Carbon Cub SS

Did I mention that I was departing from a grass strip just south of Denver called Rocky Mountain Airpark at an elevation of 6,295 feet and a temperature of 70 degrees? That makes for a rather formidable density altitude of 8,400 feet. Impressive performance for a high-altitude takeoff! There's nothing quite like a light aircraft with a big engine to put a smile on your face.

The airplane's remarkable performance is due not only to a big power plant (more about that later) but also to a very aggressive weight awareness program during its design and manufacture. The aircraft's basic structure is "tried and true" chromoly 4130 steel, still one of the very best strength-to-weight (and crashworthy) structures in aviation, but instead of just duplicating the architecture that Piper employed for the original Cub, the engineers at CubCrafters began with a nearly clean sheet to design the original "Carbon Cub," now called Sport Cub S2, with its 100 hp Continental, and subsequently this macho sibling, the Carbon Cub SS. Both models are still offered by the company, and aside from the obvious power plant differences, the two are essentially identical.

Even though there's a $35,000 difference between the Sport Cub S2 and the 180 hp Carbon Cub, since Carbon Cub SS deliveries began in June 2009, CubCrafters has sold only one Continental-powered aircraft. Everybody, it seems, loves a hot rod.

Grams, Not Pounds
Weight is the big issue with all LSA because of the 1,320-pound maximum gross weight limitation. This makes for some very difficult strength-versus-weight decisions by light-sport manufacturers when designing an aircraft. It isn't easy to design and build an aircraft that light that will carry two people and at least some fuel without careful compromises and thoughtful design choices. Company founder Jim Richmond's mantra is: "Weight is the enemy."

The empty weight of the Carbon Cub is about 350 pounds less than that of CubCrafters' FAR Part 23-certified Top Cub, a much-improved Super Cub clone, and about 250 pounds less than an average 150 hp Piper Super Cub. How did CubCrafters pull this off? Engineering. When designing the original Carbon Cub, CubCrafters' engineers used modern computer-aided design and load analysis to create a structure that is simpler, has fewer parts and is much lighter than what C.G. Taylor could even dream about in the '30s when he designed the original Piper, nee Taylor Cub; Piper's engineers still didn't have the technology to design such a sophisticated structure in the '50s, when they designed the Super Cub. Amazingly, the total parts count for this aircraft is about half that of a standard Super Cub. None of the parts are interchangeable. CubCrafters has further reduced the number of parts by making the left and right ailerons identical - same with the flaps. The Sport and Carbon Cub wings attach quite differently than the original Cub's do, and though the aluminum spar is the same as in a Piper wing, each Carbon Cub wing weighs 22 pounds less than a Super Cub's. Smaller parts and fittings are made in-house on CNC machines, resulting not only in simpler, updated designs, but also, most importantly, lighter ones. Truly, some of the parts are great examples of modern industrial artwork.

CubCrafters has six full-time engineers at the firm, and four of them are kept busy just maintaining the drawings on their Top Cub and the Sport/Carbon Cubs. The cost of maintaining these drawings is "staggering," Richmond says, but it's the cost of doing business for a manufacturer who builds aircraft both FAA-certified and to ASTM standards.

E-LSA Versus S-LSA
The empty weight on this aircraft, with the optional Alaska Bushwheels, was 932 pounds (it's 38 pounds lighter with standard 6.00 x 6 wheels). This throws it into the E-LSA, or Experimental-Light Sport Aircraft, category rather than the S-LSA, or Special-Light Sport Aircraft, category that the aircraft was delivered with. This is because there is a formula-derived maximum empty weight unique to every LSA, and the Carbon Cub's limit is 900 pounds. So to add the big tires and other options, you must change the category. There are other upsides to this change: You no longer have to stay in conformance with ASTM standards (you can modify your aircraft at will); a private pilot or higher can fly it night VFR; and perhaps most appealingly, after completing a 16-hour FAA-approved course, you can get your repairman's certificate and maintain your own aircraft. This category swap is quite common; about 50 percent of the Carbon Cubs that leave the factory are delivered that way.

To take photos of the engine, we removed a serious handful of screws to pull the upper cowl. Yes, screws. Though mechanics probably won't be happy about it when they have to pull the carbon-fiber cowl, screws were chosen over Dzus fasteners to save weight. When I lifted the upper cowl free of the aircraft, I was amazed at how light it was. Though I guessed it at about a pound, Richmond said that a complete cowling is about 6 pounds, and the upper half is probably closer to 2 pounds. That's still amazingly light. But it didn't feel flimsy at all.

The engine compartment has a large (again carbon) plenum to better direct cooling air across and down through the cylinders. The proprietary CC340 engine is a Lycoming O-320 clone, with many ECI components. CubCrafters uses ECI O-320 cylinders instead of O-360 jugs to save weight, increases the stroke slightly and adds dual electronic ignition to bump up the horsepower to 180. It has a lightweight starter, alternator, wiring harness and oil lines. The oil sump is sheet metal instead of cast aluminum, saving 11 pounds. And the unique four-into-one exhaust system, designed especially for CubCrafters by Sky Dynamics, has no welds and weighs just 6.2 pounds.

Even the paint scheme on this demonstrator is optimized for light weight. When CubCrafters went hunting for ways to make the Carbon Cub lighter, it discovered that many of the lightest airplanes in the old days had paint only on part of the aircraft, leaving the rest in silver dope. With the heavier poly paints we have today, painting only part of the aircraft makes even more sense. On the demonstrator I flew, this "performance" paint scheme is part poly paint, and the rest is a lightweight, silver Poly-Tone. According to factory numbers, this effort conservatively saves 7 pounds, though Richmond thinks it's often closer to 9 pounds on an average airplane.

But How Does It Fly?
None of this extreme effort to save pounds, ounces and even grams would have been worth it if the aircraft weren't fun to fly. It is. On my solo flight, with a bit less than half fuel — 10 gallons of gas — takeoff weight was about 1,170 pounds, or 148 pounds below maximum gross, enough to add full fuel and a small bag or another small person. The aircraft felt light and responsive — comfortable — like I'd been flying it for a long time. The airfoil is the venerable USA 35B, a solid, slow-speed design used for years by Piper on everything from Cubs to Aztecs. Vortex generators are added to the wing to further optimize low-end capabilities, and the combination makes runways unnecessary.

With full flaps, the Carbon Cub is quite comfortable in the 30 mph indicated range; adding some power, I was able to fly the airplane comfortably at nearly zero indicated airspeed. Though this low indication is obviously a result of the inaccuracies caused by the extreme nose-high attitude, the airplane is no doubt moving very slowly. When the wing finally does stall, a burst of power immediately reattaches the flow, and the airplane is flying instantly. That's an impressive quality and one that certainly gives a pilot confidence when maneuvering the machine in the low-speed end of the envelope. I couldn't help but laugh out loud when the airplane just clawed its way out of the stall with power.

Though I didn't take any long trips in the Carbon Cub SS on my evaluation, I found the adjustable front seat to be comfortable and visibility quite good for a taildragger. The electrical switches are right in front of the pilot, where they belong, and the flap handle extends from the left wing root; it's easy to find and manipulate. The rear seat, a fabric sling arrangement, promises a rather low seating position, but I didn't get a chance to experience it in flight. It easily folds up and out of the way to open up a generous cargo area, and the baggage area extends rearward to further even more the cubic-carrying capacity. The main cabin door and window open separately just as in any Cub, and the left window also opens wide, offering a near-open cockpit experience.

There's a five-minute max power restriction to comply with LSA rules, but with the generous climb rate of this aircraft (I was seeing 1,200 to 1,500 feet per minute), it's not much of a restriction at all. In five minutes, you can be at whatever altitude you want to be. And the 80 hp continuous limitation results in a speed of about 100 mph while burning roughly 6 gph with the Bushwheels installed. With smaller 6:00 tires, the speed would be closer to 120 mph.

The Carbon Cub feels solid and gives you an impression of strength; in fact, it was tested to a gross weight of 1,865 pounds, rather than the usual 1,430 pounds for which most LSAs are tested. This is because CubCrafters offers an experimental, kit-built version, called the Carbon Cub EX, with a gross weight of 1,865. Of course, you assemble that kit yourself and certify it under amateur-built experimental rules, not LSA standards. The very complete kit costs $65,000, which includes everything to build the aircraft except the engine, prop, radios and instruments. Richmond estimates that, if a kit customer bought all the components from CubCrafters including the CC340 engine, he'd have about $110,000 to $120,000 in it, plus his labor; if he scrounged aggressively for parts and put an O-320 on the machine, he could probably be flying for around $85,000.

As this was being written, CubCrafters announced a new option for Carbon Cub called the "adventure package." A heavier tailwheel is substituted, an external baggage door is added, the baggage compartment is enlarged, the rear seat and controls are removed, larger fuel tanks bring the total capacity to 44 gallons, and the landing gear is lengthened three inches and moved forward three inches to increase the angle of attack at takeoff and to make heavy braking less likely to result in a nose-over. This special-purpose, single-place LSA will certainly take you to the backcountry in style.

CubCrafters is building about one of these fun machines a week; buyers are largely older, experienced pilots, not the new faces we hoped to attract to aviation with the LSA option. Many buyers own or previously owned a high performance aircraft such as a Cirrus or Bonanza, and some are "stepping down" from turbines. For these buyers, owning an aircraft that is this much fun to fly, knowing that they can continue to fly it long after they might be incapable of holding onto their medical, is comforting. Since January 2010, the Carbon Cub has been the No. 1 selling LSA with 17 aircraft registered, according to FAA registration records. It's easy to see why.