Stephen J. Mraz firstname.lastname@example.org
Advances in laminar-flow airfoils and composites have sparked a renaissance in sailplane design. And no company seems to have exploited these two technologies more adeptly than Windward Performance in Bend, Oreg. With a team of aeronautical and tooling experts led by Greg Cole, the relatively young company has designed and manufactured the SparrowHawk, the first sailplane or glider built in the U.S. in 30 years. The firm also has another poised for production, the DuckHawk. But the company’s highest aspirations are invested in Perlan Project. Perlan is an attempt to build a unique sailplane that will ride the Polar Vortex surrounding the South Pole up to 90,000 ft or more and set an altitude record for manned flight.
Designing a sailplane is an exercise in compromise. There are many interconnected variables, both in the physical qualities of the plane and its critical airfoils. Trade-offs between the two depend on the purpose of the aircraft — whether it’s going to be an altitude or distance record setter, a high-speed racer, or perhaps just a hobbyist’s dream. The aeronautical variables, the lift-to-drag ratio, glide rate, turning radius, as well as the plane’s ability to bleed off speed and slow down, almost all vary with speed, weight, or wind conditions.
Cole’s single-seat SparrowHawk was designed to be an “optimum compromise.” It can fly at up to 123 knots, its redline limit in calm air. (Fly faster than the redline limit and any turbulence or sudden up or downdraft can overstress the wings.)
The SparrowHawk also doesn’t stall until below 32 knots. Low-speed flying is important because thermals (the masses of raising air pilots use to gain altitude), can be small, only a few thousand yards across.
If the plane cannot turn tightly enough to remain in the thermal, the pilot will miss out on some free lift. So Cole built a plane that would have good speed and climbing ability, as well as a 32:1 glide ratio for cross-country flying. (It drops 1 ft for every 32 ft it travels horizontally.)
Of course costs were important. The SparrowHawk lists for $41,250.
The key to the SparrowHawk is the use of prepreg composites, carbon-fiber cloth impregnated with epoxy that cures at high temperatures. This differs from wet-layup composites, a method favored by German sailplane makers in which technicians apply liquid epoxy to dry cloth and it cures at room temperature. When finished, wetlayup components with the same strength as a prepreg part are twice as heavy. Thanks to prepregs, the SparrowHawk weighs only 155 lb but can have a take-off weight of 415 lb (with pilots and gear). Composites also resist corrosion and let engineers accurately replicate the complex laminar- flow low-drag airfoils their software lets them design.
The first step in construction is to create plugs, exact replicas of each part. Tooling experts build molds around the plugs using wet-layup techniques and cloth with a high-carbon content. The carbon-rich cloth gives the molds the same coefficient of thermal expansion as the parts that will emerge from them, a necessity due to the 270°F curing temperature. Making this tooling, along with the accompanying assembly jigs is the most-expensive step in building a sailplane.
The finished SparrowHawk accommodates pilots as tall as 6 ft 3 in and weighing 240 lb, and handles up to 5.48 gs. It carries many of the same instruments and controls as a typical general-aviation aircraft including control stick, rudders, radio, and altimeter, as well as some specialized gear. One such instrument, the variometer, is an extremely sensitive vertical speed indicator. Pilots use it to find areas of lift (a thermal) and to see how fast they are rising (or falling). There are also GPS-based navigation systems specifically for gliders. Besides the normal features of a GPS unit, they can highlight airfields a glider can reach based on its current altitude and performance. The units even account for terrain, ruling out fields as beyond range if they are tucked behind hills.
The SparrowHawk has a wingspan that prevents it from falling neatly into any racing class. And its top speed makes it too delicate to truly take advantage of dynamic soaring (see Dynamic soaring: The lazy way to fly). The DuckHawk addresses these limitations.
“We want the DuckHawk to be able to handle dynamic soaring, which involves some pretty high speeds, but it should also do well at cross-country flying,” says Cole. “And we wanted it to cleanup on the record-setting side of flying where teams wait for just the right wind conditions as well as the race-winning side where you have to fly the conditions you’ve got.”
To make sure it fit into a racing class, Cole gave the plane a 15-m wing span, one of the most popular sizes for racing gliders. Its fuselage and tail designs are almost the same as those of the SparrowHawk. But the wings set it apart, While most 15-m sailplanes have 100 to 120 ft2 of wing surface, the DuckHawk has just 80 ft2. But these skinny wings, with an aspect ratio of 30:1, were carefully crafted to give the 300-lb (empty) craft a 50:1 glide ratio. They also handle speeds up to 200 knots in smooth air.
“I spent three years designing the airfoils for the DuckHawk,” says Cole. “And while I might have done 20 or 30 wing designs or iterations for successful planes in the past, I made over 200 for the DuckHawk while I juggled sizes, planforms, and airfoils.”
To give the pilot more control over the plane’s performance, the DuckHawk carries 400 lb (about 50 gallons) of water ballast in the slim wings. Pilots can dump some or all of it while flying and usually do so when wind conditions change. The added weight puts more loading on the wing, and at higher wing loads, planes can fly faster with lower sink rates. Changing weight shifts the lift-to-drag ratio which, in effect, changes the speed at which the aircraft gets the best performance.
The DuckHawk also has air brakes, a pair of panels the pilot manually cranks out to add drag. The further the air brakes are extended, the more drag they generate. For example, fully opened, the brakes take the DuckHawk from a 50:1 glide angle to a 7:1 angle.
The crew at Windward is currently putting together the tooling for the DuckHawk. Material costs alone top $500,000 for just the tooling. The DuckHawk will be introduced with a $105,000 price tag.
“We expect to build six DuckHawks per year for about 10 years,” says Cole. “All are built to order. We don’t build up an inventory. And if sales are good, we could increase production to 15 aircraft per year.”
It began as project between Steve Fosset, a record-setting aviator, sailor, and adventurer, and the first person to fly nonstop around the world in a balloon, and Einar
Enevoldson, a record-setting test pilot. Enevoldson was aware of mountain wave lift, a phenomenon in which air blowing over mountains creates ripples in the layers of air above it for up to 100,000 ft, depending on the strength of the wind and steepness of the incline. He knew sailplanes could use these stacked ripples to gain altitude. Then he heard about the Polar Vortex, a consistent swirling wind that circles the South Pole during its winter. As this wind passes over the Andes in southern Argentina, it creates mountain waves that climb to 110,00 ft or higher. Enevoldson put the two together, recruited Fosset, and set about carrying out a mission that would see a sailplane climb the Polar Vortex to a record 50,727 ft in 2006. The next step was to custom build a sailplane that could carry them to 100,000 ft, breaking the altitude record for manned flight set in an SR-71 spy plane.
They turned to Cole and his company to build the plane, the Perlan, Spanish for pearl. (It was named for the mother-of-pearl clouds commonly seen at high altitudes around the Polar Vortex.) Cole tackled the design issues and problems of adding a pressurized cabin, a system and backup for scrubbing CO2 out of the air so pilots needn’t take bottled air, and a heater to ward off the –100°F outside temperatures.
Cole and his team conducted experiments to help them determine how much visibility a pilot would need to fly Perlan. He used the results to position round portals around the cockpit, much like Rutan’s SpaceShipOne. The portals are more structurally sound than bulbous cockpit canopies. Cole also sized the cockpit for two passengers seated fore and aft. Although this created a larger and more-expensive aircraft, the options for a sponsor to serve as copilot, as well as future research flexibility, led Cole and the Perlan team to choose this configuration. (Unfortunately, Fosset died about this time in the project. But Morgan Sandcock, an Australian business man, stepped in to provide much needed financing. He plans on being onboard for the record setting attempt.)
After crunching the numbers, Cole discovered that at 90,000 ft, Perlan would be circling at about 44 knots indicated, but its true airspeed would be 300 knots and its Mach number would be 0.6, thanks to the thin atmosphere. (Air at that height is only 2% as dense as air at sea level. So planes must fly much faster to generate lift.) This put the plane right on the edge of transonic flight, a regime that gives aircraft designers fits. To get up to 100,000 ft, Cole would have to really push the envelope. (See Perlan Tech Specs for a comparison of a Perlan built for 90k ft and one for 100k ft.)
Realizing his limits, especially the financial ones, Cole opted to shoot for 90k ft, leaving the 100k ft mark for the next phase in Project Perlan. “I designed the plane to fly well at 60k ft, so it should do fine up to that point and through to 90k ft,” says Cole.
Cole is confident he and his team will uncover some new engineering tricks and techniques during the project. “And if we get up top 90,000 ft, which I think we will, there will be a clamoring for scientific flights to take samples and measurements. Unlike balloons, we will be able to get data from specific areas and get it back it a timely manner.”
Perhaps the biggest hurdles for the Perlan team is getting a million dollars in funding and some good weather conditions.
Dynamic soaring: The lazy way to fly
How do albatrosses manage to fly thousands of miles across the oceans while barely flapping a wing? The trick is called dynamic soaring, a method of extracting energy from the air by flying in and out of air masses moving at different speeds. The albatross flies between the relatively still air close to the water and a layer of air 60 ft up that travels about 20 mph.
Sailplanes can do it as well, but they tend to use air masses with more sharply defined shear layers and higher relative speeds. Dynamic soaring is actually more popular with sailplane enthusiasts who fly radio-controlled and scaled-down aircraft. You’ll see why shortly.
| A few 15-m Class Sailplane records |
Average speed over a 62-mile triangular course 154.78 mph — Pilot: Horacio Miranda Place: Chios Malal, Argentina Average speed over a 186-mile triangular course 117.08 mph — Pilot: Klaus Ohlmann Place: Malargue, Argentina Average speed over a 310-mile triangular course 95.96 mph — Pilot: Terrence Delore Place: Mt. Cook, New Zealand
A simple diagram (above) illustrates the principle. A glider with a lift to drag ratio of 25:1 and an initial speed of 40 knots flies up and into a layer air moving 30 knots. The 30-knot wind pushes the aircraft’s air and ground speed to around 60 knots. The sailplane then drops down into the dead zone where it’s airspeed becomes equal to its groundspeed, 60 knots. It loses speed making a 180° turn and then loses more as it turns and climbs back into the wind. But then it gets another 30-knot kick to its air and ground speed. If the plane holds together and repeats this maneuver, the glider will quickly get to the point where the speed lost in the turns equals the velocity gained from crossing the shear boundary. By then it will be going over 200 mph. (One team claims a record of 357 mph with an RC glider using dynamic soaring.)
Radio-controlled gliders do well at this technique because they carry no human pilots who must withstand the g forces. And because the aircraft often fl ies to within 50 to 5 ft of the ground, it is risky. One false move and the sailplane augers into the ground.
One goal for DuckHawk sailplane is to let pilots dynamically soar. This means the craft must be efficient and strong, capable of some fairly high speeds. If the DuckHawk meets design criteria, it should fly 165 knots even in rough air.
You say glider.
I say sailplane.
Most people would say a sailplane is the same as a glider. But aviation purists point out that every plane turns into a glider when its engine quits. Only sailplanes are designed to fly without power.
A day at the races
Pilots compete purely for the glory; there are no monetary rewards. But pilots good enough (and lucky enough) to do well in national events can also qualify for the U.S. Soaring team which competes in World Gliding Competitions held every other year.
Events last three to 10 days, with races scheduled daily. The pilot with the fastest time earns 1,000 points for each race. Other finishers earn a fraction of that based on the ratio of their average speed to the winner’s average speed. (For example, if the winner averaged 100 mph and the second-place pilot averaged 80, the second place finishers would rack up (80/100) × 1,000, or 800 points. Pilot with the most points in their class at the end of the event are declared winners.
Races are closed circuits, often triangles, with the home airfield serving as the start and finish point. Race organizers assemble in the morning, assess weather conditions, and layout a track. In the eastern U.S., races can be 150 miles, while in the west, where winds are usually stronger, a 300-mile course isn’t unusual. (The longest U.S. .race ever was a 626-mile circuit.)
Two planes haul all racers up to altitude where they go to various holding points around the field. When the race begins, pilots jockey for good starting positions and perhaps wait for course conditions to improve before starting. Pilots must go through the GPS-defined starting gate at 5,000 ft. At the end of the day, when all racers have landed, officials check the GPS track of the top finishers to ensure they hit all the way points, awarding points and trophies accordingly.
Resources: Experimental Soaring Society, esoaring.com/
Sailplane Racing Assoc., sailplane-racing.org/
Soaring Society of America, ssa.org
Windward Performance Ltd., windward-performance.com
Catch some high-speed sailplane video at http://tiny.cc/eYQY6
Or climb aboard a ridge-running flight in a sailplane at http://tiny.cc/3jhCr