HillHopper Page
HillHopper Page
By Mike Sandlin
Last Update: May 25, 2010 (updated link)
The HillHopper, an experimental airchair glider, made it's first flights in March, 2004. |
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The HillHopper was a biplane airchair, intended to be another example of what I call a "4S" glider (a glider for open air flying that is simple, slow, safe, and soarable).
The HillHopper has a light, strong, stiff airframe which can be quickly constructed from a few simple parts. The box kite structure, reminiscent of aircraft design a hundred years ago, is an open frame of tubular struts in a mostly rectilinear alignment. Concentrated loads are routed through the vertices of the structure, avoiding bending loads where possible. Cross bracing with steel cables gives the aircraft good torsional rigidity.
The HillHopper was built in my backyard shed using mainly a hacksaw and a hand held power drill. Readily available materials were used, and an effort was made to minimize the number of different parts and supplies (i.e., there is a broad commonality of parts and materials). For example, the rudder pedals are made from the same materials as other parts of the glider, and making them did not involve learning any new building tricks. No welding or special machining was performed. I lean heavily toward the use of aircraft bolts in an "erector set" fashion, so airframe construction is reversible and almost any part can be removed and replaced.
Flying slow allows casual, comfortable flying with lots of time to think and look around, as well as the ability to perform tight turns, rolling takeoffs, and to operate from small, rough fields. The ability to fly slow comes mostly from the low wing loading, which means that the ratio of the total flying weight to the wing area is small. The HillHopper has about the same wing loading as a light hang glider, and thus flies at about the same speeds and soars using similar techniques.
Most ultralight injury accidents occur during takeoff or landing at airspeeds near normal flying speed. The Hillhopper is designed to protect the pilot in these situations by surrounding him or her with frangible protective structure and limiting the crash energy by flying slow in the first place. Four point seat belts and elastic landing gear suspension are part of this crash protection system. Effective low speed control and forgiving flight characteristics help avoid impact situations, and a hand deployed emergency parachute is carried in case of loss of control at high altitude. Unlike my previous designs, the HillHopper has a monolithic fuselage that does not come apart for transport, resulting in a reduced number of assembly attachments (only the main wings are detached), so the chance of missing some critical step during assembly is greatly reduced.
This reduced burden of critical steps also applies to ground tow setup (relative to a hang glider), since no takeoff dolly is required, and one hand is always free and ready on the release handle. Also, there is little or no likelihood of a lockout or tumble, since these mishaps have not been known to occur to tailed gliders with three axis control.
The HillHopper is intended to be fully soarable and to fly high in the full range of ordinary daytime conditions, essentially in the same manner as a hang glider.
Launches might be made by car tow, ultralight aerotow, or just by rolling down a hill. The towing, launch, and landing procedures and conditions are intended to be the same as for a hang glider, although in general I have found the controllability and stability in turbulence of an airchair to be superior to those properties in a hang glider.
I expect the HillHopper to have lower glide performance than the Goat or the Bug, due mainly to its shorter wingspan, so it might seem to be a step backwards, but I hope that its overall flight qualities and practical advantages will make it a satisfying glider to operate.
Rolling downhill into a short hill hop.
The cross rigging for the front face of the central cell is routed behind the pilot, through the pilot's seat, so there are no structural cables in front of the pilot, just the two nose tubes farther forward. The glider can be entered from directly in front or from either side of the nose section.
The wheel is at the center of lift, allowing either a nose down or nose up static orientation, at the momentary whim of the pilot. I usually operate as a "nose dragger", starting and stopping with the nose on the ground. Proper static margin can be confirmed prior to flight by raising the nose to level (with the glider loaded for flight, but not moving) and requiring that it settle forward, thus confirming adequate forward balance and avoiding the need for any placards or weight estimations. There is no wheel brake, the nose skid provides dramatic stopping capability. The tire is a 13 inch diameter ultralight airplane type. The aft landing gear struts are attached to the main airframe by bungee cord to create an elastic suspension system.
The tail section is an open ended box with no wing tips. Tail surfaces are hinged by line wrapped in a figure eight around two parallel, adjacent tubes, not using any eyebolts or other metal hinges. The stick and rudder control system is conventional and the same as that used by nearly all sailplanes and stick controlled airplanes.
The HillHopper is constructed of aluminum tubing braced with flexible steel aircraft cable, in addition to small ribs made of a Styrofoam and fiberglass composite. The wing and tail surfaces are covered with heat shrinkable aircraft polyester fabric, then sealed with non-taughtening urethane (or butyrate) dope (nominally the Polyfiber process). All fabric surfaces are flat except for the tops of the main wing panels. The conventional 3 axis control system uses Spectron line routed through marine (sailboat) pulleys.
The HillHopper is a glider but there is room for a small motor and a 5 foot diameter propeller in the aft fuselage if I should choose to make it into a power glider (i.e., an airplane).
Current status:
This project will not be continued for now. Experience with car top freeway transport at high speeds makes transport seem unsafe, and thus impractical by current methods. I do not choose to use a trailer and don't have any better ideas at the present time. This aircraft has been satisfactory to fly as far as it has gone (short flights on the training hill), and I think it has a future where a simple, compact design is desired, but right now the airframe will be disassembled for other uses.
This is a private, noncommercial project (no plans, no kit, no nada). There are no drawings avilable for this design, and not really any better photos. I'm glad it inspires comment and design interest. Someday I hope to continue along this line, building a "motor floater", a light airplane built for self launch soaring, but for now I will leave this design and be working only on monoplane airchairs.
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