Efficiency Flying
With fuel prices around the world vastly outpacing already high inflation, small changes can result in big savings at the pump. While there aren’t a lot of things we can do to fly more efficiently, altitude and speed will make a difference.
Turbines
Turbine engines are more efficient at higher altitude. It’s a basic and obvious concept of operating one. Even so, it can be revealing to take a fresh look at the numbers and realize why it’s worth making the effort to climb higher.
The endurance performance chart on the JetProp models lays out the argument well. From sea level to about 17,000 feet, endurance only increases approximately 15 percent. In those altitudes, true airspeed increases about 22 percent, and fuel burn decreases about 30 percent. Which means in terms of efficiency you’re not much better off at 15,000 feet than you are at 500.
From that point the endurance curve takes a big turn and from around 17,000 feet to the low 30s, endurance increases tremendously. True airspeed suffers little with the altitude gain, which allows for big gains from reduced fuel burns. In fact, true airspeed stays within a knot or two at lower weights, although it does suffer from higher weights. At 10,000 pounds the maximum true airspeed of 278 knots occurs at 21,000 feet, and drops to 264 knots at 31,000. Regardless, fuel burns decrease substantially, from around 285 pounds per hour per engine to 189 pounds at 31,000 feet. The result is a range of roughly 5:45, or nearly 55 percent more than 17,000 feet. It pays to climb as high as possible as quickly as possible.
Pistons
With different physics at play, piston pilots don’t necessarily have the luxury of simply climbing as high as possible and staying there in order to save fuel. Usually there’s a sweet spot more in the mid-level altitudes.
More than altitude, however, speed plays a major factor. We know an airplane’s maximum range is the bottom of the total drag curve, and the maximum endurance is the tangent of the curve. But these speeds are agonizingly slow. Engineer B.H. Carson knew this and studied a third option that we now call the “Carson Speed.” Carson wrote that pilots don’t want to fly quite so slow, and he offered a faster, more practical speed that also maximized efficiency. It’s more or less 1.3 times the best glide speed.
Since best glide varies by weight, the Carson speed will as well, but even somewhere in the ballpark will result in substantial endurance gains. It’s not uncommon for pilots of traditional Spam cans to see miles per gallon rates that far outpace their cars. Not that you needed another reason to fly, but knowing you can do so on less fuel than your car is pretty good motivation.
