PROBLEMS OF AERIAL FLIGHT.





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.





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