The Second Boys' Book of Model Aeroplanes

by Francis A. Collins

CHAPTER XIII SELECTED QUESTIONS FOR BEGINNERS

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?

A beautiful model built by Stewart Easter

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?

Percy Pierce launching a prize-winning model

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.

A tractor with large plane forward built by F. W. Curtis

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.