The American Society of Mechanical Engineers
The American Society of Mechanical Engineers
The American Society of Mechanical Engineers
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ALLEN—T R E N D OF A IR TRA N SPORTATIO N 7<br />
been accomplished with only moderate increases in the weights <strong>of</strong><br />
engine components, and a small net reduction in weight per horsepower<br />
has fortunately been achieved. Some engines are now<br />
available which develop 1 hp for about 1.1 lb, including propellerdrive<br />
reduction gearing. It seems unlikely th at this figure will be<br />
much improved for current basic types <strong>of</strong> engines w ithout a prohibitive<br />
sacrifice in reliability.<br />
<strong>The</strong> widespread use <strong>of</strong> gasoline having a high-knock rating has<br />
been a primary factor in recent steps toward improvement <strong>of</strong> the<br />
aircraft engine. <strong>The</strong> first improvements, achieved by careful<br />
choice <strong>of</strong> the base crude oil, selective fractionation, and addition <strong>of</strong><br />
tetraethyl lead as a knock suppressor, provided a foundation for<br />
major advances in the engine-power ratings. <strong>The</strong> gains thus<br />
initially achieved have stim ulated the study <strong>of</strong> properties <strong>of</strong> a<br />
wide variety <strong>of</strong> synthetic fuels. This work has now resulted in<br />
the use <strong>of</strong> blends <strong>of</strong> natural gasoline having strictly controlled<br />
properties and synthetic hydrocarbons having a high order <strong>of</strong> resistance<br />
to detonation. This practice has removed some technical<br />
barriers to still higher engine performance and there is ample<br />
indication th at the limit <strong>of</strong> such improvements is still in the future.<br />
<strong>The</strong> m utual adaptation <strong>of</strong> engines and fuels has been materially<br />
advanced in this country by cooperative research in<br />
which all interested agencies actively participate.<br />
Use <strong>of</strong> fuels <strong>of</strong> high-octane value has not only perm itted large<br />
increases in maximum-power rating for take-<strong>of</strong>f, but it has had an<br />
im portant and favorable effect upon engine durability and upon<br />
cruising-power output, largely by eliminating detonation hazards<br />
at normal cruising powers with the leanest practicable fuel-air<br />
mixtures.<br />
W ith the current trend to higher flying speeds in transport<br />
operation, there has been much concern for the drag <strong>of</strong> the large<br />
radial engines. Conventional values for cooling drag can be<br />
reduced drastically by air-flow controls which proportion the<br />
pressure head and the am ount <strong>of</strong> air flowing over the cylinders to<br />
the existing need for cooling at any particular operating condition.<br />
<strong>The</strong> power used for cooling <strong>of</strong> cylinders may be held within limits<br />
heret<strong>of</strong>ore thought practicable only for liquid-cooled engines.<br />
For extremely high speeds, however, the liquid-cooled engine appears<br />
to have a definite advantage over its competitors, largely<br />
because <strong>of</strong> more favorable shape and proportions. Speeds used in<br />
air-transport operations scarcely can be expected to rise in the<br />
near future to values where the low-drag shapes, associated with<br />
liquid cooling, will bear sufficient premium to <strong>of</strong>fset the operating<br />
advantages <strong>of</strong> direct air cooling.<br />
<strong>The</strong>re appears to be a trend in aircraft engines toward the<br />
development <strong>of</strong> two principal variations <strong>of</strong> basic engine types..<br />
One <strong>of</strong> these variations provides a maximum <strong>of</strong> power output for<br />
high-performance aircraft a t some sacrifice <strong>of</strong> reliability and fuel<br />
consumption. <strong>The</strong> other variation is constructed to provide a<br />
minimum fuel consumption for long-range operation with a<br />
slightly reduced maximum-power capacity. This trend is highly<br />
significant to air transportation because <strong>of</strong> effects on the economics<br />
<strong>of</strong> long-range operations.<br />
Since the utility <strong>of</strong> an air transport (particularly for long-range<br />
service) is primarily dependent upon its pay-load capacity, every<br />
other weight involved in the airplane structure can be evaluated<br />
in terms <strong>of</strong> dollars per pound. <strong>The</strong> gross weight chargeable to<br />
the whole propulsion system consists <strong>of</strong> the engine, the propeller,<br />
structural supports, cowling, accessories, plumbing, etc., plus fuel<br />
and oil required for the schedule. A study <strong>of</strong> pertinent data<br />
shows th at fuel and oil weights are a predominant influence in this<br />
figure for nonstop flights <strong>of</strong> more than a few hundred miles. A<br />
typical air-transport engine will consume a quantity <strong>of</strong> fuel and<br />
oil about equivalent to its own weight in 4 or 5 hr <strong>of</strong> operation at<br />
the maximum power required for cruising. Thus for a 20-hr<br />
flight, weight savings on the fuel consumed are about four times as<br />
im portant in term s <strong>of</strong> pay load as are items <strong>of</strong> fixed weight, such<br />
as the basic engine. For the same reason, any items affecting<br />
the propeller efficiency at cruising power are extremely im portant.<br />
In the selection <strong>of</strong> engines for long-range transports, engine<br />
factors which are conducive to a maximum <strong>of</strong> safety and reliability<br />
are, <strong>of</strong> course, <strong>of</strong> prim ary importance. Ease <strong>of</strong> m aintenance,<br />
accessibility in flight, and proper adaptation to service requirements<br />
are much more im portant factors than in the case <strong>of</strong><br />
other aircraft types. A premium is placed upon experience accumulated<br />
in previous operations <strong>of</strong> a similar type. I t is essential<br />
th a t the aircraft shall have serviceability characteristics<br />
which will eliminate need for extensive attention between<br />
scheduled flights. As aircraft become larger, this is a factor <strong>of</strong><br />
increasing importance because <strong>of</strong> the larger investm ent and<br />
consequent overhead charges.<br />
<strong>The</strong>se factors emphasize the necessity for a degree <strong>of</strong> reliability<br />
in the long-range-aircraft power plant which approaches th at<br />
provided in marine practice. I t seems probable th a t the future<br />
will show an increasing emphasis on durability and reductic)n in<br />
fuel and oil consumption a t moderate power-output levels. I t is<br />
fortunate th at all concerned have come to realize th at these<br />
qualities cannot be obtained without moderate sacrifices in the<br />
high performance qualities associated with other classes <strong>of</strong> aircraft<br />
service.<br />
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<br />
<strong>The</strong> progress <strong>of</strong> air transportation depends in very large measure<br />
upon passenger comfort and passenger satisfaction. Volume<br />
<strong>of</strong> traffic is very sensitive to improvements in these two factors.<br />
<strong>The</strong> air lines in the United States have been very responsive to<br />
these demands on the part <strong>of</strong> the traveling public and, in turn,<br />
they have demanded <strong>of</strong> the aircraft designer more space per passenger,<br />
less crowding, better seat and berth design, larger dressing<br />
rooms, better ventilation and tem perature control, air-conditioning,<br />
and, now, air-pressure control for high altitudes and to minimize<br />
the effect <strong>of</strong> rapid changes in altitude. In addition to these<br />
demands, improved food service al<strong>of</strong>t is now required, such th at<br />
appetizing meals can be served attractively and expeditiously.<br />
Improved lighting for passengers’ accommodation in reading and<br />
writing has become such a serious problem th a t in some cases<br />
auxiliary power plants have been required to supply the current<br />
demands for luxury items as well as mechanical necessities. <strong>The</strong><br />
weight <strong>of</strong> these luxury items has increased on the average from 10<br />
lb per passenger in 1930 to approximately 180 lb per passenger in<br />
1939. Floor area has increased from approximately 5 sq ft per<br />
passenger in 1932 to 20 sq ft per passenger in 1939, and cabin<br />
volume from 25 cu ft in 1931 to 120 cu ft in 1939. This increase<br />
in roominess is in itself a prim ary comfort feature, Fig. 12.<br />
Soundpro<strong>of</strong>ing has changed from a luxury item, only a few<br />
years ago, to a prim ary requirem ent today in crew and passenger<br />
quarters alike.<br />
<strong>The</strong> increased effort to cater to passenger satisfaction has led<br />
F ig . 1 2<br />
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<br />
S t a t e s T r a n s p o r t A i r p l a n e s