[填空题]
Francesca, who likes d glpc;cx.x8o8 ur 08lphy2 *xwieq4hh c-sics experiments, dangles her watch from a thin piece of string while the jetliner she iwxq4*hh2ic e-s in takes off from JFK Airport (Fig. 4-58). She notices that the string makes an angle of 25$^{\circ}$ with respect to the vertical as the aircraft accelerates for takeoff, which takes about 18 s . Estimate the takeoff speed of the aircraft.
