Latest CubCrafters Design Looks Back and Ahead

Although Damm gave me a heading, I still wasn’t seeing the airport. “Keep going. You see that house and workshop on the top of the hill?” Yes, I do. “OK, the dirt strip is just to the west of that, aligned roughly east-west.” I kept looking to where the elevation appeared to flatten slightly, trying my best to pick out anything that looked like an airport. No luck.

Sensing my frustration, Damm helped: “You’re looking in the wrong place. See that hill?” I see Damm’s finger over my left shoulder. “That’s the runway.”

For a moment, I stopped giving the new CubCrafters UL bushplane much in the way of direction as the tumblers fell into place, and I realized that after about 30 minutes of flying, I’d be putting the company’s latest offering down on a “runway” that most pilots would try to avoid even after a full-blown engine failure.

You bush pilots out there know what happens next. The “sight picture” is really different when landing on a significant grade, making it tough to judge the roundout point. After a few circuits, I started to get the idea, understanding better each time how to judge speed, altitude, and descent rate. I really couldn’t tell you what the airspeed was—my focus was totally outside. Each landing was followed by a short taxi down the hill and then an uphill takeoff, despite the wind being calm.

Damm knew how to highlight the UL’s potential here—with two of us and nearly full fuel, the Carbon Cub handled the uphill departure with ease, climbing comfortably faster than the terrain without coming close to hanging the thing on the prop. That’s what a lot of wing, nearly 180 square feet of it, will do for you.

Horsepower helps too, and that’s perhaps the most surprising aspect of the Carbon Cub UL. In a break from the company’s long-standing adherence to Cub-style norms, the motivator up front isn’t the familiar Lycoming or Titan air-cooled engine. Instead, it’s the newest Rotax, the turbocharged, intercooled, 160 hp 916.

Why the change, and why now?

“We needed an airplane for international markets,” Damm said. “You have to have an airplane that runs on autogas. Avgas is available here, but in some places in the world, it simply isn’t. We also wanted an airplane that fit into the UL category around the world.”

While the U.S. has light sport aircraft (LSA) for now—and Modernization of Special Airworthiness Certification (MOSAIC) for the near future—other aviation authorities have different requirements for “lightweight” aircraft. In many other nations, what is broadly referred to as the UL (or ultralight) category is not the same as our version. Most countries stick to the 600-kilogram weight limit (essentially the same as our 1,320-pound limit for LSAs on wheels), though there are exceptions that go as high as 700 kg (1,543 pounds).

It’s true that other CubCrafters models fit into the LSA weight limits, but what makes the new UL compelling for the company is the multifuel (avgas and 94-octane autogas) capability of the Rotax. In the time since the airplane was introduced at Sun ’n Fun 2023, market response has proven the Rotax to be a good choice, creating new interest in the brand.

Still, this was not a small engineering project. CubCrafters had assistance from Rotax, but the grunt work was committed at the company’s sprawling Yakima facility, where it builds three distinct lines of modern Cub-style airplanes, offers a builder-assist program for those who want to create their own, and even provides service and repair.

The good news is the liquid-cooled Rotax 916 is close enough in shape to the legacy engines that the packaging is almost self-starting. Luckily, the propeller is in the same place as with the Lycomings. But everything from there to the firewall is new and different, a process still ongoing when I flew the prototype UL.

Under the spinner, the opening that normally feeds the airbox or carburetor in a Lycoming installation ducts air to the oil cooler. In the copilot-side cheek, a splitter pulls incoming air back and toward the standard intercooler. Air entering the other side of the split and the pilot-side opening are led back to a pair of coolant radiators. A duct integrated with the top of the cowling helps feed a tall radiator just behind the engine while another is positioned along the lower surface of the cowling in the exit-air path.

This arrangement is very likely to change. My flight in the airplane, on a not particularly warm day, began to stress the coolant temperatures, something CubCrafters has seen in its testing and is working to remedy as R&D continues.

On the firewall is the Rotax-supplied, power-distribution system that manages the dual engine control units (ECUs) that, in turn, control fuel injection, ignition, and wastegate functions. That’s right, the 916 iS, like its nonturbocharged 912 iS brethren, is an electronic engine, with automotive-style fuel injection and ignition. It’s not a truly fly-by-wire system since the throttle is still mechanically linked to the pilot’s left hand, but the turbo system is managed by the ECU. Rotax’s engine-management technology is well proven in the 912 iS.

The engine itself helps the overall goal of weight reduction. At just under 200 pounds, the Rotax 916 iS is 40-70 pounds lighter than the CC340 engine offered in the middle-range and LSA-qualified CubCrafters mod-els. In addition, CubCrafters put the UL on a strict diet, with a lighter composite cowling (made out of pre-pregmaterial that’s lighter), titanium (instead of steel) landing-gear legs, and titanium firewall.

“Our goal is to have the UL, at 1,320 pounds maximum gross weight, have enough useful load for a 200-pound pilot, 120-pound passenger, 20 gallons of fuel, and 20 pounds of baggage,” said Damm.

If you’ve done the math, that’s an empty weight of 860 pounds, which is 32 pounds lighter than the listed spec for the Carbon Cub SS on which the UL is based.

Pilots familiar with the Rotax 900-series engine and, in particular, the turbocharged 915 iS or nonturbo 912 iS will be immediately comfortable in the Carbon Cub UL. The UL prototype uses the smaller SS-style instrument panel that, in this case, carries a compact 7-inch Garmin G3X Touch main display with an RS Flight Systems EMU (engine monitoring unit) display just to the left. It reveals crucial engine information such as manifold pressure, rpm, oil pressure and temp, coolant temp, fuel pressure and flow, total fuel quantity, and system voltage. It also shows throttle “percentage” and the status of Lanes A and B.

In the parlance of Rotax’s dual-channel ECU, “lane” refers to each leg of the parallel computing system. The engine can run on either but typically has Lane A running the show with Lane B humming along in the background, a faithful understudy ready to step in should the lead actor stumble off the stage. (By accident, of course.) You’re aware of the ECU status on both the EMU display and through two big annunciators that warn of a fault with either lane. It’s different from conventional piston aircraft but totally learnable.

Lined up on the runway, you can slide the throttle forward smoothly but quickly. From there, the Rotax leans into the job with real vigor and no apparent throttle lag or turbo surge. It accelerates to flying speed with the customary one notch of flaps in less than four seconds and just seems to levitate. Once rotated, and if you have no need to clear terrain, you can let the nose settle not far above the horizon and enjoy the view of the runway falling away from you. In no way did the UL feel like it was down on power compared to the midrange Carbon Cubs.

In deference to the early stage of the UL’s cooling system, we pulled back to “95 percent throttle,” which is how Rotax describes engine power settings through the instruments even if the percentage isn’t always exactly related to maximum power. We dropped the nose to attain

78 knots indicated (90 mph) and 500 fpm in the climb, where the coolant stabilized at 231 degrees Fahrenheit and the oil temp at 190 degrees. At this setting, the engine is pulling 37 inches of manifold pressure at 4,800 engine rpm.

Later in the flight, we tested a full-power V_Y climb (61 kias, 70 mph) where we recorded 1,100 fpm. Fuel flow is 10.3 gallons per hour (gph) at 100 percent throttle. At V_X, the airplane is substantially nose up to 46 kias (53 mph) and still climbing better than 800 fpm. We followed that with a quick high-speed cruise check.

At 98 percent throttle, the 916 is happily churning away at 42 inches and 5,400 rpm, burning 9.9 gph. At 4,900 feet msl, we saw 115 ktas (132 mph true). I didn’t attempt to check the calibration of the pitot-static system for a couple of reasons. First, we were at a not ideal altitude for the turbo Rotax, which can produce maximum power to 15,000 feet and maximum-cruise power to 23,000 feet. Second, several aspects of the CarbonCub UL’s cooling system will change between now and when you can buy one, so high-cruise data is more likely to change than not.

A more realistic cruise speed would be 95 percent throttle and 6.4 gph, giving 104 ktas (120 mph true). Even more economical are the results at 94 percent throttle, where the UL trots along at 89 ktas (102 mph true) on 4.9 gph. To get that, the Rotax is pulling 32 inches at 4,700 rpm. Notable in this case is the engine’s smoothness and complete predictability with power changes. It moves from one thrust level to the next seamlessly.

Yes, there is a whole airplane behind the firewall, and it’s a known quantity in the Cub world. From a handling standpoint, the Carbon Cub UL is a tiny bit contradictory. It has the low-speed handling you want, tons of lift, and is capable of climb gradients to make your passengers whoop. (Best to warn them first.) These are the vaunted Cub-like characteristics that go into making it a supremely good short-field machine.

With practice, you could drop it in over a line of trees and have it stopped in just a few hundred feet. You’ll need to be on your game and willing to fly it slowly, with the angle-of-attack system beeping in your ear, but the airplane gives you tons of warning to the unloaded stall and recovers quickly. In fact, to get a serious break at altitude requires very aggressive control inputs.

But the UL also has some strong big airplane character. For one, it’s very strongly pitch stable, sticking to trimmed speed tenaciously. It’s a “rudder airplane” but not stupendously so, thus it won’t take long to get the hang of it. My sole complaint comes only because I have time in the other Carbon Cub models. For reasons of weight savings, the UL uses the earlier control system, which lacks the pleasurable lightness of the G-series flight controls found on the EX/FX and X/NX models.

In my world, where pilots of the Van’s RV are accustomed to fingertip-light controls, the Carbon Cub feels a tad stiff. It’s an airplane that wants all your fingers on the stick, but that payback is a strong sense of stability and excellent pre-stall feedback.

Back to the business side of things. CubCrafters feels strongly that its UL has worldwide potential. Since starting the program with Rotax more than two years ago, it launched the airplane, continued R&D work, and has begun building half a dozen examples as dealer demo/market survey aircraft. That’s not likely to be the final design, either.

“We like to get customers into the airplane to see what they like and don’t like,” said Damm. “And then we can iterate the design from there.”

The design should get locked in by mid-2024, with the manufacturing team refining its part of the process through the end of the year. Prices have not been set but will probably be closer to the $312,000 of the EX-3/FX-3 series than the current $237,000 of the Carbon Cub SS.

The airplane on these pages has been re-covered in the Oratex system, which is a “prepainted” concept that requires no solvents, filler, or paint after application, and then sent over to Europe for its wintertime tour. CubCrafters is trying the Oratex for that market in part because the system is lighter. But it’s also emblematic of the company’s approach to the legendary Cub design: Keep the traditional elements, but continue to try new things. New materials (hence the “carbon” in the Carbon Cub name) and even new engines.

What the Carbon Cub UL shows is that innovation can keep an airplane style so rooted in our history that it might as well have been conceived by the Wright brothers—still amazingly relevant a quarter of the way through the 21st century.

CubCrafters is unique in aviation in that it sells FAA-certified completed airplanes, provides airframe kits into the homebuilt world, and also offers a builder-assist program that blurs the lines between factory- and individual-built aircraft. To be licensed as an experimental/amateur-built aircraft, there has to be an individual builder (or group) on the paperwork who has convinced either the FAA or a representative that they’ve completed a “majority” of the work. This is colloquially known as the “51 percent rule.”

Defining what makes up a “majority” of the work is made easier by an FAA-accepted checklist of typical build tasks, including both assembly and fabrication. This list of tasks, used by all the popular builder-assist programs, helps define what the builder needs to do and what the factory is allowed to complete.

Typically, the builder is given more assembly tasks than raw fabrication because the company selling the kit is usually better at making the parts than a first-time builder. It’s important to understand that this checklist doesn’t document every single part or assembly but breaks it down into major categories—wing ribs, main spar, tail feathers, etc. What’s more, it doesn’t say how many of those things a builder has to do, so “fabricating” one wing rib gives the builder credit for all of them.

For CubCrafters, the builder-assist program is upside down from most. It’s structured as two weeklong visits to the Yakima, Washington, factory, the first totally focused on fabrication. Working alongside the team that creates the very same parts for production airplanes, the amateur builder participates in key processes that very strategically ticks all the boxes on the checklist. I have so far spent the first week helping to build one of the demonstrator Carbon Cub ULs and am due to return this spring for the second half of the project, which focuses more on assembly and inspection.

But for the first part, it’s a matter of using a massive hydraulic press to create wing ribs from accurately CNC-cut aluminum, helping to place sections of steel tube into a jig for a really good company-paid welder to assemble, forming key elements of the tail structure and assisting and helping to lay out pieces of fiberglass into the same molds used for the certified aircraft. (It’s worth noting that the amateur-built and certified parts are tracked and kept entirely separate, even as the raw materials themselves are exactly the same.)

The builder-assist program doesn’t really save money on the airplane, but it does give the owner a sense of accomplishment and a better understanding of what goes into it. CubCrafters does discourage builders from applying for a repairman certificate (available to homebuilders but only for their specific airframe). But the remaining benefit is the ability to make modifications later without needing an STC or other means of compliance. The builder in the program alongside me said this was the very reason he chose the builder-assist path.

A) Rotax’s most recent engines all have electronic ignition and fuel injection. These warning lights tell you if one of the parallel, redundant ECUs has a fault.

B) The engine monitor keeps the pilot in the loop.

C) To keep weight down, the prototype Carbon Cub UL uses Garmin’s smaller G3X Touch EFIS.

D) A special multifunction “mag” switch controls engine start and allows sequential testing of the redundant engine controls.

E) The Carbon Cub UL features a simple all-metal stick topped by a pitch-trim switch which seems to say everything about the CubCrafters no-nonsense approach.

Price, as tested: Not available

Engine: Rotax 916 iS

Propeller: E-Props, three-blade, fixed pitch

Horsepower: 160 for takeoff, 137 max continuous

Seats: 2

Length: 23 ft., 3 in.

Height: 8 ft., 4 in.

Wingspan: 34 ft., 3 in.

Wing Area: 179 sq. ft.

Wing Loading: 7.37 lbs./sq. ft.

Power Loading: 8.25 lbs./hp

Cabin Width: 30 in.

Cabin Height: 52 in.

Max Takeoff Weight: 1,320 lbs.

Standard Empty Weight: 860 lbs. (est.)

Max Baggage: 200 lbs.

Useful Load: 460 lbs.

Max Usable Fuel: 24 gal.

Service Ceiling: TBD

Max Rate of Climb: MTOW, ISA, SL: TBD

Cruise Speed at 90% Power: TBD

Max Cruise Speed: TBD

Max Range: TBD

Stall Speed, Flaps Up: NA

Stall Speed, Full Flaps: 32 mph

Takeoff: 60 ft.

Landing: 110 ft.

This feature first appeared in the April 2024/Issue 947 of FLYING’s print edition.