Pig Page

by Mike Sandlin

The Pig is my latest biplane airchair design....

Last update......

May 5, 2010 (text revision)

The Pig (Primary Instruction Glider) is my latest airchair project, a biplane ultralight sailplane with a two axis control system (rudders & elevator, but no ailerons). Its first high flight was made in September, 2008. 

The Pig, September 2008. The hand deployed emergency parachute is in the purple bag. 
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 my current version of this slow/simple/safe aircraft.

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.

Here's a video of the Pig doing a rolling launch from a training hill. This is, for now, my brightest vision of entry level flying:

Pig Launch on a Training Hill

The Pig has a lot of the usual airchair features:

        In addition, the  Pig has some special features of its own:    

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".  
    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.
    The photo at the very top of the page serves as an illustration of the mechanics of aerodynamic yaw/roll coupling as related to the dihedral angle of the wing. Imagine that the pilot is turning right and so has just yawed to the right, and the oncoming air  is reaching the wing exactly from the direction of the camera view. You can see the bottom of the left wing but only the top of the right wing, so the oncoming air has a higher angle of attack on the left wing than on the right wing. (This is exactly the condition the would have been achieved by applying right aileron, if ailerons had been present.)  Now there is more lift on the left than on the right, so the wing rolls to the right, as desired for a right turn. (A big turn is not something you would actually do at this low altitude, but I hope you can get the idea from this example.)

Walking the Pig is pretty easy.

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.

Here is an older version of the Pig, floating down the training hill at
 Crestline, California, after a rolling launch.

The wing airfoil is a simple utility type that I 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. From this I expect 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).

I'm in the Pig, at the beginning of a truck tow, up and rolling. The towline can be seen dipping off to the left, with the aft end attached to the tow hook at the nose of the glider. I'm reaching up to the middle of the nose tube, ready to pull the release handle.

Room has been reserved behind the the pilot to allow for installation of a small motor, presumably with a 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. Right now I'm not doing anything with motors, but someone with those interests might want want to take a look at it.


 The current empty weight, with parachute, is 147.5 lbs.  (This may be a few pounds heavier than my previous airchairs, but with an estimated gross weight of 308 lbs, my 194 square foot wing area gives me a wing loading of 1.6 lbs. per square foot, still well down into the hang glider range, so I'm happy with this.)

I have nothing for sale and no commercial intentions. The Pig1 technical drawings are available for on line viewing or download, see the main menu,  Basic Ultralight Glider homepage, "Pig Drawings". There are 81 drawings available in three different file formats. Also see my Airchair Update Page for the latest news.

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