1194 
droplet environmental conditions that do not result in 
sudden phase change but allow a part of the liquid to 
move slightly rearward on a wing is a more serious 
problem than ice that freezes where the droplet strikes 
near the leading edge. 
The formation of ice on the wing of an airplane has 
been observed to increase the drag of the airplane by 
35 per cent [14] during flight in natural icing conditions. 
The formation of ice on the control surfaces of an 
airplane not only increases the drag but also may 
adversely affect the control of the craft [15]. The nature 
of the problem of icing on airfoils is such that pro- 
tection equipment is considered to be necessary on 
craft expected to operate in inclement weather. 
The formation of frost on the wings causes a serious 
loss of lift and an increase in drag. It is considered to be 
extremely dangerous to undertake a take off with an 
airplane on which even a small frost formation exists. 
Propeller Blades. The formation of ice on propeller 
blades, the blade being an airfoil, has an aerodynamic 
effect similar to that previously described for wings and 
control surfaces. Furthermore, during operation with- 
out icing protection, theice formation remains on some 
blades but is shed by others. The rotation of blades, 
some with ice, others without ice, creates asymmetry 
in both the centrifugal and aerodynamic forces on the 
propeller and, therefore, will produce serious propeller 
vibrations. A further problem arising from ice forma- 
tions on propellers is due to fragments of the ice leaving 
the blade during flight and striking other airplane 
components with damaging effect. Flight observations 
made during natural icing conditions [14| show that 
the efficiency of a propeller may be reduced by as much 
as 19 per cent because of the formation of ice on the 
blades; however, most encounters with icing do not 
reduce the efficiency by more than 10 per cent. Ice 
protection for airplane propellers is provided for air- 
craft that are to be operated in inclement weather. 
Engine Induction System. The formation of ice in the 
air-induction system of aircraft engines impairs the 
engine operation by reducing the air flow to the engine 
and, in certain instances, alters the operation of the 
fuel-metering system. Ice forms on the inner walls of 
the air-inlet duct, screen, guide vanes, impact pressure 
tubes (part of the fuel-metering system), throttle plate, 
and all protuberances exposed to the air stream; in 
fact, this part of the icing problem presents the most 
serious hazard to the airplane. Durimg cruising oper- 
ations the manifold pressure may be gradually reduced 
because of the closing of the air inlet to the engine, or 
the engine may stop suddenly when changes in the 
fuel metering result in noncombustible charges being 
drawn into the engine cylinder. When the engine power 
is reduced for a glide to a landing, the formation of ice 
in the throttle area may be accelerated and prevent 
full power should it be needed for further cruising or 
climbing. If the glide path undershoots the landing 
area, therefore, an accident is almost unavoidable, be- 
cause the power for extending the glide will not be 
available and a landing must be made in an unprepared 
area outside the airport. Ice protection is provided for 
CLOUDS, FOG, AND AIRCRAFT ICING 
the induction system in all practical aircraft. The extent 
to which special devices are needed to protect the in- 
duction system is largely determined by (1) the internal 
aerodynamic smoothness of the induction system, (2) 
the extent to which the entrance excludes water drop- 
lets and still admits air, (3) the manner in which fuel is 
delivered to the cylinder Gn a mixture with air or 
liquid injection), and (4) the air- and fuel-metering 
arrangements [3, 9]. By attention to these fundamentals 
some recently developed designs have demonstrated 
increasing invulnerability to icing. 
Windshield. The formation of ice on an airplane 
windshield or other exposed transparent area impairs 
the vision through the area within a few seconds after 
icing conditions are encountered [6, 12]. Protection for 
the essential areas is provided on aircraft that are to be 
operated in inclement weather. The location of the 
windshield in the forward areas of the fuselage surface 
and the extent to which the exterior surface of the trans- 
parent area and its frame are faired in with the fuselage 
contours determine the rate at which ice forms on the 
glass and therefore the degree of protection required. 
Putot-Static Head. The formation of ice on the exposed 
total-pressure pitot tube and static vents by which the’ 
airspeed indicator and altimeter are operated renders 
these instruments maccurate or inoperative within a 
few seconds after icing conditions are encountered. 
Protection against such formations is provided on all 
aircraft in which flight in inclement weather is to be 
undertaken. It is possible to locate the static pressure 
vent in a substantially nonvulnerable position on the 
airplane. The pitot head must be exposed; however, the 
designer has some latitude in the possible shape of the 
component within which the quantitative need for 
protection may be minimized [2]. 
Radio Antennas. The formation of ice on radio an- 
tennas causes the exposed structures to fail because of 
the increased drag forces or violent vibrations induced 
by the flow of air over the distended shapes formed by 
the ice. The communication system for the airplane 
is guarded against such failure by submerging the 
antenna, by placing it in a sheltered area, or by making 
the structure sufficiently strong to withstand the in- 
creased loads or vibrations. The shape, location, and 
orientation of the antenna determine the severity of 
the icing problem on this type of component [8]. 
Vents and Air Inlets. The formation of ice on a wing 
leading-edge region that is forward and in the vicinity 
of a vent from a fuel tank will cause a loss of pressure 
in the ventilation system which may result in fuel- 
system and engine failure [16]. 
The formation of ice at the opening to air-duct inlets 
will impair the flow of air through the duct, and the 
severity of this aspect of the problem is aggravated by 
the need for a screen in the duct [1]. 
Protuberances and Other Vulnerable Components. The 
formation of ice on protuberances such as radio-an- 
tenna masts and on the forward areas of the fuselage, 
engine nacelles, propeller hub, and other exposed parts 
results in an increase in the airplane drag of as much 
as 25 per cent [14]. When large formations of ice which 
