DUCT FAN ARRANGEMENT

Duct Fan Propulsion By R. W. Hovey (EAA 64958)

BLADE LOCATVIOM

(°g CHOED) DR\ve

ee ^

POWER PLANT

Box 1074 Saugus, California 91350

KIC)M ROT ATI I N G)

SUPPORT

STRUTS

A,

.LL TRUE amateur experimental aircraft buffs harbor a secret desire to build a jet powered airplane. The astronomical costs and complexities of available jet engines however

preclude use on all but the most affluent and sophisticated project efforts. Experimental jet engines have been developed in both England and the U. S. with the intent of producing

small inexpensive engines for light aircraft. In most cases the successful programs have evolved into expensive designs aimed at useage by business jet types (where the money is). This article presents an alternate solution, namely the "Duct Fan", or poor man's

jet. Duct fan propulsion devices have been used on a variety of aircraft and surface vehicles, however, the

The motor (engine) may be coupled directly to the fan as shown, or a speed changing system may be used. In this case the designer has the option of locating the engine inside the fuselage, using pulleys or shaft and gearing to transmit power to the fan. As is shown later, the duct fan design and engine characteristics must be matched very carefully to achieve satisfactory performance. Engine location may be in front of, or behind the fan, but flow interference losses are greater with the engine located

behind the fan. HOW IS IT USED?

amateur aircraft designers and build-

An unshackled imagination can run

ers have not had much success with this intriguing approach. An elusive

wild with aircraft configurations using duct fan propulsion units. The most common application is a direct substitution for a jet engine, in a fixed

propulsion efficiency and lack of suitable powerplants have been major deterents. Improvements being made

wing design. Duct fans also have

in performance of high speed engines, however, now make a closer look at duct fan applications for light aircraft worthwhile.

unique properties useful in STOL and

WHAT ARE THEY?

VTOL designs producing thrust in either static or dynamic flight conditions. In VTOL aircraft, static thrust is used to provide vertical lift, while STOL aircraft designed to fly hori-

The term "Duct Fan" as used here, applies to any enclosed propeller device powered by conventional type

zontally use the duct fan to provide horizontal propulsion with the duct itself providing part, or all of the verti-

engines, (piston, Wankel, etc.) The propulsion assembly consists of an internal rotating prop or fan, surround-

ed by a close fitting circular wing or duct. Thrust is created by the action of air flowing through the duct. The fan itself has many characteristics of a conventional free propeller, and may be a single stage design, or

two

stages,

contra-rotating.

The

contra-rotating type is, however, seldom used. The fan, of course, must be powered (rotated) by the engine as shown in a typical installation in Figure 1.

cal lift normally provided by a fixed wing in cruise flight. In some development test aircraft, direct vertical lift fans were also tilted in flight to provide thrust and lift for horizontal flight. Vertical lift fans inherently operate best at zero vehicle speeds,

(static thrust). Aircraft designed for horizontal flight usually have a particular design speed for the best conversion of horsepower to thrust. The

take-off requirements. It should be

noted that in any aircraft design configuration, the duct fan arrangement is simply a replacement for the free propeller. It has both advantages and drawbacks which are discussed later. WHY USE A DUCT FAN?

The most obvious reason is that a duct fan powered aircraft looks somewhat like a jet and has increased asthetic appeal. The duct fan diameter can be smaller than a free prop thus allowing the designer more freedom in propulsion system location, and aircraft configuration. A less obvious but more important consideration lies in the application of light weight high speed engines to custom built aircraft. A variety of engines designed for surface vehicles are available to the aircraft designer offering both cost and power-to-weight advantages. These are usually automobile or snowmobile engine drivatives which use high crank speeds to realize high performance, and as such are unsuited for direct propeller drives, the reason be-

ing that small fast turning props are much less efficient than large slow turning props. The duct fan prop can, however, be much smaller for the same thrust and efficiency. This factor allows the high speed engine to be coupled directly to the fan without using a speed reduction device. When making a trade-off-study, to see if a duct fan should be used instead of a free prop, the weight and cost of the fan ring, or duct, can be evaluated against the weight and cost of a gear or pulley speed reducer. Figure 2 shows the relationship between prop

fan blade configuration and duct ring

rotational speeds, diameters and tip

are designed to produce the best ef-

speeds.

ficiency at this cruise speed and still retain sufficient low speed thrust for

(Continued on Next Page) SPORT AVIATION 27

,— fcO

DUCT FAN . . .

(Continued from Preceding Page)

As this indicates, an aircraft engine designed for slow rotational speeds, should never be used to power a duct fan. Shaft rotational speeds should produce tip speeds of 600 to 900 feet per second, with either duct fans or free props. Higher tip speeds crowd the sonic limit, while slower tip speeds result in excessive blade areas. Fan blade area requirements vary as a function of both rotational speeds and thrust loading (or power loading). This situation is similar to a conventional fixed wing which must produce a given amount of lift. To fly slower, the wing area must be increased. The design wing area also varies as the lift, (gross weight) requirements change, much as the fan blade area changes with thrust and power changes. The total fan blade area may be divided into any number of blades. Comparative tests have shown very little change in efficiency by changing the number of blades so long as the total area of all of the blades is the same. The one exception is a design where high thrust loadings are used, in which case a large number of blades may be used to advantage to reduce blade stall. This is known as the "Cascade Effect". What happens is that the close proximity of adjacent blades tends to control boundary layer separation much like the action of slotted flaps. Most light aircraft fan configurations will not, however, use this number of blades or total blade area relative to the disc area. So far we have not discussed the duct, shroud, or circular wing. This non-rotating device must fit closely with the fan blade tips to reduce aerodynamic losses Rigidity requirements at the fan tip location are thus important structural considerations. The remaining sections of the duct may be designed much like any fixed wing structure. The most critical aerodynamic shape is at the leading edge inlet or inside lip radius. Inflowing air creates a low pressure area over the inlet lip which, in turn, produces forward thrust acting on the duct itself. This is independent of fan thrust, and is added to it. Obviously, the larger the radius and outside diameter, the larger the duct thrust will be for a given fan. The outside shape of the duct is designed only to reduce drag in high speed flight. The inside exit duct portion, (that part of the duct down stream from the fan) will be nearly cylindrical in most designs. The intent is to take the airflow that has been slightly compressed by the fan and expand it to ambient atmospheric pressure at the exit lip. In most light aircraft fan designs, the pressure 28 JUNE 1973

0ooo

tooo

4000

£000

SHAFT ______

SPEED -

R.PM.

FIGURE 2______

FAN STATIC PERFORMANCE

DUCT

THRUST LOADING (LB/s« F T. OF asvO

Zo

DUCT FAN

PAM WITH 001 SHROUD

1-0

a

4-

6

HORSEPOWER

8 lo

do

' so|OO

. FT. OF DISK

2oo

4oo

AREA

FIGURE 3

ratio across the fan is so small that there is not much to be gained by expanding the aft duct. The over all shape of the duct ring is designed to match the operating flight speed where the best conversion of horsepower to thrust is desired (efficiency). Thus, a low speed duct fan has a large inlet lip radius and the duct ring section thickness is large compared to the fan diameter. The converse is true of a high speed design, (150 mph) where the duct ring section would be thin. Noise considerations are becoming increasingly important, and duct fans are much quieter for any given tip speed comparison. Support struts for the duct ring should not be located directly in front of the rotating fan, this condition can create a siren effect. Engine noise is, of course, independent of the fan or prop noise. Another plus consideration for the duct fan is the range of speeds

where it will function at a relatively high efficiency. Since the duct fan is designed to operate in a self induced high speed column of air, the fan pitch requirements do not change much with changes in aircraft speed. The fixed pitch free prop on the other hand has high propulsion efficiency over a very narrow range of speeds. HOW EFFICIENT IS IT?

The fan propeller does the same job as a free propeller except that it is surrounded by a close fitting duct. Prop tip losses are reduced by a close fit with the duct. Thus a short, wide chord blade can have a much higher blade aspect ratio. The total blade area, pitch and number of blades are selected in about the same way that propellers are configured, using rpm, speed and power loading factors. The same rules apply, that for a given thrust condition, a larger prop (or duct fan)

Education through Smr

By Gene R. Chase, CFII 463670 Another tragic accident has been reported to us. We will review it briefly here because it's the sort of thing that can happen to any of us when we don't use caution. The pilot (not an EAA member) was preparing to depart an EAA Chapter fly-in last fall in Oregon. He asked one of several bystanders to help him start his Taylorcraft B*C- 12D. In learning that this person was inexperienced, he decided to do it himself. The pilot stated that he did not set the parking brake because he "didn't trust it". The plane was not tied down nor were wheel chocks used. He then checked to see that the throttle was closed, turned the ignition switch to both and proceeded to hand prop the plane.

DUCT FAN . . .

(Continued from Preceding Page)

will use less horsepower. The same thrust can be obtained by moving a large diameter column at a low speed, or small diameter column of air at high speed. It is a fundamental law of nature, however, that the small amount of air moved at high speed,

will require more horsepower to get the same thrust. Conversely, the larger the prop diameter, the greater the efficiency. The duct fan has one thing going for it, however. The low pres-

sure air over the duct inlet lip creates

lift. A portion of this lift acts in a forward direction to produce thrust, which is added to the bare fan thrust. This duct lip thrust is most effective

The engine started and the plane began moving. The pilot grabbed the left wing struts, and unable to open the cabin door he hung on in an unsuccessful attempt to hold the plane. A 24 year old woman and 3 year old child were struck by the propeller and ' killed instantly. This pilot did not consider himself a careless person. In his own words, he stated, "I was completely sure in my own mind the throttle was closed on this occasion because of the danger to myself as well as others, because I have had fear of this happening, and have great respect for what a propeller can do. Since starting (airplanes) myself, I have been very cautious in regards to the throttle setting." In reading this, several instances of poor judgement on the part of the pilot stand out glaringly. Had he not committed any one of the numerous mistakes, this tragedy might have been averted.

This brings to mind the fact that several accidents have been caused, not by the throttle being unmanned, but because the pilot didn't fully understand the operation of the throttle. Some are simple push-pull knobs or levers while others are of the vernier type, and are operated by rotating the hand knob. Some throttles may only be moved when a spring-loaded button on the knob is depressed, and, of course, many have friction adjustments which restrict the movement and can lock the throttle in place. Pilots have lost control of their aircraft while taxiing, or have either undershot the runway or landed long because of mis-use of the throttle because they weren't familiar with its operation. An important part of every pilot's cockpit check out in a new or different aircraft should be the complete familiarization of the type of throttle and its operation.

under static thrust conditions where the relative speed is zero. As flight speed is increased, the aerodynamic drag of the duct tends to reduce the positive thrust effect. At some forward flight speed, the duct ring thrust and drag are equal and the fan would operate just as well as a free prop, without the duct. At speeds above this point the duct ring detracts from the

HOW DO YOU CALCULATE PERFORMANCE?

overall performance. As a general rule duct fan propulsion should not be used at cruise speeds over 150 mph. The duct, or shroud ring is designed primarily for the speed which produces best conversion of power to thrust, (i.e., efficiency). Large thick ducts are used for static or low speeds, and thin ducts are used for high speed application.

A curve showing the relationship

between horsepower, diameter of the fan, and thrust for "Static Thrust" is shown in Figure 3, this data is based on actual test results of typical fan configurations. Performance for forward flight conditions as well as design data on blade pitch, blade area, and duct design are included in a book titled "Ducted Fans for Light Aircraft" by

R. W. Hovey, which may be obtained

by sending $10.00 to:

R. W. Hovey Box 1074

Saugus, California 91350 Postage is prepaid in U. S. A. SPORT AVIATION 29