The Art of the Helicopter John Watkinson - Karatunov.net

330 The Art of the Helicopter
and density altitude. The top right section of the chart allows density altitude to be
computed by finding a point where a horizontal line from pressure altitude on the left
scale intersects the free air temperature line. A vertical line from this point will give
a reading on the density altitude scale. This line will also intersect the AUW curve on
the bottom right chart. Moving left from this point will give a reading on the hover
OGE torque scale.
The bottom left chart converts the hover OGE torque to the torque needed at various
skid/wheel heights. The HOGE line is at an angle of 45 ◦ and does not change the torque
required. However, the lines for various heights of IGE have different slopes and reduce
the torque accordingly.
The chart can be used in different ways. In the above example the pressure altitude,
air temperature and AUW were used to find the torque needed to hover. However, from
AUW and known altitude of the destination it is possible to deduce the maximum air
temperature at which a vertical landing could be made. In the case of an engine failure,
the chart can also be used to show that above a certain AUW hovering even in ground
effect is impossible and a rolling landing on a runway would be needed.
In practice, power needed to hover is affected by windspeed. In most cases the power
needed will be reduced, except if the wind is from a particular direction and at a particular
speed that causes the main and tail rotors to rob one another of inflow. This
phenomenon was considered in Chapter 5. The tandem rotor helicopter is less sensitive
to wind direction in the hover.
Turning next to forward flight, Figure 8.5(a) shows how the various power requirements
typically change with airspeed. The rotors will require induced power and profile
power and the hull will require parasitic power to overcome drag. The induced power
will be high in the hover and falls as airspeed increases. This is because forward flight
gives the rotor access to a larger mass of air per unit time and production of thrust
becomes more efficient. The induced power will also increase with AUW. Profile power
will also rise with AUW and increases slightly with airspeed as the profile drag on
the advancing blade increases more than the drag on the retreating blade lessens.
In contrast, the parasite power is substantially independent of AUW, but increases
with airspeed. The drag force will increase as the square of airspeed, but the power is
the product of the drag force and the airspeed and so will increase as the cube of the
airspeed. At very high speeds the rotor disc must be inclined forwards substantially so
that the horizontal component of thrust from the rotor can become large to overcome
hull drag. This nose-down attitude may make the hull drag even greater. As main rotor
thrust increases the tail rotor thrust will also need to increase to counteract torque and
so the tail power will also increase at high speeds.
Figure 8.5(b) shows that there is an airspeed, typically around 50 knots, at which
the total power is minimum. At this airspeed the induced power has fallen but the
parasite power has not yet risen seriously. The figure also shows the available power,
which is relatively constant, although in some machines it rises with airspeed owing
to the ram effect of dynamic pressure at the intake. The power required reaches the
available power on the right and this would determine the highest airspeed that could
be reached, assuming no structural limits or retreating blade stall effects. At all lower
airspeeds, a power margin exists and so the machine can take off vertically. At the
minimum power airspeed the machine can stay airborne for the longest time. Clearly
the greatest power margin exists at this airspeed and thus the maximum rate of climb
would be obtained at full power. The minimum rate of descent in autorotation will also
be obtained at this airspeed.
Figure 8.5(b) contrasts the conditions for a normally loaded machine at sea level,
with the conditions for a high AUW, where the available power only exceeds the required