How can I find the center of pressure of my model aeroplane? The
simplest plan is to adjust the planes so that the model flies on a
perfectly even keel, and then balance the machine. Since the center of
pressure and the center of gravity must coincide to produce horizontal
flight, this point of balance will be the center of pressure. As a rule,
the center of pressure will be found to be near the front edge of the
main plane, perhaps slightly back of this forward edge.
What is the best position for the propeller?
In the case of a model driven by a single motor, the propeller shaft
should pass through the center of gravity and center of pressure of the
machine. It is very important that the alignment should be perfect or
you will have great difficulty in securing a horizontal flight. In case
of a double propeller machine, the line midway between the two propeller
shafts must pass between the center of gravity.
My aeroplane starts off all right but often begins to rock from side to
side like a cradle and then flutters to the ground. How can I prevent
this?
[Illustration: A beautiful model built by Stewart Easter]
[Illustration: A successful model of 1910 built by E. G. Halpine. Note
contrast in plane area]
This "rocking" motion is probably caused by a too low center of gravity
in your machine. If your planes are set at a dihedral angle, they should
be brought nearer to the horizontal. Another plan is to place them lower
down. A side gust of wind will start this rocking or vibration, and in
order to right itself the machine loses momentum and falls. This
difficulty is seldom experienced with horizontal wings, especially if
their aspect ratio is high.
I have no trouble in getting my model off the ground, but it seems to
rise too fast and fall backward or uses up the power before it really
gets under way. Is the trouble with the propeller or the planes, for my
motor seems all right?
If your model rises quickly, the planes are probably well designed and
built, the motor is effective, and the propeller sufficient for all
needs. The trouble is likely to be with the angle of the planes. Perhaps
your front plane is elevated too high. The rear plane should be kept
horizontal or very nearly so. An aileron at the rear of the main plane
will help you to properly adjust same. As a rule, the angle of ascent
should be a little more than that of the front plane. If it still acts
badly, strengthen your motor and bring your large plane back a trifle.
What is the best place to put the keel?
The greatest stability is usually gained by fixing the keel at the
center of gravity. There is great difference of opinion as to whether it
should be above or below. Try both ways. What is stability for one
aeroplane may mean disaster for another.
Why should an aeroplane pitch and roll when rising or even traveling
horizontally, but sail in a bee line when it begins to descend?
The stability of a model is greater when its course is downward than
when in a horizontal flight. In this position, the center of pressure is
much less likely to shift, and the pole of gravity has rather a
steadying effect. For this reason a glider thrown from an elevation
travels much more evenly than a power-driven machine. Try reversing your
planes.
Why is it that my aeroplane sails all right, but when I increase the
power of my motor it falls off and darts about until it reaches the
ground?
It has been found in experiments of man-carrying machines that an
increase in the speed of a flight would often render an ordinarily
steady machine unstable. It is argued by some authorities that the air
is churned up, as it were, by the forward planes, and that the rear
wing, therefore, rocks just as will a boat in rough water. If your model
flies well at a certain speed, you had better stick to the motor, and
try a larger plane.
How far apart shall I place the two wings of my monoplane?
There is no general rule possible. Some designers believe that the two
planes should be separated by a distance equal to four times the width
of the main plane. An excellent model which makes long flights is
illustrated herewith, in which the distance is equal to nearly twenty
times the width of the main plane, while another successful model has an
open space of only twice the width of the main plane. Much depends, of
course, upon the speed at which the aeroplane flies. The wings may be
much further apart on a high-speed than on a slow-flying model.
Are there any simple equations for working out the relation of the
proportion of the size and weight of model aeroplanes?
There are many such equations, but none of them are simple. The formula
which obliges one to solve a complicated problem in algebra or calculus
to know where to cut off a stick is obviously absurd in the case of
small models. These formulas which may be found in technical books on
aviation seem to savor too much of certain school books to be brought in
for the builder of model aeroplanes. A good rule to remember is that the
thrust should equal the weight of the machine.
Some of the large aeroplanes are driven by propellers made up of four
blades. Would you try this arrangement on a model aeroplane?
By no means. As explained in the chapter on propellers, the work
performed by your propeller blades is entirely different from that of an
electric fan. It is true that fairly efficient propellers made up of
four blades have been used on some passenger-carrying aeroplanes. This
form might prove effective on a model aeroplane, but increased weight is
prohibitive. By carefully designing your two blades, you can get equally
as much work out of them and at the same time reduce the weight by
one-half. It is argued by some authorities that the single blades act
upon undisturbed air and are therefore more efficient than a four-blade
propeller, as the air is always churned up.
How can I calculate the speed of my machine from the size and pitch of
my propeller?
It is practically impossible to do so. If you multiply the pitch of a
propeller by the number of turns at a given time, you will arrive
theoretically at the speed.
In practice, however, the slip of an aerial propeller is from
twenty-five to forty per cent. It is very difficult to determine just
what the slip is. Your calculation is likely to be a matter of thirty
per cent off and is of little value.
I am troubled with my machine trembling a great deal during flight. What
can I do to make it steady?
The framework of your model is probably too light for your motor.
Strengthen your machine if it will stand it, or take off some strands of
your motor, if it will stand it. The frame may be strengthened by using
wire braces. This method so commonly used last year is being abandoned
by the successful builders, as it makes it necessary to attach struts at
right angles to the frame, which add to the weight and resistance. The
wire in vibrating also offers an appreciable amount of resistance to the
air. In a large passenger-carrying machine, these wire braces are
absolutely necessary, but the model being so much smaller it is better
to make the frame heavy enough to remain rigid. A great deal of extra
work and annoyance is saved by doing away with wires. This trembling
again may be caused by your propeller not being properly balanced.
What is the lightest metal I can use?
Aluminum has less specific gravity than any metal now available. Its
cheapness also makes it practicable. Magnalium is a trifle heavier than
aluminum but considerably stronger, and is preferred by many model
builders. There are still others who prefer steel to either of these.
Steel is three times as heavy as aluminum and about five times as
strong. If you can get the metal in the proper size for model aeroplane
building, steel is probably the lightest after all. Some model builders
have great success with umbrella ribs, which is probably the most
available form of steel for our purpose. As most of us have discovered,
a steel bicycle spoke is by far the best axle for propellers.
Should the propeller be in front or behind the machine?
It will require some years of experimenting to answer this question
definitely. On one hand it is argued that the forward propeller, or
tractor as it is called, works in undisturbed air and is therefore more
efficient. Most of the large passenger-carrying aeroplanes are driven by
tractors. Others argue that the propeller churns up the air and that the
planes would therefore balance themselves against the gusts thus set in
motion, which makes their flight unstable besides requiring additional
power. Practically all of the successful model aeroplanes this year are
driven by propellers.
What is the lightest practical model aeroplane?
The English aviators who excel in constructing very light models have
had great success with what are termed "one ouncers." Some of the models
of this type weigh even less than one ounce. The distance qualities of
these machines are marvelous. Mr. Burge Webb claims a record of 1,500
feet in a straight-away flight by one of his one-ounce models.
Can I make a model fly by turning the propellers in the same direction?
It is doubtful. In using twin propellers the blades should, of course,
revolve in opposite directions although they are wound up by turning
them away from the center. Be careful, of course, to mount the right and
left propeller in the proper position. Some aviators believe that the
propellers should be turned at the same time,—there are machines to do
this,—in order that the thrust may be exactly balanced.
What is the longest flight ever made by a model aeroplane?
Mr. Flemming Williams, the English expert, claims to have made a flight
of almost exactly one-half a mile. His machine, which is illustrated in
this volume, weighs ten ounces. It was launched by throwing it in the
air, and in making this record flight travelled with the wind.
What is the best weight for a model?
Here is a very difficult question. It depends entirely upon what you are
trying to accomplish. In England, where everything is sacrificed to the
distance qualities of the model aeroplane, the best models vary from
five to ten ounces in weight. In America, where much more is required of
a model aeroplane, since it must rise from the ground under its own
power and possess considerable automatic stability, the average weight
is much more.
What is the average speed of a model aeroplane in flight?
About twelve miles an hour. When sailing with the wind this speed may be
increased indefinitely. In sailing against the wind, a model aeroplane
may creep along while remaining almost stationary.
Will tandem propellers make my model swifter or steadier?
Little has been accomplished either in America or in England with the
tandem propeller. Several small French models have been flown in this
way, but without striking results. Theoretically, the torque of the
propellers balance one another in this position; the forward propeller
creates a considerable backwash, and the second propeller works at a
disadvantage. Further experiments may discover an advantage in this
arrangement
How hard should I throw my machine when starting it?
It is not a good plan to launch the machine at a higher speed than that
at which it travels under its own power. By increasing its speed, you
are likely to set up violent oscillation, and, as explained elsewhere, a
model becomes unstable with increased speed. It is better to start it
too slowly than too rapidly. In the case of a glider, of course, it is
well to throw it with all your might. This is a problem, incidentally,
which does not occur when the models rise by their own power.
My machine flies very well indeed, but I cannot make it start from the
ground. When I add more power it swerves to one side and will not fly
off. What would you advise?
[Illustration: Percy Pierce launching a prize-winning model]
[Illustration: Launching the sling-shot gliders]
This is a common experience. Our advice is to return to the original
motor and make the most of the successful flights. Your model may not be
adapted to rising at all, that is, it is too light to carry the motor
required to raise it from the ground. If your frame is made strong
enough, it may be able to stand the increase in power, if it vibrates
violently, as is probably the case with a heavier motor.
What is the best time to release the propellers, just as it starts or
before?
Try both ways. Some model builders allow their propellers to get under
way a second or so before the flight commences, and release them and
push the machine forward at the same instant. One amateur secures his
propellers by means of a thread which he breaks by touching it with a
match, but it is rather fanciful.
What is the best height for a model aeroplane to fly?
In the principal model tournaments held in New York the models were
seldom more than twenty feet from the ground at any time. Most model
builders have this altitude in mind in designing their machines. It
seems to be generally agreed that when a model rises higher than this
too much power is used up in gaining altitude and there is a
corresponding loss in the distance qualities. On the other hand in the
remarkable 1,600 foot flight made by Cecil Peoli in July, the model rose
to a height, it is believed, of more than 100 feet and gained
considerable distance in gliding down. After all it is a problem which
must be decided by the individual. Incidentally it is estimated that a
large man-carrying aeroplane on rising to an altitude of one mile can
safely soar with all power shut off for twelve miles. This means of
course that an aviator has the choice of landing anywhere within a
circle twenty-five miles in diameter, which gives him a rather wide
choice.
What is meant by "critical soaring speed?"
It is claimed by some aviators that every aeroplane has a certain speed
at which it flies best and beyond which it is unsafe to push it. If more
power is put on, they argue, it will only tend to send the machine up in
the air and will not increase its speed. This is probably true in a
measure of model aeroplanes as well. When you have found the speed at
which your aeroplane flies best do not change the number of rubber bands
of your motor or the number of turns in winding.
Do the tails of birds serve as rudders to guide them in flying?
It is an open question. Some aviators who have made very careful
observations of birds in flight deny that the tails have anything to do
with their direction. The theory is advanced by some scientists that the
birds used their heads to change their course, operating them as a
forward rudder, much as do some types of aeroplanes. They argue that
since a rudder is much more effective when placed forward the smaller
surface presented by the flat heads and necks of birds has more effect
than a comparatively large tail.
What is the relation of the wing surface to the weight and horse-power
of the biplane as compared with the monoplane?
The Wright biplane carries a trifle more than two pounds of weight for
each square foot of lifting surface. The Bleriot monoplane carries about
five pounds weight to every square foot of lifting surface. On the other
hand the Wright machine carries about forty pounds per horse-power,
whereas the Bleriot will only carry about twenty-seven pounds. These
figures do not work out for all biplanes or monoplanes but they indicate
broadly the relation between the two general types of aeroplanes.
What would be the effect of bending down the outer ends of the wings of
an aeroplane as the wings of some birds droop?
The experiment is well worth trying. It would seem that this angle would
give increased stability, although if the model were knocked off its
course it might increase the resistance considerably. An elaborate test
of this form has been made by an aeronaut named Weiss in England, who
believes he has gained automatic stability in this way. Mr. Weiss has
built and flown a number of aeroplanes, varying in size from one
weighing five pounds to one which carried a weight of 140 pounds, and
the tests are reported to have been very satisfactory.
What is meant by the phugoid path of a model aeroplane?
The line described by the machine in flight. Every aeroplane left to
itself flies in a series of waves swaying more or less up and down from
the horizontal. In a aeroplane under the control of an aviator this is
largely overcome by manipulating the rudders controlling the vertical
motion. Every model aeroplane or soaring machine has a phugoid path
peculiar to itself which is affected by the power of its motor, the form
of the wings, its ability to right itself, etc. It is obvious of course
that if the path of your model aeroplane is irregular the machine must
travel more slowly and its distance qualities are therefore reduced.
Is a variable wing surface an advantage and can it be applied to model
aeroplanes?
In theory at least there is a great advantage in the variable surface
plane. As yet but one aeroplane has been flown in which the pilot may
increase or cut down the spread of his wings at will. When such control
is possible the aviator may employ the maximum spread of the wings for
rising, for instance where it is needed, and then reduce their area when
aloft, thus gaining in speed qualities. At the present stage of the
development of the model aeroplane a variable wing does not seem
practicable, although it is reasonable to suppose that the improvement
will come in time. The control would of course have to be automatic,
which renders the problem rather complicated.
What is a vortex pack?
Literally a small cyclone or eddy of the air. The term is used by
aviators to describe a turbulent section of the atmosphere. It is very
common in flying, especially at low altitudes, to run into a very
turbulent eddy of air, such as is set in motion by high buildings or
deep valleys. They are extremely dangerous since they cause the
aeroplane to dip and roll about violently, and call for quick and
skilful handling of the aeroplane to keep it from being upset. The term
might be used to describe an eddy of air which brings a model aeroplane
to grief.
What form of propeller will give the highest efficiency?
It is impossible to lay down a hard and fast rule. Much depends upon the
form of the machine. The Voison propellers have been found in actual
practice to give only about forty per cent efficiency. This is said to
be partially due to the fact that their parts are held together by bolts
and projecting nuts which offer considerable skin friction. The Wright
propellers are believed to be the most efficient propellers for large
machines since their efficiency is about seventy per cent. The
accompanying photographs of the standard model aeroplane types in
America will show that there is great difference of opinion as to the
diameter and pitch of the screws.
What is meant by a variable speed machine?
Merely an aeroplane whose speed may be increased or reduced by degrees
at the will of the aviator. This will enable the pilot to use a slow
speed for rising, for instance — and on reaching a desired altitude
increase his speed. On encountering high winds, for instance, the speed
might be increased so that the aeroplane would cut through them almost
undisturbed. The variable speed aeroplane will doubtless soon make its
appearance. Many aviators expect that it will be the next great step in
the advancement of the science of aviation.
Has any model aeroplane been fitted with an automatic stability device
and what is it like?
No such equipment has attracted public attention up to the present
writing. An interesting method of securing automatic control has been
suggested by H. L. Twining, the well-known writer on aviation. His plan
is to attach a geared wheel to the propeller shaft in such a way that it
will not begin to move until the propeller has made about 100
revolutions. A string is then run about this wheel which is pulled back
as it turns. The pull of this string in turn is made to raise or lower
the horizontal rudder of the model, but only after the machine
propellers have made 100 turns and the model is presumably well up in
the air. In this way the propeller may be set to send the model upwards
at a sharp angle and then made to take the proper angle for a horizontal
flight. The attachment suggests very interesting possibilities. It may
be possible, if the device works, to alter the angle of the rudders
either vertical or horizontal several times during a flight.
What is the record flight for a motor-driven model aeroplane?
A flight of upwards of one mile is reported to have been made by a motor
driven model in India, while in the United States the mile mark is
claimed to have been passed. Neither of these flights are official. The
most advanced work with motor-driven models is at present being done in
France. At a recent model tournament at the Velodrome du Pare des
Princess, Paris, a number of model aeroplanes equipped with engines of
various types were flown. One of these, a biplane measuring nearly seven
feet in length, was fitted with a petroleum two-cylinder motor which
developed one-third of a horse-power. It rose beautifully, cleared a
high building and was flying well when it unfortunately collided with
some telegraph wires and came to grief. Before the accident it had flown
nearly three hundred feet in a perfectly straight line. Another model at
the same meet equipped with a carbonic acid motor flew very well for a
time but was injured in a collision.
How long has a model aeroplane remained in the air?
The American record for time aloft is held, we believe, by Cecil Peoli
of New York, whose model has remained in the air for sixty-five seconds.
Several records of from thirty to forty seconds are reported from
France. It is probable that the best record has been made in the long
distance flights in England when 26,000 feet was covered.
Has the model aeroplane any practical commercial utility?
Probably not, unless we take seriously the suggestion of a writer on
aerial warfare, who believes they will some day be so perfected that
they may be used to drop bombs or high explosives over forts or besieged
cities. His suggestion is that hundreds of model aeroplanes equipped
with miniature engines might be released in a swarm, each carrying a
deadly explosive which would be dropped automatically at a certain time.
It would be impossible, he argues, for gunners to bring down an entire
fleet of these swiftly moving machines, and so while many of them might
fall short enough would succeed in dropping their missiles to make them
an exceedingly dangerous weapon. The writer points out that the expense
of such a mosquito fleet would be trifling compared with the cost of the
ordinary engines of warfare and might be operated without risking any
lives.
[Illustration: A tractor with large plane forward built by F. W. Curtis]
[Illustration: Model built by William Robinson]
How large may a model aeroplane be made with good results?
None of the successful strand motor models at present exceed five feet
in length. This would seem to be the practical limit at which a model
may be carried with this motive power. Flights of more than 1,500 feet
have been made with models but two feet in length. When regular engines
are installed there is of course no limit to the size of models.
When was the first model aeroplane constructed?
Crude machines propelled through the air by twisted rubber strands were
used as playthings more than a century ago. The model built and flown by
Langeley in 1887, was probably the first machine to appear on the lines
now generally followed by aeronauts. Successful model aeroplanes have
been used as toys for less than five years. Their development in this
period has been remarkable.
Are the equations for calculating the thrust of propeller in terms of
wing area and skin friction and general proportions useful in designing
model aeroplanes?
Only in a general way. When the dimensions are very small these
complicated equations are misleading. They are for the most part
extremely complicated and for this reason we have avoided them entirely
in the present volume. Much better results may be obtained by proceeding
merely by the rule of thumb, and testing out the proportions of your
aeroplane by actual practice.
What is the American indoor record for model aeroplanes?
A flight of 265 feet was made at a New York tournament by Stewart
Easter, the model rising from the ground under its own power. This was
made diagonally across the hall as far as it could go. Many flights have
been made at the New York meets in which models have flown the entire
length of the hall and struck the further wall with their motors far
from run down. In other words, model aeroplanes have reached a state of
development where they have outgrown the largest indoor enclosures
available for flying.