7
$\begingroup$

I've calculated the seat-miles per gallon for three planes. This factors in the speed; it basically means how much does it cost to transport 1 person 1 mile.

Plane Seats Cruise (MPH) gph seat-mpg
Cherokee 6 5 160 16 50
Grand Caravan EX 14 207 65 44
Rutan Quickie Q2 2 155 3.6 86

This is really surprising to me; I would have expected the exact opposite to be the case. I would have thought that as you group more cabin space together, you get more efficient.

If true, why do bigger planes get proportionally less efficient and more expensive per seat mile?

But if my math is right, and you want to move 14 people from point A to point B, you'll use less fuel sending them in 3 Cherokee 6's than in 1 Grand Caravan. I picked the Caravan because it appears to be the most efficient one out there; any of the other turboprops or twins are far worse. What's going on here?

On the road, the bigger the vehicle the more efficient (buses for example). What's different about the air?

$\endgroup$
7
  • 3
    $\begingroup$ You should compare at the same speed. $\endgroup$
    – mins
    Aug 14 at 18:53
  • 2
    $\begingroup$ @mins my calculation is gallons per seat-mile, which factors in speed. It basically means, how much does it cost to transport 1 person 1 mile. $\endgroup$ Aug 14 at 18:54
  • 3
    $\begingroup$ @IsaiahTaylor Drag goes up with the square of speed, so all else being equal, fuel per hour goes up faster than miles per hour, so fuel per mile goes up as well. $\endgroup$
    – StephenS
    Aug 14 at 19:03
  • 1
    $\begingroup$ Gallons per seat mile does not take into account speed. It takes into account miles, seats and gallons. $\endgroup$
    – Jpe61
    Aug 14 at 19:49
  • 1
    $\begingroup$ And, you are coparing apples to oranges. The planes you list are very different conceptually, so the really don't tell squat about what you are asking for. You should compare different variants of the same plane family, like A320 vs A340. And that is a bit far fetched too... $\endgroup$
    – Jpe61
    Aug 14 at 19:52
8
$\begingroup$

All else being equal, your intuition is correct that a larger aircraft will be more efficient (in a gallons per seat-mile measure) than a smaller aircraft. That's why there are big aircraft like 747's, 777's, and the A-380 flying on routes that have enough traffic to support them. When you have 300 passengers, it's more efficient to put them into one aircraft that holds that many than into two 737's to fly them the same distance.

Similarly, when all else is held constant, it's more efficient to go slowly than to go fast... how much effort does it take to pedal your bike at 5 mph vs at 20 mph? If you don't have the stopping and starting of city driving, your car will get better mileage at 30 mph than at 70 mph (assuming the transmission keeps the engine in an efficient range at both speeds) -- you're spending less energy overcoming wind resistance.

And the same "all else held equal" statement can be made about heavier vs lighter aircraft. For the same aircraft at the same speed & the same altitude (and winds, and temperature, and etc), the heavier one will use more gas to get where it's going.

The problem with the computations is that all three of these variables are changing when hopping between the Rutan EZ to a Cherokee 6 to a Caravan... speed is changing, the aircraft weight is changing, and capacity is changing. When all of these are changing, it's no longer a case of "all else being equal" and it's no longer valid to draw conclusions about the relationship of one of the variables (capacity, in this case) to efficiency.

If the comparison were between a Cherokee 6 and a 4-seat Cherokee cruising at the same speed, then you'd see results matching intuition.

Put another way, the fact that a 2-seat moped is more efficient (in gal/person-mile) than a 4-seat sedan is, doesn't mean that a 60-seat bus won't be more efficient than the sedan; it means that everything else that changed going from the moped to the sedan, obliterating the "all else being equal" assumption, overwhelms the inherent efficiency of the bigger vehicle. And, in many cases, we happily accept the increased person-mile costs of the sedan, since it is faster & safer & a more pleasant ride than the moped.

Same deal with the Caravan... it's faster, it's more powerful, it's a lot of things that make it more desirable than the Cherokee, and those not-all-else-being-equal factors make it look less efficient in this case. But that's more indicative of the distinct nature of the two aircraft, than information about the general trend of more seats making for better efficiency.

(Also, unless fuel were your only cost -- pilots & maintenance are free -- you wouldn't use 3 Cherokee flights to move the 14 people... all the other costs would make the Caravan a better deal financially, even if speed weren't a factor in the decision.)

$\endgroup$
5
  • 2
    $\begingroup$ This old question addresses one of the specific differences: aviation.stackexchange.com/questions/29588/… $\endgroup$
    – Adam
    Aug 16 at 21:47
  • $\begingroup$ @Adam Nice find -- turboprops are preferred for lots of applications as being more economical to operate than piston engines overall, but not in the narrow definition of fuel consumption. By the time the operator factors in the bills for maintenance and the benefits of lighter, less complex engines, the fuel bill is just one of many factors. $\endgroup$
    – Ralph J
    Aug 16 at 21:55
  • $\begingroup$ Qantas CEO says two 787s are cheaper than one A380, then again the number (and technology) of the engines isn't the same. +1 $\endgroup$
    – ymb1
    Aug 16 at 22:02
  • $\begingroup$ Thanks so much for this answer, it clarified a lot. It seems to me that the main thing is that the big planes go faster, so they cost more. If someone built a slow-flying Caravan, the cost might scale similar to a Cherokee 6. $\endgroup$ Aug 17 at 23:53
  • $\begingroup$ @IsaiahTaylor: See related: How does the range of a single- and twin-engine for small aircraft compare? $\endgroup$
    – ymb1
    Aug 18 at 10:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.