Question

Which of the following is required condition of thrust required minimum?

(a) CL = (CD0/K)^0.5

(b) CL = (CD0)*0.21

(c) CL = (CD0/K)*1.5

(d) CL = 0.5

The question was posed to me in an interview for internship.

The question is from Flight Mechanics in chapter Performance and Flight Mechanics of Aircraft Design

Answer 1

Right choice is (a) CL = (CD0/K)^0.5

To elaborate: At cruise Thrust required is same as drag produced by the aircraft. Now, to operate the aircraft at Thrust required minimum then, aircraft should be operated with certain value of lift coefficient. Such value can be determined by using following relation: CL = (CD0/K)^0.5 Where, CL is minimum Thrust required lift coefficient, CD0 is parasite drag coefficient and K is induced drag constant.