Motors for flying machines must be light in weight, of great strength,
productive of extreme speed, and positively dependable in action.
It matters little as to the particular form, or whether air or water
cooled, so long as the four features named are secured. There are at
least a dozen such motors or engines now in use. All are of the gasolene
type, and all possess in greater or lesser degree the desired qualities.
Some of these motors are:
Renault--8-cylinder, air-cooled; 50 horse power; weight 374 pounds.
Fiat--8-cylinder, air-cooled; 50 horse power; weight 150 pounds.
Farcot--8-cylinder, air-cooled; from 30 to 100 horse power, according to
bore of cylinders; weight of smallest, 84 pounds.
R. E. P.--10-cylinder, air-cooled; 150 horse power; weight 215 pounds.
Gnome--7 and 14 cylinders, revolving type, air-cooled; 50 and 100 horse
power; weight 150 and 300 pounds.
Darracq--2 to 14 cylinders, water cooled; 30 to 200 horse power; weight
of smallest 100 pounds.
Wright--4-cylinder, water-cooled; 25 horse power; weight 200 pounds.
Antoinette--8 and 16-cylinder, water-cooled; 50 and 100 horse power;
weight 250 and 500 pounds.
E. N. V.--8-cylinder, water-cooled; from 30 to 80 horse power, according
to bore of cylinder; weight 150 to 400 pounds.
Curtiss--8-cylinder, water-cooled; 60 horse power; weight 300 pounds.
Average Weight Per Horse Power.
It will be noticed that the Gnome motor is unusually light, being about
three pounds to the horse power produced, as opposed to an average of 4
1/2 pounds per horse power in other makes. This result is secured by
the elimination of the fly-wheel, the engine itself revolving, thus
obtaining the same effect that would be produced by a fly-wheel. The
Farcot is even lighter, being considerably less than three pounds per
horse power, which is the nearest approach to the long-sought engine
equipment that will make possible a complete flying machine the total
weight of which will not exceed one pound per square foot of area.
How Lightness Is Secured.
Thus far foreign manufacturers are ahead of Americans in the production
of light-weight aerial motors, as is evidenced by the Gnome and Farcot
engines, both of which are of French make. Extreme lightness is made
possible by the use of fine, specially prepared steel for the cylinders,
thus permitting them to be much thinner than if ordinary forms of steel
were used. Another big saving in weight is made by substituting what are
known as "auto lubricating" alloys for bearings. These alloys are made
of a combination of aluminum and magnesium.
Still further gains are made in the use of alloy steel tubing instead of
solid rods, and also by the paring away of material wherever it can be
done without sacrificing strength. This plan, with the exclusive use of
the best grades of steel, regardless of cost, makes possible a marked
reduction in weight.
Multiplicity of Cylinders.
Strange as it may seem, multiplicity of cylinders does not always add
proportionate weight. Because a 4-cylinder motor weighs say 100 pounds,
it does not necessarily follow that an 8-cylinder equipment will weigh
200 pounds. The reason of this will be plain when it is understood
that many of the parts essential to a 4-cylinder motor will fill the
requirements of an 8-cylinder motor without enlargement or addition.
Neither does multiplying the cylinders always increase the horsepower
proportionately. If a 4-cylinder motor is rated at 25 horsepower it is
not safe to take it for granted that double the number of cylinders
will give 50 horsepower. Generally speaking, eight cylinders, the bore,
stroke and speed being the same, will give double the power that can be
obtained from four, but this does not always hold good. Just why this
exception should occur is not explainable by any accepted rule.
Horse Power and Speed.
Speed is an important requisite in a flying-machine motor, as the
velocity of the aeroplane is a vital factor in flotation. At
first thought, the propeller and similar adjuncts being equal, the
inexperienced mind would naturally argue that a 50-horsepower engine
should produce just double the speed of one of 25-horsepower. That
this is a fallacy is shown by actual performances. The Wrights, using a
25-horsepower motor, have made 44 miles an hour, while Bleriot, with a
50-horsepower motor, has a record of a short-distance flight at the rate
of 52 miles an hour. The fact is that, so far as speed is concerned,
much depends upon the velocity of the wind, the size and shape of the
aeroplane itself, and the size, shape and gearing of the propeller. The
stronger the wind is blowing the easier it will be for the aeroplane
to ascend, but at the same time the more difficult it will be to make
headway against the wind in a horizontal direction. With a strong head
wind, and proper engine force, your machine will progress to a certain
extent, but it will be at an angle. If the aviator desired to keep
on going upward this would be all right, but there is a limit to the
altitude which it is desirable to reach--from 100 to 500 feet for
experts--and after that it becomes a question of going straight ahead.
Great Waste of Power.
One thing is certain--even in the most efficient of modern aerial motors
there is a great loss of power between the two points of production and
effect. The Wright outfit, which is admittedly one of the most effective
in use, takes one horsepower of force for the raising and propulsion of
each 50 pounds of weight. This, for a 25-horsepower engine, would give
a maximum lifting capacity of 1250 pounds. It is doubtful if any of
the higher rated motors have greater efficiency. As an 8-cylinder motor
requires more fuel to operate than a 4-cylinder, it naturally follows
that it is more expensive to run than the smaller motor, and a normal
increase in capacity, taking actual performances as a criterion, is
lacking. In other words, what is the sense of using an 8-cylinder motor
when one of 4 cylinders is sufficient?
What the Propeller Does.
Much of the efficiency of the motor is due to the form and gearing of
the propeller. Here again, as in other vital parts of flying-machine
mechanism, we have a wide divergence of opinion as to the best form. A
fish makes progress through the water by using its fins and tail; a
bird makes its way through the air in a similar manner by the use of its
wings and tail. In both instances the motive power comes from the body
of the fish or bird.
In place of fins or wings the flying machine is equipped with a
propeller, the action of which is furnished by the engine. Fins and
wings have been tried, but they don't work.
While operating on the same general principle, aerial propellers are
much larger than those used on boats. This is because the boat
propeller has a denser, more substantial medium to work in (water), and
consequently can get a better "hold," and produce more propulsive force
than one of the same size revolving in the air. This necessitates the
aerial propellers being much larger than those employed for marine
purposes. Up to this point all aviators agree, but as to the best form
most of them differ.
Kinds of Propellers Used.
One of the most simple is that used by Curtiss. It consists of two
pear-shaped blades of laminated wood, each blade being 5 inches wide
at its extreme point, tapering slightly to the shaft connection. These
blades are joined at the engine shaft, in a direct line. The propeller
has a pitch of 5 feet, and weighs, complete, less than 10 pounds. The
length from end to end of the two blades is 6 1/2 feet.
Wright uses two wooden propellers, in the rear of his biplane, revolving
in opposite directions. Each propeller is two-bladed.
Bleriot also uses a two-blade wooden propeller, but it is placed in
front of his machine. The blades are each about 3 1/2 feet long and have
an acute "twist."
Santos-Dumont uses a two-blade wooden propeller, strikingly similar to
the Bleriot.
On the Antoinette monoplane, with which good records have been made, the
propeller consists of two spoon-shaped pieces of metal, joined at the
engine shaft in front, and with the concave surfaces facing the machine.
The propeller on the Voisin biplane is also of metal, consisting of two
aluminum blades connected by a forged steel arm.
Maximum thrust, or stress--exercise of the greatest air-displacing
force--is the object sought. This, according to experts, is best
obtained with a large propeller diameter and reasonably low speed. The
diameter is the distance from end to end of the blades, which on the
largest propellers ranges from 6 to 8 feet. The larger the blade surface
the greater will be the volume of air displaced, and, following this,
the greater will be the impulse which forces the aeroplane ahead. In all
centrifugal motion there is more or less tendency to disintegration in
the form of "flying off" from the center, and the larger the revolving
object is the stronger is this tendency. This is illustrated in the many
instances in which big grindstones and fly-wheels have burst from being
revolved too fast. To have a propeller break apart in the air would
jeopardize the life of the aviator, and to guard against this it has
been found best to make its revolving action comparatively slow.
Besides this the slow motion (it is only comparatively slow) gives the
atmosphere a chance to refill the area disturbed by one propeller blade,
and thus have a new surface for the next blade to act upon.
Placing of the Motor.
As on other points, aviators differ widely in their ideas as to the
proper position for the motor. Wright locates his on the lower plane,
midway between the front and rear edges, but considerably to one side of
the exact center. He then counter-balances the engine weight by placing
his seat far enough away in the opposite direction to preserve the
center of gravity. This leaves a space in the center between the motor
and the operator in which a passenger may be carried without disturbing
the equilibrium.
Bleriot, on the contrary, has his motor directly in front and preserves
the center of gravity by taking his seat well back, this, with the
weight of the aeroplane, acting as a counter-balance.
On the Curtiss machine the motor is in the rear, the forward seat of the
operator, and weight of the horizontal rudder and damping plane in front
equalizing the engine weight.
No Perfect Motor as Yet.
Engine makers in the United States, England, France and Germany are all
seeking to produce an ideal motor for aviation purposes. Many of the
productions are highly creditable, but it may be truthfully said that
none of them quite fill the bill as regards a combination of the minimum
of weight with the maximum of reliable maintained power. They are all,
in some respects, improvements upon those previously in use, but the
great end sought for has not been fully attained.
One of the motors thus produced was made by the French firm of Darracq
at the suggestion of Santos Dumont, and on lines laid down by him.
Santos Dumont wanted a 2-cylinder horizontal motor capable of developing
30 horsepower, and not exceeding 4 1/2 pounds per horsepower in weight.
There can be no question as to the ability and skill of the Darracq
people, or of their desire to produce a motor that would bring new
credit and prominence to the firm. Neither could anything radically
wrong be detected in the plans. But the motor, in at least one important
requirement, fell short of expectations.
It could not be depended upon to deliver an energy of 30 horsepower
continuously for any length of time. Its maximum power could be secured
only in "spurts."
This tends to show how hard it is to produce an ideal motor for aviation
purposes. Santos Dumont, of undoubted skill and experience as an
aviator, outlined definitely what he wanted; one of the greatest
designers in the business drew the plans, and the famous house of
Darracq bent its best energies to the production. But the desired end
was not fully attained.
Features of Darracq Motor.
Horizontal motors were practically abandoned some time ago in favor of
the vertical type, but Santos Dumont had a logical reason for reverting
to them. He wanted to secure a lower center of gravity than would be
possible with a vertical engine. Theoretically his idea was correct as
the horizontal motor lies flat, and therefore offers less resistance to
the wind, but it did not work out as desired.
At the same time it must be admitted that this Darracq motor is a marvel
of ingenuity and exquisite workmanship. The two cylinders, having a bore
of 5 1-10 inches and a stroke of 4 7-10 inches, are machined out of a
solid bar of steel until their weight is only 8 4-5 pounds complete.
The head is separate, carrying the seatings for the inlet and exhaust
valves, is screwed onto the cylinder, and then welded in position. A
copper water-jacket is fitted, and it is in this condition that the
weight of 8 4-5 pounds is obtained.
On long trips, especially in regions where gasolene is hard to get,
the weight of the fuel supply is an important feature in aviation. As a
natural consequence flying machine operators favor the motor of greatest
economy in gasolene consumption, provided it gives the necessary power.
An American inventor, Ramsey by name, is working on a motor which
is said to possess great possibilities in this line. Its distinctive
features include a connecting rod much shorter than usual, and a crank
shaft located the length of the crank from the central axis of the
cylinder. This has the effect of increasing the piston stroke, and also
of increasing the proportion of the crank circle during which effective
pressure is applied to the crank.
Making the connecting rod shorter and leaving the crank mechanism the
same would introduce excessive cylinder friction. This Ramsey overcomes
by the location of his crank shaft. The effect of the long piston stroke
thus secured, is to increase the expansion of the gases, which in turn
increases the power of the engine without increasing the amount of fuel
used.
Propeller Thrust Important.
There is one great principle in flying machine propulsion which must
not be overlooked. No matter how powerful the engine may be unless the
propeller thrust more than overcomes the wind pressure there can be no
progress forward. Should the force of this propeller thrust and that of
the wind pressure be equal the result is obvious. The machine is at a
stand-still so far as forward progress is concerned and is deprived of
the essential advancing movement.
Speed not only furnishes sustentation for the airship, but adds to the
stability of the machine. An aeroplane which may be jerky and uncertain
in its movements, so far as equilibrium is concerned, when moving at
a slow gait, will readily maintain an even keel when the speed is
increased.
Designs for Propeller Blades.
It is the object of all men who design propellers to obtain the maximum
of thrust with the minimum expenditure of engine energy. With this
purpose in view many peculiar forms of propeller blades have been
evolved. In theory it would seem that the best effects could be secured
with blades so shaped as to present a thin (or cutting) edge when they
come out of the wind, and then at the climax of displacement afford a
maximum of surface so as to displace as much air as possible. While
this is the form most generally favored there are others in successful
operation.
There is also wide difference in opinion as to the equipment of the
propeller shaft with two or more blades. Some aviators use two and some
four. All have more or less success. As a mathematical proposition it
would seem that four blades should give more propulsive force than two,
but here again comes in one of the puzzles of aviation, as this result
is not always obtained.
Difference in Propeller Efficiency.
That there is a great difference in propeller efficiency is made readily
apparent by the comparison of effects produced in two leading makes of
machines--the Wright and the Voisin.
In the former a weight of from 1,100 to 1,200 pounds is sustained and
advance progress made at the rate of 40 miles an hour and more, with
half the engine speed of a 25 horse-power motor. This would be a
sustaining capacity of 48 pounds per horsepower. But the actual capacity
of the Wright machine, as already stated, is 50 pounds per horsepower.
The Voisin machine, with aviator, weighs about 1,370 pounds, and is
operated with a so-horsepower motor. Allowing it the same speed as the
Wright we find that, with double the engine energy, the lifting capacity
is only 27 1/2 pounds per horsepower. To what shall we charge this
remarkable difference? The surface of the planes is exactly the same in
both machines so there is no advantage in the matter of supporting area.
Comparison of Two Designs.
On the Wright machine two wooden propellers of two blades each (each
blade having a decided "twist") are used. As one 25 horsepower motor
drives both propellers the engine energy amounts to just one-half of
this for each, or 12 1/2 horsepower. And this energy is utilized at
one-half the normal engine speed.
On the Voisin a radically different system is employed. Here we have
one metal two-bladed propeller with a very slight "twist" to the blade
surfaces. The full energy of a 50-horsepower motor is utilized.
Experts Fail to Agree.
Why should there be such a marked difference in the results obtained?
Who knows? Some experts maintain that it is because there are two
propellers on the Wright machine and only one on the Voisin, and
consequently double the propulsive power is exerted. But this is not a
fair deduction, unless both propellers are of the same size. Propulsive
power depends upon the amount of air displaced, and the energy put into
the thrust which displaces the air.
Other experts argue that the difference in results may be traced to the
difference in blade design, especially in the matter of "twist."
The fact is that propeller results depend largely upon the nature of
the aeroplanes on which they are used. A propeller, for instance,
which gives excellent results on one type of aeroplane, will not work
satisfactorily on another.
There are some features, however, which may be safely adopted in
propeller selection. These are: As extensive a diameter as possible;
blade area 10 to 15 per cent of the area swept; pitch four-fifths of
the diameter; rotation slow. The maximum of thrust effort will be thus
obtained.