Extending Your Fuel Efficiency

A few tricks of the trade for those wishing their fuel levels were a little higher.

Fuel Efficiency

Fuel Efficiency

** The best way to stretch fuel is to slow down.
It can be a case of diminishing returns, but
sometimes a few extra miles means a lot.**

It ought not to be true, but it is: In every pilot’s life there comes a moment when he wishes he had a little more fuel.

Perhaps the headwind was stronger than forecast; the gauges have dropped below a quarter sooner than you hoped they would; the descent and climb for an en route stop to drop off a passenger used up more fuel than you expected; you took a detour around weather; or your planning was careless in the first place. Whatever the reason, you find yourself in that awkward spot: a certain distance from your destination, with a certain amount of fuel and with a nagging worry about where those needles will be pointing when you arrive.

The cautious thing to do is to land at the next opportunity and get more fuel. But that is not always possible or convenient. There may be no intermediate place with suitable weather; you may have told someone to meet you at a certain time. And there is always the reluctance to lose time, and to give up altitude and then to have to claw it back — a reluctance so strong that many a pilot has run out of fuel rather than overcome it.

But nothing can be done about extreme pigheadedness. Let us stipulate that there are situations in which a pilot of normal maturity, competence and regard for safety might feel concerned, even conflicted, about his or her remaining fuel, but in which a decision to continue might depend on rational analysis rather than, say, how lucky he or she was feeling that day. These are the situations in which it is not irresponsible to “stretch” range.

How far an airplane can go on a given amount of fuel is principally determined by four factors: propeller efficiency, fuel consumption, speed and wind.

The role of wind is obvious. Any headwind, and even a side wind, increases the time to fly. The chart of wind components is familiar to pilots, though roofers, who have to cut their two-by-fours to match the run and rise of rafters, are more likely to remember the precise numerical relationships. But in the era of GPS no chart is needed; the wind component is obvious from the groundspeed.

Your flight time will be lengthened in roughly the same proportion as the headwind component stands to your airspeed. If you cruise at 150 knots and the wind component is minus 15 knots, your flight time will be increased by about one part in 10; it will take you 66 minutes — actually, 66 minutes and 40 seconds — to go as far as you would normally go in an hour. That is not likely to be a problem. But a component of 30 or 40 knots might be. To maximize your range you want as little headwind as possible, and so you should pick an altitude — if you haven’t already done so — where the wind component is least.

A headwind component works against you in two ways. First and more clearly, it increases the time needed to go a certain distance. Less obviously, it complicates the choice of a speed to fly.

Speed is a pilot’s most powerful tool for increasing range. The amount of speed you get in exchange for a given fuel flow — in other words, your miles per gallon — varies across the speed range. It is worst at very high and very low speeds, owing at the high-speed end to parasite drag and at the low-speed end to lift-related induced drag. Parasite drag increases with speed, and induced drag increases with slowness; they are equal at the speed for minimum drag. This is the speed at which the least power is needed to stay aloft, and therefore it is the speed for greatest endurance. It is typically about a third greater than the clean stalling speed.

You might guess that the minimum-­drag speed ought naturally to be the best range speed as well, but it isn’t. The reason is that, as you increase power from the minimum required to stay aloft, speed at first increases more rapidly than fuel flow does. Just how much higher the best-range speed is than the minimum-power speed depends on airframe, engine and propeller characteristics, but it’s going to be somewhere around 40 or 50 percent above the clean stalling speed. Like most aerodynamic curves, the speed-power curve is pretty flat at the bottom, and so you might as well err on the high side and call it 50 percent, especially because “half again” is easier to calculate in your head than “four-tenths more.” An airplane with a 54-knot clean stalling speed would have a best-range speed — this is an indicated, not a true, speed — of 54 plus 27, or 81 knots, and one with an 80-knot stalling speed, 120 knots. Most single-engine airplanes have clean stalling speeds below 70 knots, and so the best-range speed of the faster ones would be around 100 to 105 knots.

This is an indicated airspeed, and it is intolerably slow. That is why the choice of a speed to fly is complicated by a headwind: A low cruising speed is more strongly affected by wind, and for a longer time. A common rule of thumb is that the best-range speed should be increased by a quarter of the headwind component. Something analogous would apply to whatever reduced speed you choose in order to conserve fuel. In any case, a strong headwind is going to take a painful bite out of a reduced cruising speed.

As I mentioned before, the speed-power curve is quite flat at the bottom, and so the difference in range between the theoretical best-range speed and a speed 10 or 20 knots above it is comparatively small. In fact, even the difference between the mileage you get at best-range speed and what you get at maximum cruising speed is less than you might hope. The gain in miles per gallon between 75 percent power and best-range speed is typically around one-third; it’s something like the difference between city and highway mileage in a car. The greatest gains will be seen by pilots who habitually cruise at 75 or 80 percent of power. Reducing power from 75 percent to 55 percent might yield them a 20 percent improvement in miles per gallon for only a 10 percent loss in speed. On the other hand, the difference between cruising at 110 kias and at a throttled-back 130 kias, in an airplane capable of 150 kias, will be just a few percent.

The important number to remember is one-third. Take that as the absolute outside limit of what you can gain by slowing down. You cannot double your range, or even increase it by half; just to improve by a full third will require a huge sacrifice in speed. If you’re down to the last hour of a trip and you’re doing 180 knots at 75 percent power, even the most drastic slowing will gain you only 60 extra miles.

Besides wind and speed, another means the pilot of a nonturbine airplane has to increase range is leaning the mixture. How effective leaning can be depends on how the pilot normally does it; if you normally cruise as lean as possible, there’s nothing left to gain.

There is, and has always been, a great deal of mythology about mixture. Many pilots believe that they can harm an engine by leaning past peak EGT, and they consequently run on the rich side, blowing some unburned fuel out the exhaust pipe. It is generally untrue that lean operation is harmful, and it is particularly untrue at the kinds of reduced power settings you would use to extend your range. A pilot who habitually leans to 50 degrees F on the rich side of peak can save a gallon or two an hour, for the loss of a few knots, by leaning to 50 degrees on the lean side. Contrary to widespread belief, the cylinders will run cooler, not hotter.

When reducing power on airplanes with constant-speed propellers, reduce rpm as much as possible; at reduced power there is no problem with going “oversquare” — that is, setting the manifold pressure higher than the first two digits of the rpm. Reducing rpm both improves propeller efficiency and reduces friction losses in the engine, but these gains are comparatively slight.

A well-planned flight should not require stretching range. But in the context of modern general aviation operations, legal fuel reserve requirements — 30 minutes at cruise power VFR, 45 IFR — are minimal. At the same time, most airplanes are designed in such a way that payload and range must be traded off against one another, creating, on occasion, a powerful temptation to either overload the airplane or skimp on fuel. While it is generally best to go by the book, the potential consequences of being too heavy at the start of a flight are not nearly so grave as those of being too light at the end.