PHYSICAL AND OPERATIONAL ASPECTS OF AIRCRAFT ICING 
have accumulated on forward regions of the airplane, 
such as engine nacelles or propeller hub, become dis- 
lodged, as will be the case when the airplane encounters 
air temperatures above 32F, they move with the air 
stream in a rearward direction. If they strike other 
components of the craft, serious structural damage is 
inflicted. Protection for areas on which the formation 
of ice does not seriously impair the safety of flight is 
not provided for in current airplane design. The need 
for such protection varies over a wide range depending 
on the aerodynamic cleanliness with which the designer 
is able to render the general design layout. By elimi- 
nating protuberances, by the use of flush inlet scoops 
and sheltered vents, and by the arrangement of the main 
airplane components so as to eliminate, insofar as 
possible, leading-edge areas having small radii of cur- 
vature, the deleterious results of icing may be signifi- 
cantly minimized. 
Topics Needing Further Study 
The hazards and nuisances presented by the air- 
plane-icing problem have in part been met by the de- 
velopment of ice-protection devices and by refinements 
in airplane design which make the components less 
vulnerable. Of the various methods of removing or 
preventing ice—mechanical, chemical, and thermal— 
the thermal method has given the greatest promise of 
providing a solution that can be adapted to practical 
transport or military airplanes without undue penalty 
in weight, added complication, or airplane cost. It 
should not be construed, however, that the other 
methods are unworkable or impracticable. The sug- 
gestions for further research that follow pertain to the 
extension of knowledge on the thermal ice-prevention 
system, to several atmospheric studies, and to an in- 
strument-development program which is related to 
meteorological observations. 
Heat-Transfer Relationships. The validity and ac- 
curacy of the relationships at present employed for 
determining heat transfer, mass transfer, recovery tem- 
perature, and droplet interception should be examined 
for the full range of values of velocity, shape, and size 
of aircraft now under design or likely to be designed 
in the near future. The recovery temperature at various 
flow conditions on a wetted airfoil at velocities near 
and above the speed of sound may be of interest in 
future aircraft; data on this factor should be established. 
Of interest in this problem is the comparative signifi- 
cance of viscous dissipation, conductive effects, and 
evaporation in the regions of laminar, turbulent, and 
separated flow for the high-speed range. An under- 
standing of the nature of thermal ice prevention in 
greater detail than now exists is needed in order that 
effective protection may be provided without prohib- 
itive penalities in added equipment to operating air- 
craft. 
Variations in the Icing Problem with Altitude. An 
attractive speculation is that by flying very high the 
icing problem may be avoided. It is therefore of interest 
to learn what icing conditions are to be encountered at 
altitudes such as in the tops of cumuliform clouds, and 
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with what degree of severity and frequency icing is apt 
to be encountered. Almost all the data thus far collected 
have been obtained at comparatively low altitudes. 
In consideration of the probability that extensive future 
operations will be conducted at high altitudes, the 
altitude range of the data should be extended. Data 
should be collected at altitudes above 25,000 ft on the 
dimensions and frequency of icing conditions and on the 
liquid water content, droplet size, and temperature 
range occurring with icing at these altitudes. 
Critical Geographical Areas. Reports from military 
and commercial transport operations supply evidence 
that some geographical areas are attended by unique 
icing conditions of severe intensity; therefore, factual 
data on and a theoretical basis for the variations in the 
icing problem with geographical locations should be 
established. 
Physics of Icing-Cloud Stability. Inasmuch as water 
normally freezes at 32F, it is of prime interest to under- 
stand why the droplets in an icing cloud have not 
undergone the phase change to the solid state and 
become snow or hail. An effort should be made to 
identify and analyze the full range of conditions which 
must be met. in order that water droplets may change 
to ice. When this topic is more thoroughly explored 
and understood, the very interesting problems of modi- 
fyig the weather artificially can be evaluated more 
adequately. 
Forecasts of Icing. If airmen could receive accurate 
and complete forecasts of icing conditions, many oper- 
ations could be routed around the hazardous locality, 
thus eliminating the need for special flight equipment 
and greatly extending the usefulness of aircraft in 
which ice protection is not installed. Improvements in 
the forecasting of icing are needed as are the means for 
effectively communicating the forecast to flight-oper- 
ation centers. 
Instruments for Collecting Atmospheric Data on Icing. 
The understanding of the phenomenon of icing, the 
design of efficient anti-icing equipment, and the de- 
velopment of reliable techniques of forecasting the 
occurrence of icing require that accurate and compre- 
hensive data on the factors that produce icing be 
collected. Although instruments have been developed 
whereby data of considerable value have been obtained 
[11], still further instrumental research is needed. 
In the study of the physics of the icing cloud, in- 
struments that will reveal the existence and size of 
nuclei, water droplets, and snow crystals are needed. 
Data on such particulate substances need to be col- 
lected in order to reveal their variations with space and 
time. 
In the study of the operational aspects of the icing 
problem, instruments are needed with which data on 
the occurrence of icing over wide ranges of geographical 
location and altitude can be collected. 
REFERENCES 
1. Army Air Forcus, Report on Icing Tests for Screens in Air 
Inlet Ducts. Translation F-TS 2625RE. 
2. CHRISTENSON, C. M., ‘‘Aircraft Icing Revelations Lighten 
