Diesel v avgas – an aircraft with a fine pedigree just got better | Words & photos: Jean-Sebastien Seytre

The Dijon—Darois airfield is bordered at its southern end by a main road. Beyond the road are the workshops of Robin Aircraft.

Robin aeroplanes have been manufactured there since 1957, first by CEA (Centre Est Aéronautique), which became Pierre Robin Aircraft in 1970 (hence CEAPR), which was then bought by Apex Aircraft in 1990.

Of all the light aircraft types produced here, the ‘DR’ models (Délémontez-Robin), evolved from the Jodel models (abbreviated from Edouard Joly and Jean Délémontez), are by far the most iconic.

From the first DR100 to the current DR401 model, these aircraft are all distinguished by their characteristic ‘cranked wing’ outer panel dihedral and their wonderful all-wood construction.

However, rest assured this article is not about the history of the DR400. Rather, it is about the comparative characteristics of petrol and diesel engines within the new ‘401’ range.

The models tested for this comparison are the DR401-160A (F-GLDK) with a 160hp Lycoming O-320 petrol engine and the DR401-155CDI (F-HDSL) equipped with a 155hp Continental Diesel.

The Lycoming O-320 has powered Robin aircraft since the DR250 of 1965. Apex was one of the first manufacturers to adopt diesel technology by installing the Thielert CD135 in the DR400 airframe (the ‘Ecoflyer’).

Apex saw the future of the single engine piston aircraft as being diesel, using universally available jet, military, automotive and biofuels in any mixture, and based its production almost exclusively on this engine. But the bankruptcy of Thielert (caused by fraud, not by any engineering failiure) multiplied costs by 300%, pushing Apex into insolvency in 2008.

After four years of parts-only production at the Darois factory by CEAPR, Robin Aircraft was formed in 2012 to take up the mantle once again.

I flew both aircraft with Gérald Ducoin, test and demonstration pilot for Robin Aircraft. We started with ‘Delta Kilo’. This 160hp Lycoming version has its propeller pitch and wing profile midway between optimal climb and optimal cruise.

There are no wing tanks, so the fuel valve aft of the throttle has only two positions: open or closed. There is an additional fifty litre tank that drains into the main 109 litre tank by simply pulling a plunger.

Note, however, that the transfer rate is lower than the cruise consumption of the engine, so the pilot needs to think ahead! This configuration is a good compromise for mixed training/touring use.

The instrumentation is classic analogue, with the addition of a pair of Garmin G5s acting as primary flight display and HSI. Start-up is standard carburetted Lycoming: mixture fully rich, three pumps of the throttle to prime, and operate the starter with further pumping the throttle, as required.

To reach the runway we have to cross the main road to Dijon?unusual, but pragmatic! I find ‘DK a bit heavy on the feet for a Robin DR, maybe because I have not handled one for a long time. At the R02 hold, we run through the usual checklist, testing the magnetos and carburettor heat.

The wind is almost zero and our takeoff weight around 810kg. Rolling with 15° of flap, acceleration is good and the aircraft wants to fly between 55 and 60 knots. The runway is at nearly 1,600 feet AMSL, and the temperature is 25°C, so the density altitude is about 3,000 feet.

With these parameters, the maximum slope allows us to clear the hill beyond the end of the runway comfortably before making the first turn.

With the flaps up and 130kph indicated (approximately 74kt) our rate of climb is just under 800fpm. This equates to nearly 797fpm at sea level under ISA conditions at the 1,000kg MTOW. Cruising at 3,500ft AMSL at 75% power, we see an IAS of 200kph (about 110kt) for a consumption of around 38 lph (8.5gph).

Landing with full flap, the DR401-160A approaches at 120kph (approximately 65kt) and needs 490m to clear a fifty foot obstacle. Back to the apron to change mounts.

Externally, the diesel DR401 is distinguished from its 100LL-fuelled brothers by a redesigned engine cowl and a three-bladed propeller. More blades means better ground clearance and lower noise. The air intakes have been repositioned and the cowling houses a compact and very effective silencer.

Internally, the fuel system had to be completely revised to feed the low-pressure pump of the Continental Diesel and to return a measure of fuel, heated by the engine, to keep the main tank warm. Otherwise, the structure of ‘SL looks identical to that of ‘DK.

There is no option?and no need?for wing tanks on the diesel version: with a main tank of 109 litres and that additional ‘long-range’ tank of fifty litres, the 155CDI has an endurance of 6.75 hours without reserve.

Transfer of fuel from the long range tank to the main tank is still by gravity feed but, unlike the petrol engine, the rate of transfer exceeds the rate of consumption at 75% power?a comforting safety feature.

Originally, and just like Diamond’s Austro engine, the design of the Continental Diesel CD155 was based on an automotive engine (that of the Mercedes A-Class). Technically, however, the CD135 and 155 are very different to Thielert’s 2.0 and 2.0S.

Although Continental Motors took over Thielert’s assets following its bankruptcy filing, Continental has substantially developed the design. The Continental Diesel engines that now equip Robin aircraft are still manufactured in Lichtenstein, south of Stuttgart in Germany but, compared to the original Centurion 1.7, the time between replacement (TBR) has been increased to 2,100 hours.

Already less expensive to operate than a Lycoming, the improved TBR makes the diesel even more attractive, particularly to clubs and schools.

Adapting the basic design included a turbocharger, a constant speed propeller, and FADEC (Full Authority Digital Engine Control). Propeller pitch and fuel injection are automated via single-lever control by the pilot.

In contrast, the Lycoming versions have fixed pitch propellers and three levers?for throttle, mixture and carburettor heat. Why no fuel injection? Well, there is a demand for mogas in Europe and the fuel-injected Lycoming engines need avgas.

The CDI’s propeller is a three-blade 190cm MT ‘Scimitar’ unit. Despite its hydraulic pitch-change mechanism, it is no heavier than the 14kg two-bladed metal Sensenich that equips the 160A. On the other hand, with a dry weight of 134kg, the CD-155 installation, complete with cooling system, is twenty kilos heavier than the O-320.

But the greater energy density of Jet A-1 more than offsets the additional mass of aircraft and fuel. Also, thanks to diesel torque and the CS prop, the MTOW of the DR401-155CDI is, at 1,100kg, 100kg greater than the 160A’s. It is also fifty kilos better than the 160LR (the ‘long range’ avgas version with wing tanks and a ‘cruising’ wing profile).

This difference is of real importance since, with full tanks, the 155CDI can carry four people while the 160A will be limited to three.

Interior space is the same for all 401 models, as they share the same airframe; the MTOW is determined by engine performance. ‘SL sports a fully glass cockpit: G500 PFD and MFD to the left, back-up Mid Continent SAM centrally, and GTN750 NAV/COM (with remote control of audio panel and transponder) on the right.

This all fits perfectly on the panel, compacting all the essential information directly into the pilot’s line of sight whilst giving easy access to supplementary data for loading approaches and handling comms. Engine parameters (prop rpm, Ts & Ps, and engine load) are independently displayed on a digital ‘Compact Engine Display’. A separate quad display shows fuel volume and temperature, OAT, and battery voltage.

My first flight in a DR400 diesel was ten years ago, in an Ecoflyer powered by a Thielert 1.7 litre engine. The performance from the 135hp engine seemed tepid and the central position of the throttle on a Robin baffled me, although this configuration is standard on most aircraft (as it is now on the DR401).

In addition, I was not used to the minimal effort needed to operate the power control. Now, with more experience, these features did not bother me at all, and at the controls of ‘SL I could enjoy the manoeuvrability, and focus on the pleasure of piloting the wood and fabric aircraft of the Délémontez-Robin line.

Only the humming of the CD-155, more muted than the hoarse sound of a Lycoming and with much less vibration, will remind you that you are pulled by a diesel engine and, despite the added mass of the engine installation, the flight qualities of the aircraft are unaltered, starting with the ground acceleration, which I thought was much more eager than with the DR401-160A.

However, when taxying, the pull of the CS prop, even with the power lever at idle (6 to 7% displayed on the power meter) is significant, and it may be necessary to use the brakes to regulate speed. There is, nevertheless, no tendency to ‘nose down’ in slow flight and the flare is still easy to negotiate.

In short, diesel or petrol, piloting a DR401 remains a pure pleasure. The essential benefits of the CDI versions come from the formidable assets of their power plant, which became clear during my flight.

The FADEC makes start-up easy: fuel pump on, battery master on, engine master on, then turn the ignition key and it starts. The tests for the twin FADECs are just as simple. Hold down the ‘FADEC test’ button and the system cycles through the test sequence: both LEDs light, then one, then the other, then both extinguish and the test is successful.

Reassuringly, during the test cycle you can hear the variation in rpm and see the propeller speed vary between 800 and 1,200rpm.

This time our takeoff mass is around 915kg, 185kg less than maximum. For the first takeoff, Gérald takes the controls. Toe brakes on, full power until the digital display indicates 100% power and 2,300rpm, then off with the toe brakes and the acceleration is impressive, the rotation speed of 55kt being reached in a few seconds.

With 65kt on the ASI, the rate of climb is over 1,000fpm. The best rate of climb is at 78kt when we see 900fpm on the G500 (the flight manual gives 740fpm at sea level under ISA conditions at the MTOW of 1,100kg). In addition, turbocharging helps maintain performance at altitude. So whilst at sea level the 155CDI has only a small advantage over the 160A (at the same mass), the difference widens with altitude as the air thins.

Climbing over Burgundy, Gérald performs a full power stall. With more than 35 degrees indicated on the PFD, the angle of attack gradually increases until the aircraft literally hangs on the propeller until?with stick fully back?the aircraft finally starts to drop earthwards at fifty knots indicated?well below stall speed.

As soon as the cowling passes the horizon with the stick slightly forward, however, the aeroplane recovers and maintains altitude. Even in this (very) unusual flight attitude, there is no tendency to drop a wing, confirming the stability associated with the Jodel-Robin design.

We then level off to look at different cruising regimes. At 75% (2,000rpm), we reach 115kt IAS, or 124kt TAS at a consumption of 24 lph. The CD-155 can also operate at maximum power continuously at any altitude thanks to its constant-speed propeller and liquid cooling. The IAS then settles at around 135kt, but the fuel flow increases accordingly.

Landing from an approach at 65kt, the DR401-155CDI needs only 415m to clear fifty feet with 175m of ground roll (sea level, ISA, hard runway): it’s as capable as a Cessna 172, with its recognised STOL capabilities.

My impression is that, with the DR401, Robin Aircraft has adapted to the requirements of today’s general aviation whilst retaining its strengths. The wooden construction remains the trademark of the aircraft with its disadvantages and (especially) its advantages.

Far from being outmoded, it represents proven know-how, just like the all-metal construction used by some light aircraft models for more than sixty years. In addition, this wood-based manufacturing is part of a sustainable process.

For the diesel versions, the cost of Jet A1, the frugality of the engines, and their ability to run on biodiesel are additional economic and ecological benefits. The last point may be debated on the basis of particulates, although ejected at altitude and well away from people.

On the other hand, petrol engines produce more CO2, and avgas pollutes with lead. First and foremost, the marriage between a high-torque diesel engine, a constant speed propeller and modern, automatic engine management yields impressive performance for a given power.

Although this test compared the Continental 155hp and the Lycoming 160hp because of the similarity of their nominal powers, in fact the DR401-155CDI is closer to the 180LR in terms of performance, particularly taking into account the CDI’s ability to operate continuously at 100% power.

And the 155CDI can also compete with all the other engines in the DR401 range. Thus, it can be used for training at a cost barely higher than that of the 120hp DR401 Lite or 120A petrol engine Lycoming versions thanks to fuel savings.

So the DR401-155CDI is truly a versatile aircraft, built for touring, but also very suitable for ab initio training or glider towing. The 155CDI and the less expensive, but less versatile, 135CDI both offer many persuasive qualities for both flying clubs and individuals.

In addition to their other qualities, Robin aircraft have undeniable advantages over their direct competition: lower price, greater payload, greater range, shorter takeoff and landing distance, and equally at home on grass or tarmac. Currently, Robin Aircraft are producing two aircraft a month with the next eighteen already sold.

While Textron/Cessna has announced the discontinuation of Skyhawk JT-A production, Robin Aircraft believes in the success of its diesel range. I am surprised that Robin Aircraft mentions the lack of Jet A1 at some GA aerodromes as a possible drawback of this engine. In urgent need, you can always fill jerry cans at the nearest petrol station to the aerodrome.

More importantly, in countries where recreational aviation is not so developed, the problem of 100LL supply is not so easily solved. Thus, the diesel engine could create a new market for Robin aircraft in places where Avgas is not distributed.

Beyond the passion of all those craftsmen who are the soul of Robin Aircraft, what struck me when I visited the workshops is that all manufacturing is done on site. Nothing is outsourced except engine and avionics.

In today’s globalised world, where making stuff too often translates into ‘offshoring’, I think this is to be welcomed.

A big thank you to this Burgundian enterprise which continues to produce a flagship of the French aeronautical industry.

Acknowledgements: Robin Aircraft and Mistral Aviation

Image(s) provided by:

Jean-Sebastien Seytre