to beat. Although not the ideal travelling machine, he has made numerous trips to la belle France, where he was welcomed and helped on many occasions.

The Turbi lives at his family airstrip near Devizes, the place where Rupert grew up and has flown for 40 odd years. It’s 520 metres by 30 and is oriented 260/080.

Walk around

At 22ft 6in long, with a wingspan of 28ft 7½in, the aircraft is finished in a smart green and cream scheme. The wing is a NACA 23012 section and is notable by having pronounced differential ailerons, the downward deflection being about four times greater than the upward movement. This large movement does not allow for any fabric sealing from aileron to wing. Also the significant weighted leading edge frise ailerons are directly behind the outer wing leading edge slots promising to give good low speed handling. I couldn’t detect any visible signs of washout to aid a gentle stall, but this is rarely obvious to the naked eye. At the wing tips there are hand holds for ground handlers to have some purchase while pulling and pushing, whilst turning and taxying.

The simple undercarriage uses rubber blocks in compression. Rupert assured me they are more than enough for the job. The brakes are cable operated, while

Far left The generous tail of the fabric-covered large elevators and rudder, and rather give the low performance of the Turbi away. It’s not the dimensions of the surfaces, but the widely spaced stitching holding the polyfibre securely in place. The faster the projected machine’s performance, the closer the stitches. Perhaps you remember Biggles diving his Camel away from the enemy and being concerned that his wing fabric might balloon? Closer stitching might have solved that… W E John’s only inferred the problems and not the result… it was apparently a bad thing. The wooden 72 x 46 prop with its alternating light and dark wood laminations was made by Dave Silsbury, a name synonymous with several Turbi projects.

Left The nose profile shows the downward sloping, converging rear eyebrows to cool the hotter rear cylinders.

The tank capacity is 10 imperial gallons (45.5 litres). At cruise Rupert reckons the Continental 75 burns 18 litres an hour at 2,150rpm, giving an airspeed of 70kt. In these circumstances a two-hour flight would give a still air carburettor was overhauled by Steve Hodge of SOAR Aviation and Rupert says it ‘revitalised’ the aircraft, with much easier starting.

An uncluttered cockpit

As a straightforward VFR machine, the cockpit of ‘Bravo Oscar is simplicity itself. You clamber aboard from a right-hand wingwalk, a door for each cockpit making things easier. The rear baggage area behind the rear cockpit can be checked. Home to a ‘wooden’ (of course) fuel dipstick, oil, and some small wheel chocks. Putting the straps aside with a bit of forethought so as not to have them misplaced once seated, you have to stand on the seat then lower yourself down. Your lower legs sitting over the upright rear spar as your feet rest on the broad wooden rudder pedals. You can tell Roger Druine liked wood – a glance around the internal structure, and it’s displayed in its entirety for you to examine.

For the left-hand first, there’s the throttle lever, which I recognise as being the same item in a Tiger Moth. There’s a tell-tale inboard arm sticking out that catches the Tiger’s mixture lever when the throttle is closed, pulling the two levers back together. However, on the Turbi there is no mixture control.

The black carb’ heat lever is a plastic T-handle that is easy to grab. These lowered powered Continentals were quite prone to icing particularly over dewey grass on wet mornings with low power settings.

A green rod beneath the throttle that rotates from 12 o’clock to 3 o’clock, turns the fuel tap from off to on. Activated by a black extension that has to be swung forward to engage the rod, I liked it as it’s unlikely ever to vibrate off due to its mechanical advantage staying open. Comms with the outside world are via an 8.33 Icom handheld radio, and there’s a two-place intercom for conversation between pilot and passenger.

The instrument panel offers little chance of confusion, with just a compass, ASI, altimeter and a slip ball. The ASI though has the lowest speed marking of any I have seen, at 20kt. Having thought briefly about it, we all need to know if our own ASI is indicating before becoming airborne. If our own wings lift us into ground effect at under 40, whatever the value, of knots or mph, then we need to know it works before we lift from the runway. In our case, a needle off the stops and pointing at 20 is good news.

On the left of the main panel, there’s an rpm gauge, while on the right there’s oil temperature and pressure gauges. Keep things 100F to 225F, and 30-60 psi to stay healthy!

A circular wiggle around the cockpit with the stick confirms the ailerons and elevators rise as the stick points towards them. “Is there a cockpit trim… and where have you hidden the mag switches?” I ask. Rupert’s reply, “There’s a fixed tab on the right elevator… and I thought you flew Tiger Moths? Same place.” I found them outside on the left-hand fuselage so that the prop swinger could see them too.

Straps and helmet tight, chocks in place, fuel on, mags off, throttle closed, Rupert sucks in for 16 blades (seems a lot of swings but he knows his machine). He mentioned to me earlier that in the winter he uses one of his vet’s 20 mil’ needles of fuel squirted directly into the carb intake for a consistent cold start. Mags ‘On’, and the engine fires on the first compression.

For the first time I actually realised that, despite the fact I am in the rear cockpit, the view is particularly good, from 8 o’clock 4 o’clock. The bonus is that I can very nearly see over the nose. Looking through two sets of plexiglass windscreens allow a good view forward and only the upper edges of the eyebrows and a sliver of the engine top mask a minute area of the horizon.

Taxying is quite straightforward in technique as the toe brakes are effective, but bear in mind the aircraft has to be moving fast enough to overcome the drag of the skid. She is not short coupled or ready to spin on the proverbial pre-decimal sixpence. Her natural ground environment is, of course, from a strip, tarmac will wear the dragging tailskid with friction. A certain amount of considered anticipation is needed.

A run-up confirms the mags operation a good carb heat drop. The pre-take-off checks are minimal, but as always still so very pertinent and my straps are… tight. A further round the houses with the stick checking all the control cables are still attached. A self-brief on emergencies on departure… and the Turbi and I are off.

The ground roll is perhaps 200 metres with 2,100rpm. The first 50 with the stick held back, the skid in the grass and a pressure of slight right rudder as I counted to four, the stick eased forward to raise the tail to allow a view of the runway’s end and the world ahead. The aircraft lightens up, easing into ground effect. There was no conscious rotation, once held level on two wheels at a natural attitude, she flew herself off, very similar to levitating. Nothing exciting, just delightful.

Above right In this foreshortened view, the Turbi could almost be mistaken for a Turbulent!

Below Rounding out in the flare, about to meet her shadow.

The best climb speed is 55kt and Rupert expects around 500 feet per minute at 90% all up weight and for the Permit climb flight, 461ft per minute at the all up max weight of 500kg having taken 130 seconds to climb in still air the statutory 1,000ft gain up to 2,000ft. On such days he tends to strap a secure bag of bricks into the front cockpit rather than trying to find a light passenger to achieve an all up weight. I’ve been told both sandbags and bags of potatoes work well too…

At the top of the climb the speed increases to 70kt with 2,150rpm, giving us a splash over 20 miles to the gallon in old money. ‘Bravo Oscar will reach 85kt at 2,300rpm, which is wide open throttle, but we’re just out having fun. To manoeuvre, the Turbi doesn’t require muscular rolling inputs or heavy feet, but encourages a gentle, appreciative touch. Rolling into turns and maintaining a consistent bank emphasises empathy and coordination. She does exhibit a touch of adverse aileron drag, but sitting so far back in the fuselage, any slight yawing departure can be instantly detected and corrected. It’s neither a racer, nor a long range traveller, and she allows you to feel and taste the air. Light turbulence has her reacting to gentle bumps, rather than fight them and accept them for an easy attitude to life. Directionally she is stable, a look at the size of the fixed fin and rudder and their distance from where the weight of the crew, fuel and engine definitely suggested that.

Laterally she doesn’t trouble herself to be too prompt to return from crossed controls to wings level after side slipping, but she does at least return. Upset her from a trimmed cruise speed (Induced longitudinal pitching), and she’ll return to the same speed and nearly the same

At the edge of its slow speed regime, approaching the stall, the ailerons are still effective in raising and lowering the wings. This is helped by the built-in slots on the outboard leading wing edges. Air is being channelled through and helping an active, positive response. All very good stuff. When once in France he was told, well warned in no uncertain terms, of a pilot who taped these off before flight, with absolutely horrendous results, as the behaviour of the old girl became intolerant in roll, with an immediate beefy need to try and steer her! I can’t imagine why he did it… The stall arrives at 30kt. A light unloading of the forward stick gives minimum height loss and power can be brought in steadily. Don’t rush to whackit on, it might cough.

Bringing her home

At the other end of the envelope, the ‘book’ gives Vne as 115kt, but I didn’t go there, as with the turbulence being generated by Hambledon Hill and other Dorset friends to the south, I thought better of the demonstration. It would be bad manners of me to bring back a much-loved airframe creaking like an old sailing vessel.

The approach is flown at 55kt, slowing to 50 over the hedge to round out and float… float… float. Slowly bringing the stick back, touchdown happens eventually in a three-point attitude. A tiny bit of ruddering held her straight without the need for brakes. Thinking about it, this is perhaps only the second aircraft I have the pleasure to fly with a fixed tailskid. A short debrief, and a ride for Neil followed. His white crash helmet in the front cockpit now compromising my view forward. I really did have to swing the nose, clearing the way ahead. On landing I make a 10° offset approach to keep aiming point in sight. To make things simpler I wheeled it on to maintain an improved view of the world ahead.

The time spent with Rupert and his Turbi was a pleasure, broadening my experience of Roger Druine’s designs, too. Having topped up Rupert’s tank, drunk a coffee and eaten most of another packet of dark chocolate biscuits, I thanked him again, swung his prop, placed the chocks behind him in the cubby hole and off he went. My chance to watch her levitating into her element, Rupert’s extended arm waving.

A truly delightful Druine. ■

Druine D5 Specifications

Wingspan: 28ft 7in

Length: 22ft 6in

Wing area: 145.3 sq. ft.

Empty weight: 617lb

MTOW: 1,100lb

Wing loading: 7.57lb / sq ft.

Aerofoil: NACA 23012

Engine: Continental Motors C75-12

Power to weight ratio: 14.6lb per hp.

Fuel capacity: 45.5 litres

Cruise speed: 70kt @ 2,150 rpm @ 18 litres an hour consumption.

Over optimistic still air range to dry tanks: 175 nm

Top The pilot has a fine uncluttered scanning view. Those undercarriage legs appear unfairly unsubstantial, but work just fine.

Above A gentle formation break showing clean under surfaces.