The Importance of Wingspan Can’t Be Underestimated

Recent years have seen a general drift toward longer spans and higher aspect ratios. The Beechcraft Bonanza has a span of 33.5 feet and an aspect ratio of 6.2; the Cirrus SR22, which might be seen as today’s Bonanza, has a span of more than 38 feet and an aspect ratio of 10.1. The trend is generally toward greater aerodynamic efficiency, partly in response to fuel costs and partly because the increasing use of turbocharging leads to higher cruising altitudes, where longer wings are more at home.

The two airplanes I’ve built, Melmoth and Melmoth 2, are (or were—the first Melmoth was destroyed in an accident long ago) broadly similar, with low wings, T-tails, bubble canopies, retractable gear, and the same 200 hp Continental 360 engine and Hartzell constant-speed prop. The first Melmoth was aluminum, with 2+1 seating; the second is composite and seats four. Both were built with long-distance travel in mind and have lots of internal tankage: Melmoth’s wing and tip tanks held 155 gallons; Melmoth 2’s completely wet wings hold 142 gallons. The two Melmoths, with the same engine, propeller, empty weight, and cabin cross-section, differ significantly in one aspect: wingspan. The first began life with a wingspan of 23 feet and went through 21-foot and 28-foot iterations before its eventual demise. Melmoth 2 has a wingspan of 36 feet but only a little more wing area—106 square feet to the first Melmoth’s 93. (For comparison, the wing areas of most commercial four-seaters range from 145 to 180 square feet.) The first Melmoth’s aspect ratio was 5.75; Melmoth 2’s is 12.6.

Span and area are entangled with one another in the sense that structural strength and stiffness (not to mention space for retracting landing gear) require a certain wing thickness, and that in turn implies a minimum chord (the distance from leading to trailing edge), because airfoils shouldn’t be too thick. So you can’t just increase span willy-nilly without at some point having to increase chord and area as well. However, increasing the wing area, which was originally selected to permit a certain landing speed at a certain weight, adds drag and makes the airplane heavier.

Increased wingspan—other things remaining the same—rewards you with better efficiency and climb rate, and improved high-altitude performance. The first Melmoth had a maximum lift-drag, or L/D, ratio of about 11.8 and a “Breguet range”—a fictional, greatly exaggerated number that ignores takeoff, climb, and varying engine efficiency and assumes that you always fly at a low and ever-decreasing ideal speed—of 3,000 nm. Melmoth 2, with half again the span, has an L/D ratio of 17 and a Breguet range of 3,600 nm, despite carrying 8 percent less fuel. Rate of climb is less strongly influenced by span than L/D and range are, but Melmoth 2, climbing at 1,800 fpm at full power and a typical weight of 2,200 pounds, betters the original Melmoth by about 20 percent.

Note that I said “half again the span” and added nothing about aspect ratio. That is because, contrary to widespread belief, aspect ratio actually does not enter into it. Aspect ratio is generally thought of as the quintessential measure of efficiency, but if you could double an airplane’s wing area (thereby halving the aspect ratio) without increasing its parasite drag, the L/D ratio and Breguet range would remain the same. But you can’t increase wing area without increasing drag and weight, and that’s why aspect ratio becomes important: It’s a measure of how little wing area you can have with a given span.

Curiously, and I think unexpectedly for most pilots, altitude also does not enter into it. You might intuitively suppose that thinner air would make the airplane more efficient, but in fact neither the maximum L/D ratio nor the maximum range is affected by altitude.

You will object that at 8,000 feet you will go faster, with the same fuel flow, than at 2,000 feet. True. But that is because your indicated airspeed is lower. If you flew at the same indicated airspeed and fuel-air ratio at both altitudes, you would find your fuel flow is greater at the high altitude. The reason is that drag at a given indicated airspeed is the same at all altitudes, but the power required to overcome it is proportional to the square of the true airspeed, not the indicated airspeed. At the bestrange speed, the miles per gallon is at a maximum, however, and is unaffected by altitude except to the extent the engine’s efficiency might vary at different settings of manifold pressure and rpm.

“Best range” and “best efficiency” are not seen in normal flying. Under actual cruising conditions, Melmoth 2 is not that different from the original Melmoth. The reason is that maximum L/D and the Breguet range assume speeds that are quite low—around 40 percent above the clean stalling speed—and remote from those we actually use. At real-world speeds, 65 percent or 75 percent power, the differences shrink. Melmoth 2 will cruise at 170 knots at 12,500 feet using about 8.5 gallons an hour—about 60 percent of rated power; the first Melmoth would burn about 9.6, around 70 percent power, at the same weight and altitude. So you see that despite a 50 percent improvement in best L/D, the practical benefit of the longer wing is much smaller.

When I designed the first Melmoth, I was strongly influenced by John Thorp and his T-18 homebuilt, whose wing I copied almost exactly. Thorp, who also designed the original rectangular-wing Piper Cherokee, used to say that low aspect ratio wings perform better than theory would lead you to expect, and he was adamant there was no reason to taper the wings of any airplane weighing less than 12,000 pounds. When I designed Melmoth 2, however, I was more influenced by Burt Rutan’s derisive observation that if I intended to fly long distances, I had certainly chosen the wrong wing to do it. Aesthetics, too—hence the long, slender, tapered wing of Melmoth 2, of which Thorp might have disapproved.

For efficiency—the least fuel burned for the most work done—a large wingspan is necessary. But Melmoth 2’s long wing cost it the rollicking roll rate I enjoyed so much in the first Melmoth. Melmoth 2 rolls more like an Airbus. Sometimes, I think I would pay for the extra fuel just to have the rolls back.

This column first appeared in the November 2023/Issue 943 of FLYING’s print edition.