Aircraft Ownership

A fuel-saving announcement from your ASI

By Dave Hirschman

The airspeed indicator (ASI) can tell you a great deal about how to fly more efficiently, but few pilots know how to decode its drag-reducing, fuel-saving, and range-extending message.

According to Jack Norris, an aerospace engineer and technical director for the 1986 Voyager around-the-world flight, a simple, mechanical ASI (and an understanding of the aerodynamic drag chart and an airplane’s best rate of climb speed) is all we need to maximize speed vs. drag. Minimizing drag is the key to reducing fuel burn and extending range.

“The airspeed indicator tells us a lot more than just ram air pressure,” said Norris, author of The Logic of Flight, a self-published book on aircraft efficiency and propeller design. “Your ASI can also tell you the most logical and efficient way to fly without being wasteful of fuel or time.”

All pilots learn in ground school that any airplane’s greatest flight efficiency is found at L/D max, that point on the drag chart where the induced and parasitic drag curves meet, and total drag is lowest. Pilots seeking peak efficiency can fly at L/D max for the absolute minimum fuel burn.

But here in the real world, few of us would ever choose to fly so slowly.

“No one wants to plod along at some low speed with mushy controls,” said Norris, a private pilot for 60 years. “You do that if you’re flying the Voyager around the world. But even then, it took nine days, three minutes and 44 seconds. What we’re really looking for is flying as fast as possible with as little drag as possible.”

Norris points to what he calls the “Max Speed vs. Drag” point on the chart. There, pilots can gain 31 percent more speed while paying a paltry 15 percent drag penalty. Since true airspeed (TAS) increases with altitude, at 12,500 feet, for example, pilots can obtain an additional 21 percent payoff for a total 59 percent speed gain over L/D max.

“Who wouldn’t want to go 59 percent faster for 15 percent more drag?” Norris says. “Aerodynamics is full of tradeoffs—but this one’s a bargain.”

The best speed vs. drag point is always 1.31 times V_Y, (the best-rate-of-climb speed), Norris says. Higher speeds are possible at lower altitudes and higher power settings. But since parasitic drag increases at the square of indicated airspeed, the additional speed carries a high price in dramatically higher fuel consumption and reduced range.

“Very few pilots really understand that the shape of the total drag curve is really a leaning, lazy J,” Norris says. “There’s a place where the curve flattens out and you can fly much faster for a very small increase in drag. You don’t need any special equipment or fancy math to figure it out. All you need to know is your aircraft’s V_Y and add 31 percent.”

Max efficiency profile

Norris recommends the following profile for virtually all piston-engine, general aviation aircraft: After takeoff, simply cruise climb at (1.31 times V_Y) as high as possible with the throttle wide open. When you’ve reached the maximum altitude at which you can maintain your target IAS with the mixture properly leaned, you’re done.

The pilot’s operating handbook for the AOPA’s IO-550-powered Beechcraft Bonanza BE36 seems to bear out Norris’ IAS-based strategy.

At a total weight of 3,400 pounds, V_Y is 96 knots, making the ideal target IAS 126 knots. On a standard day, with the throttle wide open and 2,500 rpm, mixture set 20 degrees lean of peak, the Bonanza shows 129 KIAS at 14,000 feet, 157 KTAS, and a fuel burn of 10.6 gph.

That’s about 14 KTAS less than the Bonanza’s best-power setting at 6,000 feet where the airplane travels 171 KTAS at 14.4 gph. So, on a 500-mile trip, flying at high altitude and optimal IAS adds less than 15 minutes flying time and saves 8.7 gallons of avgas (or more than $52 at current prices). Put another way, optimal IAS at altitude reduces speed 8.2 percent while slashing fuel consumption 20 percent.

Norris says his IAS-based approach works equally well for planes with fixed-pitch and constant-speed propellers and all engine sizes.

“Flying is subject to the same physical laws, and the drag curves apply to all aircraft,” he said. “Airplanes only know indicated airspeed. A wing doesn’t know how fast it’s moving over the ground, and it doesn’t care. Understanding IAS allows pilots to minimize drag, fly more intelligently, and get the most efficiency and utility out of their aircraft.”

Give it a try

Try Norris’ IAS method and let us know how it works for you.

Environmental factors such as winds aloft and icing levels are sure to influence your aeronautical decisions. One rule of thumb is to climb as quickly as possible when tailwinds are present to maximize the time such favorable conditions can act upon your aircraft. In strong headwinds, lower groundspeeds at altitude can negate any gains in TAS or reductions in hourly fuel burn.

Also, physiological factors and the availability of supplemental oxygen can come into play at the higher altitudes Norris’ IAS-based strategy suggests. Federal aviation regulations mandate that pilots use of supplemental oxygen whenever they’re above 12,500 feet cabin pressure altitude more than 30 minutes, and at all times above 14,000 feet. (But studies show hypoxia can begin at significantly lower altitudes for many people, and headaches, dehydration, and fatigue are common after prolonged periods at 8,000 or 10,000 feet without supplemental oxygen.)

August 25, 2008