Effect of Pressure Variations on Speed Runs

Abstract

IT HAS BEEN NOTED on numerous occasions that, in speed runs at constant power and constant pressure altitude, a different value of the velocity is obtained if the run is made in one direction than if it is in the opposite direction. In one particular instance, a variation of 6 m.p.h. at 300 m.p.h. was so obtained. An investigation has been made in order to determine how much of the discrepancy in these values can be attributed to pressure variations in the atmosphere— in the form of highand low-pressure areas and their movements. If a speed run is made at constantpressure altitude in the direction of the pressure gradient (toward the center of a low-pressure area), the ship will be flying downhill, since the pressure in the direction of flight is continually decreasing and, hence, the airplane must lose altitude in order for the altimeter to read a constant value. Similarly, if the airplane is flying in the opposite direction (away from a low-pressure center or toward a high-pressure center), it will actually be climbing, even though the pressure altitude remains constant. Furthermore, a changing pressure field (resulting from the movement of the pressure centers) could possibly aggravate the situation. Both of these effects—that of pressure fields as well as their movements—will be considered in the following analysis.