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The Ringtail Ultralight

By Mark Stull
Ringtail Ultralight

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 included a couple fun and interesting experiments. Fabricated almost entirely from 2024-T3 aluminum, it was completed and first flown in January of 2011. I used the efficient, 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 the recent Sun ’n Fun International Fly-In & Expo. 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.

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.

Ringtail Ultralight

Everything about it had to be very precise so it would look okay and work well with a minimum of structural weight. The ring is made of 5/8-inch diameter 0.035-inch wall 2024-T3 tubing. It’s 4.5 feet in diameter and has a 1.5-foot chord. The universal joint is located at 25 percent of chord, and the two tubes that stick forward on either side contain the counterbalance weights.

Ringtail Ultralight
Close-up view of the tail showing control cables and hydraulic damper

Shrinking the fabric on the inside of the ring created a nice convex airfoil. I shrunk the outside fabric minimally to keep it from becoming concave, but it became concave in flight with chordwise 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 chordwise ripples of the inner fabric in flight, which are fine.

The aerodynamics of the ring is 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 wingtip, 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 and requiring a couple pounds of forward stick force in climb and one 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 one 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 much 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 microphone.

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 airspeed 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. 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, 340-cc Kawasaki (snowmobile) engine since this plane didn’t need the fan-cooled one that was on my pusher airframe. 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 little carburetor.

Mark Stull
Mark Stull and view of the Kawasaki engine with cog belt reduction drive.

It was a challenge to figure out what to do with the exhaust. Two-stroke engine exhaust sprays sooty oil mist that can get all over me and the plane. I used a long 2.5-inch-diameter augmenter tube to send the exhaust down by the right main gear wheel. It works great, keeping the plane completely clean. The two black rubber bands around it that you can see in the pictures help keep it from ringing.

The 5-gallon fuel tank is off a Baldor generator. I love its compact, rectangular shape and 5-gallon capacity. 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 plane’s center of gravity, allowing different weight pilots. It’s reclined about 40 degrees to reduce wind drag. In addition to the lap belt, it has a chest belt that goes across and under my armpits. The chest belt prevents the pilot from ejecting out the front if the BRS parachute is deployed, and it’s more comfortable than crotch straps. It feels nice and secure in turbulence.

I used my well-proven wheelchair tire for the steerable nose gear. The main gear has 11 x 3.50 x 5 inch tires on plastic, Azusalite wheels, turning on sturdy, 1-1/8 inch, chromoly axles. None of the gear has suspension or brakes. The tricycle gear makes ground handling easy.

The efficient, high-aspect-ratio (9.5 to 1) wings help this plane achieve 1.6 gallons/hour average fuel economy. They work well with a heavy pilot or at high altitudes. The upper surface has a Gottingen 387 airfoil. A slight undercamber 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.

Its 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. The wing flaps can deflect about 60 degrees to slow the landing and stall speeds, shorten the landing ground roll, and/or alter the glide slope for emergency landings.

Ringtail Ultralight
This bell crank provides for the differential aileron controls. All hardware fittings are handmade.

Lucky Stars III flies, handles, stalls, and works fine. It’s a legal ultralight, weighing just 225 pounds empty, not counting the parachute. That’s 29 pounds under the legal limit for Part 103 regulations. It even has enough wing area to pass the stall speed limit. I use a minimum of instruments to keep things simple and light: tachometer/hour meter, variometer/altimeter, airspeed gauge, and slip indicator ball. I use no engine or navigation instruments in flight.

The plane 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.

If I were to critique this plane like a magazine test report, I would note the lack of conventional pitch and yaw control feel from the ring tail. Even so, it’s easy and forgiving to fly. I added bungee returns on the rudder pedals that provide some artificial feel. Sitting unprotected in the prop wash is a disadvantage, particularly in colder climates, that reaffirms my preference for pushers.

Ringtail landing

Landing takes some practice to get used to the very low seat height 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 three seconds, much quicker than my previous designs. I learned to flare out extra low and quickly fly the plane onto the runway like a taildragger’s wheel landing.

I miss the gentle breeze, unobstructed view, and being able to tell my airspeed by the feel and sound of the wind that I enjoyed with my pusher designs. I saved my last pusher fuselage so I could return to it; I plan some significant changes to it for my next project. I borrowed the pusher’s wings, parachute, and several minor things for this new plane.

All in all, Lucky Stars III 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. I design, build, and fly ultralights just for fun. I sell no plans or kits.

The Lucky Stars Ringtail received the award for Outstanding Fixed Wing Ultralight at Sun ’n Fun 2011. Pictures in flight courtesy Bill Yeates. Mark Stull is from Christoval, Texas, and can be contacted at mstull@wtxs.net.

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