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Pond Racer

pond-racer


In 1988 Robert Pond (who derives a comfortable income from the manufacture of floor-cleaning products) contracted Burt Rutan’s Mojave, California, Scaled Composites, for the design and production of a prototype new-generation racing plane. Scaled engaged Nissan subsidiary Electramotive, a motorsport engine developer in Vista, California, to provide engines and gearboxes for the airplane. Rutan wanted to keep his pilot away from fuel, oil and hot coolant which meant a fuselage separate from the engine nacelles. The weight of the pilot’s pod is low, since it is supported at both ends and serves no structural purpose other than to hold up the pilot. The interference drag between the wing and the pod is nil. But most important, the pilot is as far away as he can be from the hazards of the powerplants. The airplane’s structure is mostly of graphite composites, often in the form of a sandwich with plastic foam cores. A special epoxy is used for compatibility with the engines’ methanol fuel. Despite its tiny size — 25-foot span and 20-foot length and advanced materials, the Pond Racer is a dense little airplane, weighing 4,500 pounds ready to fly. Its useful load is only 650 pounds.

Liquid-cooled, with a single over-head cam, two valves per cylinder, and 3.2-liter (200-cubic-inch) displacement, the Electramotive VG30 engines are based on the Nissan V-6 block used in the 300ZX and Maxima automobiles. Though their blocks are very compact, the engines use up a lot of space once the turbochargers and radiators and associated ducting are included (in fact, the complete installed powerplant weighs around 700 pounds versus 350 for the basic engine). They are packed incredibly tightly into the two-foot-diameter, five-foot long nacelles. The engines are mounted solidly to the airframe and the cowling skins are a load-bearing part of the engine mount, permitting the gap between spinner and cowling to be little more than a knife-slit.
More than a year was spent developing a gearbox to bring the engine’s 8,000-rpm operating speed down to a propeller-friendly 2,000.

The engines are electronically controlled. There are manual backups, but normally the throttle and prop controls move potentiometers, whose signals arrive, digitized, at each engine’s two Intel microcontroller chips along with temperature and pressure data from almost two dozen other sources. The microcontrollers consult schedules containing the desired boost and the duration and timing for the spark and injection, and adjust those parameters for each power stroke.

At the same time, airframe and powerplant data are recorded every two seconds, and can be dumped at the end of each flight to a computer and instantly displayed in graphical form. Technicians can then use the data drawn from each run to modify the laws governing engine operation until the optimum is achieved.

To deliver 1,000 hp, the engines must be boosted to 110 in. Hg and turn at 8,000 rpm. They must run continuously at that setting for the 15 minutes of a race.

On March 22, 1991, the first flight was described as a “no-brainer”. It took several flights to get the engines working well at moderate power, and even late in August they were still chronically misfiring and undergoing constant readjustment. One source of difficulty is that the engine control computers don’t monitor one variable important to airplanes though not to race cars: air density. With a wing loading of over 70 psf, touchdown is at a hot 120 knots.

The original intention had been to remove some or all of the angled “butterflies” at the tips of the horizontal stabilizer after flight-test demonstrated sufficient directional stability. Now they may be moved to a horizontal position instead, since the airplane has so little static margin that it requires no trimming between 140 and 250 knots.

A major source of trouble during testing has been the methanol fuel. Not a petroleum product, methanol, like alcohol, is derived from plant fermentation. It has about half the specific impulse of gasoline, which means that twice as much of it must be burned to produce a given amount of power. But it also burns over a far wider range of mixtures, so that engine cooling can be supplemented simply by pumping a lot of excess methanol through the engine. What doesn’t burn in the cylinders emerges from the exhaust pipes as a roaring plume of flame familiar to drag-race buffs. Methanol also doesn’t detonate, and that is what allows it to run at the astronomical levels of boost necessary to pull 1,000 hp out of an engine having fewer cubic inches than that of a Cessna 152.
Methanol has very unfriendly relations with many materials. “It eats us alive,” Dick Rutan says. After repeated episodes of corrosion and oil contamination, it became standard operating procedure to drain the fuel systems after each flight and refill them with aviation gasoline in order to protect components from the methanol.
Even if Burt Rutan scored a bull’s-eye in flying qualities, there are still other major uncertainties waiting to be resolved. One is the reliability of the engines while being operated continuously at 1,000 hp. Another is handling qualities at top speed; because the engines can’t be opened up, the Racer has not yet been flown above 333 KIAS (357 knots true airspeed). Yet another unknown is the efficiency of the propellers. Four-blade and 80 inches in diameter, they are King Air props modified by Hartzell to specifications developed by John Roncz, a longtime consultant of Rutan’s who also designed the wing and tail airfoils for the Racer. Their knife-thin tips are designed to run at 98 percent of the speed of sound — not a regime in which propellers are routinely used.
The other question mark is airframe drag. Although the Racer is small, it is complex in shape, with many intersections and much internal cooling flow whose drag is difficult to estimate. At 460 knots, air will slam into it like cinder blocks, with a force of 600 pounds per square foot. At that speed a small surplus of drag could mean the difference between success and failure for the whole project.

Its bulky fuel load notwithstanding, the Pond Racer turned out to be a tiny airplane only 20 feet long, with a wing span of slightly more than 25 feet. It weighs 4,000 pounds when fully fueled. Scaled Composites, Rutan's company in Mojave, Calif., finished the racer's airframe in June 1989, but the engines and gearboxes took another year and a half to be completed. During that time the Rare Bear pushed the official speed record to 528 mph-one mph faster than Rutan's hoped-for top speed.

At Reno 1991, a connecting rod punched through the left engine's oil pan, dumping lubricant on the hot exhaust pipes and causing a fire. Race pilot Rick Brickert trig-gered the Pond Racer's Halon fire-extinguishing system, which smothered the blaze, then flew the craft to a one engine landing. After Reno, the racer was transferred from Mojave to Pond's home airport at Palm Springs, Calif., to await further development.

 

 


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