Flying Diamond’s D-Jet

Diamond was first to announce development of a single-engine jet, and the airplane is a delight to fly.

It was odd that, as I taxied out in the number three prototype of the Diamond D-Jet, the last thing on my mind was that there is only one engine. As with any other business jet you can’t see the engines, and the sounds and sensations of one jet engine spinning away are the same as two or three. This thing is a real jet, not half a jet.

The sensations of takeoff were also like those in any of the dozens of other jets I have flown. I wasn’t sure how strong initial acceleration and climb would be, but nobody will be disappointed. It was easy to get close to 3,000 fpm initial climb rates even though the day was hot and muggy and Diamond’s home airport at London, Ontario, is at nearly 1,000 feet elevation. The slight but steady vibration and hum of the engine you expect in a jet were just right.

The third D-Jet conforms pretty closely to what Diamond expects to be the finished airplane. It is the first to have the new Williams FJ33-5A engine capable of as much as 1,900 pounds of thrust, about 200 pounds more than the version in the first two prototypes. Serial number three has no interior or environmental systems installed, but the aerodynamic shapes are close enough to the final that useful performance and flying-quality data can be collected.

Diamond announced development of the D-Jet in 2006 and had the original proof of concept airplane flying shortly after. Serial number two had a number of aerodynamic changes and conformed quite closely to the expected final, but now number three, with the production engine and redesigned engine-air inlets, provides a useful glimpse at what the production D-Jet can do.

Because the airplane is a prototype and does not have pressurization, Diamond test pilot Howard Judd and I were in full military-style flight suits, helmets, oxygen masks and parachutes. The airplane had been cleared to fly at its maximum cruise airspeed, and preliminary low-speed and stall testing had been done, but we were restricted to the normal flight envelope by the Canadian authorities with a non-essential crewman — me — flying the airplane.

There are several locations for the engine on a single-engine jet, each with its own pluses and minuses. Diamond elected to go with a centerline location, feeding air into the engine through inlets in each wing root via a Y-duct. The center engine location puts the thrust line close to the so-called waterline of the airplane, so there is little concern about power changes causing pitch forces. Among the negatives of this engine location are some ram air losses in the duct, a greater chance that foreign debris will be kicked up into the inlets and the need to heat the duct to prevent ice formation.

The Williams engine, like all recently designed jets, is electronically controlled by a full-authority digital engine computer (fadec). And, as expected in real jets, the fadec has dual channels and four independent sources of electrical power plus the ship’s battery. The fadec automates starting and also limits engine thrust so the pilot can move the throttle to any position – including full forward – without over-speeding the engine.

About the only time it crossed my mind — other than when looking at the fuel flow reading – that there was only one jet behind me was when preparing for takeoff. In twin-engine jets you are required to calculate how much runway is necessary to either stop or continue the takeoff with sufficient engine-out climb in case one engine quits at the worst possible moment. In the D-Jet the pre-takeoff calculation is like any other single. Is the runway long enough, and where will I set it down if the engine quits? The D-Jet has such low drag with no propeller and such an efficient design that it can glide for more than 65 miles from its 25,000-foot ceiling, and the test pilots have confirmed that a turn back to the runway for landing is easily accomplished from 1,000 feet agl or higher.

With all the flight test gear installed, two of us with all our gear, and 1,000 pounds of fuel, the D-Jet weighed 5,500 pounds for takeoff. Rotation speed is 85 knots, and the airplane leaves the ground instantly when you apply back pressure instead of rolling along on the mains for some time as most larger jets do. The target airspeed for the 50-foot-high point was 100 knots, which I blew right through because the airplane was accelerating so briskly. The best rate of climb is about 160 knots.

The D-Jet controls feel completely natural and just as I expected. I would categorize the stick force as medium – higher than most piston singles but lighter than the small, conventional business jets. Stability is excellent, and I found the workload to be very low in holding target airspeeds and then maintaining altitude when level. As in other jets with a wide margin between minimum and maximum airspeeds, the D-Jet has a larger pitch trim range than a piston single. That means you get your thumb on the trim switch right after takeoff and run it nose down as the airplane accelerates quickly, and reverse the process when maneuvering for landing. It’s entirely normal in any jet, but a pilot new to jets may find it surprising at first.

Toronto Center assigned several altitudes on the way up to 25,000 feet so I didn’t get a usable time to climb. However, I did get to capture several altitudes, retrim and stabilize, and each time it was remarkably easy to hit and hold the target. The last step was from 23,000 to 25,000, and the D-Jet went up at 1,300 fpm without slowing close to the best-rate-of-climb airspeed.

As we leveled at FL 250 the true airspeed was right at the 255-knot expected long-range cruise speed, so I pulled power back – a long way – to hold that speed. The fuel flow settled down around 315 pounds per hour (pph), but that even may have been a little too high because the airplane kept accelerating gradually. Diamond calculates the D-Jet can cover around 1,100 miles with full tanks at that airspeed in still air. However, the air is never still, so I think the lower long-range speed will be useful only with a tailwind.

The air was unusually warm aloft with temperatures 18 degrees C above the international standard (ISA), so the engine could not make full rated power due to lowered air density. With maximum power set, the fuel flow was about 458 pph instead of the expected 500 pph. Of course, the weight of the fuel must match the density of the air, so with less air available, less fuel was burned and thus there was less power output. The true airspeed settled in around 314 knots, leaving no doubt the airplane can make the promised maximum cruise speed of 315 knots with air temperatures anywhere near standard. In fact, 325 to 330 knots true have been typical top cruise speeds in cooler air.

The solid stability is also evident when flying at the ceiling. I did steep turns of 45 degrees bank and found it easy to hold altitude. Even when slowed to about 120 knots indicated, the wing has lots of margin above the stall to be horsed around with all you would ever want — or may not want as in the case of turbulence.

The D-Jet is so low in drag it doesn’t like to come down. With power at flight idle and observing the 215-knot temporary maximum airspeed limit, the descent rate was only about 1,000 fpm. Slowed to the best glide speed of 120 knots, the sink rate was less than 500 fpm. Of course, the engine was producing idle thrust, which helped, but the 65-nm glide radius from FL 250 does seem even a little conservative.

In the finished airplane, the maximum landing gear extension speed will be 200 knots, so the gear can be used as an air brake if you need to get down quickly. Serial number three has hydraulically actuated gear retraction but the production airplane will use electric actuators. The new gear is at Diamond and has passed the tough drop tests already.

Back at the London airport, a nice test awaited for my first D-Jet landing because the wind was 80 degrees to the runway with gusts over 20 knots. That will tell you a lot about how easy an airplane is to fly and how harmonized its controls are. Because the runway was long, it would have made sense to land with approach flaps set, but I made the first one with full flaps and it worked out perfectly with no coaching from Howard in the right seat. The Vref target approach speed is 85 knots at typical landing weights, but with the gusty crosswind I held just under 100 knots and the D-Jet floated only a little.

On the next pass I made the landing with approach flaps and it worked out even better in the crosswind. Then we changed runways as the wind cranked around more, and this time, with only about a 20-degree crosswind, I held the 85-knot target and can report that using 3,000-foot runways will be just fine, as Diamond predicts, as long as they are dry and uncontaminated.

The D-Jet has huge slotted flaps that are crucial to achieving the 61-knot maximum stall speed certification that rules require for any single-engine airplane. But the flaps are well designed and cause no noticeable pitch changes with retraction or extension. Of course, you need to retrim after the flaps change because the airspeed is changing, but you don’t need to push or pull while the flaps are in motion.

Diamond is still working out a few bugs on the nosewheel steering system, which uses a mechanical linkage to the pedals. But the brakes are already working well with smooth power and lots of authority. It is not possible to analyze cabin sound levels because, well, there is no cabin — just bare carbon fiber walls with no insulation or interior. The vibration levels are very low and should only get better in a finished airplane. There is an odd little shake during engine start that goes away as the engine accelerates, and Williams, I’m sure, will be able to solve that by adjusting the fuel flow schedule from the fadec.

To accommodate the airflow demand of the more powerful engine, Diamond enlarged and reshaped the engine air inlets on number three compared with number two. There were also subtle changes to the engine cowling and wing-to-body fairings, all aimed at cutting drag while allowing the new engine to breathe better.

Diamond has settled on pneumatic boots to de-ice the leading edges, engine bleed air to heat the inlets and ducts, and electric heat for the windshields and probes. TKS was considered early on, but the weight of carrying so much fluid to meet current requirements didn’t make sense. The boots are lighter, much lower in cost, well proven, and Goodrich has made great strides in technology that allows for a much thinner de-icer that adds very little drag but provides plenty of “snap” to fracture the ice.

The avionics are Garmin G1000 with the oversize MFD in the middle, the same as the Cessna Mustang. The cabin mockup features comfy pilot seats and a bench across the rear that can seat three. The center cockpit pedestal is cantilevered so your feet slide under it, making it easy to get in and out of the pilot seats compared with some other small-cabin-class airplanes. There are also useful baggage compartments in the nose and aft of the cabin with external access.

Diamond expects to squeeze out about a 900-nm no-wind range with full tanks when flying at max cruise speed. But that will require unrestricted climb to FL 250, a late descent at idle power and very little vectoring around the airport to achieve. In the real world of delayed climbs, early descents and lots of vectoring, effective range will be considerably less.

The financial tribulation impacting all aircraft companies has slowed D-Jet development some, but Diamond is still investing many millions per month on the project. Diamond isn’t issuing precise development schedules but expects the first D-Jets to be delivered in 2010. The engine change and other refinements have also brought a price increase to $1.89 million in today’s dollars, an increase of about 25 percent. Current order holders will get the better airplane for the price when they signed up, so they will get a more capable airplane with a longer lasting engine for no more money. In this case it pays to be early.

Diamond still has much to do before the D-Jet can enter service. Full static tests on a conforming airframe are ahead, along with flight testing at the highest and lowest speed corners of the envelope. Diamond has decided to go with a stick-pusher stall prevention system as is common in larger jets, so the vagaries of spin testing, which would otherwise be required, are not an issue.

The big question on many minds — including people at Diamond — is whether a piston single pilot can move into the D-Jet without a lot of difficulty. After my time in the third prototype, I would say the answer is clearly yes. Because it is a jet, a type rating is required, and all type-rating check rides must be flown to the ATP standard, so all D-Jet pilots will need to be good instrument pilots. But if they are good IFR pilots in their piston single, they will be better in the D-Jet because the workload is lower, power management is so simple and the good stability makes it easy to fly.

Being first to develop a new category of airplane is always tricky, but Diamond is off to a great start.

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