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Just what the question says. I want to know if aircrafts' range, loiter time, or other measures of merit can be dramatically and reliably increased using our current understanding of meteorology and aerospace engineering to make fixed-wing aircraft that use ambient meteorological conditions to remain aloft, using onboard propulsion as little as practical. If so, what realistic applications do you envision? If you know of any academic literature on this topic, please share it.

I know that recreational glider (i.e., "sailplane") pilots often use thermal columns and some other phenomena to extend their flights. I also know the Perlan project uses rather unique conditions of Patagonia to achieve impressive altitudes. I'm more interested in broad commercial, academic, and military applications: probably usable in a large number of places, autonomous or remotely piloted, and optimized for mass production.

To give you some cues, the broadest application occurring to me is atmospheric satellites for telecom, survey, navigation, etc. More specific applications would be something like loitering munitions or intelligence-surveillance-reconnaissance drones. Other ideas and their explanations are welcome.

Could one achieve loiter times of weeks or months, or would too much reliance on ambient meteorological conditions render an aircraft optimized for "soaring" unreliable?

Neither engineering nor meteorology is my trade, so patient explanations are appreciated. Thank you in advance for your answer.

Sorghum
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    Gliders don't just "extend" their flights using thermals, their entire flight (except takeoff) is entirely based on thermals – 60levelchange Oct 30 '23 at 06:08
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    Boeing SolarEagle was a project competing for the DARPA Vulture initiative, consisting in maintaining during 5 years an aircraft of 500 kg aloft at an altitude of 20/30 km , delivering 5 kW of energy to its payload. While this project was cancelled (in favor of a satellite solution), other similar projects may exist. – mins Oct 30 '23 at 19:19
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    Thanks for the reference @mins. – Sorghum Oct 30 '23 at 22:10
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    @60levelchange I'm unsure there's a meaningful difference between what you and I wrote unless I'm missing your point. Using an external power source to launch or tow a glider into the air "extends" the glider's flight time from 0 minutes to a number greater than 0 minutes. The flight entirely depends on that external power source, yet all it's doing is increasing the flight time beyond 0 minutes. – Sorghum Nov 01 '23 at 02:34

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Thermals are finite. This means they have a useful diameter which is smaller than the diameter of the smallest circle most airplanes can fly. The wing loading of a heavy (= full ballast tanks) glider is around 50 kg/m² and is already too high for thermalling competitively on cloudless days with weak thermals.

Compare that to at least 200 kg/m² for anything commercially useful (e.g. Beech King Air B200 or Do-228 with both at 205 kg/m² when at MTOW). Even the Britten Norman Islander at only 100 kg/m² could only fly around regular thermals, not continuously in them. Given an L/D of maybe 15 for light utility airplanes with retractable gear and 12 or less for fixed-gear airplanes, their minimum sink speed with a wing loading of 100 kg/m² is in the order of 2 to 2.5 m/s. In order to gain altitude, the strength of the thermal at a radius of 80 to 100 m from its core must be at least 3 m/s in order to gain altitude (slowly). Still, they would have to circle most of the time and have preciously little time left to cover some distance if their destination is not directly downwind on a regular day in Europe, or need low wind speeds in order to stay in one place.

In order to carry a meaningful payload, the unmanned airplane with a very low wing loading must be large. Since the inner wing flies a much smaller radius than the outer wing, circles of large airplanes cannot become smaller than several multitudes of their wingspan. Even a very lightweight (and fragile!) airplane could at most have 30 m wingspan and weigh at most 800 kg in order to thermal successfully.

Thermals are only possible when the atmospheric lapse rate near the ground is instable. This condition is only met on maybe 20 or 25% of the days in a year and only between noon and several hours before sunset. And between ground and 3000 m on an exceptional day at moderate latitudes. Even 2000 m are already considered good and only possible towards the end of a day. Staying airborne overnight is impossible, except if a mountain and a steady wind are at hand to stay in ridge lift until the thermals are again strong enough on the next day.

In desert areas like inner Australia, Namibia or the US Southwest, stronger and more reliable thermals are possible. Maybe you can thermal with a large and heavy airplane there, but certainly not in Canada or most of Europe. And the density of customers for the services of atmospheric satellites is rather low in desert areas ...

Atmospheric satellites need altitude in order to cover large areas. With only 2000 m everything above a radius of 150 km will already be beyond the horizon when assuming a grazing angle. This also means that many receivers will be behind the shadow of buildings or mountains, so the useful radius shrinks to 23 km with an angle of 5° to the ground. You will need many airplanes to cover a larger area, and doing so reliability will never be possible.

What is still possible is the use of ridge lift, or to fly straight below lined-up clouds on a windy day with thermals. This can help to reduce power or fly faster, and clever pilots do this already if they can.

Peter Kämpf
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  • OK, I know this is a silly idea really, but you could shrink the wingspan while keeping the wing loading low - use a bi- or tri-plane. Some of the downsides of multiplanes don't really apply here as high speed is undesirable. After all the concept is akin to an (untethered) kite, and kites have been proposed for similar applications – Chris H Oct 31 '23 at 15:34
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    @ChrisH Yes, but. Double deckers or triplanes come with a hefty increase in zero-lift drag, but will be much lighter than a cantilever-winged monoplane. This cuts the need for span and will not affect performance much at low speed, but the plane will be blown away once wind speed picks up. – Peter Kämpf Oct 31 '23 at 17:46
  • What kind of "thermodynamic limit" is those 0.65°C per 100 m supposed to be? It is a statically stable stratification, if anything. The adiabatic lapse rate existing in the mixed layer part of the CBL is ~9.8K.km^-1. – Vladimir F Героям слава Oct 31 '23 at 21:31
  • Ok, atmospheric satellites are out. Let's say you wanted something less ambitious like a small drone to patrol a 50km stretch of the Indo-Chinese border in the Himalayas. In your opinion (I realize this involves lots of assumptions), what's the maximum payload you could reliably keep aloft with a combination of ridge lift, mountain waves, and thermals? What design features and resulting performance characteristics would the drone need to do this? Will the drone likely need to stray from its 50km patrol zone or points of immediate interest within that zone to follow favorable weather? – Sorghum Nov 01 '23 at 01:52
  • 0.65°C/100m is simply the average value between the ground an the tropopause. 0.98°C/100m is the cooling rate of rising dry air. The actual conditions vary wildly from one day to another. If one does not take into account humidity - which is a huge factor in reality - then the equation GroundTemp - 0.98 * ConvectionHeight = ConvectionTopTemp holds true. – Criminal_Affair_At_SO Nov 01 '23 at 14:00
  • @VladimirFГероямслава This sentence was stupid. I have edited it. – Peter Kämpf Nov 02 '23 at 02:19
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    @Sorghum Reliably? Zero. With a good chance to be useful over some days? Maybe a couple of hundred grams (wild guess). With a good chance to be useful for some hours? Several tens of kilos. The smaller and lighter your drone, the better are its chances to stay aloft, but also the lower will be its speed. And speed is necessary to prevail against strong winds. – Peter Kämpf Nov 02 '23 at 02:24
  • It is still quite incorrect. Thermal convection exists in normal latitudes almost every day. Often even in winter. It even exists in places like Dome C in Antarctica, during the "day time", albeit very shallow, a few hundreds of meters and with the thermals being accordingly weak in vertical speed. – Vladimir F Героям слава Nov 02 '23 at 06:32
  • @VladimirFГероямслава So thermals occur in stable air? But only when moist air has a lower density than the surrounding dry air, and the resulting vertical movement is normally too small to be useful as a thermal. Maybe you propose a better sentence and I will edit it in. – Peter Kämpf Nov 02 '23 at 08:29
  • @PeterKämpf No, they happen in unstable air and continue rising through well-mixed air until stable air stops them. But the unstable air is much more common than you claim it is. It is true that useful vertical velocities only happen in summer when the boundary layer is deep and the surface heat flux is strong. The vertical velocities of thermals scale as (z_i * g * w'T' / T)**(1/3) where w'T' is the surface kinematic heat flux and z_i is the boundary layer height https://glossary.ametsoc.org/wiki/Deardorff_velocity – Vladimir F Героям слава Nov 02 '23 at 08:37
  • The moisture is actually not too important unless clouds develop, but then in shallow convection the cumulus clouds do not modify the underlying mixed layer that much, they just let the thermals rise higher inside the clouds. In the expression above the moisture flux does appear in the v subscript, but it is usually a small contribution. – Vladimir F Героям слава Nov 02 '23 at 10:10
  • @VladimirFГероямслава Thank you for the link! Moisture reduces the density of air compared to dry air of the same temperature. That is why you can find weak updrafts over lakes. Once moisture condensates and contributes its energy to the surrounding air, thermals become stronger but that happens only in clouds. An unmanned airplane might use that updraft, but optical or infrared sensors will be blind when flying in clouds. Practically useful thermals really are rare. – Peter Kämpf Nov 02 '23 at 11:48
  • Thank you @PeterKämpf. This has been immensely helpful. – Sorghum Nov 02 '23 at 22:35
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You should not forget the infrared mongolfiers. This system was invented by French Centre National d'Etudes Spatiales (national spatial studies center) in 1977. It can fly between 18000 and 28000 meters for a few weeks with a useful load of 45 kg (ref in french).

Here is an English document (credits should go to @mins)

Serge Ballesta
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    @mins: I have updated my answer with your reference... – Serge Ballesta Oct 30 '23 at 14:06
  • Worry not, I've not forgotten about balloons. I only limited my question to fixed-wing aircraft because I was specifically curious about fixed-wing aircraft. Nonetheless, aerostatic lift seems quite advantageous for increasing duration and payload. Thank you for providing the literature reference. – Sorghum Oct 30 '23 at 22:08
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Yes it is certainly possible to extend flight times almost indefinitely by using atmospheric lift, but there is a trade off between duration and other factors such as a flight plan and movement of payload. If you’re solely interested in keeping an aircraft aloft with minimal fuel then this is achievable but if you need to get payload to a destination in a set time then it’s typically better to straight-line with minimal consideration of environmental effects.

Frog
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    Last sentence... it doesn't read good. You should write "considering economic factors". We do not choose the faster way, or faster speed, but the most effective considering fuel consumption (cost), crew time (cost), opportunity costs (more flights), fees, etc. – Giacomo Catenazzi Oct 30 '23 at 13:57
  • “Doesn’t read well”, perhaps. I agree that your point is a valid one but what I wrote is what I intended, i.e. trading off flight time vs fuel as distinct from minimising the overall cost of the flight. – Frog Oct 31 '23 at 18:48