The American Society of Mechanical Engineers

ALLEN—T R E N D OF A IR TRA N SPORTATIO N 7
been accomplished with only moderate increases in the weights of
engine components, and a small net reduction in weight per horsepower
has fortunately been achieved. Some engines are now
available which develop 1 hp for about 1.1 lb, including propellerdrive
reduction gearing. It seems unlikely th at this figure will be
much improved for current basic types of engines w ithout a prohibitive
sacrifice in reliability.
The widespread use of gasoline having a high-knock rating has
been a primary factor in recent steps toward improvement of the
aircraft engine. The first improvements, achieved by careful
choice of the base crude oil, selective fractionation, and addition of
tetraethyl lead as a knock suppressor, provided a foundation for
major advances in the engine-power ratings. The gains thus
initially achieved have stim ulated the study of properties of a
wide variety of synthetic fuels. This work has now resulted in
the use of blends of natural gasoline having strictly controlled
properties and synthetic hydrocarbons having a high order of resistance
to detonation. This practice has removed some technical
barriers to still higher engine performance and there is ample
indication th at the limit of such improvements is still in the future.
The m utual adaptation of engines and fuels has been materially
advanced in this country by cooperative research in
which all interested agencies actively participate.
Use of fuels of high-octane value has not only perm itted large
increases in maximum-power rating for take-off, but it has had an
im portant and favorable effect upon engine durability and upon
cruising-power output, largely by eliminating detonation hazards
at normal cruising powers with the leanest practicable fuel-air
mixtures.
W ith the current trend to higher flying speeds in transport
operation, there has been much concern for the drag of the large
radial engines. Conventional values for cooling drag can be
reduced drastically by air-flow controls which proportion the
pressure head and the am ount of air flowing over the cylinders to
the existing need for cooling at any particular operating condition.
The power used for cooling of cylinders may be held within limits
heretofore thought practicable only for liquid-cooled engines.
For extremely high speeds, however, the liquid-cooled engine appears
to have a definite advantage over its competitors, largely
because of more favorable shape and proportions. Speeds used in
air-transport operations scarcely can be expected to rise in the
near future to values where the low-drag shapes, associated with
liquid cooling, will bear sufficient premium to offset the operating
advantages of direct air cooling.
There appears to be a trend in aircraft engines toward the
development of two principal variations of basic engine types..
One of these variations provides a maximum of power output for
high-performance aircraft a t some sacrifice of reliability and fuel
consumption. The other variation is constructed to provide a
minimum fuel consumption for long-range operation with a
slightly reduced maximum-power capacity. This trend is highly
significant to air transportation because of effects on the economics
of long-range operations.
Since the utility of an air transport (particularly for long-range
service) is primarily dependent upon its pay-load capacity, every
other weight involved in the airplane structure can be evaluated
in terms of dollars per pound. The gross weight chargeable to
the whole propulsion system consists of the engine, the propeller,
structural supports, cowling, accessories, plumbing, etc., plus fuel
and oil required for the schedule. A study of pertinent data
shows th at fuel and oil weights are a predominant influence in this
figure for nonstop flights of more than a few hundred miles. A
typical air-transport engine will consume a quantity of fuel and
oil about equivalent to its own weight in 4 or 5 hr of operation at
the maximum power required for cruising. Thus for a 20-hr
flight, weight savings on the fuel consumed are about four times as
im portant in term s of pay load as are items of fixed weight, such
as the basic engine. For the same reason, any items affecting
the propeller efficiency at cruising power are extremely im portant.
In the selection of engines for long-range transports, engine
factors which are conducive to a maximum of safety and reliability
are, of course, of prim ary importance. Ease of m aintenance,
accessibility in flight, and proper adaptation to service requirements
are much more im portant factors than in the case of
other aircraft types. A premium is placed upon experience accumulated
in previous operations of a similar type. I t is essential
th a t the aircraft shall have serviceability characteristics
which will eliminate need for extensive attention between
scheduled flights. As aircraft become larger, this is a factor of
increasing importance because of the larger investm ent and
consequent overhead charges.
These factors emphasize the necessity for a degree of reliability
in the long-range-aircraft power plant which approaches th at
provided in marine practice. I t seems probable th a t the future
will show an increasing emphasis on durability and reductic)n in
fuel and oil consumption a t moderate power-output levels. I t is
fortunate th at all concerned have come to realize th at these
qualities cannot be obtained without moderate sacrifices in the
high performance qualities associated with other classes of aircraft
service.
P r o v i s i o n s f o e C o m f o r t o f P a s s e n g e r s
The progress of air transportation depends in very large measure
upon passenger comfort and passenger satisfaction. Volume
of traffic is very sensitive to improvements in these two factors.
The air lines in the United States have been very responsive to
these demands on the part of the traveling public and, in turn,
they have demanded of the aircraft designer more space per passenger,
less crowding, better seat and berth design, larger dressing
rooms, better ventilation and tem perature control, air-conditioning,
and, now, air-pressure control for high altitudes and to minimize
the effect of rapid changes in altitude. In addition to these
demands, improved food service aloft is now required, such th at
appetizing meals can be served attractively and expeditiously.
Improved lighting for passengers’ accommodation in reading and
writing has become such a serious problem th a t in some cases
auxiliary power plants have been required to supply the current
demands for luxury items as well as mechanical necessities. The
weight of these luxury items has increased on the average from 10
lb per passenger in 1930 to approximately 180 lb per passenger in
1939. Floor area has increased from approximately 5 sq ft per
passenger in 1932 to 20 sq ft per passenger in 1939, and cabin
volume from 25 cu ft in 1931 to 120 cu ft in 1939. This increase
in roominess is in itself a prim ary comfort feature, Fig. 12.
Soundproofing has changed from a luxury item, only a few
years ago, to a prim ary requirem ent today in crew and passenger
quarters alike.
The increased effort to cater to passenger satisfaction has led
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C a b i n V o l u m e a n d F l o o r A r e a p e r P a s s e n g e r , U n i t e d
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