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Assuming that you don't need to have any large piece on the underside dedicated to storage area/people (as all planes do) and you don't care about stability, what is the most energy efficient form for an aircraft in terms of energy required to maintain altitude? I would guess that it is the flying wing.

What it should do:

  • Use computer control if needed to reduce power requirements

All other requirements take second place; efficiency is the dominant goal.

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    $\begingroup$ You do mean "just to maintain altitude"? No forward movement is necessary? $\endgroup$
    – ALAN WARD
    Jul 22, 2015 at 12:06
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    $\begingroup$ infinitely long flying wing with appropriate airfoil... $\endgroup$
    – Thunderstrike
    Jul 22, 2015 at 12:08
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    $\begingroup$ you don't care about stability so plunging down to earth would be optimal? $\endgroup$
    – Federico
    Jul 22, 2015 at 12:46
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    $\begingroup$ The most energy-efficient form for an aircraft to just stay aloft is any lighter-than-air aircraft (e.g. balloon, blimp, etc.) No energy at all is required to stay aloft. :) However, this will not necessarily be the most energy-efficient way to actually travel a significant distance, especially if you desire to do so in a timely manner. $\endgroup$
    – reirab
    Jul 22, 2015 at 15:50
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    $\begingroup$ It would probably be good if we could make this question a little more specific. For example, are we only considering heavier-than-air aircraft? Are we only considering airplanes? Are we defining 'efficiency' in terms of fuel required to get between two places or in terms of energy required to stay aloft for x amount of time? If any aircraft counts and we only care about staying aloft, the answer is just "any lighter-than-air aircraft." However, a glider on a thermal also works. :) $\endgroup$
    – reirab
    Jul 22, 2015 at 16:06

3 Answers 3

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The best shape for an airplane or glider would look like the ASH-30mi, an open class glider with an 86.9ft wingspan, and a 41:1 aspect ratio (wing length to wing chord). It has a glide ratio exceeding 60:1, and is considered the state of the art for gliders.

enter image description here

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    $\begingroup$ How much could its glide ratio be improved if there didn't have to be a pilot in there? $\endgroup$
    – Boris Verkhovskiy
    Jul 22, 2015 at 21:27
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    $\begingroup$ @boris: For how a glider would look like without a pilot look at competition RC (radio controlled) gliders and pylon racers. Specifically the F3K, F3J, F5B and F5F classes (note that interestingly the rules for RC airplane competition are governed by the same body as real piloted airplanes: FAI). $\endgroup$
    – slebetman
    Jan 15, 2017 at 14:43
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    $\begingroup$ Note that in the 60s and 70s some of the RC planes that competed were flying wings but when people started to use carbon fibre the tail got so thin that flying wings can no longer compete against conventional tailed aircraft. This trend mirrors what happened to piloted glider competitions where in the 30s and 40s the Horten brothers and other designers managed to win competitions with flying wing gliders but when fibreglass appeared on the scene flying wings lost their advantage $\endgroup$
    – slebetman
    Jan 15, 2017 at 14:45
  • $\begingroup$ @Boris Taking the pilot out = reducing the weight does not make much difference to the glide ratio but it does change the minimum sink. $\endgroup$
    – Keith Whittingham
    Feb 20, 2022 at 14:49
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For a low speed aircraft, probably just a very long wing, like NASA's Helios research vehicle.

Helios prototype, NASA image

Part of the advantage comes from the tailless design - a tail generates negative lift to aid in stability of the plane. Getting rid of this eliminates some drag but may add control problems!

Another advantage is no drag at all from the fuselage - as the question said, we don't want to carry cargo or people.

Last but not least, there's a large efficiency advantage from the large aspect ratio of the wings which reduces losses at the wingtips.

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    $\begingroup$ Long is good, but high aspect ratio is better $\endgroup$
    – rbp
    Jul 22, 2015 at 18:19
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    $\begingroup$ If the goal is to stay aloft, the zephyr succeded more thant the Helios and posses a conventional tail configuration. Thus I may imagine it is more energy efficient, but may be mistaken as it uses more recent technologies. $\endgroup$
    – Manu H
    Jul 22, 2015 at 19:27
  • $\begingroup$ Good point, Helios gets much of its advantage from the high aspect ratio wing - like a glider - not specifically from the lack of a tail. However a tail works by generating slight negative lift, so I think it might impact efficiency slightly. That's why I went for the tailless design. $\endgroup$
    – Andy
    Jul 23, 2015 at 6:35
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    $\begingroup$ @Andy tail does not necessarily produce negative lift. It just has to fly at a lower angle of attack than the wing to ensure stability, as explain here and here $\endgroup$
    – Manu H
    Jul 23, 2015 at 15:12
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I concur with Andy, but in a few more words:

The fuselage of an aircraft provides no lift (or it does so very inefficiently in lifting-body designs), therefore it only contributes to weight and drag. It's a necessary evil in most practical aircraft because the basic idea is to move people or things with the airframe, so a compromise is reached between cargo volume and acceptable fuselage drag. If you instead remove the fuselage, leaving just the wing, you reduce weight (so less lift is required) and drag (so less thrust is required, which allows smaller engines which are also a contributor to airflow inefficiencies).

This is the mentality behind Northrop's various "flying wings" (YB-35, XB-49, B-2) as well as NASA's Pathfinder-style aircraft including Centurion and Helios, and certain other designs such as White Knight (the mother ship for SpaceShipOne; it is technically a twin-boom fuselage design not unlike the P-38, but it was intended to minimize fuselage surface area).

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