In support of NASA’s Environmental Research Aircraft and Sensor Technology (ERAST) program, flight tests of the Pathfinder solar-electric-powered, remotely piloted aircraft (RPA) were conducted at the Navy’s Pacific Missile Range Facility (PMRF), Barking Sands, Kaua’i, Hawaii, from May to November 1997 and from June to August 1998. This airplane was designed to operate at low speeds and low Reynolds numbers for long duration at altitudes above 60,000 ft (18 km). Three successive altitude world records for propeller-driven aircraft were established during these tests: 67,400 ft (20.54 km) on June 9, 1997; 71,350 ft (21.75 km) on July 7, 1997; and 80,201 ft (24.445 km) on August 6, 1998.

Figure 1. The Pathfinder Solar-Powered Airplane is shown here taking off for its record-breaking flight on June 9, 1997.
Figure 2. The Altitude History of the portion of the flight that included the encounter with the wave was reconstructed from radar and GPS readings.

The 1997 Pathfinder airplane was of a flying-wing configuration (see Figure 1) with a span of 99 ft (30 m) and a chord of 8 ft (2.4 m). With a nominal gross weight of 500 lb (mass of 227 kg), the wing loading was extremely light — less than 0.64 lb/ft2 (30.6 Pa). The airplane was propelled by 6 electric motors powered by a solar-cell array on the upper surface during the day and by batteries at night. The airplane had an equivalent airspeed of 17 knots (8.7 m/s) with an overall climb and descent rate of nominally 220 ft/min (1.1 m/s). The airplane was designed to carry a payload of as much as 50 lb (23 kg) to high altitude for studies of the atmosphere and ecosystem and for development of sensors.

The extremely light wing loading made the airplane highly responsive to gusts and highly sensitive to winds during takeoff and landing as well as during pre- and post-flight ground handling. Wind speeds aloft that exceed the true airspeed of the airplane could be expected to make it difficult to navigate to desired and approved regions of airspace.

During descent from the record altitude on June 9, 1997, the airplane encountered a mountain-wave updraft near an altitude of 9,600 ft (2.9 km) approximately 3 nmi (5.6 km) west of the Kaua’i coastline. The remainder of this article describes the observations made in connection with this encounter. A brief description of the local geography is prerequisite to a meaningful report of the observations: PMRF is situated on the west side of the island of Kaua’i. The 5,200-ft (1.6-km) Mt. Waialeale and a north/south mountain ridge line divide the island in half 16 mi (26 km) to the east. The mountain and ridge line block and divert the easterly trade winds. Lihue lies to east of the Mountain and ridge line.

Early on June 9, 1997, weather conditions were considered acceptable for a Pathfinder flight: At 06:00 Hawaii standard time (HST), surface winds at PMRF were light and variable — perfect for rolling the airplane out and preparing it for takeoff. All upper-level winds were below aircraft true-airspeed limits as measured by the PMRF 02:05 HST and 04:38 HST rawinsonde balloons. The National Weather Service (NWS) had forecast that during the next 24 hours, upper-level winds would decrease slightly while “trade” winds at Lihue would increase slightly. Surface winds at PMRF were expected to increase to 10 knots (5 m/s) by noon and then slowly decrease to light and variable by 19:00 HST.

The airplane first encountered the updraft at 21:15 HST. The conditions near encounter time indicated a very stable temperature profile, winds at the top of the ridge [altitude > 5,000 ft (>1.5 km)] near 15 knots (8 m/s), and wind direction perpendicular to the ridge line. Measurements of these winds came from three sources: airplane onboard instrumentation [true airspeed indicator and a Global Positioning System (GPS) receiver], a NWS balloon released from Lihue at 02:00 HST on June 10, and PMRF balloons released at 19:14 HST on June 9 and 05:38 HST on June 10.

Data from the GPS receiver and from a ground-based radar tracking system were used to reconstruct the horizontal and vertical components of the position and velocity of the airplane. During the 15 minutes following the initial encounter, the wave slowly lifted the airplane 900 ft (274 m) at a rate of 60 ft/min (0.3 m/s), as shown in Figure 2. While in the wave updraft, the airplane maintained this rate of climb. After leaving the wave, the airplane descended at a rate of 300 ft/min (1.5 m/s), which likely included the effect of a downdraft. Thus, the overall variation in the rate of climb was 360 ft/min (1.8 m/s) — twice as great as indicated by the balloon closest to the encounter time, but considerably less than that indicated by the last balloon above an altitude of 14,000 ft (4.3 km).

These observations indicate that the climb and descent performance of the Pathfinder airplane is a sensitive observation tool for the study of weak or light wave activity. Conversely, the sensitivity of Pathfinder to weak wave motion indicates the value of closely monitoring the meteorological profile for conditions that favor the development of waves and the importance of extracting the rates of rise of balloons from rawinsonde releases. On subsequent Pathfinder flights, balloon rise rates were monitored and analyzed for indications of up and downdrafts. Advising flight planners and mission managers of the possibility of encountering waves enables them to account for anomalous climb- and descent- rate behavior. In addition, it makes them better prepared to navigate successfully through such phenomena to obtain additional flight time that may be needed prior to landing.

This work was done by Edward H. Teets, Jr., of Analytical Services and Materials, Inc., and Natalie Salazar of New Mexico University for Dryden Flight Research Center.

This Brief includes a Technical Support Package (TSP).
Wind and Mountain Wave Observations From a Flight Test of a Solar-Powered Airplane

(reference DRC-98-82) is currently available for download from the TSP library.

Don't have an account? Sign up here.