Ring Tail debuts

Takes ‘outstanding award’ at Sun ’n Fun

By MARK STULL

Lucky Stars III is my eighth original ultralight design, but my first with a tractor engine. It was a worthy challenge, significantly different from my previous designs, and includes a couple fun and interesting experiments.

First flown in January, it was fabricated almost entirely from 2024-T3 aluminum. I used the high-aspect ratio wings off my previous plane, so I only had to make a new fuselage and tail.

The ring tail has no aerodynamic advantage. It’s just for fun. People go to air shows to see something new and different. I figured I’d give them their money’s worth at Sun ’n Fun. I enjoy trying experimental things on an ultralight that you can’t get away with on a faster plane. The ring tail is exactly that. (Editor’s Note: It seemed to have worked: Lucky Stars took “Outstanding Fixed Wing” in the Ultralight category at Sun ’n Fun).

The whole tail tilts on a universal joint in any combination of pitch and yaw, serving as elevator and rudder. The ring’s structure came out amazingly stiff, strong, and light, with eight compression ribs joining the leading and trailing edge tubing hoops. I made the ring extra large to give it ample control authority. I added a small hydraulic damper to keep the tail’s high momentum from swinging beyond intended yaw deflections.

Shrinking the fabric on the inside of the ring created a convex airfoil. I shrunk the outside fabric minimally to keep it from becoming concave, but it became concave in flight with chord-wise ripples. To eliminate that, I pressurized the space between the inner and outer fabric with ram air. Ram air enters through seven holes in the front of each of the four spokes. The pressure keeps the outer fabric tight and flat, and makes the inner fabric have a slightly more convex airfoil. That creates slight chord-wise ripples of the inner fabric in flight, which are fine.

The aerodynamics of the ring are unusual. All of the air that flows through the inside of the ring is deflected to the angle of the ring. But the outside of the ring acts like a giant wing tip, spilling much of the air, rather than deflecting it. I had to make the ring large enough in diameter so plenty of air would flow through it. A ring is very stall resistant, allowing unusually high angles of attack, but has more drag than a conventional tail.

The bottom of the ring is pushed aft by the prop wash, proportionate to engine rpm, requiring a couple pounds of forward stick force in climb and 1 pound in cruise. I added a trim bungee to take that pitch force, so the controls are light all the time now.

A skid next to my seat prevents the ring from getting closer than 1 foot from the ground. The plane is balanced to stay on its nose wheel, whether or not the pilot is seated.

Another experiment was to have a tractor engine with no windshield or enclosure to protect the pilot. A streamlined cockpit enclosure would make the plane more efficient, but I prefer a wide open cockpit with just the face shield on my helmet to keep the wind out of my eyes and radio mic.

In theory, a small prop accelerates a little air a lot. But a large prop accelerates a lot of air very little, and is efficient at converting power into low speed thrust. I used a Believe Aviation, cog belt, (3.1 to 1) reduction drive (which is no longer available) to swing a large prop. But there’s still a stiff breeze proportionate to rpm sitting directly behind the prop. I’d estimate the prop wash is about 12 mph more than the plane’s air speed in cruise. Increasing the plane’s airspeed by a small amount with more power increases the prop wash a lot, discouraging flying at higher air speeds. On the ground, the prop blows dirt in my eyes sometimes, so I bought prescription goggles. As expected, the large prop can create a lot of prop braking, allowing steep descents. I set the engine’s idle slow enough to easily stop on the ground without brakes.

I bought a new, free-air cooled, Kawasaki, 340 cc (snowmobile) engine. I de-rated its 29 hp down to about 25 hp with a tiny 24 mm carburetor. I had to make my own intake manifold to adapt the carburetor.

It was a challenge to figure out what to do with the exhaust as a 2-stroke engine’s exhaust sprays sooty oil mist that can get all over me and the plane. I used a long augmenter tube to send the exhaust down by the right main gear wheel. It works great, keeping the plane completely clean.

The five-gallon fuel tank is off a Baldor generator. It fit perfectly behind the engine and parachute, close enough to the plane’s center of gravity that its fuel level won’t significantly affect the plane’s balance.

This is my first design to use a conventional seat. I mounted the modified fiberglass seat as low as possible for better stability on the ground and to see under the engine and reduction drive. It took some practice to get used to flaring out that low. The seat is very close to the center of gravity, allowing different weight pilots. It is reclined about 40° to reduce wind drag. In addition to the lap belt, it has a chest belt that goes across and under my armpits, which prevents the pilot from ejecting out the front if the BRS Parachute is deployed. It feels nice and secure in turbulence.

The efficient, high (9.5 to 1) aspect ratio wings help this plane achieve 1.6 gph average fuel economy. They work well with a heavy pilot or at high altitudes. The upper surface has a Gottingen 387 airfoil. A slight under-camber on the lower surface helps improve low speed efficiency and stall speed. The wings climb and glide most efficiently at around 40 mph. I love the way the 34.2-foot span wings seem to defy gravity, giving the plane a completely different feel from shorter winged ultralights.

The wings have aluminum spars and aluminum compression ribs in a ladder frame construction. Most of the ribs are scrimmed styrofoam with plywood rib caps on top. The ribs don’t touch the bottom fabric. The 1-inch thick tip and root ribs are plywood-balsa sandwich with plywood rib caps. Covering and paint are Stits Poly Fiber.

The ailerons are large enough to provide a good roll rate. They have a differential control system and are reflexed up a little to decrease adverse yaw.

It cruises between 40 mph and 60 mph true airspeed, and stalls gently at 29 mph with the flaps up and 27 mph with the flaps down. I normally cruise at around 47 mph. The view from the cockpit is excellent, and the engine is quiet.

I use a minimum of instruments to keep things simple and light: A tachometer/hour meter; variometer/altimeter; air speed gauge; and slip indicator ball. I use no engine or navigation instruments in flight.

Landing takes some practice to get used to the very low seat hight and tremendous prop braking. The plane doesn’t float over the runway at all at idle power, no matter how steeply you approach. After flare out, the plane will slow to stall speed in about 3 seconds. I learned to flare out extra low and quickly fly the plane onto the runway like a taildragger’s wheel landing.

All in all, this new plane was a fun, educational, and worthy challenge to design, build, and perfect. It took some significant modifications to make it all work.

Everybody smiles when they see the ring tail. I’m smiling too. Conventional designs are almost always better than unusual ones, but I love to experiment and be creative.

SPECIFICATIONS

• Empty weight: 225 lbs., not counting the parachute system
• Typical fuel consumption: 1.6 gph
• Cruise: Up to 60 mph
• Stall: 29 mph clean, 27 mph full flaps
• Maximum cross wind component: 16 mph
• Takeoff roll: 65 ft.
• Landing roll: 150 ft.
• Rate of climb: 400 fpm
• Engine: Kawasaki 340, de-rated down to about 25 hp
• Wing span: 34.2 ft.
• Wing area: 122 sq. ft.
• Maximum gross weight: 500 lbs.

Stull, of Christoval, Texas, designs, builds, and flies ultralights just for fun. He does not sell plans or kits. He can be reached at [email protected]