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Sandlin Pig
 
Sand-Pig-1
The Pig, September 2008
 
The Pig (Primary Instruction Glider) is an airchair, ultralight  biplane sailplane with a two axis control system (rudders & elevator, but no ailerons). A center stick & rudder pedals provide a traditional control system.
 
The Basic Ultralight Gliders are best characterized by their light wing loading, which is about the same as that of a hang glider (around 1.7 lb. of gross weight for every square foot of wing area). Light wing loading results in slow flight, which is safe, comfortable, and allows soaring in small thermals (because of the ability to turn tightly).
 
The Pig features:
low wing loading for forgiving flight characteristics and rolling launches (1.5 lb/sqft.)
open air pilot seating (allows hand thrown emergency parachute)
center of lift landing gear (allows simple balance check for proper center of mass)
car top transport (major sections separate, wings and tail fold up)
one person assembly (37 minute setup for me in my backyard) 
light weight airframe (about 148 lbs. with parachute)
nose skid for quick stops, nose down or tail down attitude for take off or landing
 
The Pig features simple construction from readily available materials, no molds, no welding, no special machining, no spray rig, no sail making, no sheet metal, etc. The airframe is made of bolted aluminum tubing with braided steel cables, covered with a light grade of conventional aircraft fabric (polyester fabric is cemented on, heat shrunk, then adhesive sealant is applied by brush).
 
All secondary structures (small ribs) are composites of foam, carbon rod, & fiberglass tape.
 
Sand-Pig-2
 
Low time pilots should use a glider that is easy to fly, forgiving, and robust, and which has good crash protection for the pilot. Controls and airframe must look and feel good, to engender the confidence that will reduce pilot stress and allow effective learning. The mechanics of launching and landing  should be simple and non-athletic. Things should be happening slowly so the student has time to see mistakes and react to them. The Pig is a version of this slow/simple/safe aircraft.

The Pig has two wheels (40 cm. diam. standard kite buggy type), 8 feet apart, rudder turn control, no ailerons (stick moves fore and aft only). 6 degree wing dihedral angle for yaw/roll coupling, and Box kite structure for torsional rigidity.
The wing airfoil is a simple utility type that Sandlin made up (Pigfoil 3012, 12% thick at 30% chord) with a completely flat bottom. This airfoil is similar to that of some radio controlled trainer gliders, and much like a Piper Cub. This is for good low speed flying characteristics and easy construction. This airfoil can also provide a strong and stiff trailing edge which will tolerate rough assembly on irregular terrain (the wing section is assembled with the trailing edge on the ground).
 
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Folding wing design provides a large area wing in a small, light package, allowing transport & assembly by one person.
 
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The eight main wing struts are mounted on swiveling eye bolts, so that during disassembly the struts can be detached at one end and rotated ninety degrees on the other. This allows each upper wing half to be lowered  onto the lower wing half, making a compact stack for car top transport. A Pig can be strapped down onto an ordinary hang glider rack, with no special saddles or pads.
 
Sand-Pig-5
 
Room has been allowed behind the the pilot for installation of a small motor and pusher propeller at about the trailing edge of the wings. The engine frame might replace the two centerline struts, and there are other hard points nearby if required. The  prop wash can exit through the box tail without hitting any control surface, avoiding a common source of vibration and drag. Adding a motor to the Pig  would create a "motor floater", a  self launching airchair which could play the game of minimal power flight to the nearest thermal.
 
The two axis control system has proven to be adequate, simple to use, and fun for recreational flying. Launches have been made by line towing and by rolling off open hillsides.

The release handle fot the tow hook is in the middle of the nose tube.
 
Flying the Pig is simple, a two axis system like a hang glider, "fast-slow, right-left", not requiring any coordination between the yaw and roll axes as does the three axis system. There is no yaw string, and the general instruction is:  "nose level, turn with your feet".

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The initial use of the rudder induces a skid, quickly followed by banking of the wing. Generally, the control feel is quick and stable, and the controls function just as well as on any of the three axis airchairs. There are special two axis procedures, such as keeping the nose low  while ground rolling in a cross wind, so the weight of the glider on the wheels will keep the wings level.

Its first high flight was made in September, 2008. 
 
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Nothing is for sale and there are no commercial intentions. The Pig1 technical drawings are available for on line viewing or download,  Basic Ultralight Glider homepage, "Pig Drawings". There are 81 drawings available in three different file formats.
 
 
Wingspan: 26 ft
Empty weight, with parachute: 147.5 lb
Gross weight: 308 lb
Wing area: 194 sq.ft
Wing loading: 1.6 lb/sq.ft
 
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