Page 206 - Aeronautical Engineer Data Book

Aircraft design and construction 165
Table 10.6 The electrochemical series
Gold (Au) + volts
Platinum (Pt)
Silver (Ag)
Noble metals (cathodic)
Copper (Cu)
Hydrogen (H) Reference potential 0 volts
Lead (Pb)
Tin (Sn)
Nickel (Ni)
Cadmium (Cd)
Iron (Fe)
Chromium (Cr) Base metals (anodic)
Zinc (Zn)
Aluminium (Al)
Magnesium (Mg)
Lithium (Li) – Volts

Metals higher in the table become cathodic and are protected by
the (anodic) metals below them in the table.

10.3 Helicopter design
10.3.1 Lift and propulsion
Helicopters differ from fixed wing aircraft in
that both lift and propulsion are provided by a
single item: the rotor. Each main rotor blade
acts as slender wing with the airflow producing
a high reduction in pressure above the front of
the blades, thereby producing lift. Although of
high aspect ratio, the blades are proportion­
ately thicker than those of fixed wing aircraft,
and are often of symmetric profile. Figure 10.6
shows the principle of helicopter airfoil opera­
tion.

10.3.2 Configuration
Figure 10.7 shows the four main configurations
used. The most common is the single main and
tail rotor type in which the torque of the main
rotor drive is counteracted by the lateral force
produced by a horizontal-axis tail rotor. Twin
tandem rotor machines use intermeshing,
counter-rotating rotors with their axes tilted off
the vertical to eliminate any torque imparted to
the helicopter fuselage. In all designs, lift force
is transmitted through the blade roots via the