THE AMERICAN SPIRIT SAILPLANE/AMERICA COMES BACK
Form Still Follows Function

by Len Frank

 

In 2200BC, the Chinese Emperor Shin is supposed to have strapped large reed mats to his arms and made the first successful gliding flight. About 3900 years later, Sir George Cayley built the first real man-carrying glider. Cayley, among other things: observed that curved surfaces had more lift than flat; dihedrals aided stability; a movable tailplane and rudder provided the best means of control--this more than fifty years before the Wrights. All that really remained were detail improvements. We are still making them.

Cayley also invented the cycle-type tension wheel (used as glider undercarriage) and caterpillar tractors.

After the Wright brothers, gliders remained mainly un-powered versions of powered aircraft for some thirty years--open cockpits or completely exposed pilot, external bracing, short, wide wings, thick airfoils, inaccurate control of airfoil shapes...all of the aerodynamic sins.

The Germans, forbidden by the Treaty of Versailles to build high performance powered aircraft, began to develop the inefficient gliders into high performance sailplanes. By 1935, a German sailplane had set a 500 kilometer (310 miles) distance record (low drag=speed=distance) using aerodynamics superior to all but a few specialized powered aircraft. There was still room for improvement--most sailplanes were still made with decades old fabric-over-wood construction.

Pilots learned to spiral slowly upward in thermals (rising columns of warm air), then fly at the highest speed available when out of the thermal. In effect, the thermal is the "engine" powering the sailplane.

The next real steps forward came with the development of composites and the growing understanding of low speed aerodynamics. Weight and aero drag both went down, performance up. One measure of performance is lift-over-drag (L/D)--the number of feet the sailplane moves forward compared with the number of feet altitude lost. Ratios in the best ships of the `thirties were in the 20:1 range.

In the U.S., Hawley Bowlus, Ryan's plant manager when Lindbergh's Spirit of St. Louis was built, made beautiful, high performance (for the period) sailplanes. The Bowlus Albatross had a teardrop-shaped fuselage pod with an aluminum tube boom reaching back to the tail surfaces. The pod was a slick monococque structure of beautifully laminated wood--not the first use of monococque construction--but it anticipated the post-WWII use of composites.

Bowlus wings were growing in aspect ratio (wingspan/chord) but were still fabric covered, drag was far lower than the open framework gliders or even the normal fabric-covered "streamlined" fuselages of the time.

The `fifties saw more laminated wood and plywood skins used-- L/D continued to improve. Advancements in aerodynamics came from different directions. Laminar airfoils--airflow that attaches itself to the surface of the aircraft skin--had been used on the P-51 Mustang at the beginning of WWII, finally began to work into sailplanes as the use of composites allowed smoother wrinkle and rivet-free skins.

Gradually aspect ratios grew as a result of the "slender wing theory" which seeks to reduce the "spilling" of air over the tip (and adjacent area). Tips produce a vortex, loss of lift and increase in drag. Long, narrow, tapered wings have a lower percentage of their drag-inducing, non-lift area near the tips. Theoretically the ideal wing would have a needle-like chord at the tip.

The first glass-reinforced resin sailplanes were built in Germany in the early `sixties. With the new materials, airfoils, filets, leading edge shapes, were all refined, and drag was reduced to new lows by the shapes that were now available through the use of composite construction.

As pilots learned to use the increased performance, disposable water ballast began to be used so that higher wing loadings could be used to raise speeds between thermals. The idea was to build the sailplane as light as possible, then use ballast consistent with the prevailing weather conditions.

L/D ratios grew beyond 40:1. By 1970 wingspans were over 48 ft.--now they are near 60 ft.--with aspect ratios as high as 36:1 (25/30:1 is more common). Cross country competition speed averages doubled from 50mph to 100mph.

The only viable sailplane manufacturer in the U.S., Schweizer, who built metal sailplanes, left the market in the mid-`seventies, driven out by the cost of product liability, among other things.

The U.S. has all of the elements necessary for a strong soaring community. Thanks in part to the aerospace industry, composite technology is more advanced in the U.S., We have the aerodynamicists, the engineers, the enthusiasts. There is a large and growing market, (at least 40,000 glider pilots in the U.S., several times that number in Europe) but the only manufacturers willing to fill it since Schweizer left are in Europe. Cost of the European ships is high (a Nimbus IV is about $150,000). The lower and mid-range markets have been filled by the used high-end, high performance sailplanes, but overall, the cost of soaring has increased dramatically.

Further, last year's high performance sailplane may be more cost effective but its flight characteristics are still not right for an inexperienced or intermediate pilot.

Enter Tor Jensen, president of Advanced Soaring Concepts (Camarillo, CA), a long-time soaring enthusiast who saw an opportunity to produce a home-grown alternative to the (primarily) German sailplanes. Jensen has another company that produces high tech composite assemblies for others--regularly designing and building high quality tooling, and using epoxies, E-glass, S-glass, Kevlar, different types of carbon fiber, and honeycomb structures. Jensen also has a vice-president, Dewey Northcutt, who started as a sculptor but now seems to have become the cutting edge of composites and mold technology.

Marketing studies lead to the purchase of several crashed high performance German sailplanes--buying them crashed made them easier to study. The conclusion: the Germans are five years behind in composite technology, a market exists, and Jensen and company were sure that they could do it better.

The American Spirit is not the work of a single designer--it was a group effort that involved a huge amount of engineering time and countless computer design hours. Finite element analysis was employed along with aero programs and a vast amount of composite experience not contained in any computer program.

The ASC designers are critical of the quality of the high-priced sailplanes on the market today. They point to cracking gel-coat (the surface resins sprayed into the mold first), inaccurate mold work that leads to waves and ripples in surfaces, poor composite technology that leads to warping, uneven curing, rapid UV deterioration, and a general low level of workmanship, especially considering the high prices.

Jensen and company looked at a large number of kit planes on the market here (the kit market has grown to fill the vacuum left by the decline of the lightplane industry, which, like sailplane manufacturers, suffered from product liability) They are disappointed by the often haphazard construction techniques--butt joints that don't match versus "joggle joints" that do; far too much critical construction left to amateur builders; too much low strength, heavy polyester and glass; too many components not included in the kit.

ASC uses vacuum bag molding (25" to 27" of vacuum and no gel coat), UV barrier, foam sandwich construction with a special crush resistant cell made up of Kevlar, special foam and\ aluminum honeycomb around the pilot. No practical sailplane could be crushproof (or crashproof) but the Spirit is the only one that provides for pilot crash safety. Honeycomb, sandwiched between S-glass, is also used for the shear web and bulkheads. A triangulated chromoly (4130) steel cage, pre-welded, supports the wings and landing gear. The pre-fabbed wing spar is built of high modulous carbon fiber and Nomex.

The kit contains everything except labor and paint. Normal garage tools, a bandsaw and drillpress, a scale to measure the two-part adhesives, and simple air tools (electric are OK) are all that are required to build the American Spirit or its slightly more advance sibling, the Falcon (which has flaps and the option of longer wing tips for higher performance).

The horizontal stabilizer, which uses all of the techniques necessary to build the entire Spirit, is built first. About 600 hours from crate to flight ready for the Sprit, another 100/150 for the Falcon. The Spirit kit sells for $17,980--and the people who designed and manufacture it are just a phone call away.


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