PHYSICAL AND OPERATIONAL ASPECTS OF AIRCRAFT £CING 
velocity components in the direction of the solid bound- 
ary may be brought back into contact with the wing 
surface on the areas that are adjacent to the region of 
turbulent separated flow (region 2 in Fig. 2). Experi- 
mental evidence on the path and history of individual 
droplets after the first impingement has occurred, how- 
ever, has not been obtained and discussion of this part 
of the subject must remain speculative. 
The conditions for ice formation are therefore made 
more favorable for rapid freezing by the following fac- 
tors: 
1. The degree of supercooling with which the droplet 
strikes the component surface. 
2. The conduction of heat through the boundary- 
layer air. 
3. The evaporation of water from the wetted com- 
ponent surface. 
4. Conduction of heat away from the vulnerable re- 
gion through the solid boundary. 
On the other hand, conditions for ice formation are 
made less favorable for freezing by the following fac- 
tors: 
1. The conversion of kinetic energy of the gas par- 
ticle to thermal energy on the airplane surface. 
'2. The generation of frictional heat in the boundary- 
layer air. 
3. Conduction of heat to the vulnerable region 
through the solid boundary. 
4, Minor factors such as the conversion of kinetic 
energy of the water droplet to thermal energy upon 
impact, and radiant thermal energy from the sun and 
surroundings. 
It is apparent that the velocity and ambient-air 
temperature are important parameters in most of these 
factors. 
In addition to the aforementioned factors, the rate 
at which ice forms on an airplane is determined by the 
following parameters: 
1. Liquid-water content of the icing cloud. 
2. Droplet size. 
3. Shape of airplane component. 
4. Size of airplane component. 
5. Relative humidity of the atmosphere. 
Additional considerations upon which the formation 
of ice on aircraft depends are: 
1. Loss of liquid water from component surface due 
to abrasion of air stream. 
2. Component-surface smoothness. ] 
The analysis of the flow of heat from the surfaces of 
an airplane wing, with consideration of most of the 
factors just discussed, has been reported in [4, 13, 17]. 
Special Cases of Ice Formation. When an airplane 
moves from a very high altitude at which the tempera- 
ture of the ambient air is very low to a lower altitude 
at which the temperature may be above the freezing 
point of water, the structure of the airplane and there- 
fore the surfaces of the vulnerable components may be 
below 32F. If at the lower altitude the airplane passes 
through liquid-water clouds, ice may form over all the 
wetted areas. Although such an encounter may have 
annoying consequences for a short time, as soon as the 
1193 
structure gains heat from the atmosphere the tempera- 
ture will rise and the ice will be shed. The case is of 
interest in that the solid body is for a short time a heat 
sink for the heat of fusion. 
If the surface temperature of the vulnerable com- 
ponents on a rapidly descending aircraft falls below 
the saturation temperature of the boundary air, con- 
densation will occur. For this reason fog or frosb may 
form on an airplane windshield even in the absence of 
visible moisture in the atmosphere. The greater the 
specific heat and mass of the component, the longer the 
loss of vision will persist. 
A serious operational problem is presented by the 
formation of frost which is caused by the loss of heat 
by radiation from the airplane surfaces to the sur- 
rounding environment. In this case both the heat of 
condensation and the heat of fusion are dissipated from 
the vulnerable area. The formation of frost occurs most 
commonly at night with the airplane at rest on the 
ground. 
The addition of a nonmiscible volatile liquid, such 
as aviation gasoline, to a region of droplet-air interface 
(e.g., in a carburetor) will have the effect of increasing 
the heat lost from the droplet environment owing to 
evaporation of the volatile material. The addition of 
the volatile liquid in this instance causes an increase 
in the amount of water frozen. If a volatile liquid is 
added, however, which is miscible and also acts as a 
freezing point depressant when added to water, the 
ice formation will be reduced or prevented under some 
operating conditions. 
Effects of Ice on Operation of Airplane Components 
The effect of the formation of ice on the function 
of an airplane component is determined by the magni- 
tude of the formation and the vulnerability of the 
component to icing. 
Wings and Control Surfaces. The formation of ice on an 
airfoil, whether the airfoil is used as a wing or as the 
fixed stabilizing component of a control surface, re- 
duces the maximum lift coefficient and imcreases the 
drag coefficient. The shape of the ice formation, as 
determined by the rate of freezing of the droplet and 
the area of impingement on the airfoil leading edge, has 
an important effect in the deleterious results of the 
formation. Wind-tunnel research has shown that a 
protuberance of a given height on the surface of a wing 
produces the maximum loss in aerodynamic efficiency 
when the protuberance is located near the region of 
minimum local pressure. For many airfoils the mini- 
mum local pressure region may be at about the 5 per 
cent chord: point [5]. A protuberance located in the 
vicinity of the stagnation pressure point has com- 
paratively little effect on the aerodynamic character- 
istics. It follows that ice forming on the forward ex- 
tremity of the airfoil will not impair flight as much as 
will formations located 5 per cent from the leading 
edge and on the upper surface of a wing. Icing con- 
ditions resulting from large droplets that strike over a 
wide area on the leading edge therefore cause a more 
serious effect than do small droplets. Ice formed under 
