SPEED
What speed my "No. 6" made on those Mediterranean flights was not
published at the time because I had not sought to calculate it closely.
Fresh from the troubling time limit of the Deutsch prize competition I
amused myself frankly with my air-ship, making observations of great
value to myself, but not seeking to prove anything to anyone.
The speed problem is, doubtless, the first of all air-ship problems.
Speed must always be the final test between rival air-ships, and
until high speed shall be arrived at certain other problems of aerial
navigation must remain in part unsolved. For example, take that of
the air-ship's pitching (_tangage_). I think it quite likely that a
critical point in speed will be found, beyond which, on each side, the
pitching will be practically _nil_. When going slowly or at moderate
speed I have experienced no pitching, which in an air-ship like my
"No. 6" seems always to commence at 25 to 30 kilometres (15 to 18
miles) per hour through the air. Now, probably, when one passes this
speed considerably--say at the rate of 50 kilometres (30 miles) per
hour--all _tangage_ or pitching will be found to cease again, as I
myself experienced when flying homeward on the wind in the voyage last
described.
Speed must always be the final test between rival air-ships, because,
in itself, speed sums up all other air-ship qualities, including
"stability." At Monaco, however, I had no rivals to compete with.
Furthermore, my prime study and amusement there was the beautiful
working of the maritime guide rope; and this guide rope, dragging
through the water, must of necessity retard whatever speed I made. There
could be no help for it. Such was the price I must pay for automatic
equilibrium and vertical stability--in a word, easy navigation--so long
as I remained the sole and solitary navigator of the air-ship.
Nor is it an easy task to calculate an air-ship's speed. On those
flights up and down the Mediterranean coast the speed of my return
to Monaco, wonderfully aided by the wind, could bear no relation to
the speed out, retarded by the wind, and there was nothing to show
that the force of the wind going and coming was constant. It is true
that on those flights one of the difficulties standing in the way of
such speed calculations--the "shoot the chutes" (_montagnes Russes_)
of ever-varying altitude--was done away with by the operation of the
maritime guide rope; but, on the other hand, as has been said, the
dragging of the guide rope's weight through the water acted as a
very effectual brake. As the speed of the air-ship is increased this
brake-like action of the guide rope (like that of the resistance of
the atmosphere itself) grows, not in proportion to the speed, but in
proportion to the square of it.
On those flights along the Mediterranean coast the easy navigation
afforded me by the maritime guide rope was purchased, as nearly as I
could calculate, by the sacrifice of about 7 or 8 kilometres (4 or 5
miles) per hour of speed; but with or without maritime guide rope the
speed calculation has its own almost insurmountable difficulties.
From Monte Carlo to Cap Martin at 10 o'clock of a given morning may
be quite a different trip from Monte Carlo to Cap Martin at noon of
the same day; while from Cap Martin to Monte Carlo, except in perfect
calm, must always be a still different proposition. Nor can any accurate
calculations be based on the markings of the anemometer, an instrument
which I, nevertheless, carried. Out of simple curiosity I made note of
its readings on several occasions during my trip of 12th February 1902.
It seemed to be marking between 32 and 37 kilometres (20 and 23 miles)
per hour; but the wind, complicated by side gusts, acting at the same
time on the air-ship and the wings of the anemometer windmill--_i.e._
on two moving systems whose inertia cannot possibly be compared--would
alone be sufficient to falsify the result.
When, therefore, I state that, according to my best judgment, the
average of my speed through the air on those flights was between 30 and
35 kilometres (18 and 22 miles) per hour, it will be understood that it
refers to speed through the air whether the air be still or moving and
to speed retarded by the dragging of the maritime guide rope. Putting
this adverse influence at the moderate figure of 7 kilometres (4-1/2
miles) per hour my speed through the still or moving air would be
between 37 and 42 kilometres (22 and 27 miles) per hour.
Rather than spend time over illusory calculations on paper I have always
preferred to go on materially improving my air-ships. Later, when they
come in competition with the rivals which no one awaits more ardently
than myself, all speed calculations made on paper and all disputes based
on them must of necessity yield to the one sublime test of air-ship
racing.
Where speed calculations have their real importance is in affording
necessary _data_ for the construction of new and more powerful
air-ships. Thus the balloon of my racing "No. 7," whose motive power
depends on two propellers each 5 metres (16-1/2 feet) in diameter,
and worked by a 60 horse-power motor with a water cooler, has its
envelope made of two layers of the strongest French silk, four times
varnished, capable of standing, under dynamometric test, a traction of
3000 kilogrammes (6600 pounds) for the linear metre (3·3 feet). I will
now try to explain why the balloon envelope must be made so very much
stronger as the speed of the air-ship is designed to be increased; and
in so doing I shall have to reveal the unique and paradoxical danger
that besets high-speed dirigibles, threatening them, not with beating
their heads in against the outer atmosphere, but with blowing their
tails out behind them.
Although the interior pressure in the balloons of my air-ships is
very considerable, as balloons go, the spherical balloon, having a
hole in its bottom, is under no such pressure: it is so little in
comparison with the general pressure of the atmosphere, that we measure
it, not by "atmospheres," but by centimetres or millimetres of water
pressure--_i.e._ the pressure that will send a column of water up that
distance in a tube. One "atmosphere" means one kilogramme of pressure
to the square centimetre (15 lbs. to the square inch), and it is
equivalent to about 10 metres of water pressure, or, more conveniently,
1000 centimetres of "water." Now, supposing the interior pressure in
my slower "No. 6" to have been close up to 3 centimetres of water
(it required that pressure to open its gas valves), it would have
been equivalent to 1/333 of an atmosphere; and as one atmosphere is
equivalent to a pressure of 1000 grammes (1 kilogramme) on one square
centimetre the interior pressure of my "No. 6" would have been 1/333
of 1000 grammes, or 3 grammes. Therefore on one square metre (10,000
square centimetres) of the stem head of the balloon of my "No. 6"
the interior pressure would have been 10,000 multiplied by 3, or 30,000
grammes _i.e._--30 kilogrammes (66 lbs.).
[Illustration: "SANTOS-DUMONT No. 7"]
How is this interior pressure maintained without being exceeded?
Were the great exterior balloon filled with hydrogen and then sealed
up with wax at each of its valves, the sun's heat might expand the
hydrogen, make it exceed this pressure, and burst the balloon; or should
the sealed balloon rise high, the decreasing pressure of the outer
atmosphere might let its hydrogen expand, with the same result. The
gas valves of the great balloon, therefore, must _not_ be sealed; and,
furthermore, they must always be very carefully made, so that they will
open of their own accord at the required and calculated pressure.
This pressure (of 3 centimetres in the "No. 6"), it ought to be noted,
is attained by the heating of the sun or by a rise in altitude only
when the balloon is completely filled with gas: what may be called its
working pressure--about one-fifth lower--is maintained by the rotary air
pump. Worked continually by the motor, it pumps air continually into the
smaller interior balloon. As much of this air as is needed to preserve
the outer balloon's rigidity remains inside the little interior
balloon, but all the rest pushes its way out into the atmosphere again
through its air valve, which opens at a little less pressure than do the
gas valves.
Let us now return to the balloon of my "No. 6." The _interior_ pressure
on each square metre of its stem head being continuously about 30
kilogrammes the silk material composing it must be normally strong
enough to stand it; nevertheless, it will be easy to see how it becomes
more and more relieved of that interior pressure as the air-ship gets
in motion and increases speed. Its striking against the atmosphere
makes a counter pressure _against the outside_ of the stem head. Up
to 30 kilogrammes to the square metre, therefore, all increase in the
air-ship's speed tends to reduce strain, so that the faster the air-ship
goes the less will it be liable to burst out its head!
How fast may the balloon be carried on by motor and propeller before its
head stem strikes the atmosphere hard enough to more than neutralise the
interior pressure? This, too, is a matter of calculation; but, to spare
the reader, I will content myself with pointing out that my flights over
the Mediterranean proved that the balloon of my "No. 6" could safely
stand a speed of 36 to 42 kilometres (22 to 27 miles) per hour without
giving the slightest hint of strain. Had I wanted an air-ship of the
proportions of the "No. 6" to go twice as fast under the same conditions
its balloon must have been strong enough to stand four times its
interior pressure of 3 centimetres of "water," because the resistance of
the atmosphere grows not in proportion to the speed but in proportion to
the square of the speed.
The balloon of my "No. 7" is not, of course, built in the precise
proportions of that of my "No. 6," but I may mention that it has been
tested to resist an interior pressure of much more than 12 centimetres
of "water"; in fact, its gas valves open at that pressure only. This
means just four times the interior pressure of my "No. 6." Comparing
the two balloons in a general way, it is obvious, therefore, that with
no risk from outside pressure, and with positive relief from interior
pressure on its stem or head, the balloon of my "No. 7" may be driven
twice as fast as my easy-going Mediterranean pace of 42 kilometres (25
miles) per hour, or 80 kilometres (50 miles).
This brings us to the unique and paradoxical weakness of the fast-going
dirigible. Up to the point where the exterior shall equal the interior
pressure we have seen how every increase of speed actually guarantees
safety to the stem of the balloon. Unhappily, it does not remain true
of the balloon's stern head. On it the interior pressure is also
continuous, but speed cannot relieve it. On the contrary, the _suction_
of the atmosphere behind the balloon, as it speeds on, increases also
almost in the same proportion as the pressure caused by driving the
balloon against the atmosphere. And this suction, instead of operating
to neutralise the interior pressure on the balloon's stern head,
_increases_ the strain just that much, the pull being added to the push.
Paradoxical as it may seem, therefore, the danger of the swift dirigible
is to blow its tail out rather than its head in. (See Fig. 12.)
[Illustration: Fig. 12]
How is this danger to be met? Obviously by strengthening the stern
part of the balloon envelope. We have seen that when the speed of
my "No. 7" shall be just great enough to completely neutralise the
interior pressure on its stem head the strain on its stern head will be
practically doubled. For this reason I have doubled the balloon material
at this point.
I have reason to be careful of the balloon of my "No. 7." In it the
speed problem will be attacked definitely. It has two propellers, each
5 metres (16-1/2 feet) in diameter. One will push, as usual, from the
stern, while the other will pull from the stem, as in my "No. 4." Its
60 horse-power Clement motor will, if my expectations are fulfilled,
give it a speed of between 70 and 80 kilometres (40 and 50 miles) per
hour. In a word, the speed of my "No. 7" will bring us very close to
practical, everyday aerial navigation, for as we seldom have a wind
blowing as much even as 50 kilometres (30 miles) per hour such an
air-ship will surely be able to go out daily during more than ten months
in the twelve.