The Art of the Helicopter John Watkinson - Karatunov.net

326 The Art of the Helicopter
the location in question is known, the error can be minimized by adjusting the altimeter
setting scale to that pressure. However, if instead the altimeter setting scale is set to the
ISA MSL pressure of 1013.2 hPa, the altimeter reads pressure altitude. This is defined
as the altitude in the ISA at which the same pressure exists. Pressure altitude is used
primarily for flight in airways where it assures vertical separation (see section 7.7).
Whereas pressure is primarily a navigational tool, density is what governs performance.
Engines produce power by releasing thermal energy due to combustion of
hydrocarbon fuel in air. The mass of fuel that can be burned is directly proportional
to the mass of the air available, whereas physical limitations in the construction of the
engine set the greatest volume of air that can be drawn in. Consequently the density
has a significant effect because it determines the mass of air per unit of volume the
engine has drawn in. Lift is obtained when the rotor imparts downward momentum to
the air. As the disc area is fixed, the volume of air the rotor can influence is also fixed
for a given flight regime. Thus the degree to which the rotor can impart momentum is
controlled by air density.
In the ISA the density at a given height will always be the same because ISA defines
MSL pressure and temperature and specifies zero humidity. Thus using the standard
figures, the pressure and temperature at any height can be deduced and the density must
follow from that. In practice, the pressure and temperature will differ from ISA and the
local density will not be ISA density. The concept of density altitude was introduced to
allow for such changes. Density altitude is the altitude in the ISA at which the density
is the same as the present density. In order to assess the performance of a machine
under real conditions, the pilot will calculate the density altitude for those conditions
and then consult the flight manual where the performance is specified as a function of
density altitude.
Figure 8.1 shows a standard density/altitude chart that converts pressure altitude
entered on the diagonal lines to density altitude measured along the vertical axis. The
horizontal axis is air temperature and the single line falling to the right shows the ISA
air temperature falling with height at the standard lapse rate with pressure altitude.
Thus the chart shows that the line corresponding to zero pressure altitude crosses the
line corresponding to zero density altitude at 15 ◦ C (ISA MSL conditions). In order to
use the chart, the local pressure altitude is located where it crosses the ISA lapse rate
line. Reading vertically down from this pressure altitude will give the ISA temperature
for that height. If the actual temperature is the same, the pressure altitude and density
altitude are the same. However, in practice a different temperature will often be found.
If the diagonal line sloping from bottom left to top right and corresponding to the
pressure altitude is followed until it intersects a vertical line drawn up from the actual
temperature, the vertical position of the intersection determines the density altitude.
The figure shows an example. In some flight manuals, the conversion to density altitude
may be incorporated in certain performance charts. An example will be given in the
next section.
The effect of humidity is not generally considered in density altitude charts, but a rule
of thumb used by pilots is to add 1000 feet to the density altitude if the humidity is
high. Typically helicopters have no means to measure humidity and the pilot’s estimate
has to be used.
8.5 Power management
Helicopter operation is largely concerned with power management. The power plant
supplies power and the airframe demands power. Both supply and demand are