From Avgas To Jet A

Given the shortcut the process provides, it's not surprising that about half of all Sportsman kits sold have been assembled under TWTT auspices. Now, potential Sportsman buyers have another incentive to take the TWTT route to ownership: diesel power. The newly introduced Continental CD-155 diesel engine option for the Sportsman is available only to buyers who build their airplanes under factory supervision, mandated to ensure that the complex installation is performed correctly.

Announced at the Sun 'n Fun International Fly-In and Expo earlier this year---the sample Glasair displayed there was trucked in and had zero flight time on the installation---the diesel-powered Sportsman 2+2 made its official debut at EAA AirVenture Oshkosh in July.

"It's been a fantastic engine to operate," Ben Rauk, Glasair's Aircraft Production Manager and leader of the diesel program said, standing beside N7TD on the Basler Flight Service ramp. Rauk made the first flight in the diesel on May 20, and by arrival at Wittman Regional Airport, N7TD had 70 hours on the engine. (No "break in" time is needed on a diesel engine.) Its matte-black carbon-fiber cowl, contrasting with the rest of the shiny red-and-white airframe, attested to the feverish development work. "The cowl got finished the day we left for Oshkosh," Rauk said. The performance numbers for that flight underscore some of the diesel's allure: "We cruised here at 17.5 [thousand feet MSL], had 85% power burning 7.1 gallons per hour and a 140-knot true airspeed," he said.

The TWTT Sportsman already offers a choice of a 180 hp Lycoming or a 210 hp Superior engine, both providing a lot more oomph at sea level than Continental's liquid-cooled, 155 hp CD-155. Additionally, the diesel adds almost $90,000 to the Sportsman's base price, and must be replaced after 1,200 hours of operation, but it still makes sense for many prospective buyers, Glasair believes, especially in a global market where avgas is in limited supply. (Glasair is offering a $30,000 discount on the diesel for a limited period.)

Aside from a choice of powerplants, the four-place high-wing Sportsman 2+2, a beefed-up derivative of Glasair's two-place GlaStar, already offers several appealing options. It's available in tricycle or tailwheel configuration, or in a convertible-geared version that can change between trike and taildragger in a few hours' time. (The Sportsman can also sit on floats, but Glasair doesn't envision a demand for the diesel for water ops.) About half of all buyers go with the convertible model, Rauk said, primarily to maximize future salability of their airplanes. A constant-speed Hartzell Bantam three-blade aluminum propeller is another popular upgrade. Rear seats---aft facing to maximize head- and legroom---are also available, and Rauk said their backward orientation doesn't seem to impact sales. (The rear seats are child-sized, hence the Sportsman's "2+2" moniker: two adults, two children.)

The carbon-fiber airframe option in lieu of the standard fiberglass construction offers a 150-pound useful load increase, to about 1,150 pounds of payload, or 850 pounds with full fuel, when paired with the 210 hp Superior. Though the carbon-fiber airframe is only 30 pounds lighter than the fiberglass, Glasair put the weight savings into 30 pounds of structural reinforcements, allowing the airframe to carry a larger payload. The diesel engine and its batteries, however, weigh 100 pounds more than the Lycoming, and diesel fuel is heavier than avgas. How does that affect the diesel Sportsman's useful load? More on that later.

Other noteworthy design features include the relatively large rudder, promising crisp lateral control authority, and the vortex generators on the wings, improving controllability in slow flight and eliminating the need to have a twist in the wings, simplifying construction. The wings themselves fold back at the roots for aircraft storage or transport by ground. A small wheel half-embedded in the rear of the airframe makes it easy to maneuver a trike-configured Sportsman with its wings folded, which puts the CG aft of the main gear and the tail on the ground.

Overall, the fit and finish of the aircraft is impressive, especially considering the build time. "I wouldn't call it show quality," Rauk said. "We're not going to spend five hours on one joggle line, but we work to make them very nice, whether it's a demo plane or a customer's plane. They're all going to show very well."

The interior is Spartan, but comfortable and relatively roomy. About half of buyers go with IFR panels, providing a large choice of equipment options, like the pair of Advanced Flight Systems' AF-5500EF 8.4-inch MFDs anchoring N7TD's panel.

As the single-lever control (referred to as the load selector) in the power quadrant hints, the CD-155 is a full-authority digital engine control (FADEC) powerplant, and adjusts the mixture, propeller pitch and speed automatically with power changes. That also means the engine depends on an uninterrupted supply of electricity, and it has two FADEC computers and a backup battery and alternator for redundancy. If the aircraft lost both alternators and the battery died, half an hour of emergency power remains available.

For engine start, simply toggle on the engine master, check that the glow plug light on the engine display illuminates, push and hold the start button for a second or two, and the diesel surges to life. The CD-155 turns at 3,900 rpm, while gear reduction brings the prop speed down to 2,300 rpm at full power. The whine of the high-revving engine is offset by the lower noise of the relatively slow-turning propeller, so the sound level is comparable to a typical piston single. But the small cylinder size in the two-liter engine operates more smoothly and with less vibration than most piston pilots are used to, and that will translate into less fatigue on long flights. A muffler, not installed on the demo aircraft, will further lower the noise level. Some buyers in the U.S. may opt to leave the muffler off, Rauk allowed, rather than give up the extra horsepower or two the muffler will cost in engine performance, but it will be required in Europe to meet noise standards.

No mag check is needed during the run-up, as diesels have no magnetos, but verify that the spar pins holding the folding wings in position are in place. With a modest 155 available horsepower, advancing the load selector once cleared for departure won't push you back in the seat. At our estimated weight of about 2,000 pounds, we needed about 800 feet to get airborne, our climb settling at about 900 fpm at Vy of 85 knots, with an 8.8 gph fuel burn. The 180 hp Sportsman would have used only about 650 feet of runway. But like almost all diesels, the CD-155 is turbocharged, and therein lies one major performance advantage. At about 3,000 feet, the diesel begins outperforming its Lycoming-powered sibling, and passes the performance of the Superior-powered version at about 6,500 feet, according to Rauk, even as they burn more fuel. "We are finding about 35-40% more efficiency with the diesel over the gas engine," he said. The CD-155's critical altitude, where it can no longer develop full power, is about 9,200 feet. But, as Rauk noted in the recap of his flight to Oshkosh, the CD-155 can develop 85% power just about to the flight levels. (The company expects most owners to fly no higher than the teens, as the Sportsman doesn't operate at its best.)

As for useful load, the diesel's 950 pounds is in line with the 1,000 pounds the 180 hp Sportsman can carry, but its lower fuel consumption gives it an edge. At 60% power, the diesel burns 4.9 gallons (about seven pounds each) an hour. Fill up the 30-gallon main tanks, and you can still pack 740 pounds into the cabin and fly for six hours. The Lycoming-powered Sportsman consumes 8.5 gph at 65% power, so you'd have to carry full fuel (50 gallons at six pounds each) for the same endurance, leaving only 700 pounds of payload. With full fuel, the diesel still offers a useful load of about 650 pounds along with some 10 hours of endurance.

Top cruise speed is now about 140 knots, but the company is fine-tuning the installation, and Rauk would like to reach 145 knots, equaling the Lycoming-powered Sportsman's top cruise. (Top cruise in the 210 hp Sportsman is about 155 knots.) "I've seen about 143 [knots] in straight and level at 8- to 9,000 feet," Rauk said of the diesel.

We never reached those altitudes, leveling at 4,500 and heading northwest for air work. The Sportsman gave the impression that it didn't want to merely demonstrate its flight characteristics, but rather gambol in the sky with its lucky pilot, whether doing steep turns or a chandelle. Handling was docile and reassuring in slow flight, retaining controllability and maneuverability down into the low 40s. The stall break with power off in the landing configuration came at about 41 knots indicated, the Sportsman losing about 700 fpm with the stick held back, quickly recovering when back pressure was released. Both the break and the recovery were more pronounced in a power-on stall.

In addition to the diesel's fuel economy, its liquid cooling eliminates the possibility of shock cooling and reduces the possibility of overheating the engine; the water coolant retains enough heat to keep the engine warm even during descents from high altitude at idle power and, with proper installation, absorbs enough heat to prevent cooking the engine in extreme conditions. Currently, the demo plane can handle "about 90- to 95-" degree OATs without overheating during a Vy climbout, Rauk said, and he plans to tweak the setup to handle 100- to 105-degree conditions.

Reluctantly descending toward Oshkosh while dodging a layer of scud about 1,000-foot AGL, we reviewed power settings and airspeeds for the pattern. Target numbers for entry are about 45% power and 90 knots, gradually reducing power to about 25% on final with an airspeed of 60 to 65 knots, slowing to 55 over the threshold. We landed with a solid plant at about 45 knots.

With a taste of the Sportsman experience, it didn't take much to imagine myself a prospective buyer, faced with a rather daunting choice---especially in a location where one has access to both avgas and Jet A. Should I get the Sportsman with the CD-155 under the cowl? The trusty, bullet-proof Lycoming? Or the brawny Superior? I realized any of these choices would be a good one---especially knowing I could build the Sportsman myself, with a little help, in about two weeks' time.