A research project now underway addresses the concept of utilizing stratospheric mountain waves to soar to high altitudes in sailplanes. Stratospheric mountain waves are mountain waves that propagate strongly, and with continuity, into the middle and upper stratosphere, and are not extinguished, trapped, or reflected at or near the tropopause. The historical experience of high-flying aircraft has been limited to the lower region of their domain, where large amplification leading to large vertical speeds and instability is uncommon. Amplification with increasing altitude, and the instability caused by this amplification, can lead to wave overturning, similar to waves breaking at the beach. Wave overturning originating from amplification has not been experienced by aircraft yet, as far as we know. The general impression of the stratosphere as an entirely quiet region is not, in general, justified.
Recently, high-altitude meteorological research balloons, launched in support of other atmospheric-science projects, have recorded very strong mountain waves (see figure) up to a balloon-burst altitude of 105,000 ft (32 km). The waves propagate into the middle or upper stratosphere when the outer region of the polar vortex lies above a strong tropospheric wind band, above mountainous terrain. In this situation, there is no appreciable wind maximum at the tropopause, and little evidence of a tropopause in the temperature profile. Waves propagate upward with increasing vertical wind component. Stratospheric mountain waves are most commonly found, in the Northern Hemisphere, in the 60°-to-70° latitude band. In the Southern Hemisphere, they extend further toward the equator, because of the larger extent of the polar vortex in that hemisphere. Stratospheric waves can also form (albeit very rarely) at lower latitudes.
The present project is the first step to build and demonstrate the utility of a special-purpose piloted research sailplane that can climb in strong stratospheric mountain waves to its lift-limited ceiling. For a sailplane with state-of-the-art structural and aerodynamic characteristics, the lift-limited ceiling lies between 100,000 and 110,000 ft (30.5 and 33.5 km). Flights are to be made safely and repeatedly, as justified by the need for experimental data.
Work to gather additional data on the strength, location, structure, and frequency of occurrence of strong mountain waves is now underway. The data are expected to verify that the aerodynamic performance of a sailplane will enable it to climb to 100,000 ft (30.5 km) in the waves. In addition, simulations are expected to determine what degree, if any, of stability augmentation will be necessary for the sailplane.
The meteorological part of the work will consist of identification and searching of historical sources of mountain-wave data. In addition to searching such pre-existing data, special dedicated balloon ascents with Global Positioning System (GPS) sondes will be made to augment the data normally obtained from sondes launched at regular intervals and from other special balloons that are used for research not related to stratospheric mountain waves. The meteorological profile characteristics will be summarized with respect to the characteristics of waves identified in the balloon data. Numerical modeling of mountain waves will be done for selected cases found in the data acquired from the sondes during the dedicated balloon ascents and from other sources.
The work will include an aerodynamic-performance part based on a standard "drag build up" method. This part of the work will involve the use of pre-existing basic data sources and of incremental variations on pre-existing high-performance sailplanes for which accurate performance measurements have been made.
The simulation part of the work will include assessment of the flying qualities of the sailplane. With the Dryden simulation, actual or numerical models of the wave structure can be included. The direct effect of the wave structure on sailplane control can be shown. The effect of turbulence generated in wave overturning events will not be as realistically modeled. Variations of parameters will be made to determine the most attractive combination of aerodynamic stability and augmentation.
This work is being done by Edward H. Teets, Jr., of Dryden Flight Research Center and by Einar Enevoldson of Norjen, Inc., under a Flight Test Technique grant and a Dryden Discretionary Fund grant.