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Seems that a usual glider is not limited by its glide descent rate and can fly much more than this. It uses air masses that move upward to gain the altitude.

  • Are these effects limited by the size of the plane?
  • Could an airliner gain altitude for free using these "upward blowing winds"?
  • Could an airliner glide 1,000 km?
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    $\begingroup$ Are you asking specifically about "gaining altitude" or extending the glide because there is some upwards air movement? The answer to both is yes. What do you mean by "significant"? $\endgroup$
    – Simon
    Dec 11 '16 at 11:29
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    $\begingroup$ Note that gliders can not glide for 1000km. Gliders go upwards using thermals. (This is no different from going upwards, using an engine.) The whole sport of gliding is about finding and using thermals. $\endgroup$
    – Fattie
    Dec 12 '16 at 14:24
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No, there's no way an airliner can glide 1000km, this is because its wing is designed for fast cruising speeds, and it's much heavier compared to the lift generated than a glider. Gliders are light and have huge wings for their weight, which means they can get lifted by powerful air currents. An airliner is not going to be able to do that.

The longest glide of an airliner I know of was Air Transat 236, which glided about 98 miles (almost 160km) from cruising altitude to the Azores after it ran out of fuel. The captain was an experience glider pilot. That is really about the limit.

Keep in mind that there's nothing ordinary about 1000km in a glider! 1000km is a very long distance and you'd need a very good glider and a very experienced pilot in just the right conditions.

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    $\begingroup$ The linked Wikipedia article states: "In about 19 minutes, Piché and De Jager flew their plane without power some 120 km (75 miles), further than any passenger jet in history". How does this jibe with the gliding distance mentioned in the answer? $\endgroup$
    – njuffa
    Dec 11 '16 at 21:43
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    $\begingroup$ 98 miles was from memory, looking at it again I se multiple reports, some up to 100. I think it's hard to say because the pilot did a 360 degree turn to lose altitude and had to fly farther to line up with the runway. $\endgroup$
    – GdD
    Dec 11 '16 at 21:56
  • $\begingroup$ round numbers: 30,000 feet of altitude lost for 100 miles or 528,000 feet (assuming statute miles) of ground covered = 17.6:1 glide ratio. Not as good as a glider, but not bad for something moving through the air at a couple of hundred knots or more. $\endgroup$
    – Anthony X
    Dec 11 '16 at 23:28
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    $\begingroup$ There was also the infamous Gimli Glider, which managed a glide ratio of about 12:1 - slowed for approach with a crossed-controls crab (instead of s-turns), but also a great example of an ex-glider pilot taking in a heavy from altitude with no engines. $\endgroup$
    – J...
    Dec 12 '16 at 14:47
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    $\begingroup$ For the record, the official accident report on Air Transat 236 says that the engines flamed out 65 nautical miles (75 statute miles, or 120 km) from Lajes. However, it did fly a greater distance than in order this to lose altitude. The plane was at FL 345 when the final engine flamed out. $\endgroup$ Dec 12 '16 at 18:46
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First, the 1000 km glider trip requires three prerequisites:

  • Excellent weather for the whole of the trip. Strong thermals caused by lots of sun, a high cloud base and an unstable atmospheric thermal gradient. No precipitation or shielding by cirrus clouds anywhere along the route. Or you limit your trip to a back-and-forth along a mountain ridge.
  • A good pilot. The fraction of pilots who can reliably fly a distance of 1000 km or more is less than 1% of the total glider pilot population.
  • A good glider with water ballast.

Next, the sink speed of aircraft limits their capability of sustaining flight in thermals. In Europe 5 m/s is already a very strong thermal, and ideal locations like Australia or Namibia create thermals of 8 to 10 m/s strength. If your aircraft sinks at that speed or faster, flying in a thermal will only delay the time until all altitude is used up. In order to cover some distance, the sink speed must be considerably lower so that altitude lost in a glide can be regained in the next thermal.

The factors which control sink speed are:

  • Low wing loading: Higher wing loading means higher flight speed and, consequently, higher sink speed.
  • Low span loading: The less mass must be supported per unit of wingspan, the lower the induced drag will be. At the low speed of the minimum sink rate the induced drag is dominant.
  • High maximum lift coefficient: The higher the maximum lift coefficient, the slower the flight speed and the smaller the turn radius will be. Thermals are limited in size and it is immensely useful when the turn radius fits inside the thermal.

In order to fly fast enough to cover the 1000 km in a single day it is helpful to have

  • High wing loading: This is helpful to shift the best L/D ratio to a higher flight speed, so the time spent in a glide can be shorter.
  • Low zero-lift drag: In order to reach the next thermal with minimum delay, you need to fly faster than at best L/D. Now drag is dominated by the zero-lift drag.
  • High altitude, so the lower density requires a higher true air speed.

The last condition is the absolute size of the aircraft: Smaller aircraft can fly in narrower circles. Only in ridge and wave updrafts will the size not matter. Bigger aircraft need more time and space for maneuvering, so they will be less able to adapt their flight path to local updrafts.

These conflicting demands will result in a narrow range of characteristics: The aircraft will have a wing loading somewhere around 50 kg/m², a zero-lift drag coefficient of 0.08 or less and an aspect ratio of 20 or more. Its total mass will be less than 1 ton. In short, the airliner-sized aircraft will not be able to profit from updrafts except for reducing sink when flying at the windward side of a mountain range, or the lee waves high up behind the mountains.

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    $\begingroup$ Another way to glide long distances would be to use mountain waves, though of course that severly restricts your choice of flight path. I don't think it would be that difficult to fly the length of the Sierra Nevada, given the right wind conditions. $\endgroup$
    – jamesqf
    Dec 11 '16 at 18:50
  • $\begingroup$ @jamesqf: The updrafts in mountain waves are normally much less than what can be found close to the ridge, so I did not add this case. Of course, flying in a mountain wave will also reduce sink speed. Karl Striedieck flew more than 1600 km once in one day by running his ASW-17 up and down the Appalachians. $\endgroup$ Dec 11 '16 at 22:02
  • $\begingroup$ @PeterKämpf: but Klauss Ohlmann flew more than 3000 km in the wave in Argentina. These days, most gliding records have been achieved flying wave. As for the intensity, the current 100km FAI triangle for PW5 is 137km/h, mostly flown with spoilers open. $\endgroup$ Dec 12 '16 at 7:22
  • $\begingroup$ @MartinArgerami: You are right, flying in waves allows a much higher true air speed. However, high TAS means high sink rate, so the wave must be very strong. I added wave lift for a more complete answer. $\endgroup$ Dec 12 '16 at 7:59
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Summary: (a) no; (b) not to any significant extent; (c) no, due to (b). You'd get about 80 miles / 130 km from cruise altitude.

Any aircraft can glide, whether or not it is designed to be powered. The two main factors that determine glide distance over ground are glide ratio, and sink rate.

Glide ratio is the horizontal distance travel per unit height. A modern club-level glider has a glide ratio of about 40 to 1: it will glide 40 feet forward for every foot of altitude. High performance gliders approach 60:1, old basic trainers closer to 20:1. The glide ratio can also be expressed as the ratio of lift over drag, or L/D. There is one speed at which L/D is optimised, and this is the speed at which you will cover the greatest distance for a given altitude. In gliding we call this speed "Best L/D" and in still air it is the most efficient speed to fly when you want to cover ground (such as between lift sources).

You see many unsubstantiated figures for airliner L/D ratios, anywhere from 10 to the high 20s. Given that to test this you would have to stop the engines entirely, needless to say it is very seldom tested! The famous Gimli Glider (a 767) achieved about 12:1 which sounds pretty good for dragging those huge engines through the air. That glide was flown at 220 knots based on the judgement of the captain, an experienced glider pilot.

The interesting thing about L/D is that it depends solely on the aerodynamics of the aircraft, and the airspeed. Weight does not affect L/D, so claims that "gliders are light" are misleading. Increasing weight increases the best L/D speed, and (solely as a consequence of the higher speed) the sink rate, but has no effect on the glide ratio. In fact in the right conditions, competition gliders are often ballasted with hundreds of pounds of water to increase their best L/D speed, and so get you round a course faster.

So in still air, from say 36,000 feet that 767 could glide about 12 times its altitude: 70 nautical (80 statute) miles.

But what about lift? Gliders gain altitude by flying in rising air: thermal, ridge, or wave lift. Lift can vary from 100 ft/min (a weak thermal) up to 1,000 ft/min (strong wave lift). So a 40:1 glider flying at its best L/D of about 60 knots, is going down through the air at 60/40 = 1.5 knots or about 150 ft/min. Any time you are flying through air that is rising faster than 150 ft/min, you will climb. That's how long duration gliding flights are achieved.

The catch with the airliner is that it is gliding at 220 knots at 12:1, so it is sinking at 18 knots, or 1,800 feet per minute. Except in unusual conditions the aircraft is unlikely to fly through much more than a few hundred fpm of lift on average, at very best. So lift can be largely discounted and the distance covered over the ground stays around 80 statute miles from cruise altitude.

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As it was discussed here, a Sud-Aviation Caravelle made an experimental demonstration gliding flight of many km, starting at a very high altitude and in special conditions; also, I was told by a pilot of having flown, in a very hot summer day, an Spanish made version of Ju-52 over the Pirineos mountains, with its three engines idling of at full stop, just on an ascending air current, same as gliders; sorry, I can't remember his name (Galve? Calderón?). No more accurate references right now.

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Are these effects limited by the size of the plane?

Yes and No. Thermals are usually quite small. Even gliders have to fly slow and bank hard. Squall lines and mountain waves will be good for airliner as well.

Could an airliner gain altitude for free using these "upward blowing winds"?

Yes! Why not? The only problem is that they have to fly fast compared to gliders to stay in the air. If you fly fast your descent rate is big. The same applies to gliders. So airlines will need very strong uplift currents. I think it should be possible to fly a long very strong cold front or mountain wave or maybe under super cell. But this kind of weather happens very rarely. And it's DANGEROUS

Could an airliner glide 1,000 km?

Theoretically yes. If you find extremely active cold front with squall line 1000 km in length. Though I don't know if this ever happened on planet earth.

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