page_title: Propfan
section_title: Blade design
hierarchical_section_title: Propfan / Limitations and solutions / Blade design
caption_attribution_description: English: A schematic showing several types of jet engines. Note that the first engine is the only one that only uses jet propulsion; the other ones also transmit some of the generated power from the combustion for use with other propulsion devices (ie propeller, fan, rotor, ...). The schematic was based on the images: http://commons.wikimedia.org/wiki/File:Turbojet_operation-_axial_flow.png http://commons.wikimedia.org/wiki/File:Turbofan_operation_lbp.svg http://upload.wikimedia.org/wikipedia/commons/4/4e/Turboprop_operation.png http://commons.wikimedia.org/wiki/File:Turbofan_operation.png (as a basis for the propfan) The list of engine types was chosen based on a schematic in the book "Lucht en ruimtevaart" by Hugh Johnstone. A turbojet develops a propulsionary force simply because of the thrust beam. Turbofans add a fan to this, making them more efficient. Turboprops are quite similar to turbofans, yet use a propeller and a mechanical transfer system. Maximum speed is 800km/h. Propfans have propellers with bent blades, hereby being able to generate a speed up to 1000 km/h.
context_page_description: A propfan, also called an open rotor engine, or unducted fan, is a type of aircraft engine related in concept to both the turboprop and turbofan, but distinct from both. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. A propfan is typically designed with a large number of short, highly twisted blades, similar to a turbofan's bypass compressor. For this reason, the propfan has been variously described as an "unducted fan" or an "ultra-high-bypass turbofan."
context_section_description: Turboprops have an optimum speed below about 450 mph (390 kn; 720 km/h), because all propellers lose efficiency at high speed, due to an effect known as wave drag that occurs just below supersonic speeds. This powerful form of drag has a sudden onset, and it led to the concept of a sound barrier when first encountered in the 1940s. In the case of a propeller, this effect can happen any time the propeller is spun fast enough that the blade tips approach the speed of sound, even if the aircraft is motionless on the ground.
  The most effective way to counteract this problem (to some degree) is by adding more 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 the engines were upgraded, new propellers were needed to more efficiently convert that power. The major downside to this approach is that 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 (altogether known as the helical tip speed) will again result in wave drag problems. For most aircraft, this will occur at speeds over about 450 mph (390 kn; 720 km/h).
  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 450 mph (390 kn; 720 km/h) use a swept wing. In the 1970s, Hamilton Standard started researching propellers with similar sweep. 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 have a maximum tip speed in rotation of about 750–800 feet per second (440–470 knots; 230–240 metres per second; 510–550 miles per hour; 820–880 kilometres per hour), or about half the maximum tip speed for the propeller blades of a conventional turbofan. That maximum blade tip speed would be kept constant if the engine designer chooses to widen or narrow the propeller diameter (resulting in an RPM reduction or increase, respectively).
  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 twenty percent at the spinner junction to two percent at the tips, with the ratio being only four percent 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 20 pounds (9.1 kg). (The GE36 UDF engine that was tested on the Boeing 727 had front and back blades that weighed 22.5 and 21.5 lb (10.2 and 9.8 kg) each.)
caption: A comparison of the propfan with other types of aircraft engines.
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