Fuel Efficiency of Airplanes

First lets get a few facts straight: Fuel efficiency and speed are inversely related. It doesn’t matter if it is a motorcycle, car or an airplane. You pay for speed with reduced efficiency. You also pay for weight. These are fundamental facts. The only place where those rules don’t apply is in outer space (no gravity and no air). Therefore, it is meaningless to talk about fuel efficiency without considering speed and weight. Psychologically people have accepted driving at 65 mph as an acceptable tradeoff between speed, weight, and fuel. But that is an arbitrary choice. If you really want the best fuel mileage, nothing beats walking, bicycling or riding a horse. All of these produce more miles per unit energy than other modes of transportation. So ultimately this is all about tradeoffs – how much fuel efficiency are we will to tradeoff to move faster and carry more weight.

General aviation (GA) often gets a bad rap because at first glance the gas mileage looks worse than cars. But the truth is, GA is significantly more effective than cars once you factor in speed and weight.

Assuming internal combustion engines, the differences between engine efficiency is negligibly small. They all have more or less 30% thermal efficiency. Wind resistance and rolling resistance are the biggest variables. Wind resistance increases as the square of the air speed. Rolling resistance depends on the type of wheels. Cars are worse than trains because rubber wheels have greater rolling resistance than steel rollers on tracks. But it is safe to say that airplanes don’t have rolling resistance. Additionally, transportation is all about moving a load, so weight has to be part of this discussion as well- you can’t compare the fuel efficiency of a cement truck with a motorcycle without considering how much weight is being carried. We need to combine these three factors (gas mileage, speed and load) into a single parameter that represents the efficiency of movement. We will define a transport effectiveness parameter as the product of gas mileage, square of the speed, and the useful load. We will then normalize it against an automobile as our benchmark. This number is basically an indication of how aerodynamically efficient a vehicle is compared to a car. A number larger than 1 means it is better than an automobile.

 Type Useful load True Speed Fuel Burn Fuel mileage Transport Effectiveness Typical sedan 1000 lb 65 mph 1.9 gal/hr 35 mpg 1 Cessna 172N 900 lb 124 knots (142 mph) 8 gal/hr 18 mpg 2.2 Cessna R182 1350 lb 155 knots (178 mph) 14 gal/hr 13 mpg 3.7 Mooney M20J 900 lb 160 knots (184 mph) 12 gal/hr 15 mpg 3.1 Mooney M20K 900 lb 190 knots (218 mph) 12 gal/hr 18 mpg 5.3 Motorcycle 350 lb 65 mph 1.5 gal/hr 45 mpg 0.4 Boeing 737 30,000 lb 460 knots (529 mph) 950 gal/hr 0.7 mpg 40 Freight Train 1000,000 lb 80 mph 160 gal/hr 0.5 mpg 22 Walking 200 lb 3.5 mph 0.01 gal/hr 340 mpg 0.005 Bicycling 200 lb 15 mph 0.01 gal/hr 1150 mpg 0.35

As we can see, general aviation airplanes perform 2-4 times better than an average car after you account for their speeds and useful load. A Cessna 172 is more than twice as effective as a car. The Mooney M20J is almost three time as effective. The R182 fares even better, primarily because it is carries more load. The M20K is at the top because it is turbocharged and can fly faster at higher altitudes where the air is thinner. Interestingly, motorcycles do poorly in this calculation even though they are generally thought of as very efficient. Not surprisingly, walking comes last. Although walking is not powered by gasoline, one can calculate an equivalent mileage, which has been estimated to be about 340 miles per gallon.

Mass transportation and personal vehicles serve vastly different purposes, so any comparison must be taken with a grain of salt. But if we do the same calculation, we find the Boeing 737 to be a whopping 40 times more effective than a family sedan. The surprising thing is trains. Despite how trains are promoted by some people, they are only half as effective as an airliner. But they are still significantly more effective than a car. These numbers are for freight trains. Passenger trains are probably worse because you can’t pack passengers like you can with freight. Electric trains may do slightly better, but ultimately electricity needs to be generated too, and that efficiency is not much higher than the internal combustion efficiency.

One might argue that using the square of the speed unfairly tips the scale in favor of faster modes of transportion. But this was not an arbitrary choice. It is driven by fundamental physics – drag increases as the square of the airspeed. If you drive your car at twice the normal speed (130 mph), assuming all other factors remain the same, your gas mileage will be four times worse. The only place where this rule does not apply is in outer space. Nevetherless, for argument’s sake, we could repeat the same calculation with a linear speed dependence. This would represent the cost of time instead of the cost of propulsion. This calculation results in nearly the same ordering, except for a few changes. Bicycling comes out ahead of driving, and the freight train comes out ahead of the Boeing 737. GA airplanes still remain ahead of driving.

The fundamental reason airplanes and trains are able to do much better is because they are more aerodynamically shaped. They have a much smaller frontal area for the volume they occupy. Airplanes also benefit from flying at higher altitudes where the air density is smaller so they encounter less resistance for the same speed. They also do not have rolling resistance. Cars and trains can’t avoid air, and they can’t move without wheels (except the Maglev which has no wheels, and Elon Musk’s Hyperloop which runs inside an evacuated tube). And lets not forget that airplanes can fly point to point along the shortest distance, while everything else has to follow a longer ground route. Even from an infrastructure perspective, airplanes do better. Construction of highways and railways are enormously expensive projects and have huge environmental impacts. An airport needs just a one-mile long strip of pavement. Of course, air traffic control and radar are part of the infrastructure too, but they are optional safety enhancements, not a fundamental necessity. On the down side, however, airplanes can only fly between airports, whereas cars can go door to door. That convenience should not be overlooked, and it does offset some of the benefits of airplanes and trains.

All of the above calculations assume you are carrying the maximum rated load. If you are not carrying the full load, then these numbers will be lower. Fuel use does not scale proportionally with payload. This is because every vehicle has an empty weight. You still have to pay to transport the empty weight of the vehicle. This is why airlines try to fill every seat, and why we have carpool lanes on highways.

Bottom line is this. Just talking about miles per gallon is nonsense. If we want the most energy efficient transportation without any regard to speed or weight, we should just walk or ride a bicycle. But transporation is about moving heavy things fast. In that respect, airplanes are one of the most efficient machines we currently have. With mass transit, airlines do better than trains. With personal transport, GA airplanes do better than cars.

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