Flight test: Robin DR400-180
- Credit: Keith Wilson
Tourer and tug ? just two of the many uses to which the remarkable and long-lived DR400 design has been put | Words: Dave Unwin – Photos: Keith Wilson
As Keith signals he wants me to drop lower and ease slightly further forward it occurs to me that, as much as I rate the four-seat Robin Regent tourer, when flying formation I’d rather be in the virtually identical Remorqueur tug, with its better field of view.
The DR400 is one of the most successful designs to emerge from the Robin factory at Darois in the Burgundy region of France. Robin Aviation began trading in 1956, although the progenitor of the company’s first design flew eight years earlier.
In 1948, Edouard Joly and Jean Delemontez first flew the diminutive single-seat D.9 Bébé, which was so successful (I have one!) that in 1950 the two men?who were already related by marriage?decided to form the Société des Avions Jodel.
Jodel, which is a contraction of the two men’s surnames, went on to manufacture several very successful designs which were produced under licence by other manufacturers, such as Robin.
Pierre Robin’s first aircraft was a two-seat Jodel D.11, and from this he progressed to producing several Jodel-designed low wing monoplanes, including the subject of this month’s flight test, the DR400. This classic aircraft first flew in 1972 and is still in production today as the 401.
Indeed, in many respects current examples are very similar to earlier models, for although it can be powered by a variety of different engines and fitted with modern digital avionics, the methods and materials used in its construction remain the same (Sitka spruce and plywood made from Okoumé, a type of west African hardwood also known as Gaboon).
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DR400s are practically bespoke machines and are lovingly handcrafted by artisans. I was lucky enough to visit the factory in Darois-Dijon many years ago and see the quality workmanship first-hand. Robin employs traditional craftsmen, some of whom have been with the company for many years, and this is evident in the very high build quality of the finished product.
Now, some readers, particularly those raised on a diet of Rotax-powered composite aircraft, may be wondering why we’re extolling the virtues of a 46-year-old wood and fabric aircraft. The simple reason?apart from the fact that the basic design is still in production?is that the DR400 continues to outperform many of its contemporaries.
As you approach a DR400 the first thing you notice is the ‘cranked wing’ (sometimes referred to as the ‘Delemontez trapedzoidal platform’) which is also used on several other Jodel and Robin aircraft. This is constructed in three parts, and consists of a relatively wide, flat centre section, dihedral?fourteen degrees of it?and washout (five degrees) being restricted to the tapered outer wing panels.
The net result is a wing that, while very efficient in the cruise, is also a solid performer at the slow end of the speed envelope.
Some pilots claim that the wing generates less drag in the cruise than many other designs, as the aeroplane is essentially supported by lift generated by the centre section (aerodynamicists disagree). When flying at slower speeds (and thus high angles of attack) they claim the tapered outer sections come into play, conferring the Regent with a remarkably wide speed range for a fixed-undercarriage, fixed-pitch tourer.
In truth, the really clever part is that the wing is very easy to build (as the centre-section is constant chord) while incorporating some of the advantages of the ideal elliptical wing, which is very efficient in flight but expensive to build. The cranked wing consists of a single box spar, over which the ribs are threaded, skinned with ply forward of the spar and covered in Diatex fabric.
It has a slight leading-edge droop and uses a modified NACA 23013 aerofoil, with wooden ailerons covered with Diatex and metal flaps. The flaps and ailerons are interchangeable, port and starboard. Power is provided by Lycoming’s ubiquitous O-360 air-cooled flat four, the most produced piston aero-engine of all time.
The two DR400s flown for this test?a 1975 Remorqueur G-TUGZ and G-TUGY, a 1991 Regent?are powered by the same unit, a normally aspirated O-360-A3A, producing 180hp at 2,700rpm. Both feature Chabord silencers.
That’s where the similarities end, for where ‘GZ only has a single, 110 litre fuselage tank and a wooden two blade Hoffman prop, ’GY has two additional wing tanks of forty litres each and a metal two-blade Sensenich prop. As I still fly Golf Yankee regularly (Golf Zulu having been sold a few years ago) this report is predominantly about the Regent. For clarity, the other differences between the two are detailed in the box on page 46.
Access to the engine isn’t great. The oil can be checked via a small hatch, but to do anything else you must remove the cowling. The air intake for the carburettor protrudes forwards slightly, possibly to gain a slight advantage from ram-air pressure.
The fuselage is a wooden semi-monococque box structure covered primarily in plywood, although from any distance away you’d swear it was metal, so good is the finish. The rear fuselage features a gentle taper before sweeping up into the sweptback fin. The rudder is generously proportioned and the Diatex-covered all-flying tailplane (or stabilator, for our readers in America) features metal-covered anti-balance tabs (which also provide pitch trim) on its trailing edge.
A wide track, short-wheelbase undercarriage completes a very elegant-looking flying machine. All three wheels are the same size, feature snug-fitting spats, and even the nose strut’s torque links are enclosed. The nosewheel steers through the rudder pedals while the mains carry hydraulic disc brakes.
Access to the cabin is excellent. Both wings offer a generous walkway and, having turned the roof-mounted handle clockwise through about 110 degrees, the big canopy can be slid forward a fair way. The front seats tip forward for easy access to the rear, while access to the baggage area is via a usefully-sized door/window on the port side, although it can also be accessed from the cabin.
The front seats adjust over a reasonable range and are quite comfortable. Curiously, the seat belts are the rather old-fashioned design that requires the lap strap to be secured and then the shoulder strap attached separately. I don’t like that, but I do like the fact that the DR400 is fitted with control sticks instead of yokes. Indeed, the overall layout of the instruments and controls is excellent.
As well as the sticks?and in common with many French aircraft?there are two throttle plungers, making it possible for the left-seat pilot to fly with either hand. Not only are sticks less complex than a yoke but?certainly in small aircraft?they simply feel better, at least to me. Readers who fly their 172 like a 727 may well disagree! The flap lever is between the seats and has three settings, Up, 15º and 60º.
’GY was built in 1991 and probably represents the zenith of a well-designed analogue instrument panel. There is a comprehensive annunciator panel built into the glareshield, along with rheostats for the cockpit lighting and chunky rocker switches for the external lights and pitot heat. The primary flight instruments are arranged in the classic ‘six pack’ with the main engine instruments directly below in a sort of subpanel.
These consist of a large tachometer, oil temperature, pressure and fuel pressure gauges, and fuel contents for each tank. Also located in this subpanel are the carburettor heat plunger and fuel pump switch, while contained in a small console extending down from the panel and then aft between the seats are a plunger for the parking brake, mixture control, elevator trim indicator, fuel selector and trim wheel. A nice touch is that when the fuel selector is turned to off it covers the start button.
Also on the left of the panel are a basic Century IIB autopilot, suction gauge, chunky split rockers for the battery and alternator, and a rotary switch for the magnetos.
Most of the avionics (a Bendix/King GPS, King radios and a transponder) are on the right of the panel, along with EGT, CHT and OAT gauges and three black-topped plungers that operate the cockpit heating and demist. A few of the vital electrical services are protected by circuit breakers by the pilot’s left elbow, and the others are fitted with fuses on the opposite side of the cockpit.
If I could change any of it (and regular readers will know how I love to tinker) I would move the tachometer higher up so it was just to the left of the ASI, and transfer the important CHT gauge to where the tachometer was, so as to be more readily incorporated in the scan.
The big canopy slides closed smoothly and fairs into the rear fuselage almost seamlessly, which must go a long way to reducing total drag.
A curious omission on all the DR400s I’ve flown is that none has been fitted with a primer. This means you must prime the engine by pumping the throttle, which isn’t always the best idea with updraught carburettors and a wooden airframe. Anyway, a well-maintained O-360 is generally pretty reliable, and on the day of the air-to-air shoot ’GY’s fired second blade.
Taxying is easy; the nosewheel steers through the rudder pedals and differential braking can be used to reduce the turning circle. The field of view is excellent. The windscreen is big with only a small centrepost and the side windows are huge.
Performance figures on the day of the test are essentially irrelevant. With only me, half-fuel and no baggage, ’GY is a long way below MAUW and consequently the acceleration is excellent. I’m airborne in a fraction of Saltby’s 1,200m Runway 25 as the Regent practically leaps off the ground and swiftly settles into a very impressive climb with the VSI indicating more than 1,100fpm at 80kt.
However, even when flown at the 1,100kg MAUW the distance required to clear a fifty-foot obstacle is still only 610m. Flaps up and electric fuel pump off at 500ft, then trim for the 92kt Vy (best rate of climb) and head south to look for the cameraship.
I don’t really like airborne rendezvous as the sky can be much larger than you think. However, I’ve agreed with Rutland Flying School’s Steve Waddy that we’ll meet directly over the centre of the former RAF Cottesmore at exactly 1730.
I won’t be above 2,500ft QNH, and he’s not to fly lower than 3,000ft, until we’re both happy that we’ve seen each other. This works very well, and Keith shoots some superb images over Rutland Water, although at times I find the field of view less satisfactory than the Remorqueur’s full bubble canopy.
Steve’s 172 is fitted with essentially the same engine and propeller as ’GY, yet the Regent is around ten knots faster and also has a greater useful load. Two more reasons for the Robin’s continued popularity.
With all the pictures in the can I break away and continue my assessment. As I do so it occurs to me that, despite flying DR400s so regularly, I’ve never fully evaluated the control and stability either qualitatively or quantitatively, although as the prototype DR400 first flew 46 years ago, you’d imagine any bugs have would long since been ironed out.
A couple of 360 degree turns reveal very pleasant handling and nicely harmonised controls. It’s slightly surprising how well the DR400 performs in roll, as the ailerons look quite unsophisticated.
Another pleasant surprise is how light the rudder is in flight, bearing in mind that the rudder pedals also steer the nosewheel. In fact after takeoff the nosewheel steering automatically disconnects and the airflow then aligns the nose spat with the aircraft’s longitudinal centreline for minimum drag.
Overall the general handling is fine, with good stability around all three axes (it is strongly positive directionally, positive longitudinally, and just barely positive laterally) and has reasonable control forces with acceptable ‘breakout’.
The controls do firm up at higher speeds but this is no bad thing as it makes the possibility of inadvertently overstressing the airframe with an imprudent or abrupt control input less likely. Vne is a respectable 166kt with Va (design manoeuvring speed) an equally good 140kt.
Slow flight is as predictable as the rest of the flight envelope. With full flap and a bit of power the stall warner starts bleating about ten knots before the wing finally and reluctantly quits flying at about 48. At the stall it halfheartedly drops a wing, but simply relaxing the back pressure on the stick returns it instantly to controlled flight.
Turning our attention to the other end of the speed spectrum, at 75% power (2,550rpm) and 3,500ft QNH the TAS is a perfectly respectable 130kt for a fuel burn of around 38 lit/hr. With the standard tank fit of 190 litres, the Regent has a range in excess of 600 nautical miles?an endurance greater than mine.
Bearing in mind that we are talking about a four-seat aircraft with a fixed pitch propeller and fixed undercarriage these are very respectable numbers. Ambient cockpit noise levels and vibration, even at cruise speeds, remain tolerably low. The wooden airframe clearly absorbs noise and vibration better than either metal or composites.
Back in the circuit, the 400 continues to impress, primarily because it is just so easy to fly. The field of view is excellent, and different flap settings produce only minor trim changes. To be fair, if you mostly fly a Cessna 172 the plain flaps will certainly seem inferior, and if you’re ‘as fast as a fox’ then it will float, although the flaps are certainly adequate if the speed is right.
Generally, seventy knots on final feels good in ’GY, and even in strong, gusty winds speed control is easy and I’ve never experienced any difficulty in maintaining the correct approach speed.
Bearing in mind that most of the ‘in production’ types I test for Pilot are factory new demonstrators, and that ’GY is 27 years old, you might be wondering how well it stacked up. Actually, extremely well. Indeed, some older readers would probably prefer the traditional analogue instrument layout, and the cockpit is comfortable, well ventilated, adequately heated and quite spacious.
What don’t I like about it? Very little. The pilot’s harness is poor, I don’t like not having a window that opens (or at least a DV panel), and the stick is perhaps a bit tall (although that opinion is entirely subjective). There isn’t a separate friction device for the pre-tensioned throttles, and although this isn’t an issue when tugging it can be in the cruise. Apart from these rather minor nit-picks, it really is difficult to find fault with the Regent.
So what about the practicalities of owning a wood-and-fabric aeroplane? Doubtless some pilots are anti-wooden aircraft, but in my opinion this is prejudice, founded on myths and half-truths. While wooden aircraft built sixty years ago using Casein glue should certainly be viewed with suspicion, the introduction of synthetic glues and fabrics such as Diatex, Ceconite and Oratex have ensured that a modern wooden aircraft can be as tough and durable as either metal or GRP?and also lighter.
Another reason why ‘wood is good’ is that it doesn’t have a ‘memory’ (and consequently DR400s are not blighted with a Fatigue Index or ‘Service Life’). I have heard of Remorqueurs owned by French gliding clubs that have logged a phenomenal number of hours and tows.
I was impressed with the capabilities of the DR400 the first time I flew one and, having now logged a reasonable amount of time in them, nothing has made me revise my opinion. With a useful load of 500kg and equally good numbers for range and endurance, the Regent is a true four-seat tourer.
Fast, comfortable and economical, it is an unpretentious aircraft with fine handling and simple systems that can lift a good load from a short strip. It’s no surprise that more than 3,000 have been sold, and that the design remains in production today.