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PROBLEMS OF AERIAL FLIGHT.






From: Flying Machines Construction Operation

In a lecture before the Royal Society of Arts, reported
in Engineering, F. W. Lanchester took the position that
practical flight was not the abstract question which some
apparently considered it to be, but a problem in locomotive
engineering. The flying machine was a locomotive
appliance, designed not merely to lift a weight,
but to transport it elsewhere, a fact which should be
sufficiently obvious. Nevertheless one of the leading scientific
men of the day advocated a type in which this, the
main function of the flying machine, was overlooked.
When the machine was considered as a method of transport,
the vertical screw type, or helicopter, became at
once ridiculous. It had, nevertheless, many advocates
who had some vague and ill-defined notion of subsequent
motion through the air after the weight was raised.

Helicopter Type Useless.

When efficiency of transport was demanded, the helicopter
type was entirely out of court. Almost all of
its advocates neglected the effect of the motion of the
machine through the air on the efficiency of the vertical
screws. They either assumed that the motion was
so slow as not to matter, or that a patch of still air
accompanied the machine in its flight. Only one form of this
type had any possibility of success. In this there were
two screws running on inclined axles--one on each side
of the weight to be lifted. The action of such inclined
screw was curious, and in a previous lecture he had
pointed out that it was almost exactly the same as that
of a bird's wing. In high-speed racing craft such inclined
screws were of necessity often used, but it was
at a sacrifice of their efficiency. In any case the efficiency
of the inclined-screw helicopter could not compare
with that of an aeroplane, and that type might be
dismissed from consideration so soon as efficiency became
the ruling factor of the design.

Must Compete With Locomotive.

To justify itself the aeroplane must compete, in some
regard or other, with other locomotive appliances, performing
one or more of the purposes of locomotion more
efficiently than existing systems. It would be no use
unless able to stem air currents, so that its velocity must
he greater than that of the worst winds liable to be encountered.
To illustrate the limitations imposed on the
motion of an aeroplane by wind velocity, Mr. Lanchester
gave the diagrams shown in Figs. 1 to 4. The circle
in each case was, he said, described with a radius equal
to the speed of the aeroplane in still air, from a center
placed "down-wind" from the aeroplane by an amount
equal to the velocity of the wind.

Fig. 1 therefore represented the case in which the
air was still, and in this case the aeroplane represented
by _A_ had perfect liberty of movement in any direction

In Fig. 2 the velocity of the wind was half that of the
aeroplane, and the latter could still navigate in any
direction, but its speed against the wind was only one-
third of its speed with the wind.

In Fig. 3 the velocity of the wind was equal to that
of the aeroplane, and then motion against the wind was
impossible; but it could move to any point of the
circle, but not to any point lying to the left of the tangent
_A_ _B_. Finally, when the wind had a greater
speed than the aeroplane, as in Fig. 4, the machine could
move only in directions limited by the tangents _A_ _C_
and _A_ _D_.

Matter of Fuel Consumption.

Taking the case in which the wind had a speed equal
to half that of the aeroplane, Mr. Lanchester said that
for a given journey out and home, down wind and back,
the aeroplane would require 30 per cent more fuel than
if the trip were made in still air; while if the journey
was made at right angles to the direction of the wind
the fuel needed would be 15 per cent more than in a
calm. This 30 per cent extra was quite a heavy enough
addition to the fuel; and to secure even this figure it
was necessary that the aeroplane should have a speed of
twice that of the maximum wind in which it was desired
to operate the machine. Again, as stated in the last
lecture, to insure the automatic stability of the machine
it was necessary that the aeroplane speed should be
largely in excess of that of the gusts of wind liable to
be encountered.

Eccentricities of the Wind.

There was, Mr. Lanchester said, a loose connection
between the average velocity of the wind and the maximum
speed of the gusts. When the average speed of
the wind was 40 miles per hour, that of the gusts might
be equal or more. At one moment there might be a
calm or the direction of the wind even reversed, followed,
the next moment, by a violent gust. About the same
minimum speed was desirable for security against gusts
as was demanded by other considerations. Sixty miles
an hour was the least figure desirable in an aeroplane,
and this should be exceeded as much as possible. Actually,
the Wright machine had a speed of 38 miles per
hour, while Farman's Voisin machine flew at 45 miles
per hour.

Both machines were extremely sensitive to high winds,
and the speaker, in spite of newspaper reports to the
contrary, had never seen either flown in more than a
gentle breeze. The damping out of the oscillations of
the flight path, discussed in the last lecture, increased
with the fourth power of the natural velocity of flight,
and rapid damping formed the easiest, and sometimes
the only, defense against dangerous oscillations. A
machine just stable at 35 miles per hour would have
reasonably rapid damping if its speed were increased to
60 miles per hour.

Thinks Use Is Limited.

It was, the lecturer proceeded, inconceivable that any
very extended use should be made of the aeroplane unless
the speed was much greater than that of the motor car.
It might in special cases be of service, apart from this
increase of speed, as in the exploration of countries
destitute of roads, but it would have no general utility.
With an automobile averaging 25 to 35 miles per hour,
almost any part of Europe, Russia excepted, was attainable
in a day's journey. A flying machine of but
equal speed would have no advantages, but if the speed
could be raised to 90 or 100 miles per hour, the whole
continent of Europe would become a playground, every
part being within a daylight flight of Berlin. Further,
some marine craft now had speeds of 40 miles per hour,
and efficiently to follow up and report movements of
such vessels an aeroplane should travel at 60 miles per
hour at least. Hence from all points of view appeared
the imperative desirability of very high velocities of
flight. The difficulties of achievement were, however,
great.

Weight of Lightest Motors.

As shown in the first lecture of his course, the resistance
to motion was nearly independent of the velocity,
so that the total work done in transporting a given
weight was nearly constant. Hence the question of fuel
economy was not a bar to high velocities of flight, though
should these become excessive, the body resistance might
constitute a large proportion of the total. The horsepower
required varied as the velocity, so the factor governing
the maximum velocity of flight was the horsepower
that could be developed on a given weight. At
present the weight per horsepower of feather-weight
motors appeared to range from 2 1/4 pounds up to 7
pounds per brake horsepower, some actual figures being
as follows:

Antoinette........ 5 lbs.
Fiat.............. 3 lbs.
Gnome....... Under 3 lbs.
Metallurgic....... 8 lbs.
Renault........... 7 lbs.
Wright.............6 lbs.

Automobile engines, on the other hand, commonly
weighed 12 pounds to 13 pounds per brake horsepower.

For short flights fuel economy was of less importance
than a saving in the weight of the engine. For long
flights, however, the case was different. Thus, if the
gasolene consumption was 1/2 pound per horsepower hour,
and the engine weighed 3 pounds per brake horsepower,
the fuel needed for a six-hour flight would weigh as much
as the engine, but for half an hour's flight its weight
would be unimportant.

Best Means of Propulsion.

The best method of propulsion was by the screw,
which acting in air was subject to much the same conditions
as obtained in marine work. Its efficiency depended
on its diameter and pitch and on its position,
whether in front of or behind the body propelled. From
this theory of dynamic support, Mr. Lanchester proceeded,
the efficiency of each element of a screw propeller
could be represented by curves such as were given
in his first lecture before the society, and from these
curves the over-all efficiency of any proposed propeller
could be computed, by mere inspection, with a fair degree
of accuracy. These curves showed that the tips of
long-bladed propellers were inefficient, as was also the
portion of the blade near the root. In actual marine
practice the blade from boss to tip was commonly of
such a length that the over-all efficiency was 95 per cent
of that of the most efficient element of it.

Advocates Propellers in Rear.

From these curves the diameter and appropriate pitch
of a screw could be calculated, and the number of
revolutions was then fixed. Thus, for a speed of 80 feet
per second the pitch might come out as 8 feet, in which
case the revolutions would be 600 per minute, which
might, however, be too low for the motor. It was then
necessary either to gear down the propeller, as was done
in the Wright machine, or, if it was decided to drive it
direct, to sacrifice some of the efficiency of the propeller.
An analogous case arose in the application of the steam
turbine to the propulsion of cargo boats, a problem as
yet unsolved. The propeller should always be aft, so
that it could abstract energy from the wake current, and
also so that its wash was clear of the body propelled.
The best possible efficiency was about 70 per cent, and
it was safe to rely upon 66 per cent.

Benefits of Soaring Flight.

There was, Mr. Lanchester proceeded, some possibility
of the aeronaut reducing the power needed for transport
by his adopting the principle of soaring flight, as
exemplified by some birds. There were, he continued, two
different modes of soaring flight. In the one the bird
made use of the upward current of air often to be found
in the neighborhood of steep vertical cliffs. These cliffs
deflected the air upward long before it actually reached
the cliff, a whole region below being thus the seat of
an upward current. Darwin has noted that the condor
was only to be found in the neighborhood of such cliffs.
Along the south coast also the gulls made frequent use
of the up currents due to the nearly perpendicular chalk
cliffs along the shore.

In the tropics up currents were also caused by
temperature differences. Cumulus clouds, moreover, were
nearly always the terminations of such up currents of
heated air, which, on cooling by expansion in the upper
regions, deposited their moisture as fog. These clouds
might, perhaps, prove useful in the future in showing
the aeronaut where up currents were to he found. An-
other mode of soaring flight was that adopted by the
albatross, which took advantage of the fact that the air
moved in pulsations, into which the bird fitted itself,
being thus able to extract energy from the wind.
Whether it would be possible for the aeronaut to employ
a similar method must be left to the future to decide.

Main Difficulties in Aviation.

In practical flight difficulties arose in starting and in
alighting. There was a lower limit to the speed at
which the machine was stable, and it was inadvisable to
leave the ground till this limit was attained. Similarly,
in alighting it was inexpedient to reduce the speed below
the limit of stability. This fact constituted a difficulty
in the adoption of high speeds, since the length of run
needed increased in proportion to the square of the
velocity. This drawback could, however, be surmounted
by forming starting and alighting grounds of ample size.
He thought it quite likely in the future that such grounds
would be considered as essential to the flying machine
as a seaport was to an ocean-going steamer or as a road
was to the automobile.

Requisites of Flying Machine.

Flying machines were commonly divided into monoplanes
and biplanes, according as they had one or two
supporting surfaces. The distinction was not, however,
fundamental. To get the requisite strength some form
of girder framework was necessary, and it was a mere
question of convenience whether the supporting surface
was arranged along both the top and the bottom of this
girder, or along the bottom only. The framework adopted
universally was of wood braced by ties of pianoforte
wire, an arrangement giving the stiffness desired with
the least possible weight. Some kind of chassis was also
necessary.





Next: AMATEURS MAY USE WRIGHT PATENTS.

Previous: FLYING MACHINES VS. BALLOONS.



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