Evening star., 1933-10-20
Rear.End Streamlining
Seen Needless Device


The interesting article by Mr.. Haskin
on your editorial page OF October Is
repeats several misconceptions that
seem to attach to any discussion Of
streamlining. The most obvious Of
these refers to the so-called teardrop
shape. This shape Is purely an artists
notion, the pollywog tail suggesting the
point Of reluctant departure Of the drop.
In its free fall the drop is almost per-
fectly spherical, there being no per-
ceptible suggestion Of q tail. The
Momentary appearance Of tail and
the subsequent shape Of the falling drop
are both dictated by surface tension
and the gravitational law; they have
nothing whatever to do with air re-
sistance and would occur equally in q
vacuum. So the imitation Of tear
drop in the search for minimum air re-
sistance Is rather pointless.
As to blunted front end in pref
erence to pointed or wedge-shaped
front, practical considerations dictate
front. Is certainly more efficient. Fishes
uniformly wear q blunted front and q
refined rear end, but this. apparently,
Is due to the concentration Of the SEN
sorry faculties in the bow-a condition
prevailing in almost all animal life.
Those fishes which are remarkable for
speed through the water have relatively
pointed front ends. It Is true that, in
its progress through a fluid such as air
or water, a blunted or fiat surface will
bank up the fluid ahead of 1t so that
next the surface there is body Of fluid
relatively at rest, and of an external
form adapted to cut through the air by
displacing 1t sidewise. But this banked
up air has no such displacement effl-
cheney as a rigid surface of the re-
quired shape would have, because its
boundary Is indefinite and it Is capable
of swirls and eddies which absorb
energy. In practice we recognize this
Of the streamlined vessel enables it to
run away from the flat-nosed scow. 1t
is doubtless true that a wedge or cone
is not the best shape: the sidewise
acceleration Of the. displaced air must
be accomplished gradually rather than
instantaneously, and with due regard
to the condition Of the air during and
after displacement. as regards swirls
and resultant pressures. But there Is
no inherent virtue in bluntness of the
front end.
The rear end, likewise, must be shaped
to permit return Of the air to quies-
cence In the rear. As the body passes,
the displaced air Is returned in its wake
by atmospheric pressure, and here, too.
the returning air must be accelerated
gradually and then accelerated gradu-
ally, to avoid eddies and q needlessly
low effective air pressure in rear Of the
body. theoretically, proper design Of
the rear end enables the recovery of
much Of the power lost at the head end,
ov,I fc fv,
speak of reduction of the rear-end drag
But the present significance of afr
resistance to railroads has been greatly
overemphasized. The end resistance
depends only on the size and shape of
the ends Of the body, and is independ-
ent Of its weight or prismatic length.
So this resistance is the same for
single car as for train Of any length.
But railroads of consequence cannot
operate their fast passenger service by
single cars; there is too much Of 1t.
They cannot crowd their tracks with
an indefinitely large number of single
cars. each constituting separate oper-
ating hazard as regards collision and
the occupancy Of length Of track
Train operation seems necessary. and a
streamlined unit, of duralumin and 100
feet long, is no general solution at all
DO Oh tIle HeIlllSylVdIlld, lcI Us Say, IIlc
usual fast train may have 10 cars and
weigh, in toto. some 900 tons or more.
At high speed its frictional and abrasive
losses in journals and on the track
will run some 7 pounds per ton at the
best. So the tractive force. aside from
all air resistance, must be over 5000
pounds. The head-end air resistance
will be dictated somewhat by the re-
ouirements Of locomotive design: let
them do what they can to reduce it.
But 1t is easily shown that the rear
end drag will not exceed some TOO
pounds at most. something like per
cent of the total train resistance. So
why streamline at the rear, when it
involves loss of flexibility In train
ake-up-specially-s cars for the
rear end, which must be turned at
terminals and switched when another
car is added or dropped?
Air resistance Is not. by Itself. OF
sufficient consequence to suggest the
redesign Of rolling stock. Due to lower
speed, uneconomic employment and
higher tractive requirements per ton.
the same is broadlv true of highway
vehicles. HUGH F1TZHUGH.