The American Society of Mechanical Engineers

ALLEN —T R E N D OF A IR TR A N SPO R TA TIO N 3
passengers plus a crew of 4 to 11, weighing not 1 ton b u t from 20
to 40 tons gross, and costing $250,000 to more th an $500,000 each,
Figs. 4, 5, and 6. The increase in size has been very gradual and
has been dependent a t alm ost every step upon parallel technical
development. Each forward step has been based on the successful
performance of the preceding one. Each small advance required
a successful experimental and design venture in order to
obtain the necessary continued influx of working capital to finance
the expansion.
Domestic air lines had a total investm ent of less than $2,000,000
in transport airplanes in 1927. This grew to $12,000,000 in two
years, sank for five years during the depression, but recovered
quickly and rose to $23,000,000 in 1938. T he u n it cost of a
typical transport airplane in 1927 was $23,000. T he curve of
F io . 7 C r u is in g a n d M a x im u m S p e e d s ; T y p ic a l A l t it u d e a n d
C r u is in g H o r s e p o w e r p e r A ir p l a n e , U n it e d S t a t e s D o m e s t ic
A ir L in e s
F i g . 8 C o s t a n d T im e o f D e v e l o p in g a N e w A i r -L i n e T r a n s p o r t
increasing u n it cost, Fig. 5, shows a gradual increase up to
$110,000 in 1936 and then a large and sudden increase as the 4-
engine air transports were developed. Cost per pound of useful
load shows the effect of th e dem and for luxury of travel. In
1928-1929, $12 per lb represented the cost; in 1939 it is $22 per lb.
The average transport airplane of 1924 had a cruising speed of
100 mph, using from 200 to 300 hp and flying a t a cruising altitude
of less th an 1000 ft. The 1940 airplane, equipped w ith a pressurized
cabin, cruises a t 200 mph, using cruising horsepowers of 1200
to 2000 a t altitudes ranging from 12,000 to 20,000 ft, Fig. 7.
These changes have not been uniform or gradual, b u t have occurred
in sudden spurts, as new engines, new techniques, and new
design tendencies were developed.
One of the m ost interesting trends in this connection is th a t of
the enormously increasing cost of developing a new air-liner design.
Jn 1927 this averaged $40,000, while today costs as great as
$1,500,000 are not uncommon. I t now requires not six months
as it did in 1926, b u t two years to produce th e first airplane of
a new m ajor type, Fig. 8.
The trend of air-transport design tow ard larger size and higher
power output has a very interesting parallel in the trend toward
increasing wing loadings and decreasing power loadings. The
average 1924 transport airplane had a power loading of 16 lb per
hp and a wing loading of 11 lb per sq ft. W ing loading, going up
from a low value, and power loading, going down from a high
value, crossed in 1935 a t approxim ately 13 lb each. By 1939 the
average power loading of landplanes had gone down to 10 and the
wing loading of landplanes up to 30. The combined loading has
thus through the years shown a gradual increase from 27 in 1924
to 35 in 1935, and to 40 in 1939. W ing loading is going up faster
th an power loading is coming down. This indicates the increasing
efficiency of modern design. Seaplane wing loadings are
definitely on the increase and will probably pass the 50 m ark
soon.
I t would be expected th a t landing speeds would show a very
large increase parallel to the increased wing loadings, and this is
true to a certain extent. H igher lift devices, however, have
made it possible for designers to increase gradually the weight
carried per square foot of wing area w ithout greatly increasing
the landing speed, Fig. 9. Wing loadings of long-range aircraft
no longer are an indication of landing speed, because now such
aircraft are not landed w ith the take-off wing loading. Provisional
gross weight on take-off is reducible by dum ping extra fuel
in case a landing is required soon after take-off, before this fuel has
been consumed in flight. T hus 70 m ph is required for landing a t
standard gross weight, but take-offs m ay be made with greatly
increased loads. Landing speeds of 55 m ph were typical in 1924
to 1927, while landing speeds of 70 mph have become fairly well
standardized a t the present. There is evidence however th a t
they will soon be on the rise again w ith the advent of stable landing
gears and sm ooth runways.
Governm ent regulations have for years lim ited wing loadings
of seaplanes as well as landplanes to the point where 70-mph
alighting was possible. T he perfectly apparent safety, however,
of much higher speeds on touching the w ater or land w ith aircraft
of 40 tons gross weight, and the great increase in economy and
range available by increasing the wing loadings, indicate th a t
such an extreme lim itation is no longer essential.
Fio. 9
P o w e r L o a d in g a n d W in g L o a d in g , U n i t e d S t a t e s
T r a n s p o r t A i r p l a n e s
D e v e l o p m e n t o f S a f e t y F e a t u r e s
Im provem ent of the airplane, its equipm ent, and its operation
from the standpoint of safety has been the m ost wholesome and
m ost reassuring trend of all. M uch has been accomplished already,
and much more is now being done.
The addition of proved safety devices has been a very gradual
process. O utstanding among these have been blind-flying in­