CRAFTSMAN'S CORNER

DUCTED FAN OPERATION BY MATTHEW S. KIGHT

Introduction: Why Use A Fan?

should also be possible to mount the propulsion system I have been continuing with my pro- of an amphibious aircraft in gram to build an affordable and effective the fuselage instead of up high axial flow fan intended for the experimental on a pylon. The advantage of aircraft market. This market would include this would be twofold; drag small sport planes and jet fighter replicas of would be reduced and the pitch two seats or less, with a gross takeoff weight down moment of the high of not more than 1700 pounds. mounted propeller would be The YLD-30, the fan that was intro- nearly eliminated. duced last year, has been replaced by a The final advantage is for larger and more powerful unit, the YLD- the romantics among us. An 40. As before the fan has its aerodynamics axial flow fan makes it possitailored to fit the power curve of an auto- ble to build a military jet fighter to a TABLE 1 derivative engine. An auto engined aircraft high degree of authenticity. Most of YLD-40 SPECIICATIONS using a propeller would require the instal- us would agree that an F-16 replica, Flow Rate 40 Ibm/sec. (sea level corrected flow) lation of a speed reduction unit (a gearbox even built with great attention to dePressure Ratio = 1.18 to 1 at 100% speed or cog belt). If the engine has the power at tail, would look pretty silly with a the right rpm to meet the fan's require- propeller on the back. An F-16 with Design Speed = 6,500 rpm (sea level standard day) ment, it is easy to employ direct drive with a v-8 or a rotary auto engine coupled Thrust = 682 Ibf at 100% speed (sea level static) nothing more complicated than a splined to YLD-40 would be the chance for Horsepower Required — 250-400 depending on driveshaft with U-joints. some of us to experience the sound aircraft and opertional altitude In general, the higher you go the less and at least a bit of the feel of a real Fan Module Weight = 70 Ibm approximately energy it takes to push an aircraft through military jet fighter. Fan Diameter = 21.5 inches the air for a given airspeed. The new fan's Now for the downside. I want to Fan Module Maximum Diameter = 24.00 inches higher pressure ratio makes it possible to state clearly my market intention and supercharge the induction system of a nor- expectation. I am not trying to build Fan Module Length = 33 inches overall mally aspirated engine and fly at a higher a propulsion system that will replace Approximate Cost Range = $8,000 to $12,000 altitude than would otherwise be possible. For example, if you were cruising at 250 INNER CONNECTING DUCT mph at 15,000 feet the density increase reOUTER CONNECTING DUCT sulting from the ram effect, combined with REAR STRUT CASE STATOR CASE the fan discharge pressure, would allow OUTER TAIL CONE ROTOR BLADE the engine to produce the same horseCASE power as if it were at 8,500 feet, a 6,500 foot advantage. The YLD-40 should produce a smoother, quieter ride than an equivalent NOSE horsepower propeller installation. AlCONE RONT INNER TAIL CONE though 1 have no data to support this, it BEARING TAIL CONE should be intuitive. The fan's acoustical COMPARTMENT RETAINER energy is partially absorbed by the aircraft YOKE structure; also the blade pulsations are smaller and more numerous, blending toSPLINED Ugether more evenly than the two or three JOINT YOKE large pulses of a typical propeller. Ground clearance is another potential advantage. Having a smaller diameter allows shorter landing gear for a possible REAR CONICAL ROTOR BEARING SUPPORT weight savings. Additionally, the ability to HUB operate closer to the ground could be used to generate a more pronounced ground effect, reducing landing speed somewhat. It ROTOR FIG. 1 YLD-40 BLADES

COPYRIGHT THERMODYNE

116 SEPTEMBER 1997

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propellers; they are going to be with us for a long time. A constant speed propeller is still the most efficient means of converting shaft horsepower into thrust in the speed range from 0 to 450 mph. The exact speed at which a particular propeller quits producing thrust because of shock losses depends on the propeller. My intention is to offer new variety into the marketplace. To create a new niche in the experimental aircraft arena, that will expand the number of aircraft from which to choose. This will not be the propulsion system for everyone; if you want maximum utility and minimum fuel consumption for a given level of performance, you want a constant speed propeller. (Editor's Note: A complete report on this ducted fan idea is available fmm the author.)

(This article was submitted by Matthew S. Kight, 521 SW Marion Ct, Palm City, FL 34990,561/220-2711.) Articles for Craftsman s Corner are obtained and edited by Ben Owen, Director of Information Services.

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