A Model for Predicting Diurnal Height Variations of Quasi-Constant-Density Tetroons

Abstract

A model has been developed for predicting diurnal height changes of quasi-constant-density Mylar tetroons resulting from the complex interaction between thermally induced density changes of the tetroon and height-dependent ambient diurnal density changes in the lower 600 m of the atmosphere. The effects of solar and nocturnal radiation on tetroon altitude are included in the model. These cyclic height variations would be additive to the effects of mechanical and thermal vertical air currents acting on the tetroon. The magnitudes of the height variations for flights at <300 m above ground are so large that they should be considered in flight planning to prevent premature grounding of the tetroons. A diurnal surface temperature range of 13°C (24°F) and the condition that air and tetroon temperature are always equal, gives a predicted diurnal height change of ?250 m for tetroons deployed for flight at 300 m above the surface. By applying 8°C maximum superheating by day and 7°C supercooling by night, the daily height change was decreased to ?135 m. The magnitude of the daily change in elevation of the tetroon was found to be independent of the size of tetroon provided radiation effects were not present. Tetroons released near the time of minimum air temperature had the lowest mean float elevation over a day while those released at time of maximum air temperature had the highest average elevation. Tetroons most likely approach the condition of temperature equality with the ambient air where skies are overcast because of the strong suppression of the direct component of solar radiation by day and interference with outgoing infrared radiation at night. Tetroons floating at lower levels experience larger height variations than those floating at higher levels. In particular, tetroons inflated to float below ?200 m above ground in the early morning would be in danger of grounding in a few hours. This model can aid in planning tetroon flights to prevent premature grounding and to keep tetroons nearer the desired flight level.