The Art of the Helicopter John Watkinson - Karatunov.net

24 The Art of the Helicopter
Fig. 2.2 Weight is the force a body exerts on its supports. At (a) the weight is equal to the supporting force
and there is no resultant. At (b) upward acceleration requires a greater force than in (a) so the weight appears
to increase. At (c), downward acceleration causes weight to reduce. Note that the mass does not change, but
the weight does.
Figure 2.2(b) shows that there is no longer a balance of forces, so there is no equilibrium.
The difference between the force of gravity and the greater force from the floor of the
elevator accelerates us upwards. We feel the extra load on our feet: temporarily our
weight has increased. As the elevator approaches the desired level, the motor will reduce
power, and the force it applies will be less than that due to gravity. Figure 2.2(c) shows
that the resultant force is now downwards, and our ascent is slowed until we stop.
Momentarily our weight is reduced: we feel light on our feet. Note that as the elevator
slows we are going upwards but accelerating downwards. There is no contradiction
here; acceleration is the rate of change of velocity.
Figure 2.3 shows what happens to an object having mass when forces act on it to
accelerate and decelerate it. As there is always a reaction to the application of a force,
if force is applied to an object in order to accelerate it, the reaction will attempt to
accelerate whatever is supplying the force the opposite way. Figure 2.4 shows some
examples. The recoil of a gun is the reaction to accelerating the shell. The thrust of
a ship’s propeller is the reaction to accelerating water backwards. A helicopter stays
airborne by accelerating air downwards: the reaction is upwards, and if it is equal to
the force due to gravity, the helicopter is in equilibrium. Note that the helicopter is
not weightless, its weight is acting on the air around it as the substantial downwash
indicates.