Flight Planning on a Globe
I was impressed when I read of the exploits of Bill Harrelson, the Virginia pilot who on March 1 flew his Lancair IV nonstop from Guam, in the western Pacific, to Jacksonville, Florida. The hop, if you can call it that, of about 8,000 statute miles took 38 and a half hours. If that weren’t enough, his positioning trip to Guam included a 20-hour leg from Indiana to Hawaii, and he did the whole out-and-back journey in the course of a week. My parents, who spoke German around the house mainly to keep secrets from me, used the term Sitzfleisch — “sit flesh” — as a synonym for patience or, more precisely, the ability to sit still for a long time. Harrelson has it.
Naturally, I had to do the numbers to be sure this was not an elaborate hoax. Harrelson’s Lancair (if you’re new to aviation, the “c” in Lancair, in defiance of the conventions of English pronunciation, is soft) is the recip-engined, nonpressurized version, with extra tankage for long-distance flying. Unlike those of most Lancair IVs, Harrelson’s Continental 550 is unturbocharged and has 10:1 pistons, which improve its specific fuel consumption. According to FlightAware, on which I followed the slowly lengthening green lines indicating his progress, Harrelson planned 180 knots at 7,000 feet. A Lancair IV is a pretty clean airplane, so I figured he could probably manage 180 knots at 10 gallons an hour. Assuming some help from the prevailing westerly wind, he would have to carry about 400 gallons, plus a bit more for taxi, climb and luck.
It turned out that the airplane was better than I thought. Harrelson took off with 361 gallons and landed with six for an average fuel burn of 9.2 gph. His average true airspeed was 180 knots with an overall tailwind — important to the success of the flight — of 3 knots.
The Lancair IV’s landing gear retracts into the fuselage, so most of its 98-square-foot, 30½-foot-span wing could be wet. I doubt the wing could hold much more than 100 gallons, however — probably less — so at least 260 gallons had to be in the fuselage. That seemed like a lot, but actually it’s only 35 cubic feet, which is equivalent to a 39-inch cube and, in a properly shaped tank or collection of tanks, would obviously fit in a four-seat cabin next to, and behind, the pilot.
Setting human endurance aside — Dick Rutan and Jeana Yeager’s nine-day Voyager flight in 1986 showed it to be almost limitless — the range of an airplane is a function of its aerodynamic cleanness, the efficiency of its power plant and the fraction of its takeoff weight that is fuel.
All aerodynamically clean airplanes are about equal regarding friction and pressure drag, so their flight efficiency is ruled not by their streamlining but by their induced drag. Induced drag is principally affected by wingspan. The two airplanes that have flown nonstop around the world — Voyager and Global Flyer — both had very large wingspans, with sailplane like lift-drag ratios above 30. They also had unprecedented fuel fractions. Nearly three-quarters of Voyager’s takeoff weight was fuel; Global Flyer’s was more than five-sixths.
The Lancair IV suffers, as a long-range airplane, from a shortage of wingspan. Its L/D ratio probably isn’t much higher than 15. What it lacks in that department it must make up in fuel fraction. To go 8,000 miles in a Lancair IV requires a fuel fraction of somewhere around 50 percent. With an empty weight of about 2,000 pounds and an allowance for the pilot and his survival gear and other life-support equipment like sandwiches, 2,200 pounds of fuel turns out to be about right.
Lest it seem that extremely modern and sophisticated equipment is needed for such a flight, let us not forget that way back in 1959 Max Conrad flew 7,668 sm from Casablanca to Los Angeles in a Comanche 250 — against the wind.
Two concerns arise when a great deal of extra fuel must be packed into an airplane: structural strength and takeoff distance. Voyager and Global Flyer distributed their fuel spanwise and in outboard booms to relieve wing bending stresses. When you load a lot of fuel into the fuselage of a conventionally configured airplane, you increase the wing bending stresses a great deal. For special-purpose flying, however, customary margins of safety are often ignored, and you keep your fingers crossed that you won’t run into severe turbulence.