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Flight test: Diamond DA40 NG

PUBLISHED: 15:49 12 June 2015 | UPDATED: 15:52 12 June 2015

Diamond has got the pitch stability and elevator weighting absolutely right

Diamond has got the pitch stability and elevator weighting absolutely right

Archant

Shining brighter: powered by the 168hp AE300, the ‘New Generation’ DA40 Star promises more performance than Diamond’s previous diesel-engined singles

Shining brighterShining brighter

This magazine’s first encounter with the DA40 was late in 1998, when James Gilbert − then Editor of Pilot − sent this writer over to Austria to fly the prototype. Hard to credit today, at the time it was all so new that Diamond had not even settled on a name for its new baby, a fresh four-seat development of the DA20 Katana. Nor had what they eventually decided would be called the Diamond Star been fitted with the production engine: displaying ‘Katana DA40’ graphics (see what I mean about the name confusion) the aircraft I flew for what would be the first British flight test of the type, OE-VPE was powered by a 125hp IO-240B.

Diamond had fitted the earlier proof-of concept machine with one of the 121hp turbocharged Rotax 914s that equipped production Katanas − and had already discovered by the time the Pilot man turned up that 120-odd horsepower was not going to be enough to drag the new bird over the fence, at least in the style paying customers might expect.

Lightly loaded for a magazine test flight that would be more about handling than carefully-measured performance, and aided by cool conditions and a decent breeze down Weiner Neustadt’s long tarmac runway, the prototype went ‘quiet nicely,’ as I wrote in the flight test we published in January 1999. As Diamond well knew, on a hot day and with four on board, that same aeroplane would more likely have made a hole in the hedge− which is why the production version would emerge with a 180hp IO-360. While the 125hp prototype was said to climb at 740 feet per minute (a figure that I am not sure I would still credit) Diamond was projecting a much more acceptable 1,070fpm with the bigger engine (today, the book figure for the current 360-powered model is 1,120 – so that one at least was a fair claim!)

The relevance of all this engine power/ climb performance stuff will become apparent later: what all models of DA40 share – right from the rather beautifully finished prototype of 1998 to the dieselengined ‘New Generation’ (NG) tested here – is divine handling that distinguishes them from almost any other touring aircraft. I might have been naïve about real-world performance in the past, but the hours I have flown in various DA40s since that first encounter have done nothing to change the vivid impression from that first flight test: ‘control harmony [is] every bit as my earlier experience with the DA20 had led me to expect. This is the sort of aeroplane that begs every excuse to be wheeled around from turn to turn for the simple pleasure of it.’

Power inside the cockpit - Garmin displays can be switched to show almost any conceivable combination of flight instrument and navigation infoPower inside the cockpit - Garmin displays can be switched to show almost any conceivable combination of flight instrument and navigation info

In a rather Germanic matter-of-fact way, Diamond’s then Vice President, Sales and Marketing Michael Feinig had declared that the company’s aim in producing the DA40 had been to ‘build the flying equivalent of the latest VW Golf’. If he intended to provoke with this line of masterly understatement, he succeeded. ‘I would say that in handling terms the DA40 is a BMW 3-series to a paragon like the Extra 300’s Caterham Seven,’ gushed Whiteman in print – but substitute 2015’s equivalent of the Beamer and it’s a comparison that stands: the DA40 remains a real sports saloon among four-seaters.

It’s also a very safe one, not merely on subjective judgement of its fine handling and benign flying characteristics (the Diamond Star’s ‘utterly benign stall behaviour is very J3 Cub-like − not at all what I’d have expected from a glass ship with a semi-laminar-flow wing’) but also on statistics, accumulated over more than a decade of operations. The record reveals a rate of just ‘one fatal accident per 1,000 aircraft-years... the best in the industry,’ according to Diamond ‘and a record over three times safer than that of some competing composite aircraft’. (Gosh; who could they be aiming at with that?)

If you do have the misfortune to stuff a DA40, passive safety measures including aluminium fuel cells protected by the dual carbon fibre spars, a 26G rated ‘safety cell’ cockpit and airbag seatbelts make it all the more likely you might walk away from the crash. More Volvo than BMW…

Adjustable rudder pedal assemblies incorporate toe-brakes. Ground handling is excellentAdjustable rudder pedal assemblies incorporate toe-brakes. Ground handling is excellent

New Generation?

The missing link between today’s ‘New Generation’ DA40 NG and the gasoline burning prototypes and initial production aircraft is the DA40 TDI. This aeroplane won fame as the world’s first dieselengined light aircraft to be built in serious numbers and a degree of infamy, at least in its early days, for the unreliability of its Thielert Centurion power unit. (There was a lot of very public finger-pointing in both directions between engine manufacturer and airframe builder, one saying the installation was at fault the other pinning the problems squarely on the power unit.)

Thielert’s original idea of adapting a car engine nevertheless had lasting merit. On the road, diesels had been recognised for their fuel economy − if not their power and refinement −for years. From the late 1980s onward, adding a turbocharger (a deceptively simple looking, lightweight exhaust-driven supercharger) had transformed the contemporary automotive diesel engine, giving it the kind of power one would previously have expected from a petrol engine. Other more subtle refinements to injection systems and basic engine ‘architecture’ had further improved the power to weight ratio of the passenger car diesel engine to the point that putting one in an aeroplane finally became viable.

Knowing that the development of an aero engine from scratch would likely be prohibitively expensive, and keenly aware from his motorsports background that the mass-produced car engine represented state-of-the-art technology and was available off the shelf at what was by aviation standards a knock-down price, Frank Thielert concentrated his company’s effort on designing a geared propeller drive with an integral vibration damper. The original engine manufacturer’s name was never made public − those who know about these things reckoned it was a Mercedes, as used in that company’s A-class vehicles. Nor did we discover exactly how much Thielert’s converted unit weighed, as installed in the DA40. What we did find out was that it only produced 135hp−just ten to fifteen horsepower more than the Rotax 914 and IO-240B that Diamond had found wanting in 1998.

The Andair fuel selector and nicely geared mechanical elevator trim are excellent featuresThe Andair fuel selector and nicely geared mechanical elevator trim are excellent features

No surprise then that the DA40 TDI (T for turbocharged, DI for direct fuel injection, by the way) was a bit of a slug in getting off the ground. My own first experience of this ostensible four-seater’s lacklustre takeoff performance was in a flight from Blackbushe. Two-up, the owner and I enjoyed a far closer view of the car auction site off the end of the main runway than either of us was really happy with. Still; once the aeroplane was off the ground it was clearly the fine-handling thing of yore − and it zipped along quite nicely, with impressive economy.

Thielert became Centurion and the new company has continued to develop its diesel engines to a high level of reliability and greater power output. Diamond, meanwhile, set out to develop its own line of diesels, establishing Austro Engines− and it is of course an Austro that you find under the bonnet of the NG. Austro describes the unit as ‘a liquid-cooled, in-line four-cylinder engine with double overhead camshafts’. It has four-valve heads and a very high pressure, ‘common rail’ direct fuel injection system, controlled by duplicate electronic units. Austro still aren’t saying whose engine the AE300 is based on, but our motoring correspondent contributor Colin Goodwin is convinced it is again the job you’d find under the bonnet of the latest A-class Mercedes.

The headline figure is that 168hp output, which takes the two-litre AE300 close to 5.9 litre Lycoming IO-360 power at sea level. However, in contrast to the way the naturally-aspirated Lycoming progressively runs out of breath as you climb, the turbocharged Austro maintains sea-level power all the way up to 10,000ft. Austro’s published literature shows the power output of the two engines being equal at 3,000ft, the AE300 then steadily opening up an advantage that widens to forty horsepower or so at 10,000ft – at which altitude the IO-360 is producing only 140hp.

Advantage diesel? Not quite: one thing Austro does own up to is the weight of its power unit. The AE300 tips the scales at what we would say is an honest 185kg (414lb) ‘wet’ (it is important not to neglect the mass of coolant and radiators when considering liquid-cooled engines). Depending on which variant one is looking at, the published weight of the air-cooled IO-360 is 300 to 330lb; approximately 100lb (45kg)−or the equivalent of one child passenger − less than the diesel engine’s weight. This more or less accounts for the difference in useful load between the NG and the IO-360 powered DA40. Developing ten or twelve horsepower less at typical UK airfield elevation and carrying relatively small weight penalty, you might expect the modern diesel version to get off the ground and climb away much like the petrol-burner. Let’s find out…

In accomodating the more powerful AE300 diesel, the cowl has been reprofiled and raisedIn accomodating the more powerful AE300 diesel, the cowl has been reprofiled and raised

Like mounting a horse

Getting into the DA40 is rather like climbing up to mount a horse: there are stirrups just ahead of the wingroots − much like those you’d find on many European Very Light Aircraft and US Light Sport Aircraft, but feeling more of a solid job − and recessed hand-holds moulded in the glare shield. As the wing isn’t that high off the ground and the cockpit sides are low, it’s easer than the equine equivalent and indeed most other light aircraft − and, helping keep mud off your bottom, you can board the aeroplane without standing on the seat cushion.

Rear seat passengers will appreciate having their own gull-wing door, and the extensive glazing lends the cabin an airy feel with nothing of the hemmed-in feeling one suffers in the older generation of US trainer/tourers − the usual suspects still to be found at most airfields.

The firm seats might make you wonder initially how comfortable a perch they’d offer for any long flight: when you arrive at destination without having given this a further thought − as I have now done many times −you realise Diamond’s seats are close to perfection. Similarly, the short control sticks beg the question of whether they’ll offer sufficient leverage: they do, and they have been very nicely engineered in giving the right mechanical advantage and full control movement without bumping into the legs or jabbing you in the tum with full up elevator.

The turbocharged Austro maintains sea level power all the way up to 10,000 ftThe turbocharged Austro maintains sea level power all the way up to 10,000 ft

Getting the Garmin panel set up can feel like a degree-level challenge to vintage aviators like this writer. I am assured it’s all easy-peasy, once one has made the effort to work through the computer-based tutorial but I haven’t had this luxury. Happily, today’s demo pilot, Mark flies G-SUEO regularly and has done his homework. The ease with which he summons up one screen after another, providing a five-minute guide to the impressive capabilities of the unit, is apparent. Once you’ve mastered Garmin glass, it is an extraordinarily powerful tool that makes a significant contribution to both ease of operation and flight safety.

No one should need any special training to operate the turbocharged engine and variable-pitch propeller. The single-lever control is intuitive and simple. Thinking pilots have been questioning the need for separate mixture and propeller controls for years. One of the reasons they’ve endured for so long is the feeling that you are not a ‘proper pilot’ if you can’t learn to handle such complexities. Given that inadvertent mishandling of mixture and prop levers – all too easily confused in the heat of the moment – will at best give you at fright and at worst kill you, I am all in favour of Diamond’s ‘forward for go, back for slow’ technology. This − and the way the duplicate engine control systems self-test −must play its own part in the DA40’s impressive safety record.

So how does the NG perform? The takeoff run begins with a great surge of acceleration − a real kick in the back − that recedes as speed builds, the character of the aeroplane changing rapidly from sports saloon to economy car as we progress down the runway: that BMW has metamorphosed into a wretched Fiat 500. Although we float off as the ASI needle touches sixty knots, I’ve been warned that it is all too easy to get on the back of the drag curve and advised not to rotate until we reach seventy, and this I duly do.

I have also been told that the aircraft will sink when the flaps are retracted, Mark’s usual habit being to leave them at the takeoff setting until he reaches 700ft. Not quite believing this, I flick up the switch− neatly click-stopped at retracted, takeoff and full positions − at 500ft, which rather proves his point: the modest rate of climb is washed away very noticeably as the flaps retract and I make a note that this particular idiosyncrasy is not to be forgotten if you want to make drama-free departures and don’t wish to cause unnecessary consternation among your passengers.

Once the speed has reached eighty knots or so, the rate of climb is good. It is also quickly apparent that general control feel and coordination are very nice indeed: there is no need to grasp the stick, but rather grip it lightly between thumb and forefinger as you would do when flying a DHC Chipmunk. The precise amount of longitudinal trim can be dialled up in fractions of a second using the beautifully geared mechanical trimwheel, falling to hand nicely in its location between the seats. Diamond has got the pitch stability and elevator weighting absolutely right for the pilot to be able to hold speed and attitude without giving them a thought. The ailerons are just as sweet, but I am perplexed by the need to hold on right rudder during the climbout. It had also taken quite a lot of rudder to stay near the centreline on takeoff: this is by no means unusual among Rotax-engine lightweights but is not something you generally come across in what we still would still most conveniently term ‘Group A’ aeroplanes. Even in the cruise, gentle right foot pressure is required. Mark thinks this particular aircraft may be misrigged: I would say that if it was actually typical of the type, I’d be asking Diamond to fit the aircraft with a rudder trimmer.

The view over the nose and to the sides is excellent. A good part of the potential view directly upwards through the canopy bubble has been sacrificed for the sake of some shade − a sensible compromise, even if having some kind of retractable blind would give the option of a better outlook when there’s no sun to worry about − or even when flying formation! Indeed, this activity was next on the menu, and what a pleasure it is to be doing it in such a biddable machine.

If light aeroplane handling and manoeuvrability can be rated on a scale that runs from ‘bomber’ to ‘aerobatic’ − as exemplified by the Extra 330, the DA40 sits well towards the aerobatic end (although we should stress it is not designed or cleared for aeros). It really is one of those aeroplanes you almost think into position. The controls are light − but not overly so− effective and nicely coordinated. Breaking off to complete some stalls reinforces the conclusions from my first Diamond Star flight fifteen years ago: in the upper air, this aeroplane is a paragon of fine handling and benign flying characteristics.

The approach and landing are slightly different, due to the aerodynamics and geometry of the tail end of the DA40. Best practice is to fly the aeroplane right down to the hold-off, avoiding the usual progressive round-out. This goes against instinct for dyed in the wool taildragger pilots like this writer and feels strange by normal tri-gear standards. However, it is done in the DA40 to avoid the speed being allowed to bleed off to the point that an unhealthy rate of sink develops. The aircraft should be allowed to ‘float on’ and not be put down in a tail-down attitude, for fear of walloping the tail bumper.

Living up to the promise?

So does New Generation diesel Diamond Star deliver all the performance the more powerful engine promises? I have to say I am mildly disappointed in this respect. Sporting a three-blade, VP propeller as it does, it really ought to take off and climb out better than experience or even the book figures suggest. On the other hand, it will no doubt sustain a decent rate of climb to higher cruising altitudes, where it will offer significantly better fuel consumption (as low as 21 litres per hour − and on cheaper jet fuel) than the ‘avgas’ model. As it is, whichever engine you chose, the DA40 remains very fine aeroplane − it’s just hard to escape the feeling that, fuel economy aside, it’s still going to be even better with an IO-360 up front.

Enjoy this article? Read more about our flight tests here.

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