CHAPTER XVIII. PROPELLERS.Principles and Use of Propellers. A propeller converts the energy of the engine into the thrust required to overcome the resistance of the aeroplane. To maintain flight the thrust, or force exerted by the propeller, must always equal the total resistance of the aeroplane. A total resistance of 400 pounds requires a propeller thrust of 400, and as the resistance varies with the speed, the engine revolutions must be altered correspondingly. The propeller is the most complicated and least understood element of the aeroplane, and we can but touch only on the most elementary features. The inclined blades of the propeller throw back an airstream, the reaction of which produces the thrust. The blades can also be considered as aerofoils moving in a circular path, the lift of the aerofoils corresponding to the thrust of the propeller. The reactions in any case are quite complicated and require the use of higher mathematics for a full understanding.Pitch and Velocity. When in action the propeller rotates, and at the same time advances along a straight line parallel to its axis. As a result, the tips of the propeller blades describe a curve known as "Helix" or screw-thread curve. The action is very similar to that of a screw being turned in a nut. For clearness in explanation we will call the velocity in the aeroplane path the "Translational velocity," and the speed of the tips in their circular path as the "Rotational velocity." When a screw works in a rigid nut it advances a distance equal to the "Pitch" in each revolution, the pitch of a single threaded screw being equal to the distance between the threads. Since the propeller or "Air screw" works in a fluid, there is some slip and the actual advance does not correspond to the "Pitch" of the propeller blades. The effective pitch is the distance traveled by the propeller in one revolution. The actual pitch or the angle of the blades must be greater than the angle of the effective helix by the amount of slip.If N = Revolutions per minute, P = effective pitch in feet and V = translational velocity in miles per hour, then V= NP/88. With an effective pitch of 5 feet, and 1200 revolutions per minute, the translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2 miles per hour.Excelsior PropellerExcelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.The actual pitch of the blades would be from 15 to 25 per cent greater than the effective pitch because of the slip. To have thrust we must have slip. With the translational velocity equal to the blade-pitch velocity, there is no airstream accelerated by the blades, and consequently there is no thrust due to reaction. The air thrown to the rear of a propeller moves at a greater speed than the translation when thrust is developed, and this stream is known as the "slipstream." The difference between the translational and slipstream velocity is the slip.The angle of the blade face determines the pitch. The greater the angle of the blade with the plane of propeller rotation, the greater is the pitch. This angle is measured from the chord of the working face of the table, or from that side faced to the rear of the blade. In the majority of cases the working face is flat. The front face is always heavily cambered like a wing section, with the greatest thickness about one-third the chord from the entering edge. As in the case of the wing, the camber is of the greatest importance.A uniform pitch propeller has a varying blade angle, smallest at the tip and increasing toward the hub. With a uniform pitch propeller, every part of the blade travels through the same forward distance in one revolution, hence it is necessary to increase the angle toward the hub as the innermost portions travel a smaller distance around the circle of rotation. Theoretically, the angle at the exact center would be 90 degrees. The blade angles at the different points in the length of a uniform pitch propeller are obtained as follows: Draw a right angle triangle in which the altitude is made equal to the pitch, and the base is equal to 3.1416 times the propeller diameter. The angle made by the hypotenuse with the base is the blade angle at the tip. Divide the base into any number of equal spaces and connect the division points with the upper angle. The angles made by these lines with the base are the angles of the different blade sections.Blade Form. The blade may be either straight-sided or curved. In the latter case the most deeply curved edge is generally the entering edge, and the maximum width is about one-third from the tip. Much care is exercised in arranging the outline so that the center of pressure will not be located in an eccentric position and thus harmfully distort the blade when loaded. If this is not attended to, the pitch will vary according to the load. In one make of propeller the blade is purposely made flexible so that the pitch will accommodate itself correctly to different flight speeds and conditions. This, however, is carefully laid out so that the flexure is proportional throughout the blade to the changes in the load.The Lang PropellerThe Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.A "Paragon" Propeller With a Curved Leading Edge.A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.Propeller Diameter. The largest propellers are the most efficient. The propeller should be as large as can be safely swung on the aeroplane. Large, slow revolution propellers are far superior to the small high speed type. It is more economical to accelerate a large mass of air slowly with a large diameter than to speed up a small mass to a high velocity. The diameter used on any aeroplane depends upon the power plant, propeller clearance, height of chassis and many other considerations. Approximately the diameter varies from about 1/3 the span on small speed scouts, to 1/5 or 1/6 of the span on the larger machines.Air Flow. The greater part of the air is taken in through the tips, and is then expelled to the rear. This condition prevails until the blade angle is above 45 degrees, and from this point the flow is outward. Owing to the great angles at the hub, there is little thrust given by the inner third of the blade, the air in this region being simply churned up in a directionless mass of eddies. At the tips the angle is small and the velocity high, which results in about 80 per cent of the useful work being performed by the outer third of the blade. In some aeroplanes a spinner cap is placed around the hub to reduce the churning loss and to streamline the hub. The blade section is very thick at the hub for structural reasons.The "Disc area" of a propeller is the area of the circle swept out by the blades. It is the pressure over this area that gives the thrust, and in some methods of calculation the thrust is based on the mean pressure per square foot of disc area. The pressure is not uniformly distributed over the disc, being many times greater at the outer circumference than at the hub. The average pressure per square foot depends upon the blade section and angle. Because of the great intensity of pressure at the circumference, the effective stream is in the form of a hollow tube.Number of Blades. For training, and ordinary work, two-bladed propellers are preferable, but for large motors where the swing is limited, three or four blades are often used. A multiple-bladed propeller absorbs more horsepower with a given diameter than the two-blade type. In general, a four-bladed propeller revolving slowly may be considered more efficient than the two-blade revolving rapidly. Where the swing and clearance are small, a small four-blade may give better results than a larger and faster two-blade. A three-blade often shows marked superiority over a two-blade even when of smaller diameter, and the hub of the three-blade is much stronger than the four-blade, although neither the three or four is as strong as the two-blade type.Effects of Altitude. At high altitudes the density is less, and consequently the thrust is less with a given number of revolutions per minute. The thrust can be maintained either by increasing the speed, or by increasing the pitch. For correct service at high altitudes the propeller should undoubtedly be of the variable pitch type, in which the pitch can be controlled manually, or by some automatic means such as proportional blade flexure.Effects of Pitch. Driven at a constant speed, both the thrust and horsepower increase with the pitch up to a certain limiting angle.For a given horsepower the static thrust depends both on the diameter and the pitch. If the pitch is increased the diameter must be decreased in proportion to maintain a constant speed. As the pitch is regulated by the translational speed and revolutions, the static thrust of a high speed machine is very small. As the translational speed increases, the pitch relative to the wind is less, and consequently the thrust will pick up until a certain limiting speed is reached.Thrust and Horsepower. The calculation for thrust and power are very complicated, but the primary conditions can be given by the following: Let V = the pitch velocity in feet per minute, T = thrust in pounds, and H = horsepower, then H = TV/33000E from which T = 33000HE/V, the efficiency being designated by E. Since the pitch velocity is NP, where N = revs. per minute and P = pitch in feet, then T = 33000HE/PN. Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the horsepower 100, the thrust will be:T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case is the blade pitch, and the great uncertainty lies in selecting a proper value for E. This may vary from 0.70 to 0.85. The diameter is also an unknown factor in this primitive equation.Materials and Construction. The woods used for propeller construction are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50 H. P. spruce is suitable, as it is light, and strong enough for this power. In Europe walnut and mahogany are the most commonly used, although they are very expensive. Birch is very strong and comparatively light for its strength, and can be used successfully up to 125 horsepower. Ash is strong, light and fibrous, but has the objectionable feature of warping and cannot withstand moisture. Maple is too heavy for its strength. White oak, quarter-sawed, is the best of propeller woods and is used with the very largest engines. It is strong for its weight and is hard, but is very difficult to work and glue. For tropical climates, Southern poplar is frequently used as it has the property of resisting heat and humidity.One-inch boards are rough dressed to 7/8 inch and then finished down to 13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface for the glue, they are thoroughly coated with hot hide glue, piled together in blocks of from 5 to 10 laminations, and then thoroughly squeezed for 18 hours in a press or by clamps until the glue has thoroughly set. Only the best of hide glue is used, applied at a temperature of 140°F. and at a room temperature of 100°. The glue must never be hotter, nor the boards cooler than the temperatures stated. The propeller after being roughed out is left to dry for ten days so that all of the glue stresses are adjusted. If less time is taken, the propeller will warp out of shape. The propeller is worked down within a small fraction of the finished size and is again allowed to rest. After a few days it is finished down to size by hand, is scraped, and tested for pitch, tracking and hub dimensions.The finish is glossy, and may be accomplished by several coats of spar varnish or by repeated applications of hot boiled linseed oil well rubbed in, finishing with three or four coats of wax polish. There should be at least 5 applications of linseed oil, the third coat being sandpapered with No. 0 paper. The wood should be scraped to dimension and must not be touched with sandpaper until at least two coats of varnish or oil have been applied.The wood must be absolutely clear and straight grained, and without discolorations. The boards must be piled so that the edge of the grain is on the face of the blades, and the direction of the annular rings must be alternated in the adjacent boards.Hauteur 2190 (avec helice 3:10)Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".
CHAPTER XVIII. PROPELLERS.Principles and Use of Propellers. A propeller converts the energy of the engine into the thrust required to overcome the resistance of the aeroplane. To maintain flight the thrust, or force exerted by the propeller, must always equal the total resistance of the aeroplane. A total resistance of 400 pounds requires a propeller thrust of 400, and as the resistance varies with the speed, the engine revolutions must be altered correspondingly. The propeller is the most complicated and least understood element of the aeroplane, and we can but touch only on the most elementary features. The inclined blades of the propeller throw back an airstream, the reaction of which produces the thrust. The blades can also be considered as aerofoils moving in a circular path, the lift of the aerofoils corresponding to the thrust of the propeller. The reactions in any case are quite complicated and require the use of higher mathematics for a full understanding.Pitch and Velocity. When in action the propeller rotates, and at the same time advances along a straight line parallel to its axis. As a result, the tips of the propeller blades describe a curve known as "Helix" or screw-thread curve. The action is very similar to that of a screw being turned in a nut. For clearness in explanation we will call the velocity in the aeroplane path the "Translational velocity," and the speed of the tips in their circular path as the "Rotational velocity." When a screw works in a rigid nut it advances a distance equal to the "Pitch" in each revolution, the pitch of a single threaded screw being equal to the distance between the threads. Since the propeller or "Air screw" works in a fluid, there is some slip and the actual advance does not correspond to the "Pitch" of the propeller blades. The effective pitch is the distance traveled by the propeller in one revolution. The actual pitch or the angle of the blades must be greater than the angle of the effective helix by the amount of slip.If N = Revolutions per minute, P = effective pitch in feet and V = translational velocity in miles per hour, then V= NP/88. With an effective pitch of 5 feet, and 1200 revolutions per minute, the translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2 miles per hour.Excelsior PropellerExcelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.The actual pitch of the blades would be from 15 to 25 per cent greater than the effective pitch because of the slip. To have thrust we must have slip. With the translational velocity equal to the blade-pitch velocity, there is no airstream accelerated by the blades, and consequently there is no thrust due to reaction. The air thrown to the rear of a propeller moves at a greater speed than the translation when thrust is developed, and this stream is known as the "slipstream." The difference between the translational and slipstream velocity is the slip.The angle of the blade face determines the pitch. The greater the angle of the blade with the plane of propeller rotation, the greater is the pitch. This angle is measured from the chord of the working face of the table, or from that side faced to the rear of the blade. In the majority of cases the working face is flat. The front face is always heavily cambered like a wing section, with the greatest thickness about one-third the chord from the entering edge. As in the case of the wing, the camber is of the greatest importance.A uniform pitch propeller has a varying blade angle, smallest at the tip and increasing toward the hub. With a uniform pitch propeller, every part of the blade travels through the same forward distance in one revolution, hence it is necessary to increase the angle toward the hub as the innermost portions travel a smaller distance around the circle of rotation. Theoretically, the angle at the exact center would be 90 degrees. The blade angles at the different points in the length of a uniform pitch propeller are obtained as follows: Draw a right angle triangle in which the altitude is made equal to the pitch, and the base is equal to 3.1416 times the propeller diameter. The angle made by the hypotenuse with the base is the blade angle at the tip. Divide the base into any number of equal spaces and connect the division points with the upper angle. The angles made by these lines with the base are the angles of the different blade sections.Blade Form. The blade may be either straight-sided or curved. In the latter case the most deeply curved edge is generally the entering edge, and the maximum width is about one-third from the tip. Much care is exercised in arranging the outline so that the center of pressure will not be located in an eccentric position and thus harmfully distort the blade when loaded. If this is not attended to, the pitch will vary according to the load. In one make of propeller the blade is purposely made flexible so that the pitch will accommodate itself correctly to different flight speeds and conditions. This, however, is carefully laid out so that the flexure is proportional throughout the blade to the changes in the load.The Lang PropellerThe Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.A "Paragon" Propeller With a Curved Leading Edge.A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.Propeller Diameter. The largest propellers are the most efficient. The propeller should be as large as can be safely swung on the aeroplane. Large, slow revolution propellers are far superior to the small high speed type. It is more economical to accelerate a large mass of air slowly with a large diameter than to speed up a small mass to a high velocity. The diameter used on any aeroplane depends upon the power plant, propeller clearance, height of chassis and many other considerations. Approximately the diameter varies from about 1/3 the span on small speed scouts, to 1/5 or 1/6 of the span on the larger machines.Air Flow. The greater part of the air is taken in through the tips, and is then expelled to the rear. This condition prevails until the blade angle is above 45 degrees, and from this point the flow is outward. Owing to the great angles at the hub, there is little thrust given by the inner third of the blade, the air in this region being simply churned up in a directionless mass of eddies. At the tips the angle is small and the velocity high, which results in about 80 per cent of the useful work being performed by the outer third of the blade. In some aeroplanes a spinner cap is placed around the hub to reduce the churning loss and to streamline the hub. The blade section is very thick at the hub for structural reasons.The "Disc area" of a propeller is the area of the circle swept out by the blades. It is the pressure over this area that gives the thrust, and in some methods of calculation the thrust is based on the mean pressure per square foot of disc area. The pressure is not uniformly distributed over the disc, being many times greater at the outer circumference than at the hub. The average pressure per square foot depends upon the blade section and angle. Because of the great intensity of pressure at the circumference, the effective stream is in the form of a hollow tube.Number of Blades. For training, and ordinary work, two-bladed propellers are preferable, but for large motors where the swing is limited, three or four blades are often used. A multiple-bladed propeller absorbs more horsepower with a given diameter than the two-blade type. In general, a four-bladed propeller revolving slowly may be considered more efficient than the two-blade revolving rapidly. Where the swing and clearance are small, a small four-blade may give better results than a larger and faster two-blade. A three-blade often shows marked superiority over a two-blade even when of smaller diameter, and the hub of the three-blade is much stronger than the four-blade, although neither the three or four is as strong as the two-blade type.Effects of Altitude. At high altitudes the density is less, and consequently the thrust is less with a given number of revolutions per minute. The thrust can be maintained either by increasing the speed, or by increasing the pitch. For correct service at high altitudes the propeller should undoubtedly be of the variable pitch type, in which the pitch can be controlled manually, or by some automatic means such as proportional blade flexure.Effects of Pitch. Driven at a constant speed, both the thrust and horsepower increase with the pitch up to a certain limiting angle.For a given horsepower the static thrust depends both on the diameter and the pitch. If the pitch is increased the diameter must be decreased in proportion to maintain a constant speed. As the pitch is regulated by the translational speed and revolutions, the static thrust of a high speed machine is very small. As the translational speed increases, the pitch relative to the wind is less, and consequently the thrust will pick up until a certain limiting speed is reached.Thrust and Horsepower. The calculation for thrust and power are very complicated, but the primary conditions can be given by the following: Let V = the pitch velocity in feet per minute, T = thrust in pounds, and H = horsepower, then H = TV/33000E from which T = 33000HE/V, the efficiency being designated by E. Since the pitch velocity is NP, where N = revs. per minute and P = pitch in feet, then T = 33000HE/PN. Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the horsepower 100, the thrust will be:T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case is the blade pitch, and the great uncertainty lies in selecting a proper value for E. This may vary from 0.70 to 0.85. The diameter is also an unknown factor in this primitive equation.Materials and Construction. The woods used for propeller construction are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50 H. P. spruce is suitable, as it is light, and strong enough for this power. In Europe walnut and mahogany are the most commonly used, although they are very expensive. Birch is very strong and comparatively light for its strength, and can be used successfully up to 125 horsepower. Ash is strong, light and fibrous, but has the objectionable feature of warping and cannot withstand moisture. Maple is too heavy for its strength. White oak, quarter-sawed, is the best of propeller woods and is used with the very largest engines. It is strong for its weight and is hard, but is very difficult to work and glue. For tropical climates, Southern poplar is frequently used as it has the property of resisting heat and humidity.One-inch boards are rough dressed to 7/8 inch and then finished down to 13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface for the glue, they are thoroughly coated with hot hide glue, piled together in blocks of from 5 to 10 laminations, and then thoroughly squeezed for 18 hours in a press or by clamps until the glue has thoroughly set. Only the best of hide glue is used, applied at a temperature of 140°F. and at a room temperature of 100°. The glue must never be hotter, nor the boards cooler than the temperatures stated. The propeller after being roughed out is left to dry for ten days so that all of the glue stresses are adjusted. If less time is taken, the propeller will warp out of shape. The propeller is worked down within a small fraction of the finished size and is again allowed to rest. After a few days it is finished down to size by hand, is scraped, and tested for pitch, tracking and hub dimensions.The finish is glossy, and may be accomplished by several coats of spar varnish or by repeated applications of hot boiled linseed oil well rubbed in, finishing with three or four coats of wax polish. There should be at least 5 applications of linseed oil, the third coat being sandpapered with No. 0 paper. The wood should be scraped to dimension and must not be touched with sandpaper until at least two coats of varnish or oil have been applied.The wood must be absolutely clear and straight grained, and without discolorations. The boards must be piled so that the edge of the grain is on the face of the blades, and the direction of the annular rings must be alternated in the adjacent boards.Hauteur 2190 (avec helice 3:10)Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".
CHAPTER XVIII. PROPELLERS.Principles and Use of Propellers. A propeller converts the energy of the engine into the thrust required to overcome the resistance of the aeroplane. To maintain flight the thrust, or force exerted by the propeller, must always equal the total resistance of the aeroplane. A total resistance of 400 pounds requires a propeller thrust of 400, and as the resistance varies with the speed, the engine revolutions must be altered correspondingly. The propeller is the most complicated and least understood element of the aeroplane, and we can but touch only on the most elementary features. The inclined blades of the propeller throw back an airstream, the reaction of which produces the thrust. The blades can also be considered as aerofoils moving in a circular path, the lift of the aerofoils corresponding to the thrust of the propeller. The reactions in any case are quite complicated and require the use of higher mathematics for a full understanding.Pitch and Velocity. When in action the propeller rotates, and at the same time advances along a straight line parallel to its axis. As a result, the tips of the propeller blades describe a curve known as "Helix" or screw-thread curve. The action is very similar to that of a screw being turned in a nut. For clearness in explanation we will call the velocity in the aeroplane path the "Translational velocity," and the speed of the tips in their circular path as the "Rotational velocity." When a screw works in a rigid nut it advances a distance equal to the "Pitch" in each revolution, the pitch of a single threaded screw being equal to the distance between the threads. Since the propeller or "Air screw" works in a fluid, there is some slip and the actual advance does not correspond to the "Pitch" of the propeller blades. The effective pitch is the distance traveled by the propeller in one revolution. The actual pitch or the angle of the blades must be greater than the angle of the effective helix by the amount of slip.If N = Revolutions per minute, P = effective pitch in feet and V = translational velocity in miles per hour, then V= NP/88. With an effective pitch of 5 feet, and 1200 revolutions per minute, the translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2 miles per hour.Excelsior PropellerExcelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.The actual pitch of the blades would be from 15 to 25 per cent greater than the effective pitch because of the slip. To have thrust we must have slip. With the translational velocity equal to the blade-pitch velocity, there is no airstream accelerated by the blades, and consequently there is no thrust due to reaction. The air thrown to the rear of a propeller moves at a greater speed than the translation when thrust is developed, and this stream is known as the "slipstream." The difference between the translational and slipstream velocity is the slip.The angle of the blade face determines the pitch. The greater the angle of the blade with the plane of propeller rotation, the greater is the pitch. This angle is measured from the chord of the working face of the table, or from that side faced to the rear of the blade. In the majority of cases the working face is flat. The front face is always heavily cambered like a wing section, with the greatest thickness about one-third the chord from the entering edge. As in the case of the wing, the camber is of the greatest importance.A uniform pitch propeller has a varying blade angle, smallest at the tip and increasing toward the hub. With a uniform pitch propeller, every part of the blade travels through the same forward distance in one revolution, hence it is necessary to increase the angle toward the hub as the innermost portions travel a smaller distance around the circle of rotation. Theoretically, the angle at the exact center would be 90 degrees. The blade angles at the different points in the length of a uniform pitch propeller are obtained as follows: Draw a right angle triangle in which the altitude is made equal to the pitch, and the base is equal to 3.1416 times the propeller diameter. The angle made by the hypotenuse with the base is the blade angle at the tip. Divide the base into any number of equal spaces and connect the division points with the upper angle. The angles made by these lines with the base are the angles of the different blade sections.Blade Form. The blade may be either straight-sided or curved. In the latter case the most deeply curved edge is generally the entering edge, and the maximum width is about one-third from the tip. Much care is exercised in arranging the outline so that the center of pressure will not be located in an eccentric position and thus harmfully distort the blade when loaded. If this is not attended to, the pitch will vary according to the load. In one make of propeller the blade is purposely made flexible so that the pitch will accommodate itself correctly to different flight speeds and conditions. This, however, is carefully laid out so that the flexure is proportional throughout the blade to the changes in the load.The Lang PropellerThe Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.A "Paragon" Propeller With a Curved Leading Edge.A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.Propeller Diameter. The largest propellers are the most efficient. The propeller should be as large as can be safely swung on the aeroplane. Large, slow revolution propellers are far superior to the small high speed type. It is more economical to accelerate a large mass of air slowly with a large diameter than to speed up a small mass to a high velocity. The diameter used on any aeroplane depends upon the power plant, propeller clearance, height of chassis and many other considerations. Approximately the diameter varies from about 1/3 the span on small speed scouts, to 1/5 or 1/6 of the span on the larger machines.Air Flow. The greater part of the air is taken in through the tips, and is then expelled to the rear. This condition prevails until the blade angle is above 45 degrees, and from this point the flow is outward. Owing to the great angles at the hub, there is little thrust given by the inner third of the blade, the air in this region being simply churned up in a directionless mass of eddies. At the tips the angle is small and the velocity high, which results in about 80 per cent of the useful work being performed by the outer third of the blade. In some aeroplanes a spinner cap is placed around the hub to reduce the churning loss and to streamline the hub. The blade section is very thick at the hub for structural reasons.The "Disc area" of a propeller is the area of the circle swept out by the blades. It is the pressure over this area that gives the thrust, and in some methods of calculation the thrust is based on the mean pressure per square foot of disc area. The pressure is not uniformly distributed over the disc, being many times greater at the outer circumference than at the hub. The average pressure per square foot depends upon the blade section and angle. Because of the great intensity of pressure at the circumference, the effective stream is in the form of a hollow tube.Number of Blades. For training, and ordinary work, two-bladed propellers are preferable, but for large motors where the swing is limited, three or four blades are often used. A multiple-bladed propeller absorbs more horsepower with a given diameter than the two-blade type. In general, a four-bladed propeller revolving slowly may be considered more efficient than the two-blade revolving rapidly. Where the swing and clearance are small, a small four-blade may give better results than a larger and faster two-blade. A three-blade often shows marked superiority over a two-blade even when of smaller diameter, and the hub of the three-blade is much stronger than the four-blade, although neither the three or four is as strong as the two-blade type.Effects of Altitude. At high altitudes the density is less, and consequently the thrust is less with a given number of revolutions per minute. The thrust can be maintained either by increasing the speed, or by increasing the pitch. For correct service at high altitudes the propeller should undoubtedly be of the variable pitch type, in which the pitch can be controlled manually, or by some automatic means such as proportional blade flexure.Effects of Pitch. Driven at a constant speed, both the thrust and horsepower increase with the pitch up to a certain limiting angle.For a given horsepower the static thrust depends both on the diameter and the pitch. If the pitch is increased the diameter must be decreased in proportion to maintain a constant speed. As the pitch is regulated by the translational speed and revolutions, the static thrust of a high speed machine is very small. As the translational speed increases, the pitch relative to the wind is less, and consequently the thrust will pick up until a certain limiting speed is reached.Thrust and Horsepower. The calculation for thrust and power are very complicated, but the primary conditions can be given by the following: Let V = the pitch velocity in feet per minute, T = thrust in pounds, and H = horsepower, then H = TV/33000E from which T = 33000HE/V, the efficiency being designated by E. Since the pitch velocity is NP, where N = revs. per minute and P = pitch in feet, then T = 33000HE/PN. Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the horsepower 100, the thrust will be:T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case is the blade pitch, and the great uncertainty lies in selecting a proper value for E. This may vary from 0.70 to 0.85. The diameter is also an unknown factor in this primitive equation.Materials and Construction. The woods used for propeller construction are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50 H. P. spruce is suitable, as it is light, and strong enough for this power. In Europe walnut and mahogany are the most commonly used, although they are very expensive. Birch is very strong and comparatively light for its strength, and can be used successfully up to 125 horsepower. Ash is strong, light and fibrous, but has the objectionable feature of warping and cannot withstand moisture. Maple is too heavy for its strength. White oak, quarter-sawed, is the best of propeller woods and is used with the very largest engines. It is strong for its weight and is hard, but is very difficult to work and glue. For tropical climates, Southern poplar is frequently used as it has the property of resisting heat and humidity.One-inch boards are rough dressed to 7/8 inch and then finished down to 13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface for the glue, they are thoroughly coated with hot hide glue, piled together in blocks of from 5 to 10 laminations, and then thoroughly squeezed for 18 hours in a press or by clamps until the glue has thoroughly set. Only the best of hide glue is used, applied at a temperature of 140°F. and at a room temperature of 100°. The glue must never be hotter, nor the boards cooler than the temperatures stated. The propeller after being roughed out is left to dry for ten days so that all of the glue stresses are adjusted. If less time is taken, the propeller will warp out of shape. The propeller is worked down within a small fraction of the finished size and is again allowed to rest. After a few days it is finished down to size by hand, is scraped, and tested for pitch, tracking and hub dimensions.The finish is glossy, and may be accomplished by several coats of spar varnish or by repeated applications of hot boiled linseed oil well rubbed in, finishing with three or four coats of wax polish. There should be at least 5 applications of linseed oil, the third coat being sandpapered with No. 0 paper. The wood should be scraped to dimension and must not be touched with sandpaper until at least two coats of varnish or oil have been applied.The wood must be absolutely clear and straight grained, and without discolorations. The boards must be piled so that the edge of the grain is on the face of the blades, and the direction of the annular rings must be alternated in the adjacent boards.Hauteur 2190 (avec helice 3:10)Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".
Principles and Use of Propellers. A propeller converts the energy of the engine into the thrust required to overcome the resistance of the aeroplane. To maintain flight the thrust, or force exerted by the propeller, must always equal the total resistance of the aeroplane. A total resistance of 400 pounds requires a propeller thrust of 400, and as the resistance varies with the speed, the engine revolutions must be altered correspondingly. The propeller is the most complicated and least understood element of the aeroplane, and we can but touch only on the most elementary features. The inclined blades of the propeller throw back an airstream, the reaction of which produces the thrust. The blades can also be considered as aerofoils moving in a circular path, the lift of the aerofoils corresponding to the thrust of the propeller. The reactions in any case are quite complicated and require the use of higher mathematics for a full understanding.
Pitch and Velocity. When in action the propeller rotates, and at the same time advances along a straight line parallel to its axis. As a result, the tips of the propeller blades describe a curve known as "Helix" or screw-thread curve. The action is very similar to that of a screw being turned in a nut. For clearness in explanation we will call the velocity in the aeroplane path the "Translational velocity," and the speed of the tips in their circular path as the "Rotational velocity." When a screw works in a rigid nut it advances a distance equal to the "Pitch" in each revolution, the pitch of a single threaded screw being equal to the distance between the threads. Since the propeller or "Air screw" works in a fluid, there is some slip and the actual advance does not correspond to the "Pitch" of the propeller blades. The effective pitch is the distance traveled by the propeller in one revolution. The actual pitch or the angle of the blades must be greater than the angle of the effective helix by the amount of slip.
If N = Revolutions per minute, P = effective pitch in feet and V = translational velocity in miles per hour, then V= NP/88. With an effective pitch of 5 feet, and 1200 revolutions per minute, the translational velocity of the aeroplane will be: V = 1200 x 5/88=68.2 miles per hour.
Excelsior PropellerExcelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.
Excelsior Propeller, an Example of American Propeller Construction. This Propeller Is Built Up of Laminations of Ash.
The actual pitch of the blades would be from 15 to 25 per cent greater than the effective pitch because of the slip. To have thrust we must have slip. With the translational velocity equal to the blade-pitch velocity, there is no airstream accelerated by the blades, and consequently there is no thrust due to reaction. The air thrown to the rear of a propeller moves at a greater speed than the translation when thrust is developed, and this stream is known as the "slipstream." The difference between the translational and slipstream velocity is the slip.
The angle of the blade face determines the pitch. The greater the angle of the blade with the plane of propeller rotation, the greater is the pitch. This angle is measured from the chord of the working face of the table, or from that side faced to the rear of the blade. In the majority of cases the working face is flat. The front face is always heavily cambered like a wing section, with the greatest thickness about one-third the chord from the entering edge. As in the case of the wing, the camber is of the greatest importance.
A uniform pitch propeller has a varying blade angle, smallest at the tip and increasing toward the hub. With a uniform pitch propeller, every part of the blade travels through the same forward distance in one revolution, hence it is necessary to increase the angle toward the hub as the innermost portions travel a smaller distance around the circle of rotation. Theoretically, the angle at the exact center would be 90 degrees. The blade angles at the different points in the length of a uniform pitch propeller are obtained as follows: Draw a right angle triangle in which the altitude is made equal to the pitch, and the base is equal to 3.1416 times the propeller diameter. The angle made by the hypotenuse with the base is the blade angle at the tip. Divide the base into any number of equal spaces and connect the division points with the upper angle. The angles made by these lines with the base are the angles of the different blade sections.
Blade Form. The blade may be either straight-sided or curved. In the latter case the most deeply curved edge is generally the entering edge, and the maximum width is about one-third from the tip. Much care is exercised in arranging the outline so that the center of pressure will not be located in an eccentric position and thus harmfully distort the blade when loaded. If this is not attended to, the pitch will vary according to the load. In one make of propeller the blade is purposely made flexible so that the pitch will accommodate itself correctly to different flight speeds and conditions. This, however, is carefully laid out so that the flexure is proportional throughout the blade to the changes in the load.
The Lang PropellerThe Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.
The Lang Propeller, Having Straight Edges, Slightly Tapering Toward the Tips. The Tips Are Sheathed With Thin Copper for Protection Against Spray. This Outline Is Often Known as the "Normale." Type From the French Propeller First Using This Outline.
A "Paragon" Propeller With a Curved Leading Edge.A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.
A "Paragon" Propeller With a Curved Leading Edge. The Maximum Width Is About One-Third the Blade Length from the Tip and §o. Toward the Tip So That It Is Very Narrow at the Outer End. The Steel Propeller Flange Is Shown in Place on the Hub.
Propeller Diameter. The largest propellers are the most efficient. The propeller should be as large as can be safely swung on the aeroplane. Large, slow revolution propellers are far superior to the small high speed type. It is more economical to accelerate a large mass of air slowly with a large diameter than to speed up a small mass to a high velocity. The diameter used on any aeroplane depends upon the power plant, propeller clearance, height of chassis and many other considerations. Approximately the diameter varies from about 1/3 the span on small speed scouts, to 1/5 or 1/6 of the span on the larger machines.
Air Flow. The greater part of the air is taken in through the tips, and is then expelled to the rear. This condition prevails until the blade angle is above 45 degrees, and from this point the flow is outward. Owing to the great angles at the hub, there is little thrust given by the inner third of the blade, the air in this region being simply churned up in a directionless mass of eddies. At the tips the angle is small and the velocity high, which results in about 80 per cent of the useful work being performed by the outer third of the blade. In some aeroplanes a spinner cap is placed around the hub to reduce the churning loss and to streamline the hub. The blade section is very thick at the hub for structural reasons.
The "Disc area" of a propeller is the area of the circle swept out by the blades. It is the pressure over this area that gives the thrust, and in some methods of calculation the thrust is based on the mean pressure per square foot of disc area. The pressure is not uniformly distributed over the disc, being many times greater at the outer circumference than at the hub. The average pressure per square foot depends upon the blade section and angle. Because of the great intensity of pressure at the circumference, the effective stream is in the form of a hollow tube.
Number of Blades. For training, and ordinary work, two-bladed propellers are preferable, but for large motors where the swing is limited, three or four blades are often used. A multiple-bladed propeller absorbs more horsepower with a given diameter than the two-blade type. In general, a four-bladed propeller revolving slowly may be considered more efficient than the two-blade revolving rapidly. Where the swing and clearance are small, a small four-blade may give better results than a larger and faster two-blade. A three-blade often shows marked superiority over a two-blade even when of smaller diameter, and the hub of the three-blade is much stronger than the four-blade, although neither the three or four is as strong as the two-blade type.
Effects of Altitude. At high altitudes the density is less, and consequently the thrust is less with a given number of revolutions per minute. The thrust can be maintained either by increasing the speed, or by increasing the pitch. For correct service at high altitudes the propeller should undoubtedly be of the variable pitch type, in which the pitch can be controlled manually, or by some automatic means such as proportional blade flexure.
Effects of Pitch. Driven at a constant speed, both the thrust and horsepower increase with the pitch up to a certain limiting angle.
For a given horsepower the static thrust depends both on the diameter and the pitch. If the pitch is increased the diameter must be decreased in proportion to maintain a constant speed. As the pitch is regulated by the translational speed and revolutions, the static thrust of a high speed machine is very small. As the translational speed increases, the pitch relative to the wind is less, and consequently the thrust will pick up until a certain limiting speed is reached.
Thrust and Horsepower. The calculation for thrust and power are very complicated, but the primary conditions can be given by the following: Let V = the pitch velocity in feet per minute, T = thrust in pounds, and H = horsepower, then H = TV/33000E from which T = 33000HE/V, the efficiency being designated by E. Since the pitch velocity is NP, where N = revs. per minute and P = pitch in feet, then T = 33000HE/PN. Assuming a 5-foot pitch, 1200 revs., the efficiency = 0.75, and the horsepower 100, the thrust will be:
T = 33000 x 100 x 0.75/5 x 1200 = 412.5 pounds. The pitch in this case is the blade pitch, and the great uncertainty lies in selecting a proper value for E. This may vary from 0.70 to 0.85. The diameter is also an unknown factor in this primitive equation.
Materials and Construction. The woods used for propeller construction are spruce, ash, mahogany, birch, white oak, walnut, and maple. Up to 50 H. P. spruce is suitable, as it is light, and strong enough for this power. In Europe walnut and mahogany are the most commonly used, although they are very expensive. Birch is very strong and comparatively light for its strength, and can be used successfully up to 125 horsepower. Ash is strong, light and fibrous, but has the objectionable feature of warping and cannot withstand moisture. Maple is too heavy for its strength. White oak, quarter-sawed, is the best of propeller woods and is used with the very largest engines. It is strong for its weight and is hard, but is very difficult to work and glue. For tropical climates, Southern poplar is frequently used as it has the property of resisting heat and humidity.
One-inch boards are rough dressed to 7/8 inch and then finished down to 13/16 or 3/4 inch. After a thorough tooth planing to roughen the surface for the glue, they are thoroughly coated with hot hide glue, piled together in blocks of from 5 to 10 laminations, and then thoroughly squeezed for 18 hours in a press or by clamps until the glue has thoroughly set. Only the best of hide glue is used, applied at a temperature of 140°F. and at a room temperature of 100°. The glue must never be hotter, nor the boards cooler than the temperatures stated. The propeller after being roughed out is left to dry for ten days so that all of the glue stresses are adjusted. If less time is taken, the propeller will warp out of shape. The propeller is worked down within a small fraction of the finished size and is again allowed to rest. After a few days it is finished down to size by hand, is scraped, and tested for pitch, tracking and hub dimensions.
The finish is glossy, and may be accomplished by several coats of spar varnish or by repeated applications of hot boiled linseed oil well rubbed in, finishing with three or four coats of wax polish. There should be at least 5 applications of linseed oil, the third coat being sandpapered with No. 0 paper. The wood should be scraped to dimension and must not be touched with sandpaper until at least two coats of varnish or oil have been applied.
The wood must be absolutely clear and straight grained, and without discolorations. The boards must be piled so that the edge of the grain is on the face of the blades, and the direction of the annular rings must be alternated in the adjacent boards.
Hauteur 2190 (avec helice 3:10)
Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".
Plan and Side Elevation of the S.P.A.D. Speed Scout. Courtesy "Aerial Age".