| description abstract | Results of helicopter clearing experiments conducted at the Greenbrier Valley Airport, Lewisburg, W. Va., during the period 7?29 September 1969, are presented and discussed. Thirty-five hover experiments and 18 runway-clearing experiments were performed on 10 separate days with fog layers from 125 to 525 ft in depth. The hover experiments, which were successful in virtually all cases, yielded clearings that varied from 400 to 2800 ft in length. The largest clearings occurred with the shallowest fog during tests conducted within ?1 hr of the natural dissipation time of the fog. The runway-clearing experiments were successful in clearing the full 6000 ft extent of the runway on two occasions, were partially successful on four occasions, and were unsuccessful on 12 occasions. Six helicopter landings were accomplished through artificially created clearings. Particular, quantitative results of the hover experiments are described. The wake penetration distance of the helicopters ranged from ?700?1000 ft. The steady-state clearing times varied from ?150?260 sec. The total entrainment (mixing) of environmental air into the wake air during the steady-state period was between 350 and 1000%. The clearing ratios, the ratios of cleared volume to down-transported volume, had values of 1.8 to 8.7. The clearing persistence, following helicopter departure from the test sites, varied with log depth and with the convective state of the fog from as little as 1?2 min to as much as 25?30 min. The cleared zones at the surface level were characterized by temperatures 0.3?3.0C warmer than ambient, by relative humidities near 100%, by specific humidities that were 0.1?1.2 gm kg?1 larger than the saturated specific humidities of the fog surroundings, and by downwash-groundwash wind velocities of 6?20 mph with peak gust speeds of as much as 50 mph. | |
