Designing and 3D Printing a New Kind of Turboprop

A childhood love of turbines led to the design and eventual 3D printing of an unusual aircraft engine that blends three distinct technologies. Nino Caldarola, an application specialist at Autodesk reseller Imaginit Technologies, Winnipeg, Manitoba, Canada, says “The idea for my current design arose while looking at 1940s and 50s jet engines. The Rolls-Royce RB.168 Nene’s incredible intricacy caught my eye. The front section of my initial engine used a lot of Nene elements. The challenge I had set myself was to model the complex assembly using modern 3D MCAD software. But then I decided to see what would happen in connecting the old technology to a modern mechanism more familiar to me — the gearbox.”

The gearbox thus became the second component of his engine design. Caldarola says he based the gearbox on that of Pratt & Whitney PT6-type engines. “The engines are found on a lot of modern turboprop aircraft called ‘city hoppers’,” he says. “I liked the engine because it is a tight package.” This is because the gearbox is a double-reduction system. It couples to the turbine through a smaller gear that converts the high rpm, low-torque output of a large gear to the low rpm, high-torque output needed to drive the propellers.

The third component of the design was inspired by the contrarotating propellers found on engines such as the Soviet turboprop Kuznetsov NK-12, says Caldarola. Here, two propellers are arranged one behind the other. “At first, I added the rotational component mainly to see if the design was feasible,” he says.

To put a historical perspective on his design, when the jet revolution came about, everybody was designing for turbojets, says Caldarola. Until the advent of 707s and DC8s, most of the aircraft at that time used surplus World War II piston engines, continuations of what aircraft engines were from the start. Then Rolls-Royce got the idea of replacing the heavy, out-of-date engines it was using on large transports — the engines had peaked in power.

“The idea was to get more power out of a lighter package,” explains Caldarola. So Rolls-Royce attached a gearbox in front of the turbojet engine and coupled this gearbox to a propeller. Suddenly, the company had a new product known as the Rolls-Royce Dart.

“The Dart stayed in production for a long time and is still used by a lot of transport and passenger aircraft in the world, an amazing tribute to its longevity,” says Caldarola. “For example, in the 70s, a precursor to the Gulfstream Business Class aircraft — the Gulfstream 1, originally a Grumman production — used two Dart engines And a Dutch aircraft called the Fokker F-27 uses the engines today.”

Freewheeler engines and contrarotating propellers
“In adding the gearbox, I wanted to have a single shaft, but the complexity of the design was too great,” says Caldarola. “So I reverted to a more modern design — what are called ‘freewheeler’ engines,” he says. These have two shafts and two sets of turbines. “This means one part of the engine technically is not connected to the other part. The gas generator, which is the front portion, couples to its own turbine and it rotates at its optimum speed, which can vary depending on the throttle setting. The gearbox and the propeller assembly attach to a second turbine, which is rotating at its own speed and in the opposite direction of the initial turbine. Since each turbine spins at its optimum rpm, rarely the same speed for both, the result is a highly efficient engine with a lower temperature exhaust. Also, the front turbine absorbs most of the gas energy of the combustion blast exiting the engine. This keeps the compressor running. The rear turbine provides power only to the propeller. Thus, both turbines work at peak efficiency for each purpose.

“As things progressed, the contrarotating portion of the design came into play for other reasons besides just seeing if the design were feasible,” says Caldarola. “First, I wanted something different — a single propeller rotating is a common sight, while two back-to-back is more unusual. But more importantly, a right-hand and left-hand rotational gearbox setup is required for multi-engine installations to forestall a tendency for the plane to turn in one direction as in single-engine layouts. The gyroscopic effect is eliminated because the propellers rotate in opposite directions at the same speed thus balancing things out.”

Dual propellers also absorb a larger amount of power coming out of the engine, says Caldarola. “For example, my engine puts out 3,000 hp. It required either a propeller with multiple blades to push the needed huge amount of air forward, or a propeller with fewer but extremely long blades. Had I used, for instance, a single four-bladed propeller, it would have required a diameter of 16 ft to absorb that kind of power. That size is unmanageable.”

According to Caldarola, having two contrarotating propellers also provides a mechanical advantage in the form of a supercharging effect. “Air is getting sucked in by the first set of blades and then getting fed at a higher pressure and speed to the second set of blades,” he says. “For illustration, consider two fans. Say one fan is pushing air outward and the other is pulling air forward and you point them at each other. The fan pushing air into the other fan is, in effect, giving additional energy. The resulting push-pull effect gives a quicker flow with less swirl. More energy is actually pushed out of the propellers.”

This setup has been uncommon in the past because of its complexity, says Caldarola. “The gearbox is quite a bit more intricate than a normal gearbox. But the design has been implemented in some fairly modern aircraft. Consider, for example, the extremely powerful Tupolev Tu-95MS Bear H. It has four engines attached to the front of the wings and each engine is putting out 15,000 hp. It would require 25-ft-diameter propellers to absorb that much energy. The use of contrarotating propellers lets the plane use 14-ft-diameter props.”

Unlike the Bear, Caldarola’s engine is intended to bolt on the back of the wing, not the front, and thus is classified as a pusher configuration. “I’d like to see a version of my design be put into commercial production in the near future. With newer technologies, I believe the engine could be quite a bit more efficient than the ones in play now,” he says. “In addition, the advent of new materials and the capability to manufacture new shapes should make this endeavor worthwhile.”

3D printing the engine
The 3D print built on a Stratasys FDM printer helped Caldarola prove-out his design and it provided a prototype for public showing. “The complete engineering model in Autodesk Inventor had 4,800 parts. We needed the prototype for an upcoming show, so we decided to 3D-print only the visible parts, eliminating large internal components that could not be seen anyway. We sent the files to Stratasys and it converted the simplified model to the correct format to be fed to the FDM printer. We then assembled everything using industrial bolts, which proves that the printed ABS plastic is strong enough to withstand a real-world assembly process. Better yet, looking at the prototype let me easily see that my calculations, if not spot-on, were good enough that the engine works,” he says.