This article demonstrates events that bring revolution in propeller design and innovation. Propeller design has changed in only small increments since 1903. Design changes incrementally in mainstream aviation, and major design innovations are rare. The mechanical and aerospace engineers who design propellers are innovating again. In propeller design, high power and low pitch are ideal for take-off, while higher pitch and lower power are ideal for high-altitude cruising. In order to meet these conflicting demands, a variable pitch mechanism is used to automatically control the blades’ pitch in flight. While new materials are important, engineers say faster, cheaper computers, along with the nearly limitless hardware and software resources in the cloud, have contributed the most to propeller design. Affordable technology is also allowing many small companies to compete against the big boys and take risks that were once far too expensive and risky to attempt.
A computational fluid dynamics analysis by Joby Aviation determines the optimal propeller and wing design for an electric-powered aircraft.
Aerospace engineer Jeremy Bain took one look at NASA's experimental propeller and wing designs and thought there was absolutely no way they could ever work. Bain, who runs Bain Aero in Stockbridge, Ga., is known for his expertise in analyzing rotors and propellers using computational fluid dynamics. Two years ago engineers working on the Leading Edge Asynchronous Propellers Technology, or LEAPTech, project reached out to Bain to show him some very early wing designs they were working on.
What they presented looked like nothing else in aviation. Eighteen small electric propellers were distributed along the leading edge of thin wings with just a 31-foot wing span and an area a third of those on an average commuter plane.
To take off, cruise, and land safely, fixedwing airplanes need to generate enough lift to overcome the plane's weight and its aerodynamic resistance, or drag. They typically generate that lift from the aerodynamic force created when the plane's wings move through air.
But as NASA's engineers explained to Bain, their plan was for the wing-mounted propellers to blow air backward over the wings, generating extra lift. This effect, known as induced velocity, essentially tricks the plane into thinking it's going faster than it really is. It would help the plane cruise using less power, even with lightweight, thin wings that would never support a conventional aircraft.
But Bain couldn’t get past the design. “It's just way too complicated,” he said to himself while the NASA engineers described the project.
Despite his initial skepticism, Bain soon came around. He knew that he and other propeller designers were moving beyond the traditional two- or three-bladed aluminum alloy propellers powered by gas turbines or piston engines. Instead, they were building blades out of carbon fiber composites, and using them in innovative propeller designs with four, five, and even more blades. Those propellers, thanks to new electric motors, could be placed almost anywhere on an aircraft to optimize performance.
These innovations are helping create one of the most exciting eras in aircraft design since the early 20th century.
After looking at the LEAPTech plans for a few minutes, Bain realized the project represented major trends that would continue to shape propeller design.
“You have all of this freedom to do things you never thought you could do,” said Bain, who ended up working on LEAPTech, a phase of NASA's ongoing program to build and test a new electric propeller and wing assembly that could lead to quieter, safer, less polluting, faster, and more efficient aircraft. “Now you can go back to the drawing board and do something completely different.”
In fact, he realized, by designing advanced propellers and using them in unprecedented ways, the work could help make improbable ideas about the future of flight a reality.
Propellers Take Shape
Propeller design has changed in only small increments since 1903, when Orville and Wilbur Wright crafted propellers for their Flyer from laminated spruce boards. From the 1920s through the 1940s, changing military and transportation demands drove the development of more powerful airplane engines, which moved propellers faster, placing greater stress on them. Propeller makers responded with innovations in material design and manufacturing to handle the faster rotation speeds. For example, beginning in the late 1920s, propeller makers started to replace laminated wooden blades with solid steel and eventually lighter aluminum alloys, usually surrounding a wood core to dampen vibrations.
Design changes incrementally in mainstream aviation, and major design innovations are rare. If an aircraft has a strong safety record, as the industry sees it, there's no reason to change it until an absolutely rock-solid, proven change comes along to replace or augment a standard.
This is especially true for propellers. The way they work is incredibly complex, making it difficult to customize and quickly change the design without the aid of high-powered computers. One small change in the blade can jeopardize the performance and safety of the entire aircraft.
For that reason, propeller design has changed very little for more than half a century.
Today, though, NASA and aircraft manufacturers are demanding advances in propeller design as they look to create new types of planes, helicopters, and unmanned aerial vehicles (UAVs). These different kinds of aircraft require propellers optimized to carry out specific flight profiles: super quiet for take-offs and landings in residential backyards or busy commercial districts; light but incredibly tough for combat missions; low maintenance for long-term surveillance; highly efficient for commercial travel, and more.
In response, the mechanical and aerospace engineers who design propellers are innovating again. They’re using new carbon fiber composites to make propeller blades that are stiffer, thinner, and lighter than those made from aluminum alloys. They’re using inexpensive, powerful computers for quicker design iterations, which let them experiment with a greater variety of designs. And they’re combining CAD software and advanced manufacturing tools like 3-D printers to combine composites to create prototypes, which they can tweak, scrap, and replace without worrying about the costs and time associated with manufacturing metal propellers.
Propeller design “used to be one size fits all,” said Martin Albrecht, an engineer and general manager of MT-Propeller, a German manufacturer of high-performance propellers with U.S. headquarters in DeLand, Fla. “Now we can customize any propeller for any application.”
New Twists on Propellers
A propeller blade is essentially a rotating wing. It transfers the power produced by an engine to force air to move through the diameter of the propeller or disc. When propeller blades are spinning vertically, as in a conventional fixed-wing commuter plane, that force generates the thrust that moves the plane through the air. When they’re spinning horizontally in a helicopter or UAV, that force keeps the aircraft aloft.
To move air efficiently, propeller designers hone several key elements of a blade. These include the angle of attack, which is the angle at which the air hits the blade, and the pitch, which is the angle of the blade relative to the propeller hub. The pitch works like the grooves of a screw. The steeper the groove, the faster the screw bites into wood with each turn. Similarly, the greater the blade's angle, the more force it applies to the air to create airflow. Leonardo da Vinci described the designs for his flying machines as “aerial screws” for a reason.
In propeller design, high power and low pitch is ideal for takeoff, while higher pitch and lower power is ideal for highaltitude cruising. To meet these conflicting demands, a variable pitch mechanism is used to automatically control the blades’ pitch in flight.
Small or inexpensive aircraft forgo that for a fixed-pitch design—a compromise between takeoff, climb, and cruise performance requirements. Usually, the blade is twisted, with the pitch higher near the hub and lower near the tip. Increasing the camber or curvature of the blade creates greater thrust.
As a propeller spins faster and its tip speed approaches the speed of sound, its performance greatly diminishes due to drag and vibrations. Noise inside and outside of the cockpit also increases. Stronger, thinner, better balanced, and more aerodynamically efficient blades can help mitigate these problems.
Carbon fiber composites were introduced in the mid-1990s for many of the same reasons metal began replacing wood back in the 1920s. They are typically lighter than metal blades and allow designers to add more blades to the propeller without increasing its weight. Lighter blades generally contribute to a quieter, smoother flight. They also require less energy to spin, which decreases fuel consumption and reduces force on the propeller's hub.
“You just can’t design a metal blade in the same shape as composites,” Bain said. “Composite blades are stronger, so you can make them thinner.”
For LEAPTech and other leading-edge projects, engineers are also attaching propellers to new types of motors, and placing them in different arrays and positions on the aircraft that they didn’t think were possible a few years ago. “Over the next 10 and 20 years there’ll be tremendous growth in the types and designs of propellers,” Bain said.
Technology Drives Change
While new materials are important, engineers say faster, cheaper computers, along with the nearly limitless hardware and software resources in the cloud, have contributed the most to propeller design.
Bain, who performed aerodynamics and aeroacoustic calculations on composite rotor blades for NASA and DARPA as a research assistant at Georgia Tech, remembers when only large companies and well-funded institutions could afford the hundred thousand-dollar computer systems needed to perform the analysis and simulation for cutting-edge blade design. Today, Bain's small company solves complicated computational fluid dynamic problems and performs high-fidelity and blade element analysis by running resourcehungry programs on four Supermicro $5,000 blade servers with 128 processors.
Affordable technology is also allowing many small companies to compete against the big boys and take risks that were once far too expensive and risky to attempt.
“We can do things much more aggressively than in the past,” Bain said.
“Not only can we design the propellers, but we can also build them.”
Engineers can thank 3-D printers for that.
When Joby Aviation, a Santa Cruz, Calif.-based company that's vying to be the Tesla Motors of the aviation industry, began designing a distributed electric propulsion system under a LEAPTech contract, it had to quickly produce prototype propellers for design validation and performance prediction tests. The team designed a propeller, shipped the computer model to a 3-D printing company, and had the prototype back at the office within two weeks. Based on that validated design, the team then built dozens of five-bladed, carbon-fiber propellers to use on LEAPTech.
For now, 3-D printing is suitable mainly for small prototype and test propellers that don’t require the high-performance characteristics of a propeller used for actual flight. Larger propellers, especially those made from new carbon composites, require more sophisticated manufacturing methods.
For example, Joby is developing carbon-fiber propellers for use on its S4, a four-seat electric personal aircraft designed to take off and land vertically. The S4's propellers spin horizontally for upwards thrust in hover, takeoff, and landing modes, but they also tilt 90̊̊ｱ/2 to spin vertically, which provides forward thrust as the aircraft cruises. That tilting exerts a tremendous force on the propeller, which must be extremely stable and designed to minimize vibrations. What's more, the S4's propeller includes five blades with a 10-foot diameter, which will experience even higher loads than shorter blades.
To counteract the enormous aerodynamic, centrifugal and gyroscopic loads on the propeller, Joby is leveraging high-performance composite materials like spread tow carbon fiber. These woven fabrics use thin and wide bundles of fiber or “tows” in a checkerboard pattern to create a more uniform laminate than common carbon-fiber fabric weaves. The new materials offer superior stiffness at a lighter weight, creating a higher performance propeller with greater stability.
Joby is also using an advanced carbon and metallic cocuring process to secure the blade directly to a titanium root that attaches the blade to the propeller hub. On a typical commuter plane, a propeller rotates at an average rate of 2,300 RPM, so fast that the blade tips approach the speed of sound. This creates centrifugal loads of about 10 to 20 tons per blade, enough to turn the slightest manufacturing flaw or in-service damage into a catastrophic failure. The co-curing process minimizes the number of pieces used to make a blade and reduces the chance of what's referred to as “departure,” which is exactly what it sounds like.
By the spring of 2015, just two years after Bain's skeptical first take on LEAPTech, it was time to put the new design to a test. At NASA's Armstrong Flight Research Center in Edwards, Calif., engineers mounted the propeller-lined wing on a truck, drove it across the desert to gather and measured aerodynamic data such as lift and drag. The propulsion system produced enough lift to take off and land at the same speed as a conventional commuter plane, but with a smaller wing that's more optimally sized for cruise efficiency, the test showed. The lift coefficient (a measure of how much lift a wing can generate from moving air) was close to three times that of a Cirrus SR22, a single-engine aircraft with a much larger wing mass.
To reduce drag even further, the leading-edge propellers, which are used mainly for takeoff and landing, can be folded onto the propeller's nacelle when a plane cruises. Highly efficient propellers at the wingtips then take over to provide the thrust needed to pull the plane forward. Overall, LEAPTech indicated that a plane built to the new design would be 50 percent more efficient than the SR22 and required five times less energy for comparable performance.
The new phase of the LEAPTech project is called Sceptor, short for Scalable Convergent Electric Propulsion Technology and Operations Research. For this stage, NASA is replacing the wing from a Tecnam P2006T commuter plane with a LEAPTech-type wing to create a new single-passenger experimental plane called the X-57 Maxwell. NASA plans by 2019 to transition that design into a nine-passenger plane powered by a 500 kW electric power system driving 14 propellers and generating nearly 700 horsepower, compared with 100 horsepower produced by each of the Tecnam's engines.
The multiple propeller array is also much quieter than a conventional commuter plane. The slower-spinning blades generate a weaker pressure pulse per revolution than faster propellers, and a smaller pressure pulse generally means less noise. The small blades, driven by electric motors, sound more like the steady high-pitched hum of a fan than the ripping howl of a conventional propeller powered by a fuel-burning engine. Electric motors also allow engineers to adjust the propellers to direct noise above the plane, instead of toward the ground, making them quieter still.
“There's a lot of untapped potential for propeller design in conjunction with the vehicle,” Clarke said. “As we explore more designs, we’ll uncover more opportunities for increased propeller and vehicle performance.”