Pilots flying the Airbus Perlan II see the curvature of the Earth from their cockpit. This unique research glider has set new records and made discoveries in high-altitude flight, as Mark Broadbent explains.
See the word ‘glider’ and the chances are you’ll picture a humble-looking aircraft flying lazily a few thousand feet above the countryside on a summer afternoon.
You might not automatically think an engineless aircraft could climb into the stratosphere to 60,000-70,000ft (18,288-21,336m) or higher, far above commercial airways where the only manned types to operate are the US Air Force U-2 and NASA ER-2.
So, the Perlan II glider, registered N901EE, is very special. It holds the record for the highest altitude ever reached by humans aboard an unpowered fixed-wing aircraft, climbing to 76,124ft (23,203m) in September 2018.
Perlan II did not equal this record-breaking height during a more recent series of flights between August and October 2019. However, it did acquire data on a rare stratospheric weather phenomenon, confirming its place as a unique tool for collecting insights into the Earth’s atmosphere and climate.
Mountain Waves
The Perlan II (the name means ‘pearl’ in Icelandic) is the result of work to explore a weather phenomenon called stratospheric mountain waves.
When winds of at least 15kts (27km/h) cross a mountain range perpendicularly and the atmosphere above is stable, waves form in the lee of the mountains and rise. In the 1990s, the NASA test pilot Einar Enevoldson found mountain waves could extend above the troposphere well into the stratosphere in regions closer to the Poles. Around the same time, researcher Dr Elizabeth Austin found that the polar vortex wind and one of its principal components, the stratospheric polar night jet (which exists only in winter), boosted mountain waves.
Building on these discoveries, it was concluded that stratospheric mountain waves could be strong enough to lift a sailplane to new heights. Gliders, of course, use the upward moving part of air currents to soar, just as a surfer rides a wave in the ocean.
The Perlan Project, backed by the late adventurer and pilot Steve Fossett, was formed to explore stratospheric mountain waves. The result was the two-seat Perlan I glider, an extensively modified Glaser-Dirks DG-500, which first flew in 2002.
After test flights in the United States, the Perlan I was transported to El Calafate in the Patagonia region of Argentina. In this region, the weather in the Andes mountains between mid-August and mid-October creates suitable conditions for stratospheric mountain waves to form. Fossett and Enevoldson flew Perlan I to 50,671ft (15,460m) in August 2006, the first time a glider had climbed into the stratosphere.
After Fossett’s death in 2007, the project had to secure new backers. Eventually, in 2014, after preliminary design work by Windward Performance, and manufacturing by Oregon-based RDD Enterprises, the Airbus Group became the title sponsor. Weather Extreme, United Technologies, Garmin, Intel, TRIG Aviation and BRS Aerospace joined the project supplying systems and equipment.
The decision was taken to build a second glider to replace Perlan I. The resulting Perlan II flew in September 2015 and after completing an initial sixmonth testing period, was moved to El Calafate. Perlan II subsequently climbed ever higher in annual test-flying campaigns in the mid-August to mid-October peak season for the polar vortex. After reaching 52,172ft (15,902m) in September 2017, while flown by Jim Payne and Morgan Sandercock, the aircraft ascended to 62,473ft (19,041m) on August 26, 2018, then 65,605ft (19,996m) on August 28 and 76,124ft (23,203m) on September 2. The humble Perlan II had successfully surfed the stratospheric mountain waves.
A Unique Airframe
Perlan II is a two-seat pressurised sailplane made from carbon fibre with a gross weight of 1,800lb (816kg), about the same as a 1967 VW Beetle. Its high-aspect ratio wings have an 84ft (25.6m) span and 262 sq ft (24m2) wing area to maximise efficiency.
The aircraft has lithium-ion rechargeable batteries, a high-altitude radar transponder supplied by Sandia Aerospace and data loggers. Cameras are mounted on the tail to record meteorological conditions and telemetry communicates with mission control and scientists on the ground.
Morgan Sandercock, the Perlan Project’s chief engineer and one of its pilots, told AIR International that the high-aspect ratio wings mean the glider’s performance is not impressive at lower altitudes (‘lower’ in this context meaning below 30,000-40,000ft/9,14412,192m). He said: “It feels like a big, heavy, slow and incredibly difficult glider to fly because of the very long wingspan. You’ve really got to be supercareful with use of the rudder. In test flights in 2015 and 2016, we found you need to think ahead with what you’re doing with the controls. At 30,000ft [9,144m] you feel the air density and below 30,000ft it feels like flying through sticky, heavy molasses.”
However, Sandercock said performance improves as you climb: “It’s a little bit opposite to most other aircraft. When you’re cruising at 36,00038,000ft [10,972-11,582m] in an airliner, that’s getting to the edge of where the aircraft is controllable. In an airliner if the yaw damper fails you must descend.
“[In Perlan II] when we got above 50,000ft [15,240m] it was really quite striking how [it felt as though] it really wants to be there. Anything above 50,000ft it feels light [and] easy to fly. The optimum design altitude for the aerofoil is 60,000ft [18,288m].”
Sandercock said flying so high up and seeing the curvature of the Earth is “quite incredible” but the tiny windows are positioned mainly to give the pilots visibility during the aerotow from the ground and the landing, rather than a pretty view of the planet.
Surviving at High Altitude
Atmospheric pressure is sufficiently low at high altitude to boil human blood unless it is protected, which is why pilots of very high-flying aircraft like the U-2 wear pressure suits. This occurs around 59,000–62,000ft (17,98318,897m) above sea level, a height known as the Armstrong Limit or Armstrong Line.
Anyone who has ever seen pictures or video of U-2 pilots flying or even getting into and out of the jet will recognise that wearing pressure suits imposes limited mobility. The pilots of Perlan I used suits borrowed from the US Air Force, but the project team realised they were unsuitable for long-duration flights in a small glider.
Sandercock said the Perlan Il design work started with the idea the aircraft must be pressurised so the crew could fly it without the cumbersome clothing The cabin is pressurised to 8.5psi (equivalent to 14,500ft/4,419m) and the crew breathe pure oxygen provided by a rebreather system. This means anything the pilots breathe out is maintained in a closed loop, so only oxygen metabolised by the crew is used.
This on-board pressurisation system also eliminates the need for heavy and power-hungry compressors to drive an oxygen generation system (Perlan II carries around one-eighth of the oxygen Perlan I carried), which reduces weight and aids its gliding performance.
In the wake of the record-breaking 2018 flight, Airbus said the technology used on Perlan II could have wider benefits. A company statement described the rebreather as “the lightest and most efficient system for a sealed cabin” and that “its design has applications for other high-altitude aircraft.”
A ‘wave visualisation’ system, installed in the tandem cockpit, helps pilots find areas of rising and sinking air so they can achieve optimal performance. This could have a commercial application, as Airbus stated: “Following lines of rising air would allow faster climbs and save fuel, while also helping aircraft avoid dangerous phenomena such as wind shear and severe downdrafts.”
Rare Clouds
After reaching 76,124ft in 2018, the Perlan Project announced grander ambitions to fly up to 90,000ft (27,432m) and perhaps even 100,000ft (30,480m) in the future.
Perlan II did not reach 90,000ft in the programme of seven flights undertaken from El Calafate this year, although attained 65,000ft (19,812m) during one flight.
However, the project does not exist simply to set and break records for flying as high as possible. Stemming from its origins researching stratospheric mountain waves, other objectives are to obtain knowledge about high-altitude wingborne flight and gather data about how the waves influence weather, climate and flight safety.
Since Perlan is engineless it does not affect the temperature or chemistry of the surrounding air, which makes it an ideal platform to study the atmosphere. It has carried instruments developed by partner organisations on research flights, including systems designed to measure radiation effects and space weather.
Sandercock explained to AIR International that part of the reason Perlan II did not fly as high this year as it did in 2018 “appears to be due to a phenomenon in the atmosphere called sudden stratospheric warming (SSW).”
This event disrupted and weakened the polar vortex, so the stratospheric mountain waves did not reach as high as the year earlier; Perlan II simply could not fly as high.
However, Sandercock said SSW had only been recorded once before, in 2002, so the data of the phenomenon collected in 2019 will be useful for meteorologists “for quite some time” in analysing longterm climate trends.
He said: “We were very lucky to be on site during this specific period as we were able to get up there and take measurements of the temperature in the stratosphere. Being able to compare flights with the previous year [from] that altitude [and] specific position, was a good result for us.”
Another achievement of the 2019 flying programme was the recording of polar stratospheric clouds. Sandercock explained: “These clouds glow at night in all sorts of amazing mother-of-pearl colours. They are extremely rare; they might be observed once a year over the entire planet. We were lucky enough to fly while those clouds were in the sky; we’ve got pictures and video taken from 50,000ft and it’s the first time anyone has ever got a photo.”
Looking Ahead
The scientific results achieved in 2019 show the Perlan II offers research agencies and universities an interesting capability to take payloads into the stratosphere. However, Sandercock admitted it is challenging to get support from institutions – one project initially interested in flying a payload on Perlan II did not have a budget to make it happen.
However, he remains hopeful: “It’s still the dream that we have more scientific research organisations [and] more universities coming to us. It would be great to say, ‘we can take this instrument to this altitude and do these great things with it’
Sandercock said that the Perlan Project’s mission to provide education to and inspire young people seeking careers in science, technology and engineering will continue. Presentations have been given to schools and colleges in the United States and South America over the years.
Fundraising activities are under way to return to Argentina and fly Perlan II up to 90,000ft (27,432m). This would set a new record for a wingborne flight, surpassing the 85,069ft (25,929m) record set in 1975 by an SR-71.
The project is also looking at other options such as setting off from Canada to fly the polar vortex in the northern hemisphere. “We will fly again,” Sandercock pledged.
See the Perlan II soar
Watch video of the Perlan II flying at high altitude at the Perlan Project’s YouTube page: https://www.youtube.com/channel/UCLvpNyuISQUKflCdyCYLlmw Video credit: Perlan Project
Using Wave Lift
Gliders use the natural flow of Earth’s complex atmosphere to gain height by finding air rising faster than the natural sink rate of the airframe, as illustrated in the diagram above.
There are three main types of lift used by glider pilots: thermals, ridge lift and wave lift. Thermals are columns of rising air created by the warming of the Earth’s surface. As the Sun heats air near the ground, it expands and rises. Cumulus clouds mark the location of the rising air and so-called ‘kick off’ thermals, spiralling columns of air above terrain that absorbs the sun more rapidly than surrounding areas.
Ridge lift is created by winds blowing over mountains, hills or other ridges. The terrain deflects the air upwards along the windward side of the slope, forming a ‘band’ of lift.
Like ridge lift, wave lift is created when wind meets a mountain. Wave lift, however, is created on the leeward side of the peak by winds passing over the mountain instead of up one side. Wave lift requires stable air above a ridge. As long as the wind above the ridge blows constantly at a higher and higher velocity with increased altitude, the air wave meeting the ridge will propagate upwards.
The Perlan II flew from El Calafate in Patagonia because the wave lift generated by the Andes is especially powerful. This is because the polar vortex from the South Pole generates a high-speed wind in the stratosphere that boosts the mountain waves, enabling the glider to regularly soar above 60,000ft (18,288m).
