thetical engineer, test pilot, and glider pilot who was in exactly these circum- The Perlan 2 Glider stances and had exactly this epiphany Dr. Daniel Johnson more than 25 years ago. But being a retired NASA test pilot, he did not have multiple millions of dollars to bring this vision into reality. On September 2, 2018, the Perlan 2 glider achieved a pressure altitude of 76,100 ft (that’s the altitude that mat- ters aerodynamically, though the lower GPS altitudes are used for records nowadays). Why can’t you fly a Schweizer 1-26 or a Ventus 3 to that altitude? Why build a bespoke glider for the task? The best answer is that if you want to succeed at any flying challenge, you’re best off flying an aircraft that is optimized for the task. Why? 1. The air is really wispy up there. The “Reynolds number” varies with the density (and viscosity) of the air be- Perlan 2 on the ramp in El Calafate, ready to tow out, worshipper genuflecting. ing flown through. At 100,000 ft, the (Photo by Daniel Johnson.) Reynolds numbers are bird-like rather than airplane-like. As you climb, the The design goal of Perlan 2 is to soar at What do you do? Of course, you true airspeed begins to approach the 90,000 ft above sea level. This is probably walk into the office belonging to the speed of sound, and the air wafting as high as any manned aircraft has ever bulletin board and have a detailed over the wings might exceed it – what flown, the actual record not documented. conversation with the physicist inside are you going to do about that? An SR-71 pilot named Darrell Green- about these findings. 2. It’s severely cold up there. How amyer is quoted as once having achieved will the materials of the glider, the level flight at 90,000 ft. control mechanisms, the avionics, the tire, the sealants, and the windows et’s just hypothetically suppose change their properties at -50° to -85 that you are an engineer and test °C? How can you find out? Very few pilotL with extensive experience in fly- materials or completed equipment ing stratospheric aircraft, and under- are tested at such temperatures and stand the aerodynamics there. pressures. Let’s just hypothetically suppose 3. It’s really high. How are you going that you are a very experienced moun- to get there? What is the most com- tain wave glider pilot. Figure 1: The Bulletin Board image that showed mon form of lift at each of the alti- And let’s hypothetically suppose stratospheric wave over Sweden and captured tudes that you must transition? The that nearly everyone believes that Einar Enevoldson’s imagination. stratospheric mountain wave must be mountain wave, like thunderstorms, connected to the tropospheric moun- is blocked by the tropopause from Your knowledge of aerodynamics tain wave that we’re used to in order to affecting the stratosphere. and soaring lets you instantly real- climb in a glider into the stratosphere. And then, one day, on a job as the ize it should be feasible to design a Do such connections exist? How are test pilot of yet another stratospheric sailplane capable of climbing in that they found? aircraft, you walk past a bulletin board stratospheric wave. If it is there, and 4. What’s the meteorology? When on which is a lidar image showing at- it is possible to reach it, it must be ex- does stratospheric mountain wave de- mospheric wave over the mountains plored. This is a presupposition of the velop? Where in the world do you find of Sweden extending above 80,000 ft human condition. it? How can you predict it, so as not to altitude. Einar Enevoldson is a non-hypo- waste effort and money? 24 Soaring • May 2019 • www.ssa.org 5. How do you keep the pilot safe? he refined the design, he evolved to a In about 1999, he pitched the con- The Armstrong limit is about 63,000 shorter wing with a lower aspect ratio struct to a fellow glider pilot who had ft, above which water turns to warm than the NASA-Perlan sizing study. a bankroll, Steve Fossett. Steve pre- steam at body temperature, so a pres- Einar judged that it should be a two- ferred that they first obtain and modify sure suit or pressurized cockpit is seat aircraft due to the expected high a commercial glider (a DG) and fly in essential. workload, the safety of the redundant pressure suits to assess the feasibility pilot, and the need for support sys- of stratospheric flights before building tems. Having spent much of his pro- a bespoke glider. fessional flying life in pressure suits, he The next step was to determine understood their expense, complexity, where in the world they might most and limitations, so judged that hav- likely find stratospheric wave. This ing a pressurized cockpit would be requires finding a location where the necessary. polar night jet crosses mountains at a He had only recently retired from favorable angle. NASA, and was able to persuade The first three seasons, they flew someone to write a simulation, which from New Zealand and mainly found The performance design goal of Perlan 2. refined the aerodynamics. NASA en- that the conditions did not quite meet (Reproduced courtesy Greg Cole.) gineers seized the opportunity to ex- their need. “Looking back,” Einar plore this unusual design challenge. notes, “it seems likely that we missed This graph was given to Greg Cole A test pilot named Helvey hap- at least one chance in NZ, because we by Einar Enevoldson at the beginning pened to have flown a U-2 from didn’t understand the configuration of of the design-build process for Perlan Fresno, California, into Nevada and the stratospheric wave.” 2, as a depiction of the best expected back across the Sierra at 60,000 ft in They then changed to the Andes, atmospheric environment. The green mountain wave conditions. He got the and flew from El Calafate, the south- line shows expected best lift at alti- hell beat out of him but lots of data. ernmost reasonable airport. The first tudes. The dip at 35,000 to 50,000 A brilliant graduate student named season there, they failed to reach the shows the loss of lift through the tro- Daniel Landau created a model of stratosphere, but in the second season, popause. The blue line shows typical the turbulence field that had been en- in 2006, they reached 51,500 ft. wind speeds at altitudes that would be countered. Using the fastest computer This proved the concept, and dem- encountered in the jet stream or polar then available at UCLA, computation onstrated no loss of climb with increas- night jet. It was anticipated that Per- took about a month. ing altitude, an important requirement lan 2 would release from tow in moun- They took the most “interesting” to be able to go higher. Steve Fossett tain wave at mountaintop height, and 15 miles of this flight and ran this in then agreed to build Perlan 2, on would have to climb through the tro- the simulator that had been designed. which Einar had been working for at popause to the stratosphere. This was Einar flew it often, and, as he recalls, least a decade. the case until 2018, when the Grob he invited a fellow test pilot named Egrett was first used as a towplane so Jim Payne to fly it, too. Airfoil design that release could occur above 40,000 This data indicated that +/- 3 g Prof. Dr. Richard Eppler, of Univer- ft, in the lower part of the stratosphere. would likely be the maximum expe- sität Stuttgart, decades ago developed rienced, so an airframe designed for computer code that directly connects Airframe and airfoil design +/- 8 g seemed sufficient. the boundary-layer development and Obviously, the first step is to try to In the quarter-century since then, the pressure distribution. NASA ad- design a glider that can climb to and atmospheric modeling has advanced opted his philosophy that a reliable be flown in the stratospheric atmo- greatly. The Perlan meteorologist, Dr. theoretical airfoil design method is sphere. After Einar discovered strato- Elizabeth Austin, notes, “[Landau’s] preferable to catalogs of experimental spheric wave, he began a design study, is an older model and the newer ones sections. what aeronautical engineers call a have many more parameters and are Dan Somers, a student of Eppler “preliminary sizing exercise.” more realistic in terms of their equa- and founder of Airfoils, Inc., was en- This study concluded that the glider tions, etc. We also have much better gaged to begin airfoil design studies. needed to have a wingspan of about analysis input data and much bet- In about 2001, Einar discovered the 100 ft, aspect ratio of about 30:1, and ter computing power to get to much Sparrowhawk glider at the Tehachapi best glide about 30:1. When Greg higher resolutions. This is not to say annual soaring meeting and inspected Cole was chosen to build the airplane, that his model results don’t have merit it very carefully. He then knew he’d he was also given the design task. As and I do believe the +/-3 g for sure.” found the guy who could build this www.ssa.org • May 2019 • Soaring 25 dreamed-of glider – Greg Cole. But it case, cross-country performance was than 1 g – which means that straight was years before that could begin.