The Ducted Fan 1__

By Barry H. Palmer

EAA 24772

SHARP-EDGE PIPE

FLARED PIPE

5555 NW. 5th St.

Miami, Fla. FIG.

1

FLUID OUTFLOW

HE DUCTED propeller has been around for many years, yet it could be classed as an aerodynamic curiosity. The sleekness of its physical appearance when compared to a conventional propeller could lead one to believe that, along with the jet engine, it is a device that could increase speed and efficiency of an aircraft. Yet, the ducted propeller, or fan, has been relegated to a few vertical take-off aircraft, and that veritable aerodynamic slug, the hovercraft. When one examines just what a duct does, it is easy to see why its station in life is that of the slow,

T

hard push.

sion device. This would give the optimum propulsive efficiency. The above concept can be understood if it is recognized that the air passes through the propulsion device at a greater speed than the aircraft passes through the air. The thrust of the propulsion device is the same as the drag of the aircraft, and thus the propulsion device uses more horsepower (force times speed) than the aircraft absorbs (force times lower speed). The greater the frontal arey of the propulsion device, the less is the difference in speed that the airplane and its propulsion device experience. This is the concept of propulsive efficiency. Naturally, propul-

To appreciate the operation of a ducted fan, a few concepts concerning an air propulsion device must be understood. First, in order that the propulsion device, whether it be a jet, propeller, or articulating wing, must jet air backwards faster than the propulsion device moves forward through the air. This can be easily understood when it is realized that, in order to force the plane or airboat or hovercraft forward, there has to be an equal and opposite reaction on the air stream. The air must be accelerated from zero speed to some backwards velocity. Obviously, if the air were following the propulsion device after its passage through the propulsion device, the effect would be one of braking the vehicle involved.

sion d e v i c e s are limited in size, as a trade-off exists between maximum propulsive efficiency and considerations of weight, clearance, engine/propeller speed matching, blade tip speed, etc. It should be realized that an increase in propulsion device frontal area results in a reduction in the velocity of the air through the device at a given thrust level. Consider a tank of water with two pipes of identical diameter standing on the bottom. One pipe is flared and the other is cylindrical in shape (Fig. 1). Although the pipes are of the same diameter, considerably more water will flow out of the pipe with the flared inlet than will flow from the

STREAMLINES FOR A PROPELLER (Stol.c Thrust)

_

CAPTIVE VORTEX - STREAMLINES FOR EQUIVALENT DUCTED FAN

It is also a fact that if only energy or power conservation considerations

are involved, the frontal area of the

FIG.

2

propulsion device should be infinity.

If it could be physically done, one would want all the air and water on earth passing through the propul20

AUGUST 1968

PROPELLER

DIAMETER OF EXHAUST JET THE SAME AS FOR DUCTED FAN AND PROPELLER

sharp-edged pipe. The reason for this is the water cannot turn the corner and get aligned to the sharp-edged pipe as easily as when a smooth flow transition is provided as in the case of the flared pipe. The resulting centrifugal force of the water trying to make the tight turn forces the water to the center of the pipe and causes an effective reduction in the flow area of the sharp-edged pipe. In short, the fluid stream contracts in diameter. A propeller in static thrust is somewhat analogous to the sharp-edged fluid "sink" whereas, with a flared inlet which can be radiused as little as six percent of the pipe diamef.or, the flared pipe is analogous to ;he ducted fan. A propeller and a ducted fan of equivalent static thrust per unit horsepower are depicted in Fig. 2. Although the propeller is much larger in diameter than its ducted fan equivalent, the diameter of the exhaust blast of both units is the same. The propeller, as a propulsion device, has an effective frontal area considerably less than its geometric frontal area. In static thrust, the discharge area of the propeller is approximately one-half the propeller geometric disc area whereas, if the designer is somewhat adventurous, the discharge area of the ducted fan can be slightly greater than the fan diameter, or approximately the same as the shroud outside diameter. As can be seen from Fig. 2, the air reaches the rotor in the case of the ducted fan. The propeller, being unconfined, is free to draw air in radially and, indeed, may sometimes even ingest some of its own Wast, creating an airy donut around its blade tips which further reduces the effective frontal area. Following are considerations governing one's choice of a ducted fan over the conventional propeller. The most notable purpose of the shroud is that it protects the propeller from foreign objects, such as people who are in turn protected from

the propeller. If a hovercraft or airboat is contemplated (or one just does a lot of taxiing with his very special design) and especially if operations are to be conducted around inexperienced personnel, such as the neighbor's German Shepherd, a serious look at a shroud is worthwhile. Another consideration which may be involved in design is propeller clearance. A heavily loaded ducted fan is roughly 30 percent smaller in diameter than its equivalent propeller. If one's tail-pusher design massages the runway whenever rotation is initiated, a duct may be in order. The compactness of the ducted fan answers another designer's woe . . . - engine/speed matching. A Volkswagen engine, for instance, combines low horsepower with high shaft speed. Both conditions are conducive to propeller inefficiency, as slow flight from low power makes a large diameter flat pitch propeller desirable and, at the same time, the high shaft speed requires the propeller to be designed with a very flat pitch also. The normal solution to this problem is to gear the Volkswagen. There is a possibility that a duct, and the smaller diameter propeller required, could be competitive to gears. Following are disadvantages and problems encountered through the use of the ducted fan. As vehicle speed builds, propeller slipstream contraction diminishes and propulsive efficiency increases. The ducted fan can improve performance only when there is significant propeller slipstream contraction. As a result, the duct may help low-speed performance and, even though the steeper pitched ducted propeller itself is more efficient than a propeller, the duct itself becomes parasitic, resulting in a reduction of top speed. It is true that some designers employ a duct to give them short field capability and, instead of "taking their lumps" at highspeed levels, the designers use the duct for something else . . . tail

"WATERSPORT" . . . (Continued from poge 19)

and rudder, plus raising the engine thrust line by two

degrees which cured the yaw problem and also shortened the take-off run on the water. During the summer of 1967, Joe and Nick bought out Larry Spiewak's share in the plane and now there are

only two owners. Nick plans to fly to northern Canada

and, to prepare for that trip this summer, has made a number of changes during the winter months. The 26 gal. fuel tank originally located in the hull has been removed

and two 13 gal. tanks will be installed in the wing roots.

feathers. This fact, augmented by the reduced propeller diameters, makes the ducted pusher a popular designer's configuration. Another problem associated with a ducted fan is construction. It is extremely difficult to make something big, light, rigid, and round. A duct has to be very rigid and very round. If too much blade clearance is allowed, flow separation from the duct may occur and the propeller inside will act like . . . well . . . a little propeller . . . airy donuts and all. There is a strong possibility that the propeller might have to be isolated from engine vibration so that it might run concentrically with the duct. Hopefully, propeller one-per-revolution vibration would not be too severe. In conclusion, a ducted fan in static thrust could be expected to produce 40 percent more thrust than a propeller of the same diameter in static thrust. The thrust advantage

diminishes rapidly for lightly loaded propellers, and more slowly for heavily loaded propellers. Additionally, a ducted fan in static thrust can be 70 percent of the diameter of its equivalent propeller, and the diameter anomaly diminishes as forward speed is increased. While the ducted fan is easily designed and developed by a trained fluid mechanic (aerodynamics engineer), each individual aircraft or vehicle design must be weighed as a system in many factors before the effect of a duct can be assessed on over-all performance. G e n e r a l l y speaking, a ducted fan should shorten take-off run, improve climb, but cut top speed. It should be realized that five more feet on the wing tips of one's mind's-eye F-104 will do the same thing, but a lot more simply. For slow speeds, and heavy loading, such as encountered in hovercraft design, and particularly underwater in tugboat design, where protection is paramount, a ducted fan may be wisely employed.

This will allow more room behind the seats for luggage and permit installation of a soundproof bulkhead behind the seats. Also, the manual landing gear retraction handle

between the seats will be replaced by a hydraulic retraction system using a cylinder from an automobile convertible top. This will allow use of a more comfortable and roomier bench-type seat.

If you get the chance, beg, borrow or steal a ride in the "Watersport." It's a fun plane! A recent trip over Lake St. Clair, cruising 100 ft. above the water, returning the boaters' waves and landing at one of the many islands

in the lake revealed the true "fun use" a builder can have with an amphibian.

®

SPORT AVIATION

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