Duct FanPropulsion

By R. W. Hovey (EAA 64958)

Box 1074Saugus, California 91350

A,.LL TRUE amateur experi-mental aircraft buffs harbor a secretdesire to build a jet powered airplane.The astronomical costs and complexi-ties of available jet engines howeverpreclude use on all but the most afflu-ent and sophisticated project efforts.Experimental jet engines have beendeveloped in both England and theU. S. with the intent of producingsmall inexpensive engines for lightaircraft. In most cases the successfulprograms have evolved into expensivedesigns aimed at useage by businessjet types (where the money is). Thisarticle presents an alternate solution,namely the "Duct Fan", or poor man'sjet.

Duct fan propulsion devices havebeen used on a variety of aircraftand surface vehicles, however, theamateur aircraft designers and build-ers have not had much success withthis intriguing approach. An elusivepropulsion efficiency and lack ofsuitable powerplants have been majordeterents. Improvements being madein performance of high speed en-gines, however, now make a closerlook at duct fan applications for lightaircraft worthwhile.

WHAT ARE THEY?The term "Duct Fan" as used here,

applies to any enclosed propeller de-vice powered by conventional typeengines, (piston, Wankel, etc.) Thepropulsion assembly consists of an in-ternal rotating prop or fan, surround-ed by a close fitting circular wing orduct. Thrust is created by the actionof air flowing through the duct. Thefan itself has many characteristicsof a conventional free propeller,and may be a single stage design, ortwo stages, contra-rotating. Thecontra-rotating type is, however, sel-dom used. The fan, of course, must bepowered (rotated) by the engine asshown in a typical installation inFigure 1.

DUCT FAN ARRANGEMENT

BLADELOCATVIOM(°g CHOED)

DR\ve

ee^POWER PLANT

KIC)M ROT ATI I N G)SUPPORT STRUTS

The motor (engine) may be coupleddirectly to the fan as shown, or aspeed changing system may be used.In this case the designer has the op-tion of locating the engine inside thefuselage, using pulleys or shaft andgearing to transmit power to the fan.As is shown later, the duct fan designand engine characteristics must bematched very carefully to achievesatisfactory performance. Engine lo-cation may be in front of, or behindthe fan, but flow interference lossesare greater with the engine locatedbehind the fan.

HOW IS IT USED?An unshackled imagination can run

wild with aircraft configurations us-ing duct fan propulsion units. Themost common application is a directsubstitution for a jet engine, in a fixedwing design. Duct fans also haveunique properties useful in STOL andVTOL designs producing thrust ineither static or dynamic flight condi-tions. In VTOL aircraft, static thrustis used to provide vertical lift, whileSTOL aircraft designed to fly hori-zontally use the duct fan to providehorizontal propulsion with the duct it-self providing part, or all of the verti-cal lift normally provided by a fixedwing in cruise flight. In some develop-ment test aircraft, direct vertical liftfans were also tilted in flight to pro-vide thrust and lift for horizontalflight. Vertical l ift fans inherentlyoperate best at zero vehicle speeds,(static thrust). Aircraft designed forhorizontal flight usually have a par-ticular design speed for the best con-version of horsepower to thrust. Thefan blade configuration and duct ringare designed to produce the best ef-ficiency at this cruise speed and stillretain sufficient low speed thrust for

take-off requirements. It should benoted that in any aircraft design con-figuration, the duct fan arrangementis simply a replacement for the freepropeller. It has both advantages anddrawbacks which are discussed later.

WHY USE A DUCT FAN?The most obvious reason is that a

duct fan powered aircraft looks some-what like a jet and has increased as-thetic appeal. The duct fan diametercan be smaller than a free prop thusallowing the designer more freedom inpropulsion system location, and air-craft configuration. A less obvious butmore important consideration lies inthe application of light weight highspeed engines to custom built aircraft.A variety of engines designed for sur-face vehicles are available to the air-craft designer offering both cost andpower-to-weight advantages. Theseare usually automobile or snowmobileengine drivatives which use highcrank speeds to realize high perform-ance, and as such are unsuited fordirect propeller drives, the reason be-ing that small fast turning props aremuch less efficient than large slowturning props. The duct fan prop can,however, be much smaller for thesame thrust and efficiency. This fac-tor allows the high speed engine tobe coupled directly to the fan withoutusing a speed reduction device. Whenmaking a trade-off-study, to see if aduct fan should be used instead of afree prop, the weight and cost of thefan ring, or duct, can be evaluatedagainst the weight and cost of a gearor pulley speed reducer. Figure 2shows the relationship between proprotational speeds, diameters and tipspeeds.

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SPORT AVIATION 27

DUCT FAN . . .(Continued from Preceding Page)

As this indicates, an aircraft en-gine designed for slow rotationalspeeds, should never be used to powera duct fan. Shaft rotational speedsshould produce tip speeds of 600 to900 feet per second, with either ductfans or free props. Higher tip speedscrowd the sonic limit, while slowertip speeds result in excessive bladeareas. Fan blade area requirementsvary as a function of both rotationalspeeds and thrust loading (or powerloading). This situation is similar to aconventional fixed wing which mustproduce a given amount of lift. Tofly slower, the wing area must be in-creased. The design wing area alsovaries as the lift, (gross weight) re-quirements change, much as the fanblade area changes with thrust andpower changes. The total fan bladearea may be divided into any num-ber of blades. Comparative tests haveshown very little change in efficiencyby changing the number of blades solong as the total area of all of theblades is the same. The one exceptionis a design where high thrust loadingsare used, in which case a large num-ber of blades may be used to advan-tage to reduce blade stall. This isknown as the "Cascade Effect". Whathappens is that the close proximity ofadjacent blades tends to controlboundary layer separation much likethe action of slotted flaps. Most lightaircraft fan configurations will not,however, use this number of blades ortotal blade area relative to the discarea.

So far we have not discussed theduct, shroud, or circular wing. Thisnon-rotating device must fit closelywith the fan blade tips to reduce aero-dynamic losses Rigidity requirementsat the fan tip location are thus impor-tant structural considerations. The re-maining sections of the duct may bedesigned much like any fixed wingstructure. The most critical aerody-namic shape is at the leading edge in-let or inside lip radius. Inflowing aircreates a low pressure area over theinlet lip which, in turn, produces for-ward thrust acting on the duct itself.This is independent of fan thrust, andis added to it. Obviously, the largerthe radius and outside diameter, thelarger the duct thrust will be for agiven fan. The outside shape of theduct is designed only to reduce dragin high speed flight. The inside exitduct portion, (that part of the ductdown stream from the fan) will benearly cylindrical in most designs.The intent is to take the airflow thathas been slightly compressed by thefan and expand it to ambient atmos-pheric pressure at the exit lip. In mostlight aircraft fan designs, the pressure28 JUNE 1973

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SHAFT SPEED - R.PM.______ FIGURE 2______

DUCT FAN STATIC PERFORMANCE

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HORSEPOWER

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. FT. OF DISK AREAFIGURE 3

ratio across the fan is so small thatthere is not much to be gained by ex-panding the aft duct.

The over all shape of the duct ringis designed to match the operatingflight speed where the best conversionof horsepower to thrust is desired(efficiency). Thus, a low speed ductfan has a large inlet lip radius and theduct ring section thickness is largecompared to the fan diameter. Theconverse is true of a high speed de-sign, (150 mph) where the duct ringsection would be thin.

Noise considerations are becomingincreasingly important, and duct fansare much quieter for any given tipspeed comparison. Support strutsfor the duct ring should not be locateddirectly in front of the rotating fan,this condition can create a siren ef-fect. Engine noise is, of course, inde-pendent of the fan or prop noise.

Another plus consideration for theduct fan is the range of speeds

where it will function at a relativelyhigh efficiency. Since the duct fan isdesigned to operate in a self inducedhigh speed column of air, the fan pitchrequirements do not change muchwith changes in aircraft speed. Thefixed pitch free prop on the other handhas high propulsion efficiency over avery narrow range of speeds.

HOW EFFICIENT IS IT?The fan propeller does the same job

as a free propeller except that it issurrounded by a close fitting duct. Proptip losses are reduced by a close fitwith the duct. Thus a short, wide chordblade can have a much higher bladeaspect ratio. The total blade area,pitch and number of blades are select-ed in about the same way that propel-lers are configured, using rpm, speedand power loading factors. The samerules apply, that for a given thrustcondition, a larger prop (or duct fan)

Education through Smr

By Gene R. Chase, CFII 463670

Another tragic accident has beenreported to us. We will review it brief-ly here because it's the sort of thingthat can happen to any of us when wedon't use caution.

The pilot (not an EAA member) waspreparing to depart an EAA Chapterfly-in last fall in Oregon. He askedone of several bystanders to help himstart his Taylorcraft B*C- 12D. In learn-ing that this person was inexperi-enced, he decided to do it himself.

The pilot stated that he did not setthe parking brake because he "didn'ttrust it". The plane was not tied downnor were wheel chocks used. He thenchecked to see that the throttle wasclosed, turned the ignition switch toboth and proceeded to hand prop theplane.

The engine started and the planebegan moving. The pilot grabbed theleft wing struts, and unable to openthe cabin door he hung on in an un-successful attempt to hold the plane.A 24 year old woman and 3 year oldchild were struck by the propeller and 'killed instantly.

This pilot did not consider himselfa careless person. In his own words,he stated, "I was completely sure inmy own mind the throttle was closed onthis occasion because of the danger tomyself as well as others, because I havehad fear of this happening, and havegreat respect for what a propeller cando. Since starting (airplanes) myself,I have been very cautious in regardsto the throttle setting."

In reading this, several instances ofpoor judgement on the part of the pi-lot stand out glaringly. Had he notcommitted any one of the numerousmistakes, this tragedy might havebeen averted.

This brings to mind the fact thatseveral accidents have been caused,not by the throttle being unmanned,but because the pilot didn't fully un-derstand the operation of the throttle.Some are simple push-pull knobs orlevers while others are of the verniertype, and are operated by rotating thehand knob. Some throttles may onlybe moved when a spring-loaded but-ton on the knob is depressed, and, ofcourse, many have friction adjust-ments which restrict the movementand can lock the throttle in place.

Pilots have lost control of their air-craft while taxiing, or have eitherundershot the runway or landed longbecause of mis-use of the throttle be-cause they weren't familiar with itsoperation. An important part of everypilot's cockpit check out in a new ordifferent aircraft should be the com-plete familiarization of the type ofthrottle and its operation.

DUCT FAN .. .(Continued from Preceding Page)

will use less horsepower. The samethrust can be obtained by moving alarge diameter column at a low speed,or small diameter column of air athigh speed. It is a fundamental law ofnature, however, that the smallamount of air moved at high speed,will require more horsepower to getthe same thrust. Conversely, the largerthe prop diameter, the greater theefficiency. The duct fan has one thinggoing for it, however. The low pres-sure air over the duct inlet lip createslift. A portion of this lift acts in a for-ward direction to produce thrust,which is added to the bare fan thrust.This duct lip thrust is most effective

under static thrust conditions wherethe relative speed is zero. As flightspeed is increased, the aerodynamicdrag of the duct tends to reduce thepositive thrust effect. At some for-ward flight speed, the duct ring thrustand drag are equal and the fan wouldoperate just as well as a free prop,without the duct. At speeds above thispoint the duct ring detracts from theoverall performance. As a general ruleduct fan propulsion should not be usedat cruise speeds over 150 mph. Theduct, or shroud ring is designed pri-marily for the speed which producesbest conversion of power to thrust,(i.e., efficiency). Large thick ducts areused for static or low speeds, and thinducts are used for high speed applica-tion.

HOW DO YOU CALCULATEPERFORMANCE?

A curve showing the relationshipbetween horsepower, diameter of thefan, and thrust for "Static Thrust" isshown in Figure 3, this data is basedon actual test results of typical fanconfigurations.

Performance for forward flight con-ditions as well as design data onblade pitch, blade area, and duct de-sign are included in a book titled"Ducted Fans for Light Aircraft" byR. W. Hovey, which may be obtainedby sending $10.00 to:

R. W. HoveyBox 1074Saugus, California 91350Postage is prepaid in U. S. A.

SPORT AVIATION 29