What's a Robin DR400 with a Rotax 912iS engine like to fly?

The basic concept is simple: a fuel-injected Rotax 912 iS Sport installed at the front end of a DR400 airframe. The objectives are clear: a reduction in fuel cost; less noise; easy access for maintenance; higher safety; reduction in CO2 emissions; simple operation; plus better cross-country performance. In a nutshell, a reliable aeroplane at an affordable price – an aeroplane that is economical and reduces noise for neighbours.

A number of people were already working along these lines, but French aircraft engineer Gilles Aurensan has actually done it.

By nature, Gilles is reserved, but he is readily forthcoming when talking about this subject. Together with fixed-wing and helicopter private pilot Karine Baillou, they run aircraft maintenance company Nogaro Aviation.

For the last 10 years, Gilles noted how French aero-clubs had suffered under the increasing price of fuel and noise complaints from neighbours. Further, although VLAs (very light aircraft) looked promising, they were often  found to be less robust when used in the club environment. So, when the fuel-injected Rotax was announced in March 2012, Gilles at once saw an opportunity to match the well-proven DR400 wooden airframe with a quiet engine combining economy with reliability.

Gilles said, “We had both the DR400 fuselage and a Rotax available. It seemed to us that the Rotax iS was a modern design, so we worked towards creating an interface between the two with the minimum of changes.”

Since the Rotax with accessories is 60kg lighter than the Lycoming, they had to move the engine forward to keep the centre of gravity in the right place. Gilles decided to develop a Supplemental Type Certificate with EASA, permitting the modified DR400 to continue under its current certificate of airworthiness. He hired certification specialist Velica SAS to do the STC, and Bordeaux engineering consultants Innovamech to design an intermediate engine bearer.

He explained, “We decided to keep the original Robin engine mount and the same Rotax fittings, linked by an intermediate bearer moving the Rotax about 20cm forwards to keep the C of G in the right place.” 

The intermediate unit has been made stronger than necessary to take an engine delivering up to 150hp and weighing 150kg – Gilles is already working on the installation of a turbocharged, injected, 135hp Rotax 915.

The final installation still leaves space for add-ons, and mechanics like the fact that, with the cowling off, access for maintenance is easy. Gilles has now reduced the empty weight by nearly 100kg (60kg on the aircraft structure, and 40kg less fuel for the same maximum endurance).

Nogaro is a small regional airport in the Armagnac region of France, and on the ramp stands DR400-120 Dauphin F-GORZ. It’s hard to believe the aircraft has already flown 10,500 hours, because it looks to be brand new. But then I should add that Nogaro specialises in rebuilding Robin aircraft.

Ever since its first flight powered by the Rotax iS on 14 December 2015, ’RZ has progressed with no problem to complete about 15 hours and 30 test flights. At present, the aircraft has completed the testing for the STC, and Nogaro is in the process of completing the EASA certification paperwork.

An engineer’s improvements

The cowling is more rounded than that of the DR400, not unlike the Aquila, with discreet air inlets, and immediately sets it apart from its Lycoming-powered siblings. The 912iS Sport graphics and green colour on the aircraft highlights it’s both an environmental improvement and more economical than its predecessor.

An access hatch for the oil dipstick also allows visual inspection of the coolant expansion tank. Its location means no more crunch noises if it’s left open when you slide the canopy forward, as could happen on the original. Definitely something imagined by an aircraft engineer!

Romeo Zulu has a Duc Flash-R three-blade, ground-adjustable prop. Says Gilles, “Rotax engines don’t like heavy props. This one will soon be certificated and gives excellent results.” On the left, the Rotax exhaust is a little stub and extends a few centimetres from the cowling; there’s no longer any need for an unsightly silencer extension in the quest for a quiet aircraft.

In the cockpit, there are a good number of differences. The aircraft is no longer a 2+2 machine, but a three-seater. This is a deliberate choice by Gilles, who wanted to reduce the chance of errors with the centre of gravity or gross weight.

The rear cockpit has been laid out for flying school use. The DR400 rear left seat has been taken out and replaced by a baggage locker/map table with a USB socket for a GPS or iPad to stay charged. The panel is still Robin-familiar, but the mixture and carburettor heat controls have gone (though there is a manual alternate air control) – long live fuel-injection!

Also gone are the EGT, CHT, oil temperature, oil pressure, fuel pressure and tachometer instruments, replaced by a single EFIS, an EMU 912 made by Stock Flight Systems, which will be installed as standard by Nogaro Aviation on all Rotax retrofits.

The screen is colour-coded green, yellow and red, signalling normal and abnormal values. There are no more magnetos, instead there are two buttons to activate the ignition, protected by a cover to prevent inadvertent operation during cockpit entry or egress.

Although it might be redundant on the Rotax 912 iS (two generators and two regulators) Gilles fits a third backup alternator to supply an additional circuit to the computer and in an emergency to power the radio and landing lights, if for example the main power should fail during a night flight.

Gilles said, “In this aircraft a reliable electrical supply is particularly important, which is why there is no compromise on circuit-breaker quality, the wiring and connectors.”

The key activates the DR-Rotax master switch and therefore ensures that withdrawing the key will eliminate the risk of a flat battery for the next pilot. The key cannot be removed unless the master switch is OFF. An emergency electrical supply is operated by a switch under a red cover, and in the event of a failure of the main system will maintain electrical power to the ignition from the battery.

The engine will continue operating, even if the generator should fail. The battery ensures more than 30 minutes continued operation in the event of total failure of electrical circuits A and B, allowing the pilot good time to return to a local airfield or make a precautionary landing.

Head to head

We’re lucky to have the DR400 Rotax iS and a classic Lycoming-powered DR400-120, kindly loaned by the Béarn flying club. Our tests will not be flown at max weight (which would be unfair to the Rotax Robin) but at the mission weight.

This puts both aircraft in exactly the same configuration: one pilot, one passenger and four hours of fuel. At this point, I must emphasise that the performance figures quoted in this article are not definitive because we were flying a prototype that is still subject to modification. Nevertheless, the differences are unlikely to be significant.

After we had ‘burped’ the oil of the Rotax by turning it through, we moved to a new fire-up procedure: key turned to ON, check battery connected, the EMU self-tests, and after ten seconds the parameters are displayed.

Main fuel pump ON, the two ignition circuits ON (A and B); the red indicator lights light up and go off after three seconds. The fuel pressure is in the green between 2.8 and 3.2 bar.

The display gives an indication of the optimum power setting for start-up: magic. Push the starter button, same position so covered when the fuel is switched off, the engine fires up. The sound is incomparable with that of the Lycoming as we sit at 2,500rpm until the oil temperature is past 50°C.

Our flight test with the Rotax Robin was with two pilots (83kg and 78kg), and four hours (43kg) fuel giving a take-off weight of 735kg. We had 21kg less fuel than the Lycoming Robin, which for this comparative test had two pilots (83kg and 68kg), four hours (64kg) fuel, for a take-off weight of 813kg. For the same mission, the Rotax Robin take-off weight was 78kg less than that of the Lycoming Robin. The Rotax Robin was fitted with 15kg of test flying instruments, an extinguisher, and the crew weighed 10kg more than the crew of the Lycoming Robin.

In flight

Lined up on R32 at Nogaro, we would try to compare the take-off distance between the two aircraft. The Rotax DR was off in 220m to clear a 15m obstacle in less than 500m. Climbing at 78kt, with an indicated rate of climb of 720fpm. Take-off with the Lycoming felt more laboured. We estimated the take-off run to be 340m and 500m to clear a 15m obstacle. Rate of climb indicated 620fpm. During the test, the Lycoming was fitted with a standard 56in prop that is the same as that fitted to three-quarters of the DR400 2+2 aircraft registered in France.

At 75% cruise power the Rotax Robin was 10kt faster, while consuming five litres per hour less than the Lycoming Robin. The good take-off and climb performance, plus faster cruise speed, are primarily due to two principal factors: the reduced mission weight, and the Duc propeller, which delivers virtually all the available power (at least 95%).

The 56in Sensenich prop fitted to the Lycoming stagnates in the range 2,300/2,450rpm and does not resume full power delivery until reaching maximum cruise (2,800rpm). The Duc propeller has an additional virtue: its natural torsion makes for a constant-speed-like effect enabling it to maintain a regime close to the nominal before ‘releasing the brakes’ at speeds up to the cruise at 75%.

It would be a mistake to believe that we are comparing a 100hp engine with a 118hp engine. The flight manual for the Lycoming O-235 L2A used in this test says full power isn’t developed until it reaches 2,800rpm.

Take-off power is delivered at 2,250rpm, at which point the engine is only giving 100hp. This puts it in perfect competition with the Rotax, which at 5,500rpm (the equivalent reduced to 2,260rpm), gives 97hp.

We did a number of stalls on both aeroplanes, with the result that there was a difference in favour of the Rotax of only one or two knots. The Rotax Robin has a lower wing-loading, but that is hardly significant.

It stalls clean at about 45kt, 43kt with one notch 43kt and 42kt with full flap. The approach to the stall is identical in both aircraft. However, with its significant weight reduction there is a difference in the handling; the Rotax Robin feels more responsive and playful with better aileron and elevator response than the Lycoming Robin.

Circuits and performance

Flying circuits, the Rotax Robin used less than 12 litres per hour, as opposed to about 20 for the Lycoming. One should note that because of normal club rules, student pilots are required to fly the DR-Lycoming fully rich below 5,000ft, which accounts for the additional fuel consumption.

Further, use of the carburettor heat also puts up consumption by three litres per hour. But with the fuel-injected Rotax the mixture is always perfect in all phases of the flight. Sensors measure the exhaust gas temperature, and the cylinder heads on each cycle (this enables computers to calculate the exact amount of fuel required for each combustion, e.g. 2,500 injections/min at 5,000rpm).

Said Gilles, “On the iS we inspect the plugs at each maintenance interval, and they are always impeccable.”

With 109 litres full fuel on the DR-Rotax we would have been able to fly circuits for 8hr 30min, or seven hours cross-country. An aero club instructor could fill up once in the morning and then work all day without refuelling.

Gilles plans to maintain a stock of spare parts and provide a maintenance kit for scheduled maintenance. His philosophy is to apply automotive principles to light aircraft.  “When arriving at the end of service life, the engine change will be done in record time. There will be no comparison with the downtime required for a Lycoming.”

Great combination

The first DR400 flew in early 1972, powered by a 180hp Lycoming; 44 years later its exceptional aerodynamic qualities render it suitable to take a lighter and more modern engine.

This STC goes in the right direction: safety (three seats, elimination of carburettor icing); mechanical efficiency (more efficient combustion, reduction in maintenance costs); flying characteristics identical and better performance (bigger usable load, greater endurance, reduced take-off distance).

The adaptation of the 912 iS to the DR airframe makes a great combination, perhaps only to be improved if you were to add a Garmin 500 into the panel. The reduction in fuel consumption and its ability to use a variety of fuel grades are invaluable, and the cut in noise levels is a matter of survival for some airfields.