Chapter 7. Accelerated Performance: Takeoff and Landing
Chapter 7. Accelerated Performance: Takeoff and Landing
Introduction
To this point, all of our discussion has related to static or unaccelerated flight where F = ma = 0. Even in climb and descent we assumed “quasi-level” conditions where the forces on the aircraft summed to zero. If we are to look at the performance of an airplane during take‑off and landing we must, for the first time, consider acceleration (during takeoff) and deceleration (during landing). We will also have a couple of new forces to consider in the ground reaction force and ground friction.
In take-off, the airplane accelerates from zero groundspeed (but not necessarily zero airspeed!) to a speed at which it can lift itself from the ground. The thrust must exceed drag for acceleration to take place and the lift won’t equal weight until the moment of liftoff. The plane may accelerate along the ground at a given angle of attack (or lift coefficient) until the speed reaches the point where the dynamic pressure combines with the lift coefficient to give lift equal to the weight or it may accelerate at some angle of attack determined by its landing gear height until it reaches a speed which will give lift equals to weight when the aircraft is then rotated (tail down, nose up) to a higher angle of attack and lift coefficient.
Any pilot will tell you that take‑off and landing are what flight is all about. The thrill of full throttle and maximum acceleration as the plane roars down the runway, followed by the freeing of the soul which comes from cheating gravity and breaking the bond with the earth is incomparable. Of course the pilot hopes this occurs before the end of the runway is reached and in such a way as to allow clearance of the water tower at the end of the strip!
In landing, deceleration must be provided through braking, aerodynamic drag, ground friction and possibly reverse thrust to slow the plane to zero speed; hopefully before it reaches the end of the runway!
Landing is the ultimate challenge of person against nature as the pilot once again attempts to remain in control of a planned encounter with the ground in a vehicle moving at speeds which can result in instant mutilation and death if there is the slightest miscalculation of crosswind or downdraft. Of course, all of this must be done in such a manner as to assure the passenger that every move is as safe and natural and controlled as a Sunday afternoon drive to the golf course.
Wind will be a factor in take‑off and landing and one would think it would be obvious that the pilot should position the aircraft at the end of the runway which will result in operation into the wind. This will result in a reduction in the length of the ground roll in either take‑off or landing. To some, however this may not be obvious.
The author once sat on a graduate committee of a student in Transportation Engineering who had taken several courses in airport design. When asked what role the prevailing winds played in the design of airport