Blade design Turboprops have an optimum speed below about , because
propellers lose efficiency at high speed, due to an effect known as
wave drag that occurs just below
supersonic speeds. This powerful
drag has a sudden onset, and it led to the concept of a
sound barrier when first encountered in the 1940s. This effect can happen whenever the propeller is spun fast enough that the blade tips approach the speed of sound. The most effective way to address this problem is by adding blades to the propeller, allowing it to deliver more power at a lower rotational speed. This is why many
World War II fighter designs started with two or three-blade propellers but by the end of the war were using up to five blades; as engines with more power were introduced, new propellers were needed to more efficiently convert that power. Adding blades makes the propeller harder to balance and maintain, and the additional blades cause minor performance penalties due to drag and efficiency issues. But even with these sorts of measures, eventually the forward speed of the plane combined with the rotational speed of the propeller blade tips (together known as the helical tip speed) will again result in wave drag problems. For most aircraft, this will occur at speeds over about . A method of decreasing wave drag was discovered by German researchers in 1935—sweeping the wing backwards. Today, almost all aircraft designed to fly much above use a
swept wing. Since the inside of the propeller is moving slower in the rotational direction than the outside, the blade is progressively more swept back toward the outside, leading to a curved shape similar to a
scimitar – a practice that was first used as far back as 1909, in the
Chauvière two-bladed wood propeller used on the
Blériot XI. (At the blade root, the blade is actually swept forward into the rotational direction, to counter the twisting that is generated by the backward swept blade tips.) The Hamilton Standard test propfan was swept progressively to a 39-degree maximum at the blade tips, allowing the propfan to produce thrust even though the blades had a helical tip speed of about Mach 1.15. The blades of the GE36 UDF and the 578-DX had a maximum tip speed of about . This speed would be kept constant despite bigger or smaller propeller diameter by changing the maximum RPM. Drag can also be reduced by making the blades thinner, which increases the speed that the blades can attain before the air ahead of them becomes compressible and causes shock waves. For example, the blades of the Hamilton Standard test propfan had a
thickness-to-chord ratio that tapered from less than 20% at the spinner junction to 2% at the tips, and 4% at mid-span. Propfan blades had approximately half the thickness-to-chord ratio of the best conventional propeller blades of the era, thinned to razor-like sharpness at their edges, and weighed as little as . (The GE36 UDF engine that was tested on the Boeing 727 had front and back blades that weighed each.)
Noise One of the major problems with the propfan is noise. The propfan research in the 1980s discovered ways to reduce noise, but at the cost of reduced fuel efficiency, mitigating some of the advantages of a propfan. General methods for reducing noise include lowering tip speeds and decreasing blade loading, or the amount of
thrust per unit of blade surface area. A concept similar to
wing loading, blade loading can be reduced by lowering the thrust requirement or by increasing the amount, width, and/or length of the blades. For contra-rotating propfans, which can be louder than turboprops or single-rotating propfans, noise can also be lowered by: • increasing the gap between the propellers; • keeping back propeller blade lengths shorter than those of the front propeller, so that the back propeller blades avoid cutting through the blade
tip vortices of the front propeller (
blade-vortex interaction); • using different numbers of blades on the two propellers, to avoid
acoustic reinforcement; and • turning the front propeller and back propeller at different speeds, also to prevent acoustic reinforcement.
Community noise Engine makers expect propfan implementations to meet community (as opposed to cabin) noise regulations without sacrificing the efficiency advantage. Some think that propfans can potentially cause less of a community impact than turbofans, given their lower rotational speeds. Geared propfans should have an advantage over ungeared propfans for the same reason. In 2007, the Progress D-27 was modified to meet the United States
Federal Aviation Administration (FAA) Stage 4 regulations, which correspond to
International Civil Aviation Organization (ICAO) Chapter 4 standards and were adopted in 2006. A 2012 trade study by NASA projected that noise from existing open rotor technology would be 10–13 cumulative
EPNdB quieter than the maximum noise level allowed by the Stage 4 regulations. The newer Stage 5 noise limits (which replaced the Stage 4 regulations for larger aircraft in 2018 and mirrored the ICAO Chapter 14 noise standard established in 2014) are more restrictive than the Stage 4 requirement by only seven cumulative EPNdB, so current propfan technology should not be hindered by the Stage 5 standards. (The term "cumulative" is used to combine takeoff lateral, takeoff flyover and approach EPNdB margins relative to certification noise levels.) The study also projected that at existing technology levels, open rotors would be nine percent more fuel-efficient but remain 10–12 cumulative EPNdB louder than future aircraft with advanced ultra-high
bypass ratio turbofans.
Snecma estimates from open-rotor tests that its propfan engines would have about the same noise levels as its
CFM LEAP turbofan engine, which entered service in 2016. Further reductions can be achieved by redesigning the aircraft structure to shield noise from the ground. For example, another study estimated that if propfan engines were used to power a
hybrid wing body aircraft instead of a conventional tube-and-wing aircraft, noise levels could be reduced by as much as 38 cumulative EPNdB compared to ICAO Chapter 4 requirements. In 2007, the British budget airline
easyJet introduced its ecoJet concept, a 150–250 seat aircraft with V-mounted open rotor engines joined to the rear fuselage and shielded by a U-tail. It unsuccessfully initiated discussions with Airbus, Boeing, and Rolls-Royce to produce the aircraft.
Size A twin-engine aircraft carrying 100–150 passengers would require propfan diameters of , and a propfan with a propeller diameter of would theoretically produce almost of thrust. These sizes achieve the desired high
bypass ratios of over 30, but they are approximately twice the diameter of turbofan engines of equivalent capability. For this reason, airframers usually design the
empennage with a
T-tail configuration in order to avoid the turbulent propwash adversely influencing the elevators and causing vibration issues therein. The propfans may be attached to the upper part of the rear
fuselage. For the
Rolls-Royce RB3011 propfan prototype, a pylon of about long would be required to connect the center of each engine to the side of the fuselage. If the propfans are mounted to the wings, the wings would be attached to the aircraft in a high
wing configuration, which allows for ground clearance without requiring excessively long
landing gear. For the same amount of power or thrust produced, an unducted fan requires shorter blades than a geared propfan, although the overall installation issues still apply.
Output rating Turboprops and most propfans are rated by the amount of shaft
horsepower (shp) that they produce, as opposed to turbofans and the UDF propfan type, which are rated by the amount of
thrust they put out. The
rule of thumb is that at sea level with a static engine, is roughly equivalent of thrust, but at cruise altitude, that changes to about thrust. That means two engines can theoretically be replaced with a pair of propfans or with two UDF propfans. ==List of propfans==