Ducted propeller
As a ducted propeller ( English ducted fan ) is referred to a drive device in which a specially adapted shape of a propeller is mounted in a cylindrical housing. The advantage of this arrangement compared to a "free moving" propeller is that the housing reduces the thrust losses due to turbulence at the blade tips of the propeller. The performance in relation to the diameter is increased, but not the efficiency of the propeller. Ducted propellers are used as a drive in aviation in many ways, such. B. in aircraft , hovercraft and flat-going boats ( swamp boats ). The fan of a turbofan engine is a special form of application .
history
The ducted propeller is first mentioned in 1918 in a patent from Mercur Flugzeugbau GmbH. This describes a "device for improving the efficiency of propellers", which is based on the use of guide vanes with adjustable pitch and a ring surrounding the propeller.
Before the Second World War , the Stipa-Caproni was best known, a barrel-shaped aircraft in which the ring around the propeller also formed the fuselage. The experience with this construction then flowed into the design of the Campini-Caproni CC2 . Both designs are seen as milestones on the way to developing the jet engine .
In the 1950s and 1960s, the main applications were as slew thrusters in VTOL aircraft. Examples are the Doak VZ-4 , the Nord 500 and the Bell X-22 . It was not until the 1970s that it was considered and implemented in general aviation aircraft.
technology
With a ducted propeller, the propeller (rotor) is surrounded by a profiled shroud ring. Compared to the uncovered, "free-moving" rotor, this leads to a different form of flow. The jet of accelerated air is not constricted, zones with reduced pressure are created on the entry lips of the mantle, which contribute to the thrust. In the ideal case, this increases the static thrust to double the value of the free-moving rotor. Since the mass flow through the rotor increases at the same time, the necessary drive power also increases. If you compare the free-moving and the jacketed rotor on the basis of equally large input powers, the gain in thrust is significantly smaller, but can still be up to 26%. In the practical application of the ducted propeller, the gain in thrust is often used to reduce the diameter of the propeller while maintaining the power and thrust of a free-moving propeller. Additional advantages are that the casing of the propeller also reduces noise and increases safety.
conditions
In order to be able to use the advantages mentioned, the desired flow shape must be established reliably during operation. On the one hand, the rotor of a ducted propeller must be given a different shape than a normal propeller. The number of propeller blades is usually also significantly larger. The propeller has an odd number of blades in order to avoid resonance vibrations in the jacket. Furthermore, the gap between the rotor tips and the jacket must be very small (max. Approx. 1% of the diameter), otherwise pressure equalization occurs around the tips. Otherwise a flow separation can occur in the jacket behind the rotor. So it has to be worked with very small manufacturing tolerances in the manufacture of the jacket .
disadvantage
Especially in the case of VTOL aircraft, the additional weight due to the necessary casing, including the suspension and drive of the rotor inside it, can mean a reduction in thrust gain and thus considerably limit the performance spectrum. The full gain in thrust is only achieved when the vehicle is stationary. In forward flight the rotor is flown against from the front, which inevitably leads to a reduction in the gain in thrust. In order to prevent flow separation even at higher horizontal speeds, an adapted shape of the inlet area of the jacket is necessary. In extreme cases, this can require a "variable inlet geometry". In addition, the surface of the jacket creates additional air resistance in forward flight. Overall, these problems have led to the ducted propeller being used in aviation, outside of engine construction, only in individual cases.
Applications
By far the most common technique of mantle propeller is in aviation turbofan engines used in the form of the turbofan. In German usage, the term “turbofan” or simply “fan” is mostly used for this special form of ducted propeller, as in English. There is no standardized German translation for this, but the terms "Bläser" or "Gebläse" are usually used. The power for driving the fan is provided by a gas turbine. Turbofan engines are used today on virtually all larger passenger aircraft and many military aircraft. However, the ducted propeller can be driven by any machine that can deliver power via a shaft.
A special application is found in helicopters, where the ducted propeller is used to replace the tail rotor and to ensure the torque compensation of the main rotor. The trade name Fenestron is known with application z. E.g. the Eurocopter EC 135 .
Furthermore, the principle of the ducted propeller is often used in model flying to imitate model- like jet drives with electric motors. The ducted propeller is driven by a powerful brushless electric motor. One speaks here of an impeller . The efficiency is usually worse than with a conventional propeller.
Selected applications of the ducted propeller in aircraft
| Application target | Type example | Installation type |
|---|---|---|
| Generation of propulsion in hovercraft | LCAC | vertical with adjustable blades |
| Torque compensation and generation of propulsion in helicopters | Sikorsky Piasecki X-49 | rigidly vertical in the tail boom |
| Torque compensation in helicopters | Eurocopter EC 135 | rigidly vertical in the tail boom |
| V / STOL capability | Lockheed Martin F-35 | rigidly horizontal behind the cockpit |
| V / STOL capability | Bell X-22 | movable on fixed sash |
| V / STOL capability | North 500 Cadet | movable on fixed sash |
| V / STOL capability | Doak VZ-4 | movable on fixed sash |
| STOL capability | Custer Channel Wing | Fixed wing with "half" ducted propeller |
| Jet-like flight behavior in a piston engine-powered aircraft | Rheinflugzeugbau Fantrainer | Rigid vertically integrated in the fuselage |
| STOL capability, improved visibility (270 °) for aerial mapping | Edgley EA7 | Rigid vertically integrated in the fuselage |
| Reduction of noise emissions | Dowty Ducted Propulsor | Replacement of the original engines on Britten-Norman BN-2 Islander |
American air cushion landing craft "LCAC"
Piasecki X-49-3 with ducted propeller for level flight
8-blade Fenestron on a Eurocopter
The "fan" or "blower" has a significantly larger diameter than the rest of the engine part ( General Electric CF6 )
See also
Individual evidence
- ↑ The fan is the first stage of the low-pressure compressor ( memento of the original from September 24, 2015 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.
- ^ Mercury (1918) . In: Karl R. Pawlas: Aviation Lexicon . Entry ID: 3080-100-3.
- ↑ Fan-in-a-ring . In: AIR International, February 1977, pp. 59-66.
- ↑ Theory on the ducted propeller (PDF; 184 kB)
- ^ RH Barnard, DR Philpott: Aircraft Flight: A Description of the Physical Principles of Aircraft Flight . S. 152 f . ( google.ch ).
- ↑ textronmarineandland.com (PDF; 4.2 MB) LCAC technical description on the manufacturer's website ; Retrieved April 20, 2013.
- ↑ vstol.org ( Memento of the original from March 20, 2008 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Photo of the Nord 500 Cadet
- ↑ custerchannelwing.com photo of the Custer Channel Wing CCW-5
- ↑ fantrainer.de Drive principle of the Fantrainer
- ↑ aviastar.org Photo and data from Edgley Optica
