The Art of the Helicopter John Watkinson - Karatunov.net

64 The Art of the Helicopter
blades in the power turbine are taking energy out of the gas flow. Given this reversibility,
we can immediately see how in a glider or in an autorotating helicopter the power flow
through the actuator reverses. In descending through the air the wing or rotor delivers
power extracted from loss of potential energy and this power is used to overcome profile
drag. The glider can maintain airspeed and the helicopter can maintain rotor speed
with no engine power.
The act of turning the airflow also results in rotational energy being imparted to the
passing air. Conceptually, the airflow can be divided into two flows. One is the steady
flow past the blade and the other is the rotational component that in the absence of
the steady flow would rotate around the blade. This is known as circulation and it is
proportional to the lift. In vortex theory, the circulation of an airfoil is calculated and
the lift follows from that.
A further consequence of the change of direction is that the horizontal component
of the relative velocity is reduced. It follows from the rearward inclination of the blade
reaction that there must be a forward component of the acceleration imparted to the air.
Air does not go straight down from a hovering rotor; it also revolves in the same direction
the rotor turns, but much more slowly: a phenomenon known as swirl. Swirl is
considered in section 3.14.
It is more efficient gradually to accelerate the air down rather than have it suddenly
find an inclined plate. Air passing the top edge of a flat plate cannot change direction
quickly enough and the flow separates and becomes turbulent. These are the reasons for
curving or cambering an airfoil. Furthermore, practical wings must contain structural
members such as spars, and the thickness will be increased to accommodate them. The
thickness is contained within a streamlined shape to reduce drag.
Figure 3.3(a) shows a streamlined cambered section. The mean camber line is half
way between the upper and lower surfaces. The chord line joins the ends of the camber
line. The angle of attack is the angle between the chord line and the relative airflow.
The camber of the airfoil can be optimized for the speed range of interest. For high
lift at low speeds, the camber will be heavy in order to make Va as large as possible.
Fig. 3.3 Both cambered and flat airfoils will produce lift. In practice they will be thickened to reduce separation
and to allow structural components to be incorporated. Cambered airfoils work best at low speeds. In the
cambered section (a) the camber line will curve away from the straight chord line. In the symmetrical section (b)
the chord line and camber line coincide. A symmetrical airfoil is equally capable of producing lift upwards or
downwards. With zero angle of attack there is no induced drag, only profile drag.