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 be 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 be found. Another 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.