PRINCIPALLY RELATING TO SCREWS.
In 1887 I was approached by several wealthy gentlemen who asked me if I
thought it was possible to make a flying machine. I said, “certainly;
the domestic goose is able to fly and why should not man be able to do
as well as a goose?” They then asked me what it would cost and how long
it would take, and, without a moment’s hesitation, I said it would
require my undivided attention for five years and might cost £100,000. A
great deal of experimenting would be necessary; the first three years
would be devoted to developing an internal combustion engine of the
Brayton or Otto type, and the next two years to experimenting with
aeroplanes and screws and building a machine. Even at that time I had a
clear idea of the system that would be the best. However, nothing was
then done, but in 1889 I employed for the purpose two very skilful
American mechanics, and put them to work at Baldwyn’s Park, Kent. At
that time the petroleum motor had not been reduced to its present degree
of efficiency and lightness; it was not suitable for a flying machine,
and I saw that it would require a lot of experimental work in order to
develop it. After taking into consideration all the facts of the case, I
decided to use a steam engine. Had I been able to obtain the light and
efficient motors which have been recently developed, thanks to the
builders of racing cars, I should not have had to experiment at all with
engines and boilers, as I could have obtained the necessary motors at
once. As it was, I was obliged to content myself with the steam engine.
[Illustration: Fig. 9.--Group of screws and other objects used in my
experiments.]
[Illustration: Fig. 10.--Some of the principal screws experimented
with--_h_, a screw with very thick blades, and _g_, a screw made after a
French model.]
I found that there was a great deal of misunderstanding regarding the
action of aeroplanes, and also of screws working in the air. I procured
all the literature available on the subject, both English and French,
and attempted to make a thorough study of the question; but I was not
satisfied, on account of the wide difference in the views of the writers
and the conflicting formulæ that were employed. I therefore decided to
make experiments myself, and to ascertain what could be done without
the use of anybody’s formula. Although this was nearly twenty years ago,
I find that there is still a great deal of discussion regarding the
action of aeroplanes and screws, in which the majority taking part in
the discussion are in the wrong. However, several good works on the
subject have recently been published.
[Illustration: Fig. 11.--The three best screws. The screw on the right
has a uniform pitch throughout, the middle screw has increasing pitch,
and the left screw compound increasing pitch.]
Having designed and put my boiler and engine in hand, I commenced a
series of experiments for the purpose of ascertaining the efficiency of
screw propellers working in the air, and the form and size that would be
best for my proposed machine. The illustration Fig. 9 shows a
photographic group of the screws and other objects with which I
experimented. Fig. 10 shows some of the leading types which, as will be
seen, have blades of different shape, pitch, and size. Fig. 11 shows
three of the best screws employed. It will be observed that one has
uniform pitch, another increasing pitch, and the third compound
increasing pitch. In order to test the efficiency of my screws I made
the apparatus shown in Fig. 12. The power for running the screw was
transmitted by means of a belt to the straight cylindrical pulley _c_,
_c_. Shaft _b_, _b_ was of steel, rather small in diameter, and ran
smoothly, and practically without friction, through the two bearings
_d_, _d_. When the first screw, _a_, _a_, was run at a high velocity,
the axial thrust pushed the shaft _b_, _b_ back and elongated the spiral
spring _e_. The degree of screw thrust was indicated in pounds by the
pointer _g_. The power was transmitted through a very accurate and
sensitive dynamometer, so that the amount consumed could be easily
observed by a pointer similar to the one employed for indicating the
screw thrust. A tachometer was also employed to observe the number of
turns that the screw was making in a minute. The whole apparatus was
carefully and accurately made and worked exceedingly well. I was thus
enabled, with my various forms of screws and other objects, to make very
accurate measurements, some of which are exceedingly interesting.
[Illustration: Fig. 12.--Apparatus for testing the thrust of
screws--_a_, _a_, the screw; _b_, _b_, shaft sliding freely in the
bearing _d_, _d_; _c_, cylindrical pulley; _e_, spiral spring; _f_,
steel rod; _g_, pointer for indicating the thrust in pounds.]
[Illustration: Fig. 13.--Apparatus for testing the direction of air
currents caused by a rapidly rotating screw. Silken threads were
attached to the wire _c_, _c_, which indicated clearly the direction in
which the air was moving.]
In many of the treatises and books of that time it was stated that a
screw propeller, working in the air, was exceedingly wasteful of energy
on account of producing a fan-blower action. Some inventors suggested
that the screw should work in a stationary cylinder, or, better still,
that the whole screw should be encased in a rotating cylinder, to
prevent this outward motion of the air. In order to ascertain what the
actual facts were, I attached a large number of red silk threads to a
brass wire, which I placed completely around my screw (see Fig. 13).
Upon starting up I found that, instead of the air being blown out at the
periphery of the screw, it was in reality sucked in, as will be seen in
the illustration. I was rather surprised to see how sharp a line of
demarkation there was between the air that was moving in the direction
of the screw and the air that was moving in the opposite direction. The
screw employed in these experiments was 18 inches in diameter and had a
pitch of 24 inches. It was evident, however, if the pitch of the screw
was coarse enough that there would be a fan-blower action. I therefore
tried screws of various degrees of pitch, and found when the pitch was a
little more than three times the diameter, giving to the outer end of
the blade an angle of 45°, that a fan-blower action was produced--that
is, part of the time when the screw was running, the air would
alternate; sometimes it would pass inwards at the periphery and
sometimes outwards. The change of direction, however, was always
indicated by a difference in the pitch of the note given out, and also
by the thrust. In Fig. 14 I have shown the extremities of the blades of
some of the different forms of screws experimented with, in which _a_
shows a plain screw, the front side being straight and of equal pitch
from the periphery to the hub; _b_ is a screw of practically the same
pitch, but slightly curved so as to give what is known as an increasing
pitch; _c_ shows the extremity of a screw in which the curve is not the
same throughout--that is, it is what is known as a compound increasing
pitch; _d_ is the shape of the screw that gave the angle of 45° above
referred to.
[Illustration: Fig. 14.--This drawing shows the ends of screw blades in
which _a_ is a plain screw; _b_, screw with increasing pitch; _c_, screw
with compound increasing pitch; _d_, end of screw blade 45°; _e_, screw
with very thick blade; _f_, blade with no pitch at all; _g_, blade which
gave a thrust in the direction of the convex side, no matter in which
direction it was revolved; _h_, screw said to have been used in the
French Government experiments.]
The first screw experimented with was _a_. This screw was run at a high
velocity--about 2,500 revolutions per minute--until a screw thrust of 14
lbs. was obtained, and then the governor of the engine was set so that
all screws of the same diameter could be run at the same speed. Wishing
to ascertain the efficiency of the screw and how much was lost in skin
friction, I multiplied the thrust in pounds by the pitch of the screw in
feet and by the number of turns it was making in a minute. This, of
course, gave the exact number of foot-pounds in energy that was being
imparted to the air. I was somewhat surprised to find that it
corresponded exactly with the readings of the dynamometer. I thought at
first that I must have made some mistake. Again I went very carefully
over all the figures, tested everything, and made another experiment and
found, even if I changed the number of revolutions, that the readings of
the dynamometer were always exactly the same as the energy imparted to
the air. This seemed to indicate that the screw was working very well
and that the skin friction must be very small indeed. In order to test
this, I made what we will call, for the moment, a screw without any
pitch at all--that is, the blades were of wood and of the exact
thickness and width of the blades of the screw _a_, but without any
pitch at all. The extremity of the blade is shown at _f_. I placed this
screw on my machine in place of _a_, and although my dynamometer was so
sensitive that the pointer would move away from the zero pin by simply
touching the tip of the finger to the shaft, it failed to indicate, and
thus the screw appeared to consume no power at all. These experiments
were repeated a considerable number of times. I then obtained a sheet of
tin the same diameter as the screws, 18 inches, and upon running it at
the same speed, I found that it did consume a measurable amount of
power, certainly more than the two blades _f_. This no doubt was due to
the uneven surface of the tin. Had it been a well-made saw blade without
teeth, perfectly smooth and true on both sides, it probably would not
have required power enough to have shown on the dynamometer. However, it
is quite possible that there is a little more skin friction with a
polished metallic surface, than with a piece of smooth evenly lacquered
wood. The screws which I employed were of American white pine such as
used by patternmakers. This wood was free from blemishes of all kind,
extremely light, uniform, and strong. When the screw had been formed, it
was varnished on both sides with a solution of hot glue, which greatly
increased the strength of the wood crosswise of the grain. When this
glue was thoroughly dry, the wood was sand-papered until it was as
smooth as glass; the whole thing was then carefully varnished with
shellac, rubbed down again and revarnished with very thin shellac
something like lacquer. In this way the surface of the screw was made
very smooth. The screws, of course, were made with a great degree of
accuracy and as free as possible from any unevenness. Having tested
screw _a_, I next tested screw _b_. I found with the same number of
revolutions per minute that this screw produced more thrust, but it
required more power to run it, and when the energy imparted to the air
was compared with the readings of the dynamometer, it was found that it
did not do quite so well as _a_; still as the thrust was greater and the
efficiency only slightly less, it appeared to be the better screw. Upon
trying screw _c_, under the same conditions, the thrust was very much
increased, but the power required was also increased to a still greater
degree, showing that this form was not so favourable as either _a_ or
_b_. All the screws experimented with had very thin blades, and it
occurred to me that the difference between _a_ and _b_ might arise from
the fact that, when _a_ was running at a very high velocity, the working
side instead of being flat might have become convex to a slight extent,
whereas with _b_, a slight bending back of the edges of the blade would
still leave the working side concave. I therefore made the screw shown
at _e_, which had the same pitch as the other three, but the working
side was of the same shape as _a_. Of course the additional thickness of
the blades made it impossible to give an easy curve to the back.
Curiously enough I found that _e_ did nearly as well as _a_, and quite
as well as _b_. The additional thickness did not interfere to any
appreciable extent with its efficiency. I then made another propeller,
shown at _g_, which was of the same thickness in the middle as _e_. Upon
running this, I found that it required considerable power, and no matter
which way it was run, the thrust was always in the direction of the
convex side, which was quite the reverse from what one would have
naturally supposed.
[Illustration: Fig. 15.--The manner of building up the large screws.]
About the time that I was making these experiments, my duties called me
to Paris, and while there I called on my old friend Gaston Tissandier.
Through his influence I was permitted to see some models of the screws
that were alleged to have been used by Captain Renard in his experiments
for the French Government, and I was somewhat surprised to find the form
of the blades, the same as shown at _h_, Fig. 14, and completely without
any twist. On my return to England, I made a screw of this description.
It is also shown in the photographic illustration, Fig. 9. Upon testing
this screw, I found that its efficiency was only 40 per cent. of that of
_a_--that is, the energy or acceleration imparted to the air was only 40
per cent. of the readings of the dynamometer. It then occurred to me
that this particular form of screw was probably the one that the French
had for exhibition purposes, but not the one they intended to use.
Having tried all the various forms of screws and other objects shown in
Fig. 9, I made some sheet metal screws; also a screw which consisted of
a steel frame covered with woven fabric, and which was identical with
screws that I had seen described in various works relating to aerial
navigation. It was found quite impossible to keep the fabric taut and
smooth, and the results were very bad indeed, it being only 40 per cent.
as efficient as a well-made wooden screw.
[Illustration: Fig. 16.--A fabric-covered screw with a very low
efficiency.]
Having thus ascertained the best form of a screw, I built up my first
large screws, which were 17 feet 10 inches in diameter, after the
well-known manner of making wooden patterns for casting steamship
propellers. Fig. 15 shows the form of the end of the blade, the middle
of the blade, and the hub. My first pair of large screws had a pitch of
24 feet, but these were too great a drag on the engine. I therefore made
another pair with 16 feet pitch which greatly increased the piston
speed, and permitted the engines to develop much more power; the screw
thrust was also increased just in an inverse ratio to the pitch of the
screws. Another pair of screws was tried with 14 feet pitch and 12 feet
in diameter, but these did not do so well. My large screws were made
with a great degree of accuracy; they were perfectly smooth and even on
both sides, the blades being thin and held in position by a strip of
rigid wood on the back of the blade. In order to prevent the thrust from
collapsing the blades, wires were extended backwards and attached to a
prolongation of the shaft. Like the small screws, they were made of the
very best kind of seasoned American white pine, and when finished were
varnished on both sides with hot glue. When this was thoroughly dry,
they were sand-papered again and made perfectly smooth and even. The
blades were then covered with strong Irish linen fabric of the smoothest
and best make. Glue was used for attaching the fabric, and when dry
another coat of glue was applied, the surface rubbed down again and then
painted with zinc white in the ordinary way and varnished. These screws
worked exceedingly well. I had means of ascertaining, with a great
degree of accuracy, the thrust of the screw, the number of turns per
minute, the speed of the machine, and, in fact, all the events that were
taking place on the machine. It was found that when the screw thrust in
pounds was multiplied by the pitch in feet, and by the number of
revolutions made in a minute of time, it exactly corresponded to the
power that the engines were developing, and that the amount of loss in
skin friction was so small as to be practically negligible.
[Illustration: Fig. 17.--The hub and one of the blades of the screw on
the Farman machine. The blade _c_, is a sheet of metal riveted to the
rod _b_, and forms a projection on the back of the blade which greatly
reduces its efficiency. The peculiar form of hub employed makes it
possible to change the diameter and pitch of this screw at will.]
In connection with this subject I would say that many experimenters
claim to have shown that the skin friction on screws is considerable, in
fact, so great as to be a very important factor in the equation of
flight. I am, however, of the opinion that these experimenters have not
had well-made screws. If the surface of the screw is uneven, irregular,
or rough, a considerable amount of energy is lost, as shown in the
French screw and the fabric covered screw. It is simply a question of
having a screw well-made. In those recently employed in France (see Fig.
17), the blades are of hammered sheet metal, the twist is not uniform or
true, and what is worst of all, the arm _b_ projects on the back of the
blade and offers a good deal of resistance to the air. This form of
screw, however, is very ingenious; as will be seen by the drawing, the
pitch and diameter can be changed at will. It is, however, heavy,
wasteful of power, and altogether too small for the work it has to do.
The skin friction of screws in a steamship has led inventors to suppose
that the same laws relate to screws running in air, but such is by no
means the case. In designing a steamship, we have to make a compromise
in regard to the size of the screw. If the screw is too small, an
increase in diameter is, of course, an advantage, and it may also be an
advantage, not only to increase the diameter, but also to reduce the
pitch; however, a point is soon reached where the skin friction will
more than neutralise the advantages of engaging a larger volume of
water. This is because the water adheres to the surface; in fact, the
skin friction of a ship and its screw consumes fully 80 per cent. of the
total power of the engines, but with an air propeller its surface is not
wetted and the air does not stick to its surface. If made of polished
wood, the friction is so extremely small as to be almost unmeasurable.
The diameter of a well-made screw running in air is therefore not
limited in any degree by skin friction, as is the case with a screw
running in water; in fact, it is rather a question of its weight, and
its efficiency ought to increase in direct ratio with its diameter,
because the area of the disc increases with the square of the diameter.
The screw slip is therefore reduced by one-half by simply doubling the
diameter of the screw. It will be understood that by doubling the
diameter of the screw, four times as much air will be engaged. If we
push this back at half the speed, we shall have the same screw thrust,
because the resistance of the air is in proportion to the square of the
velocity that we impart to it, so that one just balances the other, and
the diminution of wasteful slip is just in proportion to the increase in
diameter. In all cases, the screw should be made as large as possible.
[Illustration: Fig. 18.--Section of screw blades having radial edges.
With screws of this form, the blades, of course, become narrower as the
hub is approached, and if it is a true screw and the edges radial, the
sine of the angle will be the same at all points. It is 2 inches in this
case.]
[Illustration: Fig. 19.--Shows the form of the blade of a screw
propeller made of sheet metal. It is riveted at the edges and also to
the arm of a screw with a stiffening piece at the extreme end. However,
it is not necessary to rivet edges together. They may be welded with a
name of acetylene oxygen gases.]
[Illustration: Fig. 19_a_.--Shows the manner of welding and the finished
edge.]
[Illustration: Fig. 20.--A new form of hub, of great strength and
lightness, for use on flying machines.]
In the drawing (Fig. 18) I have shown screw blades of a proper shape to
give the best results--that is, providing a metallic screw is employed.
Instead of having the arm of the screw on the back of the blade to offer
resistance to the air, the arm should be tubular, flattened, and covered
on both sides with sheet metal. This particular formation not only
prevents the air from striking the arm, but, at the same time, prevents
the pressure of the air from deforming the blade, so, if a metallic
screw is to be used, the form of blade which I have shown will be found
much superior to that employed at the present time on continental flying
machines. We should not lose sight of the fact that weight tells very
seriously against the success of a flying machine, and that no expense
should be spared to reduce the weight, providing that it is possible to
do so without reducing the factor of safety. Suppose, for example, that
we use an ordinary hub secured to a solid shaft by a common key. All the
parts have to be made heavy in order to be sufficiently strong to
withstand the strain. In the drawing (Fig. 20) I have shown a hub which
I think is quite as light and strong as it is possible to make it. The
action of the motor is often spasmodic and puts very great strain upon
the parts, and there is a very strong tendency for the shaft to turn
round in the hub. If a key is used, the hub has to be large and strong,
and the key of considerable size, otherwise the parts would be deformed.
In my own experiments, I have found considerable difficulty in securing
a shaft to wooden screws. However, it will be seen in the drawings that
a series of grooves is cut in the shaft and that the hub has internal
projections, so that the one fits the other. This makes a very strong
connection and is of extreme lightness. Both the hub and the shaft
should be of tempered steel. The spokes should be hard drawn steel tubes
with long fine threads, so as to withstand centrifugal force. To prevent
them from rotating in the hub, the nuts _d_, _d_ are provided, which
compress the arms of the steel hub so as to grip the tube with any
degree of force required. It will be seen that with this system the
pitch of the screw may be adjusted to some extent; however, it is better
to have all parts of the screw, from hub to centre, of the same pitch. A
slight deviation from this is admissible in the experimental stage, so
long as the deviation from a true screw, caused by rotating the arm, is
not greater than one half of the slip while in flight.
Many experimenters have imagined that a screw is just as efficient
placed in front of a machine as at the rear, and it is quite probable
that, in the early days of steamships, a similar state of things
existed. For several years there were steamboats running on the Hudson
River, New York, with screws at their bows instead of at their stern.
Inventors of, and experimenters with, flying machines are not at all
agreed by any means in regard to the best position for the screw. It
would appear that many, having noticed that a horse-propelled carriage
always has the horse attached to the front, and that the carriage is
drawn instead of pushed, have come to the conclusion that, in a flying
machine, the screw ought, in the very nature of things, to be attached
to the front of the machine, so as to draw it through the air. Railway
trains have their propelling power in front, and why should it not be
the same with flying machines? But this is very bad reasoning. There is
but one place for the screw, and that is in the immediate wake, and in
the centre of the greatest atmospheric disturbance. While a machine is
running, although there is a marked difference between water and air as
far as skin friction is concerned, still the conditions are the same as
far as the _position_ of the screw is concerned. With a well-designed
steamship, the efficiency of the screw is so great as to be almost
unbelievable; in fact, if a steamship had never been made, and the
design of one should be placed before the leading mathematicians of
to-day, with the request that they should compute the efficiency of the
screw, none of them would come anywhere near the mark. They would make
it altogether too small. As before stated, when a steamship is being
driven through the water, the water adheres to its sides and is moved
forward by the ship--that is, it has acceleration imparted to it which
exactly corresponds to the power consumed in driving the ship through
the water. This, of course, retards it and we find in a well-designed
ship, not run above its natural speed, that about 80 per cent. of the
power of the engine is consumed in skin friction, or in imparting a
forward motion to the water. Suppose that we should take such a ship,
remove the screw, and tow it through the water with a very long wire
rope at a speed of, say, 20 miles an hour; we should find that the water
at the stern of the ship was moving forward at a velocity of fully 6
miles an hour--that is, travelling in the same direction as the ship. By
replacing the screw, and applying engine power sufficient to give the
ship the same speed of 20 miles an hour, identical results would be
produced. The skin friction still impels the water forward, so that the
screw, instead of running in stationary water, is actually running in
water moving in the same direction as the ship at a velocity of 6 miles
an hour. If the slip of the screw should only be equal to this forward
motion, the apparent slip would be nothing; in fact, the ship would be
moving just as fast as it would move if the screw were running in a
solid nut instead of in the yielding water. Curiously enough there have
been cases of negative slip in which the actual slip of the screw in the
water was less than the forward movement of the water, and in such cases
a ship is said to have negative slip. A very noticeable case of this
kind occurred in the Royal Navy in the sixties.[1] I was at the time
engaged in a large shipbuilding establishment in New York, and remember
distinctly the interest that the case created amongst the draughtsmen
and engineers of that establishment. Of course, this apparently
impossible phenomenon created a great deal of discussion on both sides
of the Atlantic. It appears that this ship had been built under an
Admiralty Specification which called for a screw of a certain diameter
and pitch with a specified number of revolutions per minute, and for a
certain number of knots per hour, also that the boiler pressure should
not go above a certain number of pounds per square inch. When the ship
was finished and went on her trial trip, it was found impossible to make
the full number of turns called for in the specification with the boiler
pressure allowable; nevertheless, the speed was greater than the
specification called for, and as speed was the desideratum, and not the
number of revolutions, the contractors thought their ship should be
accepted. Then arose a discussion as to the diameter and pitch of the
screw. It was claimed that a mistake must have occurred. A careful
measurement was made in the dry dock, and all was found correct. Once
more the ship was tried, and again her speed was in excess of the
specification, notwithstanding that it was still impossible to get the
specified number of revolutions per minute. Mathematicians then took the
matter in hand, and it was found that the ship actually travelled faster
than she would have done if the screw had been running in a solid nut.
Instead of a positive slip, the screw had in reality a negative slip;
but this was not believed at the time, and the discussion and
controversy continued. The ship was tried again and again, and always
with the same results. This apparently inexplicable phenomenon was
accounted for in the following manner:--The hull of the ship was said to
be rather imperfect and to cause a considerable drag in the water, so
that, when the ship was moving at full speed, the water at the stern had
imparted to it a forward velocity greater than the actual slip.
[1] The particulars relating to this event are taken from accounts
published at the time in American papers.
What is true of ships is true of flying machines. Good results can never
be obtained by placing the screw in front instead of in the rear of the
machine. If the screw is in front, the backwash strikes the machine and
certainly has a decidedly retarding action. The framework, motor, etc.,
offer a good deal of resistance to the passage of the air, and if the
air has already had imparted to it a backward motion, the resistance is
greatly increased. The framework will always require a considerable
amount of energy to drive it through the air, and the whole of this
energy is spent in imparting a forward motion to the air, so if we place
the propelling screw at the rear of the machine in the centre of the
greatest atmospheric resistance, it will recover a portion of the lost
energy, as in the steamship referred to. It will, therefore, be seen
that when the screw is at the rear, it is running in air which is
already moving forward with a considerable velocity, which reduces the
slip of the screw in a corresponding degree. I have made experiments
with a view of proving this, which I shall mention further on, and which
ought to leave no chance for future discussion.
[Illustration: Fig. 21.--Small apparatus for testing fabrics for
aeroplanes, the material being subjected to an air blast in order to
test its lifting effect as compared with its tendency to travel with the
blast.]
My first experiments had shown that wooden aeroplanes did much better
than any of the fabric covered aeroplanes that I was able to make at
that time, but as wood was quite out of the question on my large machine
on account of its weight, it was necessary for me to conduct
experiments with a view of ascertaining the relative values of different
fabrics. For this purposes, I made the little apparatus shown (Fig. 21).
This was connected to a fan blower driven by a steam engine having a
governor that worked directly on the point of cut-off. The speed was,
therefore, quite uniform and the blast of air practically constant. I
had a considerable number of little frames cut out of sheet steel, and
to these I attached various kinds of fabric, such as ordinary satin,
white silk, closely woven silk, linen, various kinds of woollen fabrics,
including some very coarse tweeds, also glass-paper, tracing linen, and
the best quality of Spencer’s balloon fabric. The blast of air was not
large enough to cover the whole surface of the aeroplanes, so that the
character of the back of the frames was of no account. The first object
experimented with was a smooth piece of tin. When this was placed at an
angle of 1 in 14, it was found that the drift or tendency to travel in
the direction of the blast was just one-fourteenth part of the upward
tendency, or lift. This was exactly as it should have been. Upon
changing the angle to 1 in 10, a similar thing occurred; the lift was
ten times the drift. I, therefore, considered the results obtained
with the sheet of tin as unity, and gave to every other material
experimented with, a coefficient of the unity thus established. Upon
testing a frame covered with tightly-drawn white silk, a considerable
amount of air passed through, and with an angle of 1 in 14, the lift was
only about double the drift. A piece of very open fabric, a species of
buckram, was next tried, and with this the lift and drift were about
equal. With closely-woven, shiny satin the coefficient was about ·80;
with a piece of ordinary sheeting the coefficient was ·90; with
closely-woven, rough tweeds, ·70; and with glass-paper about ·75. With a
piece of tracing linen very tightly drawn, results were obtained
identical with those of a sheet of tin, and with Spencer’s balloon
fabric the coefficient was about ·99. I, therefore, decided to cover my
aeroplanes with this material. It will be observed that the apparatus is
so arranged that both the lift and the drift can be easily measured.
[Illustration: Fig. 22.--Apparatus for testing the lifting effect of
aeroplanes and condensers in an air blast. _k_, _k_ show two aeroplanes
in position for being tested.]
[Illustration: Fig. 23.--Apparatus for testing aeroplanes, condensers,
etc., in an air blast. The opening is 3 feet square. Thin brass
sustainers are shown in position for testing.]
[Illustration: Fig. 24.--Cross-sections of bars of wood employed for
ascertaining the coefficient of different forms.]
[Illustration: Fig. 25.--Transverse sections of bars of wood
experimented with for the purpose of ascertaining their coefficients as
relates to a normal plane.]
[Illustration: Fig. 26.--A flat aeroplane placed at different angles.]
In order to ascertain the resistance encountered by various shaped
bodies driven at various speeds through the air, the best form of
aeroplanes, and the efficiency of atmospheric condensers, I made the
apparatus shown in Figs. 22 and 23. The smaller and straight portion of
this apparatus was 12 feet long and exactly 3 feet square inside, and
was connected as shown to a shorter box 4 feet square. Two strongly made
wooden screws _b_, _b_ and _d_, were attached to the same shaft. These
screws had two blades each, and while one pair of blades was in a
vertical position, the other was in a horizontal position. I interposed
between the screws, slats of thin wood arranged in the manner shown at
_d_, _d_; this was to prevent rotation of the air. At _e_ I placed
vertical slats of thin wood, and horizontal slats of the same size at
_f_. At _g_ two wide and thin boards, sharp at both edges and made in
the form of the letter X, were placed in the box as shown in section XY.
An engine of 100 H.P. with an automatic variable cut-off was employed
which gave to the screws a uniform rate of rotation, and as the engine
had no other work to do, the governor could be arranged to give varying
speeds such as were required for the experiments. The objects to be
tested were attached to the movable bars. In the drawing, the aeroplane
_k_, _k_ is shown in position for testing. This apparatus was provided
with a rather complicated set of levers, which permitted not only the
measurement of the lift of the objects experimented with, but also that
of the drift. The principle employed in this apparatus was a
modification of the ordinary weighing apparatus used by grocers, etc.
The first object tested was a bar of wood exactly 2 inches square shown
in Fig. 24. This was placed in such a manner that the wind struck
squarely against the side as shown in the drawing, and with a wind of 49
miles per hour, it was found that the drift or tendency to move with the
air was 5·16 lbs.; at the same time, the wind on my instrument gave a
pressure of 2 lbs. on a normal plane 6 inches square. The velocity of
the wind was ascertained by an anemometer of the best London make. Upon
turning the same bar of wood in the position shown at _b_, the drift
mounted to 5·47 lbs. A round bar of wood, 2 inches in diameter, shown at
_c_, gave a drift of 2·97 lbs. These experiments were repeated with a
wind velocity of 40 miles per hour, when it was found that the drift of
_a_ was 4·56 lbs., and that of the round bar, 2·80 lbs. It will be seen
from these experiments that the power required for driving bars or rods
through the air is considerably greater than one would have supposed.
The next object experimented with was _a_, Fig. 25. When this was
subject to a wind of 40 miles an hour, the drift was 0·78 lb. Upon
reversing this bar--that is, putting the thin edge instead of the thick
edge next to the wind--the drift mounted to 1·22 lbs.; _b_ showed a
drift of 0·28 lb. with the thick edge to the wind, and 0·42 lb. with the
thin edge to the wind; _c_ showed a drift of 0·23 lb. with the thick
edge to the wind, and 0·59 lb. with the thin edge to the wind; and _d_,
which was the same thickness as the others and 12 inches wide, both
edges being alike, showed a drift of only 0·19 lb. These experiments
show in a most conclusive manner the shapes that are most advantageous
to use in constructing the framework of flying machines. Aeroplane _e_,
Fig. 26, when placed on the machine in a horizontal position showed
neither lift nor drift, but upon placing it at an angle of 1 in 20, as
shown at _f_, the lift was 3·98 lbs. and the drift 0·30 lb. with a wind
velocity of 40 miles per hour. At this low angle the blade trembled
slightly. Upon placing the same plane at an angle of 1 in 16 as shown at
_g_, the lift was 4·59 lbs. and the drift 0·53 lb. It will be observed
that the underneath side of this plane is perfectly flat. The next
experiment was with planes slightly curved, as shown in Fig. 27. The
aeroplane _a_ was 16 inches wide, very thin, and only slightly curved.
When set at a very low angle, it vibrated so as to make the readings
very uncertain, but when set at an angle of 1 in 10 it lifted 9·94 lbs.
with a drift of 1·12 lbs. By slightly changing the angle it was made to
lift 10·34 lbs. with a drift of 1·23 lbs., the wind velocity being 41
miles per hour. Aeroplane _b_, 12 inches wide, Fig. 27, when placed at
an angle of 1 in 14 with an air blast of 41 miles per hour, gave a lift
of 5·28 lbs. with a drift of 0·44 lb.; at an angle of 1 in 12 the lift
was 5·82 lbs. and the drift 0·5 lb.; at an angle of 1 in 10 the lift was
6·75 lbs. and the drift 0·73 lb.; with an angle of 1 in 8 the lift was
7·75 lbs. and the drift 1 lb.; with an angle of 1 in 7 the lift was 8·5
lbs. and the drift 1·25 lbs.; at an angle of 1 in 6 the lift was 9·87
lbs. and the drift 1·71 lbs. Aeroplane _c_, Fig. 27, which had more
curvature than _b_, when run in a horizontal position, gave a
considerable lift, and when raised to an angle of 1 in 12 it gave a lift
of 6·12 lbs. with a drift of 0·54 lb. In another experiment at the same
angle, it gave a lift of 6·41 lbs. with a drift of 0·56 lb.; at an angle
of 1 in 16 it gave a lift of 5·47 lbs. with a drift of 0·37 lb.; at an
angle of 1 in 10 it gave a lift of 6·97 lbs. and a drift of 0·70 lb.; at
an angle of 1 in 8 it gave a lift of 8·22 lbs. with a drift of 1·08
lbs.; at an angle of 1 in 7 it gave a lift of 9·94 lbs. with a drift of
1·45 lbs.; at an angle of 1 in 6 it gave a lift of 10·34 lbs. and a
drift of 1·75 lbs. This plane was then carefully set so that both the
forward and aft edges were exactly the same height, and with a wind
blast of 41 miles per hour it gave a lift of 2·09 lbs. with a drift of
0·21 lb. It was then pitched 1 in 18 in the wrong direction, and at this
point, the lifting effect completely disappeared, while the drift was
practically nothing.
[Illustration: Fig. 27.--Group of aeroplanes used in experimental
research. Although shown the same size in the drawing, aeroplane _a_ was
16 inches wide, and _b_ and _c_, 12 inches wide.]
[Illustration: Fig. 28.--An 8-inch aeroplane which did very well. This
aeroplane gave decided lifting effect when the bottom side was placed
dead level, as shown at _a_.]
When the aeroplane _a_ (Fig. 28) was placed in a horizontal position,
and the apparatus carefully balanced, it showed at a wind velocity of 40
miles an hour, a lift of 1·56 lbs., and a drift of 0·08 lb.; at an angle
of 1 in 20, a lift of 3·62 lbs. and a drift of 0·21 lb.; at an angle of
1 in 16, a lift of 4·09 lbs. with a drift of 0·26 lb.; at an angle of 1
in 14, a lift of 4·5 lbs. and a drift of 0·33 lb.; at an angle of 1 in
12, a lift of 5 lbs. and a drift of 0·43 lb.; at an angle of 1 in 10, a
lift of 5·75 lbs. and a drift of 0·60 lb.; at an angle of 1 in 8, a lift
of 6·75 lbs. and a drift of 0·86 lb. The blast was then increased to a
velocity of 47·33 miles an hour, when it was found that the lift at an
angle of 1 in 16 was 5 lbs. and the drift 0·33 lb. It will be observed
that this aeroplane was only 8 inches wide, while the others were 12
inches or more. They were all rather more than 3 feet long, but the
width of the blast to which they were subjected was exactly 3 feet, and
they were placed as near to the end of the trunk as possible.
[Illustration: Fig. 29.--Resistance due to placing objects in close
proximity to each other.]
The next experiments were made with the view of ascertaining what effect
would be produced when objects were placed near to each other (see Fig.
29). Two bars of wood 2 inches thick, and shaped as shown in the
drawing, were placed on the machine and subjected to a blast of 41 miles
per hour; the drift at various distances from center to center was as
follows:--
24 inches centers, drift 6 ozs.
22 „ „ „ 6 „
20 „ „ „ 6 „
18 „ „ „ 6-1/8 „
16 „ „ „ 6-1/8 „
14 „ „ „ 6-1/4 „
12 „ „ „ 6-1/2 „
10 „ „ „ 7 „
8 „ „ „ 7-3/4 „
6 „ „ „ 8-1/2 „
4 „ „ „ 9-1/4 „
It will be seen by this that the various members constituting the frame
of a flying machine should not be placed in close proximity to each
other.
A bar of wood similar in shape to _d_ (Fig. 25), but being 9 inches wide
instead of 12 inches, was mounted in a wind blast of 41 miles an hour,
with the front edge 3·31 inches above the rear edge, and this showed a
lift of 7·08 lbs. and a drift of 3·23 lbs. When the angle was reduced to
2·31 inches, it gave a lift of 4·53 lbs. with a drift of 0·78 lb., and
with the angle reduced to 1·31 inches, the lift was 3·37 lbs. and the
drift 0·5 lb. It will, therefore, be seen that even objects rounded on
both sides give a very fair lift, and in designing the framework of
machines advantage should be taken of this knowledge. The bar of wood
_c_ (Fig. 25) was next experimented with. With the sharp edge to the
wind, and with the front edge 2 inches higher than the rear edge, the
lift was 2·54 lbs. and the drift 0·76 lb. By turning it about so that
the wind struck the thick edge, the lift was 4·45 lbs. and the drift
0·47 lb. This seemed rather remarkable, but, as it actually occurred, I
mention it for other people to speculate upon. It, however, indicates
that we should take advantage of all these peculiarities of the air in
constructing the framework of a machine, which in itself is extremely
important, as I find that a very large percentage of the energy derived
from the engines is consumed in forcing the framework through the air.
It is quite true that a certain amount of this energy may be recovered
by the screw, provided that the screw runs in the path occupied by the
framework. Still, it is much better that the framework should be so
constructed as to offer the least possible resistance to the air, and,
as far as possible, all should be made to give a lifting effect.
[Illustration: Fig. 30.--Cross-section of condenser tube, made in the
form of Philipps’ sustainers, in which _c_ is the steam passage.]
[Illustration: Fig. 31.--The grouping of condenser tubes, made in the
form of Philipps’ sustainers. This arrangement is very effective,
condenses the steam or cools the water, and gives a lifting effect at
the same time. The shape and arrangement of tubes shown at _b_, _b_,
although effective as a condenser, produce no lifting effect, but a
rather heavy drift.]
Having ascertained the lifting effect of wooden aeroplanes of various
forms and at varying velocities of the wind, and, also, the resistance
offered by various bodies driven through the air, I next turned my
attention to the question of condensation. I wished to recover as much
water as possible from my exhaust steam. I had already experimented with
Mr. Horatio Philipps’ sustainers, and I found that their lifting effect
was remarkable. A curious thing about these aeroplanes was that they
gave an appreciable lift when the front edge was rather lower than the
rear. I therefore determined to take advantage of this peculiar
phenomenon, and to make my condenser tubes as far as possible in the
shape of Mr. Philipps’ sustainers. Fig. 30 shows a section of one of
these tubes, in which _a_, _a_ is the top surface, _b_ a soldered joint,
and _c_ the steam space. These were mounted on a frame as shown at _a_
(Fig. 31). I had already found that bodies placed near to each other
offered an increased resistance to the air, but by placing these
sustainers in the manner shown this was avoided, as the air had
sufficient space to pass through without being either driven forward or
compressed. It was found by experiment that the arrangement of tubes or
sustainers, shown at _d_, _d_ (Fig. 31), was very efficient as a
condenser, but it gave a very heavy drift and no lifting effect at all;
whereas, on the other hand, the arrangement shown at _a_ was equally
efficient, and, at the same time, gave a decided lifting effect. When
twelve of these tubes or sustainers were placed at an angle of 1 in 12,
the lifting effect was 12·63 lbs. and the drift 2·06 lbs. It was found,
however, that a good deal of the drift was due to the wind getting at
the framework that was used for holding the sustainers in position. With
a wind velocity of 40 miles an hour and a temperature of 62° F., 2·25
lbs. of water were condensed in five minutes, and, while running, the
back edge of the sustainers was quite cool. At another trial of the same
arrangement under the same conditions, the lift was 11 lbs. and the
drift 2·63 lbs. It is quite possible on this occasion that the metal was
so extremely thin that the angles were not always maintained;
consequently, that no two readings would be alike. It was found at this
point that the belt was slipping, and a larger pulley was put on the
driving shaft of the screws; and under these conditions, with a wind of
49 miles per hour and an angle of 1 in 8, the lifting effect ran up to
14·87 lbs. with a drift of 2·44 lbs., and the condenser delivered 2·87
lbs. of water from dry steam in five minutes. The weight of metal in
this condenser was extremely small, the thickness being only about 1/500
of an inch. This condenser delivered the weight of the sustainers in
water every five minutes. They should, however, have been twice as
heavy. Cylinder oil was now introduced with the steam in order to
ascertain what effect this would have. After seven minutes’ steaming,
2·25 lbs. of water were condensed in five minutes. It will be seen from
these experiments that an atmospheric condenser, if properly
constructed, is fairly efficient. Roughly speaking, it requires 2,400
times as much air in volume as of water to use as a cooling agent. With
the steam engine condenser only a relatively small amount of water is
admitted, and this is found to be sufficient; but in an atmospheric
condenser working in the atmosphere, it must be as open as possible, so
that no air which has struck one heated surface can ever come in contact
with another.