CHAPTER VI SCIENTIFIC PROPELLER BUILDINGEver since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood.In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane.A beautiful monoplane built by R. MungokeeA beautiful monoplane built by R. MungokeeDetail of a model built by R. MungokeeDetail of a model built by R. MungokeeAn ingenious application of the dihedral angleAn ingenious application of the dihedral angleThe propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller's work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute.There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down.Diagram Showing How To Make A Propeller From A Wooden BlankDiagram Showing How To Make A Propeller From A Wooden BlankThere is much difference of opinion as to the proper modeling of the propeller. It has been worked out by elaborate equations that the blade should be concave and yet in actual tests it has been found that some machines are driven faster by a flat blade propeller. By a flat screw we mean a straight pitch propeller, or one in which the angle does not vary from the hub to the tip. When Mr. Glenn H. Curtiss made his famous record flight at Rheims, he used a straight pitch propeller, and when, later, his machine was equipped with propellers scientifically curved, his aeroplane lost speed. Evidently the exact relation of propeller forms to the machine still remains much of a mystery.Design of Metal PropellerDesign of Metal PropellerA very simple test of the efficiency of propellers of various modeling may be made by running them in heavy smoke. By burning a piece of oily cotton waste, you may watch the action of the propellers on the smoke, while, at the same time, this greasy smoke will leave its mark on the section of the propeller blade which does the most work. The speed of the blades near the hub of the propeller is, of course, much less than at the tips, and consequently the work they perform in sending the aeroplane forward is small. At the extreme end of the propellers, the air, of course, tends to slip off.The most efficient part of the blade, according to these tests, is about one-third of the radius distant from the center. Less than two-thirds of the propeller seems to do effective driving work. On the propellers driven against greasy smoke, the blades near the hub remain comparatively clean while the portion most soiled extends outward from this point. The test would seem to indicate that a broad blade narrowing to the hub would develop the maximum thrust. It would also seem that it is unnecessary to carry the lines of the blade close to the axle, thereby possibly weakening the propeller.To understand the theory of the propeller, bear in mind the principle of the action of the wings, for the analogy between the two is very close. The forward, or entering, edge of the propeller is curved so that it will cut its way smoothly and offer less resistance than a straight entering edge. The blade of the propeller is made slightly concave for exactly the same reason that the plane is curved. Like the plane, such a surface takes advantage of the resistance of the air.The curve of the propeller blade near the hub is made much higher than further on because this part travels more slowly, and it is important that the cylinder of air set in motion by the blade should have the same velocity throughout its diameter. The blade is made widest at its outer end, since this is the most effective surface and is expected to do the greatest amount of work. The other end of the propeller blade is rounded off in order that the air may slip away, thus avoiding skin friction, which at this point is naturally high.A test of high aspect ratio planesA test of high aspect ratio planesA modified Bleriot built by Cecil PeoliA modified Bleriot built by Cecil PeoliThe width of the propeller blade has been the subject of an immense amount of investigation and discussion. The friends of both the wide and narrow blade back up their arguments with complicated equations, which it would only be confusing to repeat. It is argued by some authorities that since the narrow blade does not stir up the air as long a time as the wide blade, therefore one blade does not stir up the air enough to interfere with the action of the second blade.Langley Propeller BladeLangley Propeller BladeA small blade may be driven by a much lighter motor, and is, of course, capable of much higher speed. On the other hand, the wide blade drives the model much further ahead per turn than the narrow blade, while making a much greater demand upon the motor.Briefly a narrow propeller is best for speed and the wide blade propeller for power. There is an immense amount of difference of opinion concerning the form and position of the propeller so that it is impossible to lay down any hard and fast rules. It is argued by several well-known aviators that a propeller is more effective when driven with its straight edge forward and there is scarcely a point not in dispute.One of the most novel propeller designs, the Cowley, is a blade bent in the form of an arc of a circle, the radius of the curve being equal to the diameter of the propeller. The propeller is mounted with the convex surface forward. The theory of this propeller is that it focuses the air, as it were, which it throws back forming a cylinder of air which travels at a higher speed than one set in motion by the horizontal blades.The tendency for the air to slip off the ends of the propeller blades is probably reduced. This form of propeller may be made by steaming the blades and bending them into position. A mould may be prepared and the steamed blades forced to take their shape and held in position until they have dried.A series of experiments have been made in England with boomerangs to discover the effect of curved surfaces on flight. The Langeley propeller, which embodies the information gained in this way, has a flat back while the face is concave, following the general stream line form. The ends of the propeller blades are practically square. Some of the new propellers are covered with a thin canvass glued smoothly over the greater part of the blade. The covering guards somewhat against splitting and splintering.In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform.Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction.Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem.Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling.A combination of several interesting featuresA combination of several interesting featuresA skilful adjustment of the front plane and skid built by Percy PierceA skilful adjustment of the front plane and skid built by Percy PiercePropeller blanks are easily prepared in case you find it inconvenient to buy them. The following directions refer to a propeller eight inches in length, but the same proportions hold good for any size. Select a piece of some straight-grained wood, white pine is best, which will not split readily, and is easy to work. The original block for an eight-inch propeller should be eight inches in length, two inches in width, and three-fourths of one inch thick. Now draw a line lengthwise, exactly bisecting the block, and mark off the middle of the line, and two points one inch from either end. With one of these outer points as a center, describe a quadrant of a circle above the line, and from the corresponding point, draw a similar circle below the line. From the center of the blocks draw a complete circle one-half of one inch in diameter. Draw straight lines from the ends of the arcs to the vertical diameters of the circle, and saw away the wood to these lines. In carving your propeller, first cut away the wood from the longer straight lines of the block on opposite sides. The blades should be slightly concave. It will be found a good plan to finish one side of the blade before cutting away the opposite side. Cut away the wood until the blade is less than one-eighth of an inch thick, and sandpaper away all marks of the knife or chisel. The wood should then be oiled or covered with a light coat of varnish. It is very important that the two ends of the propellers should be uniform both as to their modeling and weight. To mount the axle, drill a hole at the center just large enough to admit the wire. The outer end may be bent over and inserted into the hole to keep it rigid. If the axle does not fit tightly, drive in small wedges of wood, such as a toothpick, at both sides.The propeller used by the Wright Brothers on their machines is very simple to construct. Prepare a propeller blank eight inches in length, two in width and three-fourths of an inch in depth. Draw two lines parallel with the longer sides, the first seven-eighths of an inch and the second one and one-eighth inches back. Now at the upper right-hand corner mark off a point one and one-half inches from the end, and from the opposite corner on the lower base the same dimension. Connect these two points.Wright Propeller BladeWright Propeller BladeThe blank is completed by cutting away to these lines, leaving the blades each one and one-eighth inches in width. The axle should be left a little full, say three-eighths of an inch across. Round off the outer corners. In modeling your propeller cut away or bevel the sides formed by the two intersecting lines, which will form the entering edge of the propeller. The blade should be cut to a very slight concave, although some prefer a flat blade. The propeller is mounted by drilling a hole at the center and mounting in the usual way.The propellers of a model aeroplane are subject to more wear and tear than those of a regular passenger-carrying machine. At the end of every flight, they face a possible catastrophe. In the search for some durable form of screw, a number of interesting discoveries have been made. One builder has succeeded in coating a wooden propeller with bronze by subjecting it to an electroplating process, but this is much too complicated for the amateur. The lighter metals, aluminum and magnalium, naturally suggest themselves for the purpose. Such propellers weigh no more than wood and may be readily bent to the required shape.Procure a thin sheet of aluminum, or, if this cannot be had, a smooth piece of tin will do. It must, however, be heavy enough to hold its shape. The design of the propeller may be laid out on the tin, and the metal trimmed away. To make an eight-inch propeller, draw a rectangle eight inches in length and two inches broad, and draw a line joining the middle of short sides. At the center, draw two vertical lines half an inch on either side of the center lines, half an inch above and below the center, forming a small inner rectangle. Now from a point on the bisecting line, one inch from either end, draw two semicircles. Next, connect the top of one of these circles with the nearest point of the inner rectangle and draw another line from the point below to the corresponding corner of the large rectangle. Repeat the diagram on the other end of the rectangle, reversing the curve as indicated in Fig. A.In cutting out the design, allow the straight sections running to the sides of the larger rectangle to remain. They will be needed to hold the central piece in position. This consists of a block of wood measuring one inch by one-half an inch and one-quarter of an inch in thickness. The strips at the center should be bent tightly over the corners, overlapped, and nailed firmly down with brads. Next, at the center, punch a small hole and drill through the block a shaft large enough to hold the axle of the propeller which is then firmly imbedded in it. One great advantage of the metal propeller is the fact that you may readily alter its pitch.An efficient propeller may be made by mounting metal blades on a wooden shaft. Procure a stick one quarter of an inch square and three inches in length, and saw through both ends for a distance of three quarters of an inch. Prepare your propeller blades by plotting them out on a sheet of aluminum, as described above, and cut away the middle section. The blades may then be inserted in the open ends of the stock and nailed securely in position. The edges of the wood may then be rounded off and the axle inserted firmly at the center. The metal sheet should be bent into the proper pitch as in the case of other metal propellers.FABRIC PROPELLERSThe most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.An efficient model, showing excellent construction, designed by John CaresiAn efficient model, showing excellent construction, designed by John CaresiOne of the best minimum plane models of 1911One of the best minimum plane models of 1911THE LANGELEY BLANK.Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.
CHAPTER VI SCIENTIFIC PROPELLER BUILDINGEver since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood.In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane.A beautiful monoplane built by R. MungokeeA beautiful monoplane built by R. MungokeeDetail of a model built by R. MungokeeDetail of a model built by R. MungokeeAn ingenious application of the dihedral angleAn ingenious application of the dihedral angleThe propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller's work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute.There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down.Diagram Showing How To Make A Propeller From A Wooden BlankDiagram Showing How To Make A Propeller From A Wooden BlankThere is much difference of opinion as to the proper modeling of the propeller. It has been worked out by elaborate equations that the blade should be concave and yet in actual tests it has been found that some machines are driven faster by a flat blade propeller. By a flat screw we mean a straight pitch propeller, or one in which the angle does not vary from the hub to the tip. When Mr. Glenn H. Curtiss made his famous record flight at Rheims, he used a straight pitch propeller, and when, later, his machine was equipped with propellers scientifically curved, his aeroplane lost speed. Evidently the exact relation of propeller forms to the machine still remains much of a mystery.Design of Metal PropellerDesign of Metal PropellerA very simple test of the efficiency of propellers of various modeling may be made by running them in heavy smoke. By burning a piece of oily cotton waste, you may watch the action of the propellers on the smoke, while, at the same time, this greasy smoke will leave its mark on the section of the propeller blade which does the most work. The speed of the blades near the hub of the propeller is, of course, much less than at the tips, and consequently the work they perform in sending the aeroplane forward is small. At the extreme end of the propellers, the air, of course, tends to slip off.The most efficient part of the blade, according to these tests, is about one-third of the radius distant from the center. Less than two-thirds of the propeller seems to do effective driving work. On the propellers driven against greasy smoke, the blades near the hub remain comparatively clean while the portion most soiled extends outward from this point. The test would seem to indicate that a broad blade narrowing to the hub would develop the maximum thrust. It would also seem that it is unnecessary to carry the lines of the blade close to the axle, thereby possibly weakening the propeller.To understand the theory of the propeller, bear in mind the principle of the action of the wings, for the analogy between the two is very close. The forward, or entering, edge of the propeller is curved so that it will cut its way smoothly and offer less resistance than a straight entering edge. The blade of the propeller is made slightly concave for exactly the same reason that the plane is curved. Like the plane, such a surface takes advantage of the resistance of the air.The curve of the propeller blade near the hub is made much higher than further on because this part travels more slowly, and it is important that the cylinder of air set in motion by the blade should have the same velocity throughout its diameter. The blade is made widest at its outer end, since this is the most effective surface and is expected to do the greatest amount of work. The other end of the propeller blade is rounded off in order that the air may slip away, thus avoiding skin friction, which at this point is naturally high.A test of high aspect ratio planesA test of high aspect ratio planesA modified Bleriot built by Cecil PeoliA modified Bleriot built by Cecil PeoliThe width of the propeller blade has been the subject of an immense amount of investigation and discussion. The friends of both the wide and narrow blade back up their arguments with complicated equations, which it would only be confusing to repeat. It is argued by some authorities that since the narrow blade does not stir up the air as long a time as the wide blade, therefore one blade does not stir up the air enough to interfere with the action of the second blade.Langley Propeller BladeLangley Propeller BladeA small blade may be driven by a much lighter motor, and is, of course, capable of much higher speed. On the other hand, the wide blade drives the model much further ahead per turn than the narrow blade, while making a much greater demand upon the motor.Briefly a narrow propeller is best for speed and the wide blade propeller for power. There is an immense amount of difference of opinion concerning the form and position of the propeller so that it is impossible to lay down any hard and fast rules. It is argued by several well-known aviators that a propeller is more effective when driven with its straight edge forward and there is scarcely a point not in dispute.One of the most novel propeller designs, the Cowley, is a blade bent in the form of an arc of a circle, the radius of the curve being equal to the diameter of the propeller. The propeller is mounted with the convex surface forward. The theory of this propeller is that it focuses the air, as it were, which it throws back forming a cylinder of air which travels at a higher speed than one set in motion by the horizontal blades.The tendency for the air to slip off the ends of the propeller blades is probably reduced. This form of propeller may be made by steaming the blades and bending them into position. A mould may be prepared and the steamed blades forced to take their shape and held in position until they have dried.A series of experiments have been made in England with boomerangs to discover the effect of curved surfaces on flight. The Langeley propeller, which embodies the information gained in this way, has a flat back while the face is concave, following the general stream line form. The ends of the propeller blades are practically square. Some of the new propellers are covered with a thin canvass glued smoothly over the greater part of the blade. The covering guards somewhat against splitting and splintering.In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform.Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction.Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem.Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling.A combination of several interesting featuresA combination of several interesting featuresA skilful adjustment of the front plane and skid built by Percy PierceA skilful adjustment of the front plane and skid built by Percy PiercePropeller blanks are easily prepared in case you find it inconvenient to buy them. The following directions refer to a propeller eight inches in length, but the same proportions hold good for any size. Select a piece of some straight-grained wood, white pine is best, which will not split readily, and is easy to work. The original block for an eight-inch propeller should be eight inches in length, two inches in width, and three-fourths of one inch thick. Now draw a line lengthwise, exactly bisecting the block, and mark off the middle of the line, and two points one inch from either end. With one of these outer points as a center, describe a quadrant of a circle above the line, and from the corresponding point, draw a similar circle below the line. From the center of the blocks draw a complete circle one-half of one inch in diameter. Draw straight lines from the ends of the arcs to the vertical diameters of the circle, and saw away the wood to these lines. In carving your propeller, first cut away the wood from the longer straight lines of the block on opposite sides. The blades should be slightly concave. It will be found a good plan to finish one side of the blade before cutting away the opposite side. Cut away the wood until the blade is less than one-eighth of an inch thick, and sandpaper away all marks of the knife or chisel. The wood should then be oiled or covered with a light coat of varnish. It is very important that the two ends of the propellers should be uniform both as to their modeling and weight. To mount the axle, drill a hole at the center just large enough to admit the wire. The outer end may be bent over and inserted into the hole to keep it rigid. If the axle does not fit tightly, drive in small wedges of wood, such as a toothpick, at both sides.The propeller used by the Wright Brothers on their machines is very simple to construct. Prepare a propeller blank eight inches in length, two in width and three-fourths of an inch in depth. Draw two lines parallel with the longer sides, the first seven-eighths of an inch and the second one and one-eighth inches back. Now at the upper right-hand corner mark off a point one and one-half inches from the end, and from the opposite corner on the lower base the same dimension. Connect these two points.Wright Propeller BladeWright Propeller BladeThe blank is completed by cutting away to these lines, leaving the blades each one and one-eighth inches in width. The axle should be left a little full, say three-eighths of an inch across. Round off the outer corners. In modeling your propeller cut away or bevel the sides formed by the two intersecting lines, which will form the entering edge of the propeller. The blade should be cut to a very slight concave, although some prefer a flat blade. The propeller is mounted by drilling a hole at the center and mounting in the usual way.The propellers of a model aeroplane are subject to more wear and tear than those of a regular passenger-carrying machine. At the end of every flight, they face a possible catastrophe. In the search for some durable form of screw, a number of interesting discoveries have been made. One builder has succeeded in coating a wooden propeller with bronze by subjecting it to an electroplating process, but this is much too complicated for the amateur. The lighter metals, aluminum and magnalium, naturally suggest themselves for the purpose. Such propellers weigh no more than wood and may be readily bent to the required shape.Procure a thin sheet of aluminum, or, if this cannot be had, a smooth piece of tin will do. It must, however, be heavy enough to hold its shape. The design of the propeller may be laid out on the tin, and the metal trimmed away. To make an eight-inch propeller, draw a rectangle eight inches in length and two inches broad, and draw a line joining the middle of short sides. At the center, draw two vertical lines half an inch on either side of the center lines, half an inch above and below the center, forming a small inner rectangle. Now from a point on the bisecting line, one inch from either end, draw two semicircles. Next, connect the top of one of these circles with the nearest point of the inner rectangle and draw another line from the point below to the corresponding corner of the large rectangle. Repeat the diagram on the other end of the rectangle, reversing the curve as indicated in Fig. A.In cutting out the design, allow the straight sections running to the sides of the larger rectangle to remain. They will be needed to hold the central piece in position. This consists of a block of wood measuring one inch by one-half an inch and one-quarter of an inch in thickness. The strips at the center should be bent tightly over the corners, overlapped, and nailed firmly down with brads. Next, at the center, punch a small hole and drill through the block a shaft large enough to hold the axle of the propeller which is then firmly imbedded in it. One great advantage of the metal propeller is the fact that you may readily alter its pitch.An efficient propeller may be made by mounting metal blades on a wooden shaft. Procure a stick one quarter of an inch square and three inches in length, and saw through both ends for a distance of three quarters of an inch. Prepare your propeller blades by plotting them out on a sheet of aluminum, as described above, and cut away the middle section. The blades may then be inserted in the open ends of the stock and nailed securely in position. The edges of the wood may then be rounded off and the axle inserted firmly at the center. The metal sheet should be bent into the proper pitch as in the case of other metal propellers.FABRIC PROPELLERSThe most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.An efficient model, showing excellent construction, designed by John CaresiAn efficient model, showing excellent construction, designed by John CaresiOne of the best minimum plane models of 1911One of the best minimum plane models of 1911THE LANGELEY BLANK.Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.
CHAPTER VI SCIENTIFIC PROPELLER BUILDINGEver since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood.In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane.A beautiful monoplane built by R. MungokeeA beautiful monoplane built by R. MungokeeDetail of a model built by R. MungokeeDetail of a model built by R. MungokeeAn ingenious application of the dihedral angleAn ingenious application of the dihedral angleThe propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller's work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute.There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down.Diagram Showing How To Make A Propeller From A Wooden BlankDiagram Showing How To Make A Propeller From A Wooden BlankThere is much difference of opinion as to the proper modeling of the propeller. It has been worked out by elaborate equations that the blade should be concave and yet in actual tests it has been found that some machines are driven faster by a flat blade propeller. By a flat screw we mean a straight pitch propeller, or one in which the angle does not vary from the hub to the tip. When Mr. Glenn H. Curtiss made his famous record flight at Rheims, he used a straight pitch propeller, and when, later, his machine was equipped with propellers scientifically curved, his aeroplane lost speed. Evidently the exact relation of propeller forms to the machine still remains much of a mystery.Design of Metal PropellerDesign of Metal PropellerA very simple test of the efficiency of propellers of various modeling may be made by running them in heavy smoke. By burning a piece of oily cotton waste, you may watch the action of the propellers on the smoke, while, at the same time, this greasy smoke will leave its mark on the section of the propeller blade which does the most work. The speed of the blades near the hub of the propeller is, of course, much less than at the tips, and consequently the work they perform in sending the aeroplane forward is small. At the extreme end of the propellers, the air, of course, tends to slip off.The most efficient part of the blade, according to these tests, is about one-third of the radius distant from the center. Less than two-thirds of the propeller seems to do effective driving work. On the propellers driven against greasy smoke, the blades near the hub remain comparatively clean while the portion most soiled extends outward from this point. The test would seem to indicate that a broad blade narrowing to the hub would develop the maximum thrust. It would also seem that it is unnecessary to carry the lines of the blade close to the axle, thereby possibly weakening the propeller.To understand the theory of the propeller, bear in mind the principle of the action of the wings, for the analogy between the two is very close. The forward, or entering, edge of the propeller is curved so that it will cut its way smoothly and offer less resistance than a straight entering edge. The blade of the propeller is made slightly concave for exactly the same reason that the plane is curved. Like the plane, such a surface takes advantage of the resistance of the air.The curve of the propeller blade near the hub is made much higher than further on because this part travels more slowly, and it is important that the cylinder of air set in motion by the blade should have the same velocity throughout its diameter. The blade is made widest at its outer end, since this is the most effective surface and is expected to do the greatest amount of work. The other end of the propeller blade is rounded off in order that the air may slip away, thus avoiding skin friction, which at this point is naturally high.A test of high aspect ratio planesA test of high aspect ratio planesA modified Bleriot built by Cecil PeoliA modified Bleriot built by Cecil PeoliThe width of the propeller blade has been the subject of an immense amount of investigation and discussion. The friends of both the wide and narrow blade back up their arguments with complicated equations, which it would only be confusing to repeat. It is argued by some authorities that since the narrow blade does not stir up the air as long a time as the wide blade, therefore one blade does not stir up the air enough to interfere with the action of the second blade.Langley Propeller BladeLangley Propeller BladeA small blade may be driven by a much lighter motor, and is, of course, capable of much higher speed. On the other hand, the wide blade drives the model much further ahead per turn than the narrow blade, while making a much greater demand upon the motor.Briefly a narrow propeller is best for speed and the wide blade propeller for power. There is an immense amount of difference of opinion concerning the form and position of the propeller so that it is impossible to lay down any hard and fast rules. It is argued by several well-known aviators that a propeller is more effective when driven with its straight edge forward and there is scarcely a point not in dispute.One of the most novel propeller designs, the Cowley, is a blade bent in the form of an arc of a circle, the radius of the curve being equal to the diameter of the propeller. The propeller is mounted with the convex surface forward. The theory of this propeller is that it focuses the air, as it were, which it throws back forming a cylinder of air which travels at a higher speed than one set in motion by the horizontal blades.The tendency for the air to slip off the ends of the propeller blades is probably reduced. This form of propeller may be made by steaming the blades and bending them into position. A mould may be prepared and the steamed blades forced to take their shape and held in position until they have dried.A series of experiments have been made in England with boomerangs to discover the effect of curved surfaces on flight. The Langeley propeller, which embodies the information gained in this way, has a flat back while the face is concave, following the general stream line form. The ends of the propeller blades are practically square. Some of the new propellers are covered with a thin canvass glued smoothly over the greater part of the blade. The covering guards somewhat against splitting and splintering.In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform.Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction.Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem.Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling.A combination of several interesting featuresA combination of several interesting featuresA skilful adjustment of the front plane and skid built by Percy PierceA skilful adjustment of the front plane and skid built by Percy PiercePropeller blanks are easily prepared in case you find it inconvenient to buy them. The following directions refer to a propeller eight inches in length, but the same proportions hold good for any size. Select a piece of some straight-grained wood, white pine is best, which will not split readily, and is easy to work. The original block for an eight-inch propeller should be eight inches in length, two inches in width, and three-fourths of one inch thick. Now draw a line lengthwise, exactly bisecting the block, and mark off the middle of the line, and two points one inch from either end. With one of these outer points as a center, describe a quadrant of a circle above the line, and from the corresponding point, draw a similar circle below the line. From the center of the blocks draw a complete circle one-half of one inch in diameter. Draw straight lines from the ends of the arcs to the vertical diameters of the circle, and saw away the wood to these lines. In carving your propeller, first cut away the wood from the longer straight lines of the block on opposite sides. The blades should be slightly concave. It will be found a good plan to finish one side of the blade before cutting away the opposite side. Cut away the wood until the blade is less than one-eighth of an inch thick, and sandpaper away all marks of the knife or chisel. The wood should then be oiled or covered with a light coat of varnish. It is very important that the two ends of the propellers should be uniform both as to their modeling and weight. To mount the axle, drill a hole at the center just large enough to admit the wire. The outer end may be bent over and inserted into the hole to keep it rigid. If the axle does not fit tightly, drive in small wedges of wood, such as a toothpick, at both sides.The propeller used by the Wright Brothers on their machines is very simple to construct. Prepare a propeller blank eight inches in length, two in width and three-fourths of an inch in depth. Draw two lines parallel with the longer sides, the first seven-eighths of an inch and the second one and one-eighth inches back. Now at the upper right-hand corner mark off a point one and one-half inches from the end, and from the opposite corner on the lower base the same dimension. Connect these two points.Wright Propeller BladeWright Propeller BladeThe blank is completed by cutting away to these lines, leaving the blades each one and one-eighth inches in width. The axle should be left a little full, say three-eighths of an inch across. Round off the outer corners. In modeling your propeller cut away or bevel the sides formed by the two intersecting lines, which will form the entering edge of the propeller. The blade should be cut to a very slight concave, although some prefer a flat blade. The propeller is mounted by drilling a hole at the center and mounting in the usual way.The propellers of a model aeroplane are subject to more wear and tear than those of a regular passenger-carrying machine. At the end of every flight, they face a possible catastrophe. In the search for some durable form of screw, a number of interesting discoveries have been made. One builder has succeeded in coating a wooden propeller with bronze by subjecting it to an electroplating process, but this is much too complicated for the amateur. The lighter metals, aluminum and magnalium, naturally suggest themselves for the purpose. Such propellers weigh no more than wood and may be readily bent to the required shape.Procure a thin sheet of aluminum, or, if this cannot be had, a smooth piece of tin will do. It must, however, be heavy enough to hold its shape. The design of the propeller may be laid out on the tin, and the metal trimmed away. To make an eight-inch propeller, draw a rectangle eight inches in length and two inches broad, and draw a line joining the middle of short sides. At the center, draw two vertical lines half an inch on either side of the center lines, half an inch above and below the center, forming a small inner rectangle. Now from a point on the bisecting line, one inch from either end, draw two semicircles. Next, connect the top of one of these circles with the nearest point of the inner rectangle and draw another line from the point below to the corresponding corner of the large rectangle. Repeat the diagram on the other end of the rectangle, reversing the curve as indicated in Fig. A.In cutting out the design, allow the straight sections running to the sides of the larger rectangle to remain. They will be needed to hold the central piece in position. This consists of a block of wood measuring one inch by one-half an inch and one-quarter of an inch in thickness. The strips at the center should be bent tightly over the corners, overlapped, and nailed firmly down with brads. Next, at the center, punch a small hole and drill through the block a shaft large enough to hold the axle of the propeller which is then firmly imbedded in it. One great advantage of the metal propeller is the fact that you may readily alter its pitch.An efficient propeller may be made by mounting metal blades on a wooden shaft. Procure a stick one quarter of an inch square and three inches in length, and saw through both ends for a distance of three quarters of an inch. Prepare your propeller blades by plotting them out on a sheet of aluminum, as described above, and cut away the middle section. The blades may then be inserted in the open ends of the stock and nailed securely in position. The edges of the wood may then be rounded off and the axle inserted firmly at the center. The metal sheet should be bent into the proper pitch as in the case of other metal propellers.FABRIC PROPELLERSThe most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.An efficient model, showing excellent construction, designed by John CaresiAn efficient model, showing excellent construction, designed by John CaresiOne of the best minimum plane models of 1911One of the best minimum plane models of 1911THE LANGELEY BLANK.Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.
Ever since windmills were first set up, men have been studying the merits of different propellers. By the time steamships came to be driven through the water by rotary blades or screws, their modeling had become a science. The builders of rotary fans in turn contributed still further to our knowledge on the subject. Drawing largely upon all this experience, the aviator has learned to build fairly efficient propellers, although there is probably no department of aeronautics to-day so little understood.
In a windmill a current or cylinder of air flows, of course, against the propeller. The blades must be shaped and spaced with this in view. Reverse the action of the windmill, and the propeller proves inefficient. The broad blades will stir up a current of air, to be sure, but a very weak one. A revolving fan solves a very different problem in detaching a cylinder of air from the atmosphere and propelling it with the greatest possible momentum. Here, again, the propellers must be differently modeled and spaced. Neither the reversed windmill propeller nor the electric fan, however, would serve to drive an aeroplane.
A beautiful monoplane built by R. MungokeeA beautiful monoplane built by R. Mungokee
A beautiful monoplane built by R. Mungokee
Detail of a model built by R. MungokeeDetail of a model built by R. Mungokee
Detail of a model built by R. Mungokee
An ingenious application of the dihedral angleAn ingenious application of the dihedral angle
An ingenious application of the dihedral angle
The propeller of an aeroplane must cut its way smoothly, pressing the air backward without splashing. It is only when an aeroplane is held fast that its propellers kick up such a fuss and blow your hat off. The aeroplane propeller's work is much the same as that of a steamship, although the air through which it travels has many tricks not yet understood. The density of the air compares to that of water as one to eight hundred, but the friction encountered by the air propellers is much greater than 1-800th that of water. It may be laid down as a general rule, however, that the driving force of an aeroplane propeller varies as the square of the number of revolutions per minute.
There is at present no standard form of propeller for the man-carrying or model aeroplane. One school of designers favors a small blade revolved at high speed, while others claim that a larger propeller driven more slowly is more efficient. As a general rule it may be laid down that a model with a span of thirty inches should be driven by twin propellers eight inches in length or diameter. They should have a speed of about 1,200 revolutions per minute, or at the rate of some 200 turns every ten seconds. To test the strength of your motor, give the propeller 200 or 400 turns, and watch in hand, find how long it takes to run down.
Diagram Showing How To Make A Propeller From A Wooden BlankDiagram Showing How To Make A Propeller From A Wooden Blank
Diagram Showing How To Make A Propeller From A Wooden Blank
There is much difference of opinion as to the proper modeling of the propeller. It has been worked out by elaborate equations that the blade should be concave and yet in actual tests it has been found that some machines are driven faster by a flat blade propeller. By a flat screw we mean a straight pitch propeller, or one in which the angle does not vary from the hub to the tip. When Mr. Glenn H. Curtiss made his famous record flight at Rheims, he used a straight pitch propeller, and when, later, his machine was equipped with propellers scientifically curved, his aeroplane lost speed. Evidently the exact relation of propeller forms to the machine still remains much of a mystery.
Design of Metal PropellerDesign of Metal Propeller
Design of Metal Propeller
A very simple test of the efficiency of propellers of various modeling may be made by running them in heavy smoke. By burning a piece of oily cotton waste, you may watch the action of the propellers on the smoke, while, at the same time, this greasy smoke will leave its mark on the section of the propeller blade which does the most work. The speed of the blades near the hub of the propeller is, of course, much less than at the tips, and consequently the work they perform in sending the aeroplane forward is small. At the extreme end of the propellers, the air, of course, tends to slip off.
The most efficient part of the blade, according to these tests, is about one-third of the radius distant from the center. Less than two-thirds of the propeller seems to do effective driving work. On the propellers driven against greasy smoke, the blades near the hub remain comparatively clean while the portion most soiled extends outward from this point. The test would seem to indicate that a broad blade narrowing to the hub would develop the maximum thrust. It would also seem that it is unnecessary to carry the lines of the blade close to the axle, thereby possibly weakening the propeller.
To understand the theory of the propeller, bear in mind the principle of the action of the wings, for the analogy between the two is very close. The forward, or entering, edge of the propeller is curved so that it will cut its way smoothly and offer less resistance than a straight entering edge. The blade of the propeller is made slightly concave for exactly the same reason that the plane is curved. Like the plane, such a surface takes advantage of the resistance of the air.
The curve of the propeller blade near the hub is made much higher than further on because this part travels more slowly, and it is important that the cylinder of air set in motion by the blade should have the same velocity throughout its diameter. The blade is made widest at its outer end, since this is the most effective surface and is expected to do the greatest amount of work. The other end of the propeller blade is rounded off in order that the air may slip away, thus avoiding skin friction, which at this point is naturally high.
A test of high aspect ratio planesA test of high aspect ratio planes
A test of high aspect ratio planes
A modified Bleriot built by Cecil PeoliA modified Bleriot built by Cecil Peoli
A modified Bleriot built by Cecil Peoli
The width of the propeller blade has been the subject of an immense amount of investigation and discussion. The friends of both the wide and narrow blade back up their arguments with complicated equations, which it would only be confusing to repeat. It is argued by some authorities that since the narrow blade does not stir up the air as long a time as the wide blade, therefore one blade does not stir up the air enough to interfere with the action of the second blade.
Langley Propeller BladeLangley Propeller Blade
Langley Propeller Blade
A small blade may be driven by a much lighter motor, and is, of course, capable of much higher speed. On the other hand, the wide blade drives the model much further ahead per turn than the narrow blade, while making a much greater demand upon the motor.
Briefly a narrow propeller is best for speed and the wide blade propeller for power. There is an immense amount of difference of opinion concerning the form and position of the propeller so that it is impossible to lay down any hard and fast rules. It is argued by several well-known aviators that a propeller is more effective when driven with its straight edge forward and there is scarcely a point not in dispute.
One of the most novel propeller designs, the Cowley, is a blade bent in the form of an arc of a circle, the radius of the curve being equal to the diameter of the propeller. The propeller is mounted with the convex surface forward. The theory of this propeller is that it focuses the air, as it were, which it throws back forming a cylinder of air which travels at a higher speed than one set in motion by the horizontal blades.
The tendency for the air to slip off the ends of the propeller blades is probably reduced. This form of propeller may be made by steaming the blades and bending them into position. A mould may be prepared and the steamed blades forced to take their shape and held in position until they have dried.
A series of experiments have been made in England with boomerangs to discover the effect of curved surfaces on flight. The Langeley propeller, which embodies the information gained in this way, has a flat back while the face is concave, following the general stream line form. The ends of the propeller blades are practically square. Some of the new propellers are covered with a thin canvass glued smoothly over the greater part of the blade. The covering guards somewhat against splitting and splintering.
In the latest Percy Pierce models, for instance, the blade is carried out in a semicircle at the end of the propeller, thus practically doubling its surface. The driving power of this blade is very high. It is argued for this design that the blade being very thin is forced slightly backward at the beginning of the flight, while the model is gathering motion, but later, when the tension is removed, springs back thus increasing its effective surface and the thrust. The propeller thus automatically adjusts itself for the work it has to perform.
Since it is so difficult to fix upon the right pitch of a propeller, the builder of model aeroplanes had best work out this problem for himself. The propeller blank described later on, with a depth of three-fourths of one inch to an eight inch diameter, will give you a comparatively low-pitch propeller. An eight-inch propeller cut from a block one inch in depth will give you as high a pitch as you are likely to need. As you increase the pitch, you, of course, increase the power of your aeroplane, while at the same time you make a greater demand upon your motor. Try the propellers of different pitch until you find the one which gives you the greatest stability and the highest speed. It is well to remember that in increasing the width of your propeller blade you add to the skin friction.
Some designers carry the curve of the propeller blade to the center of the axle, while others leave the center blank. It is argued by the former that the longer the blade the greater is the thrust. Others believe that the blade exerts little or no thrust near the center and is weakened by being cut away too much. The builder of model aeroplanes has one great advantage over the designer of passenger-carrying craft. The model does not have to carry fuel. After all, the difference in the power required for the various models is so slight that an extra strand or two on the motor will probably solve the problem.
Many successful builders of model aeroplanes now carve their propellers from solid blocks of wood. This method, to be sure, allows the designer to shape the propeller blades with more freedom than with the ordinary or built-up propeller, and of course does away with much of the preliminary work. So great is the demand for the one piece propellers that the manufacturers of accessories now prepare "propeller blanks" or pieces of wood in a variety of sizes ready to be carved. The one-piece propeller is likely to split, but they are easy to make, and this work is a very fascinating kind of whittling.
A combination of several interesting featuresA combination of several interesting features
A combination of several interesting features
A skilful adjustment of the front plane and skid built by Percy PierceA skilful adjustment of the front plane and skid built by Percy Pierce
A skilful adjustment of the front plane and skid built by Percy Pierce
Propeller blanks are easily prepared in case you find it inconvenient to buy them. The following directions refer to a propeller eight inches in length, but the same proportions hold good for any size. Select a piece of some straight-grained wood, white pine is best, which will not split readily, and is easy to work. The original block for an eight-inch propeller should be eight inches in length, two inches in width, and three-fourths of one inch thick. Now draw a line lengthwise, exactly bisecting the block, and mark off the middle of the line, and two points one inch from either end. With one of these outer points as a center, describe a quadrant of a circle above the line, and from the corresponding point, draw a similar circle below the line. From the center of the blocks draw a complete circle one-half of one inch in diameter. Draw straight lines from the ends of the arcs to the vertical diameters of the circle, and saw away the wood to these lines. In carving your propeller, first cut away the wood from the longer straight lines of the block on opposite sides. The blades should be slightly concave. It will be found a good plan to finish one side of the blade before cutting away the opposite side. Cut away the wood until the blade is less than one-eighth of an inch thick, and sandpaper away all marks of the knife or chisel. The wood should then be oiled or covered with a light coat of varnish. It is very important that the two ends of the propellers should be uniform both as to their modeling and weight. To mount the axle, drill a hole at the center just large enough to admit the wire. The outer end may be bent over and inserted into the hole to keep it rigid. If the axle does not fit tightly, drive in small wedges of wood, such as a toothpick, at both sides.
The propeller used by the Wright Brothers on their machines is very simple to construct. Prepare a propeller blank eight inches in length, two in width and three-fourths of an inch in depth. Draw two lines parallel with the longer sides, the first seven-eighths of an inch and the second one and one-eighth inches back. Now at the upper right-hand corner mark off a point one and one-half inches from the end, and from the opposite corner on the lower base the same dimension. Connect these two points.
Wright Propeller BladeWright Propeller Blade
Wright Propeller Blade
The blank is completed by cutting away to these lines, leaving the blades each one and one-eighth inches in width. The axle should be left a little full, say three-eighths of an inch across. Round off the outer corners. In modeling your propeller cut away or bevel the sides formed by the two intersecting lines, which will form the entering edge of the propeller. The blade should be cut to a very slight concave, although some prefer a flat blade. The propeller is mounted by drilling a hole at the center and mounting in the usual way.
The propellers of a model aeroplane are subject to more wear and tear than those of a regular passenger-carrying machine. At the end of every flight, they face a possible catastrophe. In the search for some durable form of screw, a number of interesting discoveries have been made. One builder has succeeded in coating a wooden propeller with bronze by subjecting it to an electroplating process, but this is much too complicated for the amateur. The lighter metals, aluminum and magnalium, naturally suggest themselves for the purpose. Such propellers weigh no more than wood and may be readily bent to the required shape.
Procure a thin sheet of aluminum, or, if this cannot be had, a smooth piece of tin will do. It must, however, be heavy enough to hold its shape. The design of the propeller may be laid out on the tin, and the metal trimmed away. To make an eight-inch propeller, draw a rectangle eight inches in length and two inches broad, and draw a line joining the middle of short sides. At the center, draw two vertical lines half an inch on either side of the center lines, half an inch above and below the center, forming a small inner rectangle. Now from a point on the bisecting line, one inch from either end, draw two semicircles. Next, connect the top of one of these circles with the nearest point of the inner rectangle and draw another line from the point below to the corresponding corner of the large rectangle. Repeat the diagram on the other end of the rectangle, reversing the curve as indicated in Fig. A.
In cutting out the design, allow the straight sections running to the sides of the larger rectangle to remain. They will be needed to hold the central piece in position. This consists of a block of wood measuring one inch by one-half an inch and one-quarter of an inch in thickness. The strips at the center should be bent tightly over the corners, overlapped, and nailed firmly down with brads. Next, at the center, punch a small hole and drill through the block a shaft large enough to hold the axle of the propeller which is then firmly imbedded in it. One great advantage of the metal propeller is the fact that you may readily alter its pitch.
An efficient propeller may be made by mounting metal blades on a wooden shaft. Procure a stick one quarter of an inch square and three inches in length, and saw through both ends for a distance of three quarters of an inch. Prepare your propeller blades by plotting them out on a sheet of aluminum, as described above, and cut away the middle section. The blades may then be inserted in the open ends of the stock and nailed securely in position. The edges of the wood may then be rounded off and the axle inserted firmly at the center. The metal sheet should be bent into the proper pitch as in the case of other metal propellers.
FABRIC PROPELLERSThe most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.An efficient model, showing excellent construction, designed by John CaresiAn efficient model, showing excellent construction, designed by John CaresiOne of the best minimum plane models of 1911One of the best minimum plane models of 1911
The most nearly indestructible propellers are the fabric screws. They are also doubtless the lightest form. The blades will, of course, be perfectly flat, making straight pitch propellers. You will need a small cylindrical piece of wood one half an inch in diameter, and one half an inch in height, of some tough, hard wood. The blades may be made of reed or cane, or, still better, of wire. Aluminum wire being very light is probably the best for the purpose. Bend the wire into the form of a triangle two inches in width and four inches in length. Determine at what angle you wish them to be set, and bore holes in the hub and fix wires of each frame firmly in them. Cover the frames neatly with cloth and mount it in the usual way.
An efficient model, showing excellent construction, designed by John CaresiAn efficient model, showing excellent construction, designed by John Caresi
An efficient model, showing excellent construction, designed by John Caresi
One of the best minimum plane models of 1911One of the best minimum plane models of 1911
One of the best minimum plane models of 1911
THE LANGELEY BLANK.Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.
Many model builders still retain the Langeley propeller. It is a very simple one to build. To prepare a blank secure a block, as before, eight by two inches and three-fourths of an inch in depth. Connect the four corners with diagonal lines. Parallel to the longer side draw two lines, one three-fourths of an inch inside and the second one-half inch below it. Cut away the block forming a double fan-shaped piece. Some prefer a wider center and the hub may be made a trifle broader if desired.
In shaping the propeller cut away from opposite sides of the blank. The original Langeley is a flat blade propeller so that the modeling is very simple. You may use your own judgement as to the thickness of the blade, although about one-eighth of an inch is suggested. The Langeley is mounted in the usual way. To heighten the pitch of your propeller secure a thicker blank.