Printed headline: Synonym for Efficiency
“We took a plane with propellers; I thought these things were no longer flying,” is a comment often overheard in arrivals concourses. Even so, “nothing in aviation is more beautifully drawn than a propeller,” some private pilots and design engineers like to say.
The perception of propellers as an obsolete form of propulsion marks a chasm between those passengers without a particular interest in aeronautics and those with an aviation bug—professionals, enthusiasts or those curious about technology.
ATR, now the main manufacturer of regional turboprops, has even felt compelled to launch a dedicated advertising campaign this year. To improve the image of propellers, it is comparing their movement to the shape of a DNA double-helix. ATR executives are concerned that passengers continue to see propeller-equipped aircraft as old- fashioned.
When introduced, turbofans brought so much to commercial air transport in terms of power, speed and lower noise that they supplanted turboprops. The vast majority of commercial aircraft today are jets. But a jet engine is not always the best alternative, and commercial aircraft operators are becoming more aware of this.
After all, a turboprop is only a slightly different mechanism. The propeller is connected to a turbine engine. Schematically, a turbofan is similar, but with a fan instead of the propeller—as the name suggests.
A turboprop will not provide the same kind of cruise speed as a turbofan. However, in many cases, it makes sense commercially since a turboprop engine is cheaper to buy and operate.
On short routes, the higher cost of owning and operating a turbofan may be prohibitive, and the greater airspeed would not reduce flight time very much. In some locations, a jet’s takeoff requirements would be incompatible with a short, possibly unpaved runway.
A propeller is much more efficient than a fan, thanks to the resulting greater bypass ratio. ATR claims a 40% fuel-burn advantage over similarly sized jets on a 300-nm route. On a short flight of about one hour, “you don’t have time to reach high altitudes and high speeds anyway,” ATR officials emphasize.
That means a propeller can make flying more affordable—or simply possible—as well as less harmful for the environment.
For all these reasons, propellers are here to stay. Moreover, their design has continued to improve. Passenger experience aboard a modern regional turboprop is very good by today’s standards, in terms of vibration and noise. We can also expect propeller technology to progress further.
The current state of the art includes composite-material blades, digital controls and individual blade replacement. Propellers have become complex, and a propeller system’s type certificate (TC) is distinct from the engine’s TC.
So what are the design drivers for a propeller?
“The starting point for designing a propeller will come from aircraft architecture,” says an expert at Collins Aerospace. The propeller’s diameter is generally limited, based on clearance to the ground—depending on the size of the landing gear and the fuselage—for passenger comfort considerations. Collins’ propeller manufacturing subsidiary is Ratier-Figeac, based in Figeac, France.
Blade-tip speed should not exceed Mach 0.92, “otherwise you get compressibility losses at the tip,” Martin Albrecht, MT Propellers’ vice president and general manager, adds.
MT Propellers, based in Germany, offers propellers for the smaller end of commercial aviation. These are for aircraft such as the Cessna Caravan, the Beech 1900 Series, the Mitsubishi MU-2B and the BAE Jetstreams 31/32/41. “Ninety-nine percent of turboprops can use our propellers, up to 5,000 shp,” says Albrecht. This means they could be fitted on ATRs and De Havilland Dash 8-400s (formerly known as Bombardier Q400) regional aircraft.
Another design driver is that the propeller has to accommodate fatigue loading through its structural design, the Collins expert adds.
Part of the advancements on propeller systems comes from the control unit. “It has full authority on propeller speed and blade pitch angle,” Dowty’s technical director, Jonathan Chestney, explains. It interfaces very closely with engine control, which, especially in case of emergency, enables interaction between the two control systems to maximize responsiveness. “We have experience in integrating our controller with the engine control unit, embedding our algorithms,” adds Henry Johnston, services executive at Dowty Propellers.
Dowty, a GE Aviation company headquartered in the UK, supplies R408 propellers to De Havilland for the Dash 8-400. In regional aircraft, the Antonov An-132D demonstrator and the in-development Avic MA700 also use the R408. Other commercial programs for the company include the out-of-production Saab 2000, Saab 340 and Fokker 50.
Also equipped with Dowty propellers are some military and amphibious aircraft as well as hovercraft. Given their severe operating environments, Dowty has developed more robust external coatings. In turn, their technology has transferred to civil applications, says Johnston.
A particularly elaborate subassembly is the pitch-change system. The linear movement of a piston is translated into a rotating movement, Albrecht explains. MT Propellers uses an electric system up to 500 shp and a hybrid hydraulic-electric system for more powerful engines. Inside the hub, the required force may be in the 4-6 metric ton range. A hydraulic system has a better power-to-weight ratio.
Individual blade removal has become standard since Dowty introduced the concept on its six-blade propeller for the Q400. This makes the propeller system easier to maintain. “You can remove one single blade in case of damage from a bird strike or a collision with a truck on the ground,” says Johnston.
However, this also creates a production challenge. A slight difference in weight distribution or aerodynamic lines can result in vibration. “You need a high level of consistency in manufacturing,” Chestney emphasizes.
To improve current propeller designs, Collins is betting on fleet data analysis. Last June, the company launched an operating cost-reduction program, aimed at a 20% improvement on the ATR 42/72’s six-blade propeller, the 568F. The effort will make the most of field support but also will include “life-cycle technology upgrades.”
Based on more than 30 million flight hours of 568F propeller service experience, field data collection including direct feedback from operators, intensive research and technology investments, Collins says it will incrementally introduce advanced technologies and materials for the propeller in the areas of deicing, protective layers and treatment, pitch-change systems and chromium-free materials.
In addition to the ATRs’ 568F, Collins provides Avic with propellers for its MA60 program. The France-based subsidiary also has military programs, such as the massive, eight-blade FH386 on the Airbus A400M transport.
At its new propeller center of excellence in Figeac, due to open in 2020, Collins intends to design a propeller with health monitoring and prognostic capabilities. It will be part of what the company says is an “overarching focus on developing more intelligent aircraft of the future.” The payoff is expected to be reduced maintenance.
Meanwhile, Dowty is looking at infusing more 3D aerodynamics into its designs. Computational fluid dynamics would be supported by wind-tunnel testing, Chestney suggests. Fuel efficiency would be enhanced through improved propeller and aircraft efficiency, along with reduced propeller noise, a minimization of vibration and enhanced propeller system reliability, he adds.
MT Propellers’ engineers believe they have found a way to increase the number of blades per propeller and, consequently, the aircraft’s speed. Albrecht says the company has found an undisclosed application for the seven-blade propeller it has developed. With the seven-blade system, “you have two propellers in one to transfer the power so you can fly faster,” he says. While it has been designed for general aviation, MT Propellers’ documents suggest it could be used on much more powerful engines in regional aviation.
A nine-blade propeller is being flight-tested for research purposes. With such a system, a top speed of 420 kt. can be envisaged, Albrecht explains. The cruise speed of the Dash 8-400, the fastest in-production regional turboprop, is 360 kt.
The limiting factor is the centrifugal force the hub can withstand. MT Propellers has designed a blade that weighs 3-5 kg (6.6-11 lb.) instead of the conventional 8-10 kg, says Albrecht. The blade is based on MT Propellers’ signature technology, which uses beech and spruce wood fiber as well as synthetic-carbon fiber.
For the longer term, propeller system manufacturers have begun thinking of a path toward hybridization. A hybrid-electric aircraft may be more efficient, but it will have to carry a heavier load with the necessary batteries. Therefore, designing lighter-weight propellers will be even more critical, Dowty’s Chestney notes.