An instrument used to measure the airspeed on many early low-speed airplanes, principally during 1919 to 1930, was the venturi tube. this simple device is a convergent–divergent duct. (the front section’s cross-sectional area a decreases in the fl ow direction, and the back section’s cross-sectional area increases in the fl ow direction. somewhere between the inlet and exit of the duct, there is a minimum area called the throat .) see fi gure below. let a 1 and a 2 denote the inlet and throat areas, respectively. let p 1 and p 2 be the pressures at the inlet and throat, respectively. the venturi tube is mounted at a specifi c location on the airplane (generally on the wing or near the front of the fuselage) where the inlet velocity v 1 is essentially the same as the free-stream velocity—that is, the velocity of the airplane through the air. with a knowledge of the area ratio a 2 / a 1 (a fi xed design feature) and a measurement of the pressure difference p 1 − p 2 , we can determine the airplane’s velocity. for example, assume aa 21 a 1 4/ a a a = and p 1 − p 2 = 80 lb/ft 2 . if the airplane is fl ying at standard sea level, what is its velocity?