Ducted fan
As a ducted propeller (English ducted fan ), a drive device referred to in a specially adapted form of a propeller is mounted in a cylindrical housing. The benefit of this arrangement compared to a " free-running " propeller is that the housing reduces the thrust losses at the blade tips of the propeller due to turbulence. It will increase the power based on the diameter, but not the efficiency of the propeller. Ducted propeller are used as drive in aviation varied, such as in aircraft, hovercraft and flat-bottomed boats (bottom boats). A special application form, the supporter, the first compressor stage of a turbofan engine dar.
History
Probably the first mention of the coat propeller is found in 1918 in a patent specification Mercur Flugzeugbau GmbH. Described herein is an " apparatus for improving the efficiency of the propeller ", which is based on the use of vanes with an adjustable slope, and of a ring surrounding the propeller.
Before the Second World War is particularly known the Stipa - Caproni, a barrel-shaped aircraft in the ring at the same time formed the hull around the propeller. The experience with this design flowed then in the draft Campini - Caproni CC2. Both designs are regarded as milestones on the way to the development of Jettriebwerks.
In the 1950s and 1960s, the applications were mainly in use as a pivot engines in VTOL aircraft. Examples are the Doak VZ- 4, 500 North and Bell X- 22nd Only in the 1970s was drawn into the incorporation of general aviation aircraft into consideration and implemented.
Technology
When ducted propeller, the propeller (rotor) is surrounded with a profiled outer ring. Compared to the unclad, " freely mobile " rotor, this will lead to a different shape of the flow-through. The beam of the accelerated air is not constricted to the inlet lips of the shell, zones of reduced pressure which contribute to the thrust. Ideally, by the static thrust increased to twice the value of the free-running rotor. Since at the same time but also increases the mass flow through the rotor, also the necessary drive power is increased. Comparing the free-running and the jacketed rotor based on equal input powers, then the thrust gain is much smaller, but can still be as high as 26%. In the practical application of the jacket propeller but is often the profit used in the thrust to reduce the diameter of the propeller to give the power and the thrust of a free-running propeller. An additional advantage is that also adjusts for increased safety in ground operations through the jacket of the propeller.
Requirements
To use the above-mentioned advantages, the desired flow form must adjust the operation safely. This requires on the one hand the rotor of a propeller shell have a different shape than a conventional propeller. The number of propeller blades is also significantly greater in the rule. The propeller has an odd number of blades in order to avoid resonant vibrations in the shell. Furthermore, the gap between the rotors and the casing must be very small (within about 1 % of the diameter ), otherwise a pressure equalization occurs at the tips around. Otherwise, flow separation in the casing behind the rotor can be done. It must be used in the manufacture of the shell with very small tolerances so.
Disadvantages
Especially for VTOL aircraft may in its interior mean a reduction in the thrust gain and thus significantly limit the range of services that extra weight due to the necessary sheath including suspension and drive the rotor. The full thrust profit arises only in the state. In forward flight, the rotor is flown from the front, which inevitably leads to a reduction of thrust profit. In order to prevent even with larger horizontal velocities flow separation, an adapted form of the inlet area of the jacket is necessary. This may require, in extreme cases a "variable inlet geometry ". In addition, the jacket in forward flight produced by its surface additional air resistance. Overall, these problems have led to the ducted propeller in aviation, outside of the engine design, only in individual cases applies.
Applications
By far the most common technique of the jacket propeller is used in turbofan engines in the form of turbofans in aviation. In German usage is for this particular form of a coat propeller, as in English, most of the term " Turbofan " or simply " fan " is used. A unified German translation for this purpose does not exist, but are commonly used the terms " wind " or " fan ".
The power for the drive of the fan is powered by a gas turbine. Turbofan engines used today in virtually all major airliners and many military aircraft. The ducted propeller, however, can be driven from any machine that can deliver power through a shaft. A particular application is found in helicopters where the ducted propeller is used to replace the rear rotor and to guarantee the torque balance of the main rotor. The trade name fenestron is known with application eg when Eurocopter EC 135