J-4 Sportster (oz16366)

About this Plan

J-4 Sportster. Radio control scale model ultralight. Wingspan 114.6 in, wing area 1834 sq in. Scale is 1/3.

Quote: "1/3 scale J-4 Sportster, by Emil Agosta.

The J-4 Sportster is a spin-off of the J-3 Kitten Ultralight which won the 'Best New Ultralight Design' Award, at the Sun 'N Fun in 1984 at Lakeland, Florida. The Kitten has a cabin, while the J-4 is a parasol-winged craft with an open cockpit. The J-4 was also a winner. It won the Grand Champion Award at Oshkosh in 1984. The J-4 uses the same J-3 outer panels, but the overall span is reduced by about 16 inches due to the narrow center section.

I developed an interest in Ultralights back in 1983 when I caught a fleeting moment on our local TV news of the J-3 making a landing. I didn't know it was going to fire me up as much as it did. I'm sure that the plethora of questionable R/C trainers did play a part. If the J-3 Kitten, or J-4 Sportster, sporting a 28 hp engine can be flown by a novice without licensing restrictions, one can then surmise that the piloting skill is not of that required for faster airplanes. The Kitten cruises at 60-mph and power-on stall is 18 mph. Translating these simple parameters for an R/C model may possibly result in a slow-flying, stable, and docile scale model for a novice R/C scale pilot.

While looking for an R/C exhibit at the Blue Ridge Shopping Mall in Hendersonville NC, I was directed to the Hendersonville Airport. I had the wrong date for the exhibit, but I really lucked out when I arrived at the airport. The place was teeming with people, and things were quite festive. The occasion turned out to be the official unveiling of the J-3 Kitten Ultralight!

This cleverly engineered jewel was sitting like a pretty homecoming queen in front of a modest hangar. A cursory inspection revealed that engineer Jesse Anglin's aircraft construction and materials were what old reliable Cubs and Champs were made of. And his design conformed to the 250-pound maximum requirement of the FAA. I was impressed with this achievement since so many Ultralights resemble aluminum lawn chairs. A band was playing in the hangar while the public was admiring the subject celebre. I dove for a brochure, but no 3-views were shown. It took me months to get anything.

Demonstration flights were three-wheels-off-the-ground takeoffs. None of that getting the tail off the ground stuff. It made me think of my earlier youth. I kept seeing a blown-up version of a 25-cent rubber-powered kit advertised in Open Road For Boys. Like most modelers, the ideas were surging through my head in many directions. One thought did finally gel - a 1/3 scale airplane and trainer in one swoop.

The availability of factory drawings were light years away. The draftsman was still struggling with the rough sketches. A 3-view finally evolved, but the side view was out of scale! I worked with photos, blow ups, the finagle factor, and measurements at the factory. After test flying the 1/3 scale Kitten, I received a set of factory drawings. This got me going on a 1/4 scale version of the Kitten.

The J-4 came as a complete surprise to me during a casual visit to the hangar. I knew I wanted to model this sporty-looking Ultralight, but my drawing board was loaded, and I didn't want to cast aside the 1/4 scale Kitten, as I did with my Quadra-powered Kinner Bird.

The smaller J-3 flew flawlessly with a Saito 45 and I was now ready to tack-le the J-4. The 1/3 version would span 114.625", chord 16", with a wing area of 1,834 sq in - 86 sq in less than the Kitten. Shooting for a weight of 192 ounces would produce a wing loading of 15 ounces. That would be nice, I thought. Structural integrity had to be considered, but it was those heavy Du-Bro wheels and the upholstered pilot's seat that brought everything to within an ounce of 13 lbs. Why get greedy? 12.74 sq ft of wing area came in with 16.33 oz loading. It proved to be more than satisfactory.

All balsa construction was the way to go with a minimum of plywood and pine. The basic fuselage structure is 3/8 sq. balsa with 3/16 x 3/8 diagonals. The plug-in wings have deep balsa spars and are supported with scale size aluminum tube struts. A fiberglass cowl was laid up like the full scale J-4. Covering was with Solartex and sprayed with Cessna Vestal White Butyrate dope. The paint scheme is close to scale. Other than 1/4 scale Du-Bro devises at the wing struts attaching to the fuselage, no special 1/4 scale hardware was purchased. Regular control horns and devises are adequate. Ditto for servos..."

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