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A propfan, also called an open rotor engine, open fan engine [1] [2] or unducted fan (as opposed to a ducted 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.
In aeronautics, a ducted fan is a thrust-generating mechanical fan or propeller mounted within a cylindrical duct or shroud. Other terms include ducted propeller or shrouded propeller . [ 1 ] When used in vertical takeoff and landing ( VTOL ) applications it is also known as a shrouded rotor .
An axial fan is a type of fan that causes gas to flow through it in an axial direction, parallel to the shaft about which the blades rotate. The flow is axial at entry and exit. The fan is designed to produce a pressure difference, and hence force, to cause a flow through the fan. Factors which determine the performance of the fan include the ...
The propulsive efficiency is always less than one, because conservation of momentum requires that the exhaust have some of the kinetic energy, and the propulsive mechanism (whether propeller, jet exhaust, or ducted fan) is never perfectly efficient. It is greatly dependent on exhaust expulsion velocity and airspeed.
[4] [5] [6] Consequently, the exhaust jet produces about 10% of the total thrust. [7] A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds. [8] Turboprops have bypass ratios of 50–100, [9] [10] although the propulsion airflow is less clearly defined for propellers than for fans. [11] [12]
The Pratt & Whitney/Allison 578-DX was an experimental aircraft engine, a hybrid between a turbofan and a turboprop known as a propfan.The engine was designed in the 1980s to power proposed propfan aircraft such as the Boeing 7J7 and the MD-91 and MD-92 derivatives of the McDonnell Douglas MD-80.
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