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So, sailing boats take the kinetic energy of a crosswind and turn it into thrust by using their sails as lifting surfaces, and the discussion I had in the comment section of this question made me wonder why nobody has ever built any (working) sail-powered airplanes.

Basically, imagine a glider, but with sails on it (maybe on both top and bottom of the aircraft, to avoid imbalances in the center of thrust tipping it over) to catch the crosswind and provide forward thrust. Since these vehicles don't exist, there are presumably problems with them that would prevent them from working.

What are these problems? Do the aerodynamics of the sails interfere with the aerodynamics of the wings or something? Even if sails don't provide enough thrust to get the plane off the ground, that wouldn't be a problem for a glider that can't get off the ground by itself, right?

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    $\begingroup$ Won't anyone mention dynamic soaring? $\endgroup$
    – Abdullah
    Dec 3, 2020 at 14:17
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    $\begingroup$ @quietflyer: Not quite. Quartering wind can produce sailing speeds in excess of wind speed. But that works only because the sailboat has two media at its disposal and can extract energy from the difference in their respective speeds. $\endgroup$
    – Peter Kämpf
    Dec 3, 2020 at 15:53
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    $\begingroup$ It's disappointing to see downvotes without mention of why the voters feel this question isn't valuable. This is a fair and interesting aerodynamics question, which is non-obvious to those who don't understand how sailboats use their keel and rudder to create lift in the direction of travel. Indeed, most of the answers so far to this question betray a certain unsophistication about the hydrodynamics involved (although they're spot on about the mandatory intersection of two reference frames). $\endgroup$
    – Kenn Sebesta
    Dec 3, 2020 at 17:07
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    $\begingroup$ Re "It's disappointing to see downvotes without mention of..." -- even if one has no idea how sailboats work, a rudimentary understanding of the fact that an aircraft flies within the surrounding airmass should be sufficient to make it clear why sails would not be beneficial on an aircraft. $\endgroup$
    – quiet flyer
    Dec 3, 2020 at 19:57
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    $\begingroup$ Downvoted because once an aircraft is airborne, there is no wind. Wind is nothing more than an air mass which is moving relative to the ground. There is no such thing as wind if you remove the ground as a factor. Imagine a hot air balloon floating along in an air mass. You can hang on it all manner of sails, but it isn't going anywhere, other than where the air mass takes it. $\endgroup$
    – Mike Sowsun
    Dec 3, 2020 at 19:58

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High performance sailboats can sail faster than wind speed in certain conditions. Therefore, your question is not as absurd as it might seem at first glance.

However, the technique that enables sailboats to do so is not available to airplanes, at least not in flight. Sailboats use the speed difference between water and wind. Their keel keeps them from being blown with the wind. A glider in the air has no such second medium to resist being blown away by the wind.

Historically, the Andrée ballon used for artic exploration in 1897 tried to "sail" by combining sails and trailing ropes. However, this did not work in practice.

The only way to extract energy from the wind is when this wind changes. Wind gradients allow albatrosses to soar dynamically, and changing updrafts can be exploited by changing the load factor.

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Short version of (by now) two other answers: a sailing vessel needs a sail in one medium and a keel in another: a foil in the water, conventionally; ropes dragging on the ground; electromagnets in the storm wall of Jupiter's Great Red Spot, in a science fiction story; reaction wheels in inertia, in a solar sail.

Slightly longer: it needs two things (airfoil, "waterfoil", etc.) that exert different forces or at least torques. When a force is due to fluid (gas or liquid) moving past a foil, lift (in whatever direction is transverse to the foil) is more efficient than drag.

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    $\begingroup$ FWIW, it would be more accurate to say it needs a "wing" in one medium and a "wing" in another. A sail is a wing, and some of the more interesting cutting edge sailboats have symmetric wings. Furthermore, the daggerboard/centerboard/keel is a wing as well, and it works specifically because of it's AoA through the water. $\endgroup$
    – Kenn Sebesta
    Dec 3, 2020 at 19:29
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    $\begingroup$ Actually, it needs wings in different velocity streams -- don't have to be the same medium. A submarine could sail against the Gulf Stream, for instance, by floating at the bottom of the flow and extending a keel down into the "still" water below. Same medium, different velocities. $\endgroup$
    – Zeiss Ikon
    Dec 3, 2020 at 19:50
  • $\begingroup$ Excellent point! $\endgroup$
    – Kenn Sebesta
    Dec 4, 2020 at 3:17
  • $\begingroup$ Oops, I pocket voted this down (yes, it is possible) and can't reverse that unless this answer is edited... $\endgroup$
    – Jpe61
    Dec 4, 2020 at 7:41
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    $\begingroup$ Was checkin out this question, got into a discussion with a colleague and slipped the phone into my pocket without locking the screen. You paid the price 😃 $\endgroup$
    – Jpe61
    Dec 4, 2020 at 20:45
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For a glider to move forward using vertical sails, you would have to fix it to the ground somehow, as if it was on tracks, to provide the lateral resistance that allows the reaction force between lateral air movement and the vertical sail to occur. Something like a kite surfer. Replace the kite with a regular glider, with a tether line running from the glider's center of gravity to the surface of a lake, and a floating element with the required keel to resist lateral movement at the bottom of the tether line, and there you go. You have a glider that can move along powered solely by lateral winds, as long as it's anchored to the surface. Until you run out of lake...

Gliders do sail like sailboats, just in the vertical plane. The wing is doing double duty as the supporting element and the "sail". A sailboat's sail makes forward thrust from air moving laterally across its path. A glider's wing, in level flight, needs additional energy from air moving vertically across its path to allow it to move forward without descending, or to climb. Gravity is the energy source up to the point of the glider's flattest still-air glide angle; additional energy to maintain level flight, or to climb, comes from vertical air motion.

The sailboat needs lateral winds. The glider needs "vertical winds". I've always called soaring "vertical sailing" and gliders are called "sailplanes" after all.

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  • $\begingroup$ In a sense, you're right. There is a reason why gliders are also called sailplanes. $\endgroup$
    – Federico
    Dec 3, 2020 at 14:13
  • $\begingroup$ A wingsail makes a sailboat into a bit more of a cross between a plane and a boat. $\endgroup$
    – CGCampbell
    Dec 3, 2020 at 14:32
  • $\begingroup$ Related-- aviation.stackexchange.com/questions/76894/… -- when can the lift vector be viewed as having a "forward" component relative to the glider's flight path as viewed from the ground? Only when when the glider's achieved glide ratio relative to the ground is better than the L/D ratio. Strictly speaking, this does not require vertical wind. It can also happen when there is a tailwind, and no vertical wind. $\endgroup$
    – quiet flyer
    Dec 3, 2020 at 15:26
  • $\begingroup$ Note that I said "move forward without descending". My point was that to move forward without descending, or to climb, requires a vertical wind supplementing gravity. $\endgroup$
    – John K
    Dec 3, 2020 at 16:04
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Sail-powered vehicles cannot be faster than the wind they travel in (along the wind direction) and need another medium (ground or water) that offers resistance.

An airplane, to be in flight, needs to move forward w.r.t. the air sorrounding it and cannot be in contact with another medium.

The two are mutually exclusive.

A similar question got a similar answer.

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    $\begingroup$ Sail-powered iceboats can travel three times faster than the ambient wind. $\endgroup$
    – Camille Goudeseune
    Dec 3, 2020 at 15:58
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    $\begingroup$ @CamilleGoudeseune in the direction of the wind? $\endgroup$
    – Federico
    Dec 3, 2020 at 16:04
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    $\begingroup$ Yes, they can go downwind faster than the wind $\endgroup$
    – DeltaLima ♦
    Dec 3, 2020 at 16:32
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    $\begingroup$ 1) The wheels drive the propeller, 2) the propeller forces the thing to move forward with respect to the surrounding air, 3) The whole thing ends up moving faster than the wind. $\endgroup$
    – DeltaLima ♦
    Dec 3, 2020 at 16:39
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    $\begingroup$ PS that thing is not "sail powered iceboat", and anything powered by a simple sail cannot travel in a straight line in the straight-downwind direction any faster than the wind is blowing. $\endgroup$
    – quiet flyer
    Dec 4, 2020 at 0:04
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For the steady-state, uniform condition-- i.e. no changes in wind speed either over time or over space-- we can consider a glider flying within a moving airmass viewed from a reference frame fixed to the ground, as exactly equivalent to a glider flying within a stationary airmass (relative to the ground) viewed from a moving reference frame. There's no way to distinguish the physics at play between the two cases.

In other words, in an airmass that is uniform over time and space, if you can't actually see the ground, there is no way to tell which way the wind is blowing. There is no maneuver you can do where the aircraft will respond differently when flying upwind versus downwind versus crosswind.

If you are on the ground looking at a glider flying in a still airmass, it will be obvious to you that putting sails on the glider won't accomplish anything. If the glider continues to fly in the same airmass but you are now riding in a airplane passing by and viewing the glider from that perspective, would you think that putting sails on the glider would now accomplish something? Clearly not.

Expecting sails to work on a glider in the presence of a steady, uniform wind, is like expecting sails to work as you watch the glider from the moving airplane as it flies in still air.

Any theory that predicts that the forces acting on an aircraft are different when it is flying upwind versus downwind versus crosswind-- regardless of whether the aircraft has special "sails" or not-- is selecting the ground as a privileged reference frame. This violates the principle of Galilean invariance.

A sailboat is completely different than a sailplane (glider). A glider flies inside of the airmass. A steady, uniform motion of the airmass cannot exert any force on the glider. (This is true of vertical motions of the airmass, as well as horizontal motions of the airmass.) A sailboat exists on the boundary between two different fluids-- the water and the air-- so motion of one fluid relative to the other can and does exert a force on the sailboat.

There has been an attempt to "sail" an unpowered balloon in a direction different from the direction of the wind, but this attempt involved ropes dragging on the ground, so that the balloon could no longer be viewed as being "at rest" within the airmass. An equivalent would be a glider rising on a fixed length of cable that is tethered to the ground at one end, which is certainly possible on a windy day.

And then we have Dynamic soaring, which is a completely different "ball of wax", exploiting shear lines, wind gradients, the boundaries of thermal updrafts and downdrafts1, and other spatial or temporal variations in the airmass.

Footnotes:

  1. Links relating to extracting energy from the boundaries between downdrafts and still air via "dynamic soaring" techniques:

https://journals.sfu.ca/ts/index.php/ts/article/view/296

https://www.semanticscholar.org/paper/CALCULATIONS-ON-SOARING-SINK-Kiceniuk/94e63d06de60110c05916a5c4fed1a2197dd8245

https://journals.sfu.ca/ts/index.php/ts/article/view/298

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There are some great answers already here, and I normally frown on “me-too” answers, but since I have been critical of the question I wanted to offer a very simple and practical comparison that might facilitate that “aha” kind of clarity in case it hasn’t happened yet...

Picture two things in your mind:

  1. The first is an old fashioned water wheel attached to a mill built alongside a river.
  2. The second is an old fashioned, stern-wheeled, Mississippi river boat.

In the first, the waterwheel is passive. It extracts kinetic energy from the river to do work. It harnesses the power of the moving water to turn a grindstone. (or electric generator, or belt that drives machinery...)

The stern wheel of the riverboat, however, is active. Functionally it might appear similar, but it requires an engine, a power source, outside energy applied to it in order to make it turn and thereby perform work. Without energy applied to the wheel it simply drifts along in the river just as the boat does. If the engine quits the boat is dead in the water.

Asking why adding a passive sail to an airplane wouldn’t propel it forward is like asking why you couldn’t just put a prop or wheel in the water and expect it to go. No advanced engineering or aerospace explanation is needed to understand why this wouldn’t work!

And again, per my comments I don’t intend this as an insult at all. The question is reasonable, and I have blanked on seeing obvious things before, until something shifted my perspective and the obvious answer came into focus. Sometimes all that is needed is a different point of view.

I hope this helps.

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