The Art of the Helicopter John Watkinson - Karatunov.net

66 The Art of the Helicopter
the local centre of pressure. As Figure 3.4(b) shows, this results in a stable blade, since
an increase in angle of attack producing more lift tends to generate a couple reducing
the angle of attack.
The airfoil section selected for a helicopter blade will be a compromise to satisfy a
number of conflicting requirements. One of these is a minimal migration of the centre
of pressure over the normal range of operating conditions so that excessive feathering
couples are not fed back into the controls.
In a cambered airfoil, the centre of pressure moves fore and aft with changes in angle
of attack. The downward twist at high speed was enough to twist the blades against the
pilot’s efforts on early machines and caused some crashes. For some time, helicopters
used little or no camber in the blade section, but subsequently cambered sections
having reflex trailing edges were developed which reduced the centre of pressure movement.
This along with the development of structures with greater torsional stiffness
allowed cambered blades to return to use, although almost invariably in conjunction
with powered controls.
3.3 The coefficient of lift
Lift is equal to the rate of change of momentum induced downwards into the surrounding
air. For unit airfoil area, it is proportional to the air density, the square of
the relative air velocity and, as was seen in section 3.1, it is a function of the angle
of attack. When an airfoil section is tested, the reaction is resolved into horizontal
and vertical directions with respect to RAFso that two factors of proportionality,
called the coefficient of lift CL and the coefficient of drag CD are measured.
Figure 3.5(a) shows how these coefficients change as a function of the angle of attack.
For small angles CL is proportional as it was seen in Figure 3.1(c) that the acceleration
of the airflow is proportional to the angle through which it is deflected.
As the angle of attack increases the reaction rotates back and the lift component
of the reaction will be smaller than the reaction. The graph curves away from proportionality.
At a large angle of attack, separation takes place, and the CL drops
sharply, accompanied by a sharp increase in CD. When this occurs the airfoil is said to
be stalled.
An airfoil does not stall immediately the angle of attack is raised, and if the rate of
angular change is sufficiently high the lift momentarily available might be double the
amount available in steady-state conditions. This is variously called the Warren effect
or dynamic overshoot and is shown in Figure 3.5(b). In the case of a helicopter the
angle of attack is changing at rotor speed and dynamic overshoot has a considerable
effect. In practice it means that helicopter rotors may not lose significant amounts of
lift through stall.
In this respect the helicopter has the advantage over the fixed-wing aircraft. The
latter has to be flown at all times with the probability of a stall in mind. Even when
the pilot has sufficient airspeed, a sudden gust due to wind shear or a microburst can
reduce that airspeed and cause a stall. It was this concern that led Juan de la Cierva to
develop the gyroplane. He correctly argued that a rotary wing aircraft that could not
stall would be safer.
The coefficient of lift cannot be controlled directly; in order to obtain a certain CL
the airfoil must be set to the appropriate angle of attack. The drag is also a function of
angle of attack. It is more useful to know the ratio of lift to drag, since the peak value
of this, L/Dmax., indicates the most efficient mode in which the airfoil can be used.