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Building the MachNone

A Sky Pup Construction Project

Sky Pup

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During the short history of the ultralight movement, production craft have dominated the influx of new machines into the hands of owners. Homebuilding has comprised an insignificant portion to the total number of ultralights now flying. Yet, as the average price of production machines tops $6000, many who wish to be active in the sport remain on the outer fringe. as spectators only. Some simply cannot afford the available production birds, while others refuse to pay such a price when they can own a two-seat classic aircraft for the same investment.

The manufacturers of ultralights are not to be blamed for the prices they require for their products. Most are operating on a very average profit margin, and indeed they must, or competitors will undercut their prices and take away their share of the market. The economic facts-of-life of manufacturing businesses are that labor and overhead costs keep the product price at the level it is. Since home- builders provide the labor and have little or no overhead, the monetary cost of a homebuilt project is basically the sum of the material costs for building the machine.

Sadly, a few opportunists have pieced together vague and incomplete “building plans” to capitalize on prospective homebuilders. These ventures have weakened the credibility of the sport, while souring the distraught plans-buyers toward new designs which appear. In many instances, “plans” have been sold for aircraft which have either never flown, or were structurally unsound.

These observations of the sport, combined with a conviction that flying should be as inexpensive as possible, have led to the development of a new homebuilt ultralight design, the Sky Pup.

The Sky Pup  The Sky Pup is an outgrowth of an earlier design, the “Blue Light Special,” which was built by some fellow engineers and myself in Wichita in 1980. It was a forerunner of today’s current crop of airplane-like ultralight designs, and proved to be a successful design exercise, exceeding original performance estimations on only 10 hp. (Many readers will remember the article on the Special in the August 1981 issue of ULTRALIGHT.) Response to the Special was remarkable, despite the designers’ statement that no construction plans would be made available for the bird. It seemed that the Special had hit an urgent need in the growing ultralight movement: the need for a simple and inexpensive homebuilt ultralight.

Using insights gained during development and flying of the Special, I began work on a similar follow-up design, incorporating changes and improvements to add durability and buildability. The Sky Pup is the result these refinements.

From its conception, the Sky Pup has been developed for the sole purpose of being reproduced by a builder working in a minimally equipped home shop. Many dimensions in the structure were determined by stock sizes of building materials. Even the chord of the wing was sized to allow use of standard fabric width during the covering process, thus eliminating the need for time-consuming seams. Similarly, the D-skins of the forward part of the wing were dimensioned to allow use of 2’ x 4’ plywood sheets with no waste, no waste, and to save procurement the time since they can be shipped UPS and larger ones cannot. These may appear to be minor design details, but their effect on cutting building time and project cost is very significant.

As with the Special, primary construction materials are wood and foam. Throughout the structure, foam is used as shear web material, while wood capstrips are used to carry bending loads. This construction method lends itself to homebuilding, since it eliminates the need for lots of jigs and fixtures. As with the popular foam-and-glass construction process, the foam is the jig.

The wing is fully cantilever, having no struts or wires. It features a single spar, and is comprised of three parts: a center section which is permanently attached to the fuselage, and two detachable outboard panels. The center section width was sized to allow the disassembled bird to be trailered down the road with adequate clearance. The outboard panels are attached through sets of upper and lower attach fittings in typical airplane fashion.

The Spar is constant depth in the outboard panels, but increases in depth from the wing joint to the fuselage. It features a large cross-section foam shear web with wood caps on the upper and lower surfaces. Ribs are also made of foam with wood caps. The result is a wing structure which is very light, yet has a positive ultimate load factor of 6.

Construction of the tail feathers is similar to the wing, except with two spars in the stabilizers and one spar in the movable control surfaces. Each control surface employs diagonal ribs to provide torsional rigidity. The tail group weighs only 8 pounds, complete with fabric and finish, and like the wing, is entirely cantilever.

Fuselage construction follows that of the Special, consisting primarily of a foam box structure with wood longerons at the corners of the cross-section. The box assembly is reinforced at concentrated load points with plywood gussets of the appropriate thickness. The seat back and floorboard are covered with plywood to add strength and durability. The landing gear of the Pup is a tapered maple beam, which is, in fact, a wooden spring. It has proven to be light, durable and inexpensive . . . in keeping with the overall design concept. Since the Pup has not been flown off of anything remotely resembling a paved surface, the gear has received extensive service testing in rough conditions without showing any signs of the hard usage.

The Sky Pup prototype is powered with a Cuyuna 215RR engine with a California Power Systems 2.25:1 reduction drive . This package swings a 58-inch propeller, which provides ample thrust without unduly taxing the engine. Engines of considerably less power will provide good performance, especially at the lower elevation. Larger engine will not be allowed because of the danger of inadvertently exceeding the FAA-imposed maximum speed.

The rudder and elevator control systems are extremely simple. The left and right elevators are operated individually through separate cables on each side of the airplane. The rudder is actuated by a rudder bar, and both the rudder cables and the elevator cables run through fairleads on the sides of the fuselage. No pullies [sp] are needed anywhere in either system, which saves cost and building time. The control stick is mounted in typical airplane fashion between the pilot’s legs.

Entry to the cockpit is achieved by stepping into the fuselage and standing on the ground through a toilet-lid-sized hole in the floor. After sitting down, the pilot’s feet are retracted to the rudder bar area and the hole is zipped shut. The fit is comfortable even for tall folks, with access to the controls being very natural. The plans for the Pup allow tailoring the head­- room to accommodate the builder’s physical proportions.

The Sky Pup prototype has been flown by many people of various piloting abilities with no problems encountered. Most of these have been regular aircraft pilots with no previous ultralight experience. Without exception, they have four comments upon introduction to the Pup. When they first see it, they exclaim that it looks like an airplane and chide me for telling them it was an ultralight. Upon flying it, they have three comments: first, that they were not expecting an ultralight to have the performance and “feel” of a regular airplane; second, that they were expecting to fly something which was marginally controllable rather than crisp and responsive; and third, that the Pup is absolutely the easiest machine to fly that they’ve ever been in.

The small size of the horizontal tail and elevators, combined with the relatively short tail arm and well-forward C.G., make the Pup elevator power limited. This prevents the pilot from being able to induce extremely nose- high attitudes from any normal flight situations. The elevator power is sufficient to provide ample flairing ability during the landing maneuver and complete control capability in flight. Approaches and landings have been made with the stick in the full-aft position, the result being a slow, mush descent.

The Sky Pup possesses light controls and responds readilto to input from the pilot. Wind in the cockpit is noticeable but not severe, with goggles being optional equipment except when bugs are thick or the air is very cold. An optional windscreen could be incorporated easily, and would undoubtedly aid the overall appearance of the machine.

The major difference a person will notice between flying the Pup and flying other ultralight designs is its aerodynamic cleanness. The craft has an honest 12:1 glide ratio, which is between two and three times the glide performance of the standard tube-and- wire machines. This characteristic shows itself in many ways, but primarily in penetration, reserve power, and the ability to glide for a reasonable distance power-off, in fact, the most enjoyable pastime in the Sky Pup is to climb to altitude under power and then glide or soar deadstick (without the sound of the engine). The engine can be restarted in the air quite easily.

The Sky Pup’s lack of ailerons, spoilers, or spoilerons has been of concern to some people who have never flown it or seen it fly. Some discussion about why they were not included is probably in order.

Because of their limited empty weight and maximum stall speed, ultralights are by edict very low wing- loading aircraft. As such, they are much more weather limited than heavier, faster aircraft, and will not venture subjection to severe weather conditions. Due to their size and speed parameters, they will not be noted as having high roll rates or crisp roll response, no matter what type of lateral control device is used. Although I have flown many 3-axis ultralights, I’ve yet to fly one where the lateral control device was more effective as a control benefit than as a selling feature.

Contrary to what some would advertise, an ultralight does not need the ability to land in 90 degree crosswinds of 20+ mph. Anyone who has flown ultralights or has watched someone else fly in bad weather conditions will testify that this is no time to be out flying around, no matter what type of ultralight you have. If winds are substantial enough to pose crosswind problems, they are substantial enough to cut ground speed and roll-out to almost nothing, and allow landing across the normal axis of the runway or field.

We make no apologies about the Pup’s 2-axis control system. It flies beautifully with rudder and elevator, having a roll rate (through dihedral effect) which will rival anything around. The Pup has been flown safely in conditions when many 3-axis ultralights were “sitting it out”. When weighed against the extra cost, building time, weight, and complexity of adding these devices, leaving them off was much more attractive. To a man, all the pilots who have flown the Pup agree that ailerons or spoilers are not needed.

The Sky Pup is not intended to be the ultralight for everyone. Although it is more portable than many designs, it cannot be “rolled-up” and carried on top of the family station wagon during vacation. Since it involves more work than a weekend assembly of pre-made parts, it will not attract those people who must fly now, regardless of cost. The Pup has been designed to fill a growing void in the ultralight movement......, the need for an easy-to-fly, easy-to-build, LOW-COST ultralight. We feel that these objectives have been achieved, and are confident that the proliferation of the Pup will prove its success.

Steven K. Wood
Sports Flight Engineering
Ultralight May 10, 1983

The Case for the Homebuilt Ultralight