Pipistrel Virus

And it has Pipistrel.

Pipistrel builds airplanes. Its first products were weight-shift ultralights or “trikes” — those things with the pilot and engine hanging in a frame below a fabric wing. Next, begun in the mid-1990s, was the Sinus (rhymes with Venus), a two-seat motorglider with a 15-meter wing, an 80 hp Rotax engine and a 28:1 glide ratio. Both an aerodynamic and a commercial success, Sinus spawned Virus (VEER-oos). Virus is basically Sinus with a shorter wing — in fact two different spans are available on Virus, the shorter, at 36 feet, getting the tag SW for “short wing” — and, optionally, a 100 hp engine.

In a parallel line of development, around 2000 another Slovenian company, Albastar, which was fabricating wings for Sinuses, mated a conventional single-seat midwing fuselage to a 13-meter version of the wing to create a 40:1 ultralight-class sailplane, which it called Apis. This project was sold to a third Slovenian company, AMSFlight, which provided it with a retractable engine and marketed it as a kit. Pipistrel bought the project back from AMSFlight in 2008, updated it, and now sells it under the names Apis and, in some markets, Bee. In the meantime, in 2002 Pipistrel had developed its own side-by-side, two-seat, self-launching sailplane, Taurus, with a 15-meter version of the same wing, a 41:1 glide ratio and a retractable Rotax engine on a pylon behind the cockpit. Taurus is available in an electric version as well.

So the Pipistrel product line now includes trikes, single- and two-seat self-launching sailplanes, a motorglider and two versions of what is, with some caveats that I’ll return to later, a light-sport aircraft. It also includes propellers, by the way — just a little sideline.

All of the Pipistrel airplanes are remarkable performers. I first learned how remarkable when I was a judge at the CAFE (Comparative Aircraft Flight Efficiency) Foundation’s NASA-funded 2008 efficiency contest. One of the entrants was a 100 hp Virus SW, which I evaluated at the time. I got a particular kick out of its upper-surface sailplane-style airbrakes, which provide direct lift control during the landing approach. It won the contest, as it had the year before. One of the surprising performance points it turned in was a level speed of 142 knots. It will, in fact, cruise at 140 ktas (the factory specs say 148) making 40 nautical miles per gallon — exceptional for a factory-made 100 hp two-seater.

How Do They Do It?
Many LSA airframes have bulging cabins with a strongly cusped tail cone. Designers' reasons for using this teardrop shape are several. One is simply to save weight and surface area; a thin tail boom is lighter than a thick one and has less area to produce frictional or "scrubbing" drag. … Another is to reduce drag by exploiting the rising pressure against the converging sur-faces behind the cabin. But the shape has risks. If you allow the tail cone to converge too rapidly, you can end up with flow separation and a drag increase, particularly with any sideslip. And the curvature aft of the cabin can be destabilizing in yaw if flow does remain attached to it. Finally, the intersection of the cabin blob and the wing can create additional separation issues, though those tend to be more of a problem for low-wing airplanes.

Pipistrel designers managed the teardrop shape successfully. The Virus is slenderer and more rounded than other similarly configured airplanes, such as the Flight Design CT, Jabiru or Remos. The wing-fuselage intersection appears to have been carefully studied, as does the thick laminar-profile wing with its tapered outer panels and full-span camber-changing flaperon. (That wing has proved remarkably versatile, appearing, in various incarnations, on half a dozen different airplanes.) The wing, which, like the rest of the extremely light (625 pounds empty) and strong (12 G ultimate) carbon-fiber airframe, is perfectly smooth and true, contributes a lot to the cleanness of the design. Modern composite airframes attain an aerodynamic perfection that Eastman Jacobs, the 1940s pioneer of the laminar-flow airfoil, could only despair of achieving. On airplanes like these, extensive laminar flow actually exists.

The full-span trailing-edge flap is controlled by a lever on the floor between the seats. It has four positions: reflexed (minus 5 degrees), neutral, takeoff (9) and landing (18). This is one of the complications that make the Virus a more sophisticated aircraft than others of its general size and shape. Another is the airbrake, which is operated by a handle that drops down from the overhead between the seats.

A third refinement is the variable-pitch propeller. Not a self-regulating constant-speed, it is adjusted by a large knob on the instrument panel; you rotate it to change pitch and pull it outward several inches to feather. The controllable-pitch prop enables a small engine to achieve a high cruising speed as well as strong takeoff acceleration and climb.

Reinfected by the Virus
In September, at the NASA/Google/CAFE-sponsored Green Flight Challenge at Santa Rosa, California (which was won, to the tune of $1.35 million, by the Pipistrel entry), I had a chance to renew my acquaintance with the Virus SW.

The surfaces are flawless and wave-free, and the control-surface gaps are covered with sailplane gap-sealing tape. The cowling is compact and generally conical in shape; it’s hard to believe that a 100 hp engine fits inside. Several intakes admit cooling air: The main one, on the top of the cowling, feeds the radiator for the liquid-cooled cylinder heads, while two others, at 4 and 8 o’clock, feed the air-cooled cylinder barrels and oil cooler.

Large doors on both sides of the cabin swing upward and are held open by snaps on the undersurface of the wing. Almost the entire door is a window. You climb aboard by sitting sidesaddle on the edge of the seat — taking care not to kick or step on the very light and definitely non-load-bearing wheelpants — and swinging your legs aboard. The slightly reclined seats are fixed, and the rudder pedals move forward and aft along a rail for pilots of different sizes. Behind the passenger’s seat is the ballistic recovery parachute, behind the pilot’s a space for baggage. Throttle and pitch trim are on a low console between the seats.

The engine is a Rotax 912 ULS, the ULS standing for “uncertified light sport.” With a 9:1 compression ratio, a 2.43:1 propeller reduction and a redline of 5,800 rpm, it delivers 100 takeoff horsepower and 95 continuous from a package weighing around 140 pounds — 40 pounds less than a Continental O-200. It has no pilot-operated mixture control, but it does have a choke to enrich the mixture for starting, like an old car. It can run on 100LL but prefers mogas, so it not only uses very little fuel — 2.5 to 3.5 gph in cruise — but what it uses costs considerably less than avgas does.

I flew with Pipistrel’s imposingly built U.S. sales rep, Michael Coates. No sylph myself, I was surprised by the strong acceleration on takeoff and the steady climb rate of 1,200 fpm at 80 kias on a day when the temperature at 3,000 feet was 100 degrees. This was not even the maximum rate of climb, which can reach to 1,650 fpm; we kept the nose down for visibility. Nevertheless, the oil temperature rose to a surprising 260 F during the climb.

It was a beautiful fall afternoon. We flew westward across the sinuous hills and the innumerable vineyards of Sonoma County, arriving all too soon at the rugged seacoast. The view downward through the huge side windows was spectacular. Over the water, I flew a few steep turns and wingovers to get a sense of the ailerons and rudder and of the general slipperiness of the airplane. The control forces are light in both roll and pitch; Michael observed that the best way to get to know the airplane is to fly it with two fingers on the stick. There is not a lot of aileron differential; if you turn with feet on the floor, some adverse yaw sets in shortly after the bank begins. Michael suggested leading turns with a touch of rudder, but I found it more natural to begin turns with aileron and follow with rudder after the nose started to swing. Roll-yaw coupling is strong, and the Virus has excellent lateral stability; you can comfortably cruise hands-off, correcting right and left with tiny nudges of rudder.

Stalls, such as they were, felt similar regardless of flap setting, except that with more flap deflection the deck angle got progressively lower. With reflex or neutral flap it was possible to maneuver gently at full aft stick without a break; with full flap, the nose bobbed a bit. The mild stall, which occurs at an indicated airspeed below 40 knots, is preceded by mild buffeting. I did not attempt any of the more daring kinds of stalls — accelerated, crossed-control and so on — but I expect that the Virus’ behavior would remain on the safe side of conventional. The airplane is, however, placarded against intentional spins.

The airbrake, which is operated by unlatching the overhead handle and pulling downward, may not be extended above 87 kias; once extended, however, it is good right up to the Vne of 163 kias and can produce a prodigious rate of descent. The reason for the limitation on initial extension is chatter or snatch that occurs as the outer cover strip, which is spring-loaded to the brake structure to ensure a flush surface when retracted, first encounters fast-moving air. The limitation could be annoying; one might like to be able to begin a rapid descent directly from cruising speed.

The airbrake, which produces no pitch trim change, comes into its own on landing. Michael’s technique was to cut power to idle at the key position on the downwind leg and leave it there. The Virus glides extremely flat, and 18 degrees of flap does not produce enough drag to make it come down steeply. The airbrake provides easy and powerful control of descent rate. Holding 60 knots and with the airbrake lever in my hand, I could adjust the aim point with great precision. Flaring with the airbrake retracted, I could then extend it to plant the wheels on the runway.

If I Were King
There is little to criticize in the Virus, but as usual I would change something if I were king. I think the locations of the flap and airbrake handles should be reversed. The flap handle is seldom used, whereas the airbrake is used continually during the landing approach and would feel more natural coming out of the center console. That would also put it closer to the throttle for a go-around. I would also like to see some sort of dead-man arrangement on the airbrake grip that would allow it to stay put when you let go of it, though I see the potential problem — not insoluble — of a pilot's forgetting during a balked landing that he has the airbrake out.

Visibility while maneuvering is as bad as in any other high-wing airplane. There is an oval window in the overhead behind the wing spar, but it is of very limited usefulness. I suspect that a bit more of the roof could be made transparent, but there is a lot of clutter up there in any case. Limited visibility in turns is inherent in the configuration; it can’t be helped, except perhaps by moving the wing root aft and sweeping the wing forward.

The Virus in the form in which I flew it is not an LSA; it is too fast, and its variable-pitch prop is not allowed under U.S. rules (European rules for the generally similar “ultralight” category are more permissive). The one I flew is licensed in the Experimental category as a sales demonstrator. The Virus is also available as a kit; that would make it Experimental Amateur Built, which is a practical option. To qualify as an LSA, it has to be equipped with a fixed-pitch climb propeller that limits its level speed to 120 knots.

Not resting on its laurels, Pipistrel has announced a very ambitious, 200-knot, retractable-gear four-seater called Panthera to be certified under Part 23. Three power plants are envisioned: a 200 hp Lycoming IO-390, a hybrid and a pure electric system. (The twin-fuselage Taurus G4 that won the Green Challenge was powered by the 195 hp electric motor that is planned for the Panthera.) I would dismiss the claims made for the Panthera as pie in the sky, but I would have said the same thing about the Virus if I hadn’t flown it, and I would have been wrong. Part 23 is a shoal on which many a noble project has foundered. But it would be a mistake to sell those Slovenians short — they’re on a roll!

View our Pipistrel Virus photo gallery.

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