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Would it be possible to use pure gliders as an eco-friendly mode of transport?

From what I have read so far, gliders can travel at a speed of 100-170 km/h. That would not be able to compete with a jet plane, but it could easily be faster than a car, and possibly even a high-speed train, since it can fly directly and doesn't have to follow train tracks.

So, would it be possible to build a glider that fills this arbitrary set of requirements that would qualify it for regular travel:

  • It needs to be able to carry at least 30 people plus luggage. Otherwise the pilots are much too expensive.
  • It needs to be able to fly regularly, only being inhibited by severe weather.
  • It needs to qualify for international, commercial passenger service in all matters (safety equipment, radio, ...)
  • It doesn't need to be able to fly around the year, pre-plannable months without service are acceptable.
  • It doesn't need to work with current infrastructure, e.g. it would be acceptable if the plane needs special runways and/or launching mechanisms.

Would it theoretically be possible to build something like this?

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There's one thing that will forever prevent an unpowered aircraft from entering commercial passenger service: the inability to make a second attempt at a landing.

Airliners, with all their instrumentation, very experienced pilots, and such don't make go-arounds frequently -- but especially during weather, they do make go-arounds. A passenger sailplane absolutely can't make a go-around, so if it gets crossed up due to a crosswind gust low and slow, instead of advancing throttle and climbing out, all the pilot(s) can do is pick where and how hard to crash.

This complete lack of second chances, as much as any of the other objections raised in other answers, will ensure that unpowered aircraft will never see commercial service. It can also lead to "landing out" where you wind up having to retrieve your passenger glider from a farmer's field, freeway, or worse, along with its payload and crew.

The other big issue is how to launch these immense sailplanes -- with a common sport glider or competition sailplane, you're towed into the air by either another airplane, a winch, or occasionally a car or truck. The winch that could launch something the size of a 737 high enough to fly off in search of its first lift would be so big and expensive it would severely limit the number of airports that could service such craft. The airplane that could do it would be at least as big as that 737.

Comments suggested a one-time use rocket engine as emergency power to allow go-around -- but this isn't practical.

The engine required would need to have thrust comparable to maximum power for both 737 turbofans, and burn time long enough for the 737 sized glider to climb from low final to at least pattern altitude (ideally to the same altitude it would initially tow to). This gives an engine comparable to the Castor 4A-XL boosters used on the Japanese HII-A orbital launcher -- engines a meter in diameter and about twelve meters long. These would be bigger and heavier than both 737 engines combined (though lighter than those engines plus full fuel), weight and drag that would be carried on every tow and for the full duration of every flight, but which would contribute only in the case of needing a go-around. And this engine, once ignited, can be neither throttled nor even shut down before it has burned all its fuel.

You're surely better off with the two turbofans now used for takeoff, climb, and cruise.

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  • $\begingroup$ "inability to make a second attempt at a landing": It looks like the definitive answer! Even Greta would agree. $\endgroup$
    – mins
    Sep 19, 2020 at 2:31
  • $\begingroup$ A one-time-use rocket motor might be kept in reserve as a simple and inexpensive way to address the second-chance-at-landing issue- $\endgroup$ Jul 1 at 13:44
  • $\begingroup$ Interesting thought. How long does it take for a 737 to climb from low final to pattern altitude? How much thrust is needed? Since your description requires a solid propellant type ("simple and inexpensive") it can't be shut down once ignited, and I think you'll find it won't be either simple or cheap (hint: it's going to be bigger than the Castor boosters on Atlas orbital launchers, and you have to carry that weight from takeoff to landing on every flight). Not going to stand in for having an actual engine, I think; probably heavier and would require replacement any time you go around. $\endgroup$
    – Zeiss Ikon
    Jul 1 at 13:58
  • $\begingroup$ @quietflyer Current Castor 4A-XL engines are about 12 m long a 1.02 m diameter, making them bigger than both 737 engines combined. Not to mention what happens if it fails (much worse than a turbofan failure). $\endgroup$
    – Zeiss Ikon
    Jul 1 at 14:03
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    $\begingroup$ Sink speed scales with forward speed. To make regular thermals useable, the flight speed and wing loading would be that of a regular glider. Also, thermals have a limited diameter and adding passengers will make the resulting airplane too large and too fast to gain any height even in strong thermals. $\endgroup$ Jul 1 at 19:49
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Theoretically, yes. Practically, no.

  • Day to day weather is a huge factor in powered aviation. It is an even larger factor for non-powered aviation. You would not be able to satisfy the predictability and consistency needed to be a commercial scheduled carrier. At best, you may be a charter or sightseeing tour carrier.
  • No lift means no flight. Days of no or not enough vertical airmass movement would require canceling flights. Night time flights would be out of the question.
  • Faster flight and consistent lift may (I say again, may) be available in the jetstream currents. But, then you would need a pressurized cabin which adds more weight.
  • To make it commercially viable, the speed of transport would have to be significantly more than a car. Consistently faster than 100-120 miles per hour would be required to get people who already own cars to consider the added expense and hassle of air travel.
  • To make it commercially viable, after speed is considered, distance has to be factored in to get people who already own cars to consider the added expense and hassle of air travel. You would only attract people who are flying more than twice the distance of your clientele servicing diameter. For example, if it takes me 30 minutes or 30 miles to drive to the airport of departure, I would not fly to a destination that is less than 120 miles or 2 hours drive time away.
  • Competition must be considered in any commercial venture. Private land carriage like private cars would be a huge area of competition. So would public carriage like buses and trains. With increased technology reducing their ecological impact, these forms of transportation would be far more attractive than a proposed glider.
  • Most gliders efficient enough for consistent cross-country flights have interiors that are small, cramped, not climate controlled, and require the pilot and passenger to sit in an almost reclined position for the duration of the flight. Larger gliders like those used in WWII were larger, cumbersome, and required huge aircraft to lift them airborne. The military has already abandoned such projects for troop transport.
  • Trans-oceanic flights would be not at all practical if at all feasible.

This does not mean that it should not be attempted. You should know the obstacles in front of you and try to overcome them. Your failure might lead to someone else’s success.

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    $\begingroup$ Good answer. I would just add two things: it still takes fuel to get the glider to altitude, (so not clean) and the probable lack of willingness on the part of passengers to fly regularly on a craft with no go around capability. Also, too encouraging at the end! ;) $\endgroup$ Sep 8, 2020 at 15:44
  • $\begingroup$ @MichaelHall I would argue that 1) a glider is still cleaner 2) gliders don't explode on impact $\endgroup$
    – Abdullah
    Sep 8, 2020 at 15:52
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    $\begingroup$ @Abdullah, if you were to calculate the theoretical fuel required to tow a 30 passenger glider to altitude, and plot the range achieved in a glide against a turboprop covering the same range in the most fuel efficient manner you might find a small advantage. But likely not enough to negate my point or prove this as a viable idea. Good point about fire safety, but I’m still betting against this idea “taking off”... $\endgroup$ Sep 8, 2020 at 15:59
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    $\begingroup$ “A chance” is the key word here... How many paying passengers would be willing to sit for hours as the pilot seeks out the thermals that just might allow them to reach their destination?! $\endgroup$ Sep 8, 2020 at 16:04
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    $\begingroup$ @Abdullah - Gliders have a theoretical chance of unlimited endurance during daylight hours when there are still thermals. Their range is less practical. To get the theoretical endurance, you would have to chase the thermals in whichever area and direction they happen to be. That’s not much certainty and consistency for a paying customer who has expectations and deadlines. $\endgroup$
    – Dean F.
    Sep 8, 2020 at 20:36
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A pure sailplane is probably impractical for commercial aviation, for all the reasons others have given. But, just as there are proposals to add sails to commercial shipping to reduce fuel use, maybe flight paths of commercial aircraft could be actively modified to allow them to make use of soarable atmospheric conditions, such as high-altitude waves. Increasingly accurate meteorological forecasts combined with artificial intelligence could perhaps make this routine and effective.

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    $\begingroup$ Eastbound flights will take longer routes to take advantage of the jet stream, so this is already factored in. Westbound flights will take longer routes to avoid the disadvantage of the jet stream headwind. Plus, with pax demands for flights to be on time, especially for annoying little things like making a connecting flight, there often isn't time to loiter or wander looking for thermals. Hard to imagine this being able to be efficiently factored into commercial flight schedules. $\endgroup$
    – FreeMan
    Jul 1 at 17:43
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Well new twin engines airliners are already some impressive gliders... The glide ratio (GR) of the A320 is around 17 and the new 787 GR is above 20. Those numbers are quite impressive considering the two big engines shut down adding a lot of drag. Without engines they could probably reach the lower limit of aerobatic gliders with a GR of around 25 to 30.

I could also add a safety issue to the previous answer. Every landing in a glider is mandatory. Without engines you don't have any opportunities to go around or wait in the pattern for the runway to become available. This restrict even more the weather in which you can fly, too much cross wind and it is way to dangerous to try.

Finally saying that a glider can fly directly is only partially true as it still requires to find thermal ascending current to extend it's range. In a glider you know when and where your taking off, but it's hard to say the same for the landing, not really good for commercial ops.

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