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14.3. TEXT
Jet Transport Takeoff
Takeoff is one of the highest performance maneuvers of an entire flight. The
aircraft is quietly positioned onto the runway, the throttles are advanced smoothly,
and the engines roar to life.
Passengers feel the firm acceleration as they are pushed back into the cushions
of their seats. The speed of the aircraft is considerable as the ground rushes backward
in a blur until liftoff occurs. The nose of the plane climbs at an impressive angle
upward toward the sky and objects on the ground fall rapidly away until they appear
insignificant. Takeoff may appear to be an uncontrolled release of raw power, but it is
actually a meticulously calculated, controlled, and evaluated event.
To fully understand takeoff performance, we must examine some basic
physics. From a strictly physical interpretation, the jet engines perform work on the
aircraft. When work is done on an object there is an increase in either potential or
kinetic energy. On takeoff, there is no change in potential energy, so the work of the
engines increases the aircraft's kinetic energy. The kinetic energy of an object is
found by multiplying one-half its mass by the square of the velocity. Because work is
defined as force multiplied by the distance through which the force acts and this is
equal to the kinetic energy, the two equations can be set opposite one another.
Next we must remember Newton's second law of motion, which states that an
applied force is equal to the mass of an object multiplied by its acceleration. By
replacing the term for force with this identity, the equation becomes mass multiplied
by acceleration, with distance equaling one-half the mass multiplied by the square of
the velocity. As mass now appears on both sides of the equation, it is canceled, so
acceleration multiplied by distance is equal to one-half the velocity squared. The final
equation is found by solving for distance. Thus, takeoff distance is equal to the square
of the velocity divided by twice the acceleration.
Because the takeoff distance is proportional to the square of the takeoff
velocity, the takeoff distance would increase by a factor of four if the takeoff speed
were doubled. The distance is also inversely proportional to the takeoff acceleration.
The greater the acceleration, the shorter the takeoff distance. The more an aircraft
weighs, the faster it must be moving to create adequate lift. This means the heavier an
aircraft is, the more runway it will need. To minimize the takeoff speed and distance
required, wing flaps and slats are used. The thrust of the engines provides the
accelerating force.
On warm days, or when taking off from high-elevation airports, the reduced
density of the air reduces the engines' thrust. The higher the elevation and
temperature, the higher the power setting of the engines on takeoff.
It is comforting to know that according to Federal Aviation Regulations the
aircraft must be able to climb to an altitude of 35 feet by the end of the runway or
clearway even if one engine is not working. Is it any wonder why transport aircraft
can take off and climb with such impressive strength?
