Introduction

These notes are intended to describe the ideas behind Goat4 ultralight glider. Mainly I want to discuss things I think are important or which are frequently asked about. As always, this is a description of what I have done for my own purposes not intended as advice or best practice.

 

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Introduction

This page is intended to describe the design ideas behind Goat4 ultralight glider. Mainly I want to discuss things I think are important or which are frequently asked about. As always, this is a description of what I have done for my own purposes and is not intended to represent a conventional or proven design process.

D1. Goat Design Concepts............(April 2007)

1. Open Air Soaring

The pilot sits in an open chair, belted firmly in place but out in the breeze, in order to experience fully the adventure of soaring flight. The burden of dealing with a pilot enclosing structure is dispensed with, although an insulated flight suit is usually worn. Open air soaring will generally involve a higher parasite drag penalty than enclosed pilot flying, so the glide ratio will be less and gliding speeds and distance limited, but this is just another tradeoff that calls for the development of the right techniques and attitudes.

2. Crash Safety

As a survivor of many hang glider crashes, and a friend of pilots who did not survive them, I have considered the issue of ultralight glider crash safety as a matter of particular personal interest. Because of the inherent risks of aviation, crashes happen despite our best efforts, but the risk and extent of injury is to a large extent a matter of equipment design. The two central goals of ultralight soaring crash safety are: (1) minimize the energy of impact, and (2) protect the pilot.

To minimize the energy of impact (which is in proportion to the mass of the aircraft and the square of the speed relative to the ground), let the aircraft be inherently slow and draggy, as well as capable of very slow flight. Our best chance for a slow crash is to be flying slow in the first place.

Protecting the pilot means minimizing bodily deceleration, distributing loads over the body, and preventing direct contact of the body with external objects. In other words, the glider should do for the pilot's whole body what the helmet does for the head. To do this, let the pilot be secured in place with multipoint safety belts in the center of the airframe and surrounded by a substantial depth of collapsible structure (crumple zones). Impact from any direction will now result in localized structural collapse which will absorb crash energy and reduce the deceleration shock experienced by the pilot. Thus we adhere to the dictum: maximum damage to the airframe results in minimum injury to the pilot.

In the event of an emergency parachute deployment, let the parachute bring down the pilot and the aircraft together, in accordance with standard ultralight practice. This allows quick and reliable parachute deployment and makes use of the airframe to protect the pilot on impact. Many ultralight emergency parachute systems are rigged for descent with the aircraft in a nose down orientation, but I rig for a nose up position, so that the wing or tail structure will contact the ground first and collapse, absorbing crash energy while the upward facing pilot is protected by padded seating.

Additional crash protection can be provided by a big, bouncy pneumatic wheel and a deep skid structure under the pilot. The kind of crash we want to see is a "bust the glider and walk away" crash.

3. Docile Flying Qualities

Many aerodynamic aspects of the Goat are intended to provide forgiving low speed flying characteristics with mild stalls and spin entries (or maybe even an absence of unacellerated stall breaks or spins). These design points include a light wing loading, main wing washout (accomplished by aileron rigging), ailerons that taper to nothing before reaching the wing tip, and an untapered main wing plan form. Elevator size has been kept to a minimum to reduce trouble from pilot induced pitch oscillations and other misjudgments.

4. Static Margin Check

The main landing wheel has been placed slightly forward of the presumed aircraft center of lift (wing neutral point), so before any flight the pilot can be sure of having enough forward weight by doing a simple static balance test. With the pilot strapped in place and the nose released in the level position, the nose of the glider must drop to the ground, thus confirming adequate static margin. If the nose does not drop to the ground, the glider is too tail heavy and corrective measures must be taken before flight (to prevent a spin you can't get out of, basically). This system of balancing eliminates the need for load placarding, pilot weight estimates, etc. A pleasant side effect of this setup is that on the ground, the Goat is a nose dragger or tail dragger at the option of the pilot. I find it convenient to start and end most flights with the nose down, but for a stylish finish I sometimes roll to a stop with the nose up.

5. Easy Towing

The single point tow hookup is simple to use and the pilot always has one hand available just to pull the release. As compared to hang glider tows, no takeoff dolly is needed, and lockouts should be prevented by use of mechanical controls and a tail with an elevator. The tow hook is intended to conform to hang glider practice and incorporates a loop of line as a weak link.

6. Convenient Transport and Assembly

The Goat is designed for quick assembly and disassembly by one person, in order to be transported on a car top rack. My rack is an ordinary flat and padded hang glider rack, with no special saddles, straps, or additional padding, exactly the same setup I use to carry hang gliders.

Glider assembly involves handling five or six main parts, the heaviest of which weighs 35 to 42 pounds (wing panel). Assembly begins by unfolding and joining the wing halves to form the complete wing structure. The nose and tail structures are folding tube assemblies which are then pinned onto the wing (there is no continuous fuselage structure). The fifth part is the folding horizontal tail plane, which is put on last. The time and effort required for assembly is about the same as that required for a high performance hang glider, perhaps twenty minutes depending on conditions and available help. A pilot landing in a remote field should be able to quickly disassemble the aircraft and put the parts over a fence without assistance.

Ideally, all fasteners should be physically attached to the glider, so we can't walk away with parts in our pockets. The fasteners should be quick to attach and detach, and easy to check for preflight inspection. Quick pins have a handle and a tapered end so they can be used as "drift pins" to draw parts into position as they are assembled, eliminating the need for exact positioning of large sections prior to fastener insertion.

7. Garage-Level Technology Construction and Repair

The Goat can be built or repaired with hand tools on a garage floor using materials and processes that are conventional and readily obtainable. The main structure is aluminum tubing bolted together in traditional hang glider/ultralight fashion. A hand held electric drill, a file, and a hack saw are the major tools used, not using any molds, jigs, welding, machining, or a spray rig. I think of this not as "low technology" but as "low burden technology", allowing us to pursue the ideal of "build it in the winter, fly it in the summer".

The fabric covering is a conventional aircraft process. Light aircraft polyester fabric is cut from a roll and cemented onto the airframe and onto itself so as to form a continuous envelope, shrunk taut with an ordinary electric clothes iron, and then sealed with untinted Polybrush (Polyfiber process) or nontautening butyrate dope.

The control lines are made of 7/64 inch Samson "Lightning Rope" braided line (a blend of Vectran and Dyneema) routed through marine pulleys.

8. Traditional Stick & Rudder Controls

The objective is to have conventional, sturdy controls with "good control feel", which is difficult to define. The main control stick is in the center and operates conventional ailerons and an elevator, and the rudder is foot pedal operated.

9. Aesthetic Appeal

Goat styling leans toward functional simplicity and traditional forms when possible.

D2. Main Wing Airfoil ............(April 2007)

The nominal Goat4 main wing airfoil is an adapted version of a flex wing hang glider airfoil ("nominal" describes a reference or approximate value or characteristic which can be used for design estimates but which is not used for fabrication or inspection.) This airfoil is adapted to the tubular ladder frame main structure, and must allow folding of the trailing edge panels. All surfaces are flat except for the front half of the top surface and the wetted tube surfaces ("wetted" refers to those surfaces exposed to the outside airflow). This style of airfoil is seldom documented, but it has been used by hang gliders (in the airchair speed range) with satisfaction for many years and it is easy to make.

A more traditional airplane wing airfoil might be expected to be about 12% thick (12% of the 60 inch wing chord, or about 7.2 inches) with the thickest point at 30% (30% of chord back from the leading edge, about 18 inches back in this case). I use that traditional thickness but I put the high point further forward to accommodate the fit to the ladder structure (spar tubes). I won't use an airfoil thicker than about 12% because it would stack up higher than I consider desirable for transport and storage.

The lower part of the airfoil diagram shows how I define the contour of the leading edge upper ramp (as you can see, there isn't much of an underside ramp). First I draw an ellipse which is horizontally tangent at the high point, and vertically tangent at the very front of the leading edge tube. Then I draw a circle which is horizontally tangent at the high point and tangent to the upper front slope of the leading edge tube. The mean line between these curves, from the leading edge tube to the high point, is my model for the front ramp. This method was developed to allow quick and easy generation of a ramp that looked like those in use by hang gliders and other aircraft. Practical application of this method pretty much requires CAD (Computer Assisted Design) software, which I find almost indispensable for all phases of design.

This is actually the rib airfoil, but the mean airfoil in flight (the fabric between the ribs) is expected to stay close to this profile.

In flight the bottom surface fabric of the wing can be quite slack and may flap loosely where unsupported. Fabric panels that are too large or too loose can make a rumbling noise like a big loose tarp in the wind. Remember that he Goat4 is and airchair with a wing loading close to the hang glider range (about 1.7 to 1.8 lbs. per square foot), so an ordinary flying speed will often be less than 30 miles per hour, which many people would barely call a strong wind.

D3. Airchair Wing Loading ............(April 2007)

The Goat was designed to have the same wing loading as a hang glider, so it can fly in the same way, staying up all through the day, able to penetrate winds when necessary, mixing with hang glider traffic and using the same soaring techniques.

Let's assume that the average recreational hang glider flies at an empty weight of about 110 lbs. (airframe, harness, parachute, instruments, body armor, cover bags, and all), has a 160 lb. pilot, and has a wing area of 156 square feet. Now the gross weight is 270 lbs., and the wing loading is 1.73 lbs./square foot. The empty weight of the Goat4 is 135 lbs., so with the pilot, instruments, and a flight suit, it might have a gross weight of 300 lbs. The result is that the Goat4 has a wing loading of 1.72 lbs. per square foot, almost exactly the same as an average hang glider.

The critical benefit of a low wing loading is the ability to slow down. Besides contributing to safety and comfort, low airspeeds permit a low sink rate, even with high parasitic drag, and tighter turns can be sustained, so a slower glider can stay up better in light conditions, other things being equal. (The assumption here is that as the lift gets weaker, the thermals get smaller.)

It might be said that I am really talking about "wing maximum lift per pound of gross weight", which involves both the wing loading and the maximum wing coefficient of lift. "Wing coefficient of lift" includes wing geometry such as airfoil shape, flap settings, twist, etc., but I think the simplistic approach of being very serious about wing loading is still valid. I suspect that the differences between practical soaring geometries tend to become small when applied to the rough, crude, fabric covered wings of an airchair, but experience has indicated that the effect of wing size is strong.

Additional design topics are anticipated....