CHAPTER XIII. CHASSIS CONSTRUCTION.General Notes. The chassis or landing gear carries the weight of the aeroplane when resting on or running over the ground, and is subjected to very heavy shocks, especially when landing. It is provided with pneumatic tired wheels, an elastic shock absorbing device, and the structural members that connect the axle with the fuselage. In some forms of landing gear, the wheels are supplemented by long horizontal skids which serve to support the machine after the elastic shock absorbers are fully extended or when the wheels collapse. The skids also protect the aeroplane in cases where the wheels run into a ditch and also prevent the machine from nosing over in a bad landing. Since the skids and their structural members cause a high resistance they are now seldom used except on the larger and slower machines. In running over the ground, or in making a hard landing, part of the shock is taken up by the deflection of the tires and part by the deflection of the shock absorber. The greater the movement of the tires and absorber, the less will be the stress in the frame.In the majority of cases, the shock absorbers consist of rubber bands or cords, these being wound over the axle and under a stationary part of the chassis members. Since rubber is capable of absorbing and dissipating a greater amount of energy per pound of weight than steel, it is the most commonly used material. Rubber causes much less rebound or "kick" than steel springs. The principal objection to rubber is its rotting under the influence of sunlight, or when in contact with lubricating oil. The movement of the axle tube is generally constrained by a slotted guide or by a short radius rod.The design of a suitable chassis is quite a complicated problem, for the stresses are severe, and yet the weight and resistance must be kept at a minimum. In running over rough or soft ground for the "Get off," the shocks and vibration must be absorbed without excessive stress in the framework, and without disturbing the balance or poise of the machine. There must be little tendency toward nosing over, and the machine must be balanced about the tread so that side gusts have little tendency in throwing the machine out of its path. It must be simple and easily repaired, and the wheels must be large enough to roll easily over moderately rough ground.Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Types of Chassis. The simplest and most extensively used landing gear is the "Vee" type shown by Fig. 1, and is equally applicable to monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists of two wheels, an axle, a rubber shock absorber, and two sets of Vee form struts. The chassis shown by Fig. 1-a is that of the Hansa-Brandenburg and is typical of biplane chassis. The winding of the rubber cord and the arrangement of the chassis struts are clearly shown. The two struts are connected at the bottom by a metal fitting, and the rubber is wound over the axle and under this fitting. No guiding device is used for the axle, the machine being freely suspended by the chord. The struts are made as nearly streamline form as possible.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view of the same device, the same reference letters being used in each view. The vertical struts C run from the fuselage at F to the connecting axle guide plate G. The wheels W-W are connected with the steel tube axle A, and the struts are braced against side thrust by the cross-tube D and the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with the guiding slot S for the axle A. The elastic rubber cord absorber passes over the axle and is fastened to the plate G by the studs I. Fig. 4 is a side view of the chassis of the Lawson trainer, which like many other primary training machines, uses a front pilot wheel to guard against nosing over. The rear two wheels (W) are elastically supported between the Vee struts C and F, while the front wheel X is attached to the fuselage by the vertical strut E, and to the rear wheel frame by the tube G. It will be noted that the front wheel is smaller than the rear main wheels, as this wheel carries but little load. The tail skid T is hinged to the fuselage and is provided with elastic cord at the upper end so that the shock is reduced when the tail strikes the ground. Fig. 5 shown directly above the Lawson trainer, is the complete assembly of the Hansa-Brandenburg already described. The tail skid of the Hansa-Brandenburg is indicated by T.Figs. 2-3. Typical "V" Chassis With Axle Guide.Figs. 2-3. Typical "V" Chassis With Axle Guide.The metal shod ash skid stick is hinged to the lower face of the fuselage, and at the upper end is attached to a stationary fuselage member through four turns of elastic cord. When the skid strikes an obstacle the rubber gives and allows the tail to move in relation to the ground. By this arrangement the greater part of the device is enclosed within the fuselage and, hence, produces little head resistance.Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig. 7 is the skid chassis of the Farman biplane which shows clearly the arrangement of the skids and the shock absorbing suspension. A metal bridge is attached to the axle, and a series of short rubber bands are used in connecting the axle bridge, and the bridge on the skid. A triangular tubular radius rod is attached to the axle and hinged to the skid. This restrains the travel of the axle in a fore and aft direction. Another form of skid shock absorber is given by Fig. 8, in which the rubber rings pass over a spool on the axle. The guiding links or radius rods on the inside of the skids regulate the axle travel. In general, the use of a radius rod is not desirable as it transmits a percentage of the shock to the machine.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 9. Chassis Details of the Nieuport Monoplane. Fig. 10 Is a Detail of the Nieuport Spring.Fig. 9. Chassis Details of the Nieuport Monoplane. This Has a Central Skid and Uses an Automobile Type Steel Spring Instead of Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)Fig. 9 is an older form of Nieuport monoplane chassis, a steel cross spring being used in place of the usual rubber bands. This is simple, but comparatively heavy, and is subject to frequent spring breakage. To guard against spring failure, a long ash skid is placed under the axle. The spring system is connected with the body by three sets of oval steel struts. An old type of Curtiss chassis is given by Fig. 11. This has been widely used by amateurs and exhibition flyers, but requires a fairly smooth landing ground as there are no shock absorbers. The only shock absorption is that due to the deflection of the tires. The extreme forward position of the front wheel effectually prevents any tendency toward nosing over when landing.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.A Standard H-3 shock absorbing system is given by Fig. 12. This has a bracket or hanger attached to the axle over which the elastic cord is wrapped. The cord is wrapped in continuous turns between the axle hanger and the bottom of the Vee support members. As shown, the upper streamlined bar is the axle, while the lower is the cross bar brace which serves to hold the lower ends of the U's. I am indebted to "Aerial Age" for this cut. In order to guide the axle in a straight line in its up and down movement, two radius links are attached between the axle and the front vertical strut. One decided advantage of the "Standard construction" is that the cords are wound without crossing the strands, thus reducing cutting and wear between the cord turns. Fig. 13 is a variation of Fig. 12, the cord being wound directly around spools on the axle and the lower stationary cross tube. The axle is guided by a slot in the guide plate at the right, while end motion is controlled by a radius link. Fig. 14 is the double wheel arrangement of a large "Twin" bombing plane. Two wheels are placed directly under each of the motor units so that a portion of the load is communicated to the chassis by tubes. Diagonal tubes transmit the body load to the chassis.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Folding Chassis. Owing to the great relative resistance of the chassis it has been suggested by many designers to provide a folding frame which will automatically fold up into the body after the machine has left the ground. This would be a decided advantage but the gear is complicated and probably not altogether reliable.Height of Chassis. The height of the chassis is made as small as possible with a sufficient clearance for the propeller tips. It is usual to have the tips of the propeller blades clear the ground by from 10 to 12 inches when the aeroplane is standing with the body in a horizontal position. Any smaller clearance is almost certain to result in broken blades when landing at a sharp angle or when running through high grass. If the chassis is excessively high the resistance will be high and the machine is also likely to be top heavy.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Location of Wheels. The exact location of the wheels, in a fore and aft direction, is of the greatest importance. If they are too far ahead of the center of gravity, too much weight will be placed on the tail skid and excessive running will be required to get the tail off the ground. If the wheels are too far back, the machine will be likely to nose over when landing or running over the ground. In any case, the wheels must be well ahead of the center of gravity so that the weight will resist a forward overturning moment. In the majority of orthogonal biplanes, in which the leading edges of the upper and lower wings are on the same vertical line, the center of the wheel is from 3 to 6 inches back of the leading edges. In staggered biplanes the wheel center is from 6 inches to one foot in front of the lower leading edge. This difference is caused by the fact that the center of gravity is nearer the leading edge of a staggered wing than with the Orthogonal type, and hence the wheels must be further forward.Fig 15 (upper diagram) shows the conditions when the machine is running over the ground with the body horizontal. The vertical line a-a passing through the center of gravity C G is a distance N from the center of the wheel. The weight acting down has a tendency to pull the tail down, this moment being equal to the weight of the machine multiplied by the distance N, or W x N. The elevator flap M exerts a lifting force Ky which acts through the lever arm L, and opposes the moment due to the weight. The force K must be equal to K = WN/L. The distance I is the distance of the wheel center line from the entering edge of the wing. The weight on the tail skid S when the machine is resting on the ground will be equal to S = WN/M, and this may range anywhere from 40 to 200 pounds, according to the size of the aeroplane.Fig. 16 illustrates a principle of wheel location advanced by Capt. Byron Q. Jones, and published in "Aviation and Aeronautical Engineers," Nov. 16, 1916. The body is shown in a horizontal position with the propeller axis X-X horizontal. The center of gravity is at G on X-X, the weight acting down as at P with the line prolonged meeting the ground line at B. A line E-E is a line drawn tangent to the wheels and the tail skid at D, the angle of this line with the ground determining the maximum angle of incidence. E-E is the ground line when the machine is at rest. For the best conditions, Capt. Jones finds that the line connecting the point of tangency C, and the center of gravity at G, should make an angle of 13 degrees and 10 minutes with the vertical GB dropped through the center of gravity. With the line GA drawn perpendicular to the resting line E-E, the angle BGA should be 10 degrees as nearly as possible. This is for a two-wheel Vee chassis, but with a third front wheel as with the training of type the angle CGB can be made less. Capt. Jones has found that with the wheels in the above location there will be no tendency to nose over even with very poor landings, and this method has been applied to the training machines at the San Diego Signal Corps aviation school. If the angle BGA is greater than 10 degrees it is difficult to "taxi" the machine on the ground, this tending to make the machine spin or turn into the wind. Capt. Jones claims that a two-wheel chassis arranged according to these rules is superior to the three-wheel type for training purposes since the tendency toward spinning is less.The location of the tail skid S should be such that the elevator and rudder surfaces are well off the ground with the skid fully deflected, and yet the skids must be low enough to permit of the maximum angle of incidence or an angle of EXX = 10 degrees. To a certain extent, the maximum angle of incidence determines the chassis height. If the angle EXX is made greater than the greatest angle of incidence, the wings can be used as air brakes in bringing the machine to a quick stop after landing.The track, or the distance between the centers of the wheels measured along the axle, must be about 1/7 or 0.15 of the span of the lower wing. This makes the track vary from 5 to 7 feet on the usual types, and as high as 15 feet on the large bombing planes. The track must be great enough to prevent overturning when making a landing on soft ground or with a cross wind. If the track is excessive, there will be a heavy spinning moment in cases where one wheel strikes a depression or soft spot in the ground.Shock Absorbers. The axle movement allowed by the elastic shock absorbers and guiding appliances averages from 5 to 6 inches. The greater the movement, the less will be the stresses induced by a given drop, but in practice the movement is generally limited by considerations of chassis height and propeller clearance. It can be proved that a movement of 5 inches will produce a maximum stress equal to 8.6 times the weight of the machine under conditions of a one-foot drop, while with an absorber movement of 6 inches the stress is reduced to 7.5 times the weight. This calculation takes the tire deflection into consideration. With the absorber movement limited to one inch, the stress may be as high as 35 times the weight of the machine.F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress produced by the fall, and x = the absorber movement in inches.Landing Gear Wheels. The wheels are generally of the tangent laced wire spoke type, and are enclosed with discs to reduce the resistance. They must have very wide hubs to resist the heavy end stresses caused by landing sidewise. The length of the hub should be at least twice the diameter of the tire and a greater width, say three times the tire diameter, is preferable. The narrow hubs used on motorcycle wheels are not safe against side blows, although they may be capable of withstanding the vertical load. The wheels are rated according to the outside diameter over the tire, and by the diameter of the tire casing. A 26" x 4" wheel signifies that the outside diameter is 26 inches with a casing diameter of 4 inches.Wheel TableThe 26 x 4 tires are used on the majority of training machines of the two-wheel type, while a 20 x 4 wheel is used for the front wheel of the three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been used to some extent, particularly on the Ackerman spring wheels.
CHAPTER XIII. CHASSIS CONSTRUCTION.General Notes. The chassis or landing gear carries the weight of the aeroplane when resting on or running over the ground, and is subjected to very heavy shocks, especially when landing. It is provided with pneumatic tired wheels, an elastic shock absorbing device, and the structural members that connect the axle with the fuselage. In some forms of landing gear, the wheels are supplemented by long horizontal skids which serve to support the machine after the elastic shock absorbers are fully extended or when the wheels collapse. The skids also protect the aeroplane in cases where the wheels run into a ditch and also prevent the machine from nosing over in a bad landing. Since the skids and their structural members cause a high resistance they are now seldom used except on the larger and slower machines. In running over the ground, or in making a hard landing, part of the shock is taken up by the deflection of the tires and part by the deflection of the shock absorber. The greater the movement of the tires and absorber, the less will be the stress in the frame.In the majority of cases, the shock absorbers consist of rubber bands or cords, these being wound over the axle and under a stationary part of the chassis members. Since rubber is capable of absorbing and dissipating a greater amount of energy per pound of weight than steel, it is the most commonly used material. Rubber causes much less rebound or "kick" than steel springs. The principal objection to rubber is its rotting under the influence of sunlight, or when in contact with lubricating oil. The movement of the axle tube is generally constrained by a slotted guide or by a short radius rod.The design of a suitable chassis is quite a complicated problem, for the stresses are severe, and yet the weight and resistance must be kept at a minimum. In running over rough or soft ground for the "Get off," the shocks and vibration must be absorbed without excessive stress in the framework, and without disturbing the balance or poise of the machine. There must be little tendency toward nosing over, and the machine must be balanced about the tread so that side gusts have little tendency in throwing the machine out of its path. It must be simple and easily repaired, and the wheels must be large enough to roll easily over moderately rough ground.Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Types of Chassis. The simplest and most extensively used landing gear is the "Vee" type shown by Fig. 1, and is equally applicable to monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists of two wheels, an axle, a rubber shock absorber, and two sets of Vee form struts. The chassis shown by Fig. 1-a is that of the Hansa-Brandenburg and is typical of biplane chassis. The winding of the rubber cord and the arrangement of the chassis struts are clearly shown. The two struts are connected at the bottom by a metal fitting, and the rubber is wound over the axle and under this fitting. No guiding device is used for the axle, the machine being freely suspended by the chord. The struts are made as nearly streamline form as possible.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view of the same device, the same reference letters being used in each view. The vertical struts C run from the fuselage at F to the connecting axle guide plate G. The wheels W-W are connected with the steel tube axle A, and the struts are braced against side thrust by the cross-tube D and the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with the guiding slot S for the axle A. The elastic rubber cord absorber passes over the axle and is fastened to the plate G by the studs I. Fig. 4 is a side view of the chassis of the Lawson trainer, which like many other primary training machines, uses a front pilot wheel to guard against nosing over. The rear two wheels (W) are elastically supported between the Vee struts C and F, while the front wheel X is attached to the fuselage by the vertical strut E, and to the rear wheel frame by the tube G. It will be noted that the front wheel is smaller than the rear main wheels, as this wheel carries but little load. The tail skid T is hinged to the fuselage and is provided with elastic cord at the upper end so that the shock is reduced when the tail strikes the ground. Fig. 5 shown directly above the Lawson trainer, is the complete assembly of the Hansa-Brandenburg already described. The tail skid of the Hansa-Brandenburg is indicated by T.Figs. 2-3. Typical "V" Chassis With Axle Guide.Figs. 2-3. Typical "V" Chassis With Axle Guide.The metal shod ash skid stick is hinged to the lower face of the fuselage, and at the upper end is attached to a stationary fuselage member through four turns of elastic cord. When the skid strikes an obstacle the rubber gives and allows the tail to move in relation to the ground. By this arrangement the greater part of the device is enclosed within the fuselage and, hence, produces little head resistance.Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig. 7 is the skid chassis of the Farman biplane which shows clearly the arrangement of the skids and the shock absorbing suspension. A metal bridge is attached to the axle, and a series of short rubber bands are used in connecting the axle bridge, and the bridge on the skid. A triangular tubular radius rod is attached to the axle and hinged to the skid. This restrains the travel of the axle in a fore and aft direction. Another form of skid shock absorber is given by Fig. 8, in which the rubber rings pass over a spool on the axle. The guiding links or radius rods on the inside of the skids regulate the axle travel. In general, the use of a radius rod is not desirable as it transmits a percentage of the shock to the machine.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 9. Chassis Details of the Nieuport Monoplane. Fig. 10 Is a Detail of the Nieuport Spring.Fig. 9. Chassis Details of the Nieuport Monoplane. This Has a Central Skid and Uses an Automobile Type Steel Spring Instead of Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)Fig. 9 is an older form of Nieuport monoplane chassis, a steel cross spring being used in place of the usual rubber bands. This is simple, but comparatively heavy, and is subject to frequent spring breakage. To guard against spring failure, a long ash skid is placed under the axle. The spring system is connected with the body by three sets of oval steel struts. An old type of Curtiss chassis is given by Fig. 11. This has been widely used by amateurs and exhibition flyers, but requires a fairly smooth landing ground as there are no shock absorbers. The only shock absorption is that due to the deflection of the tires. The extreme forward position of the front wheel effectually prevents any tendency toward nosing over when landing.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.A Standard H-3 shock absorbing system is given by Fig. 12. This has a bracket or hanger attached to the axle over which the elastic cord is wrapped. The cord is wrapped in continuous turns between the axle hanger and the bottom of the Vee support members. As shown, the upper streamlined bar is the axle, while the lower is the cross bar brace which serves to hold the lower ends of the U's. I am indebted to "Aerial Age" for this cut. In order to guide the axle in a straight line in its up and down movement, two radius links are attached between the axle and the front vertical strut. One decided advantage of the "Standard construction" is that the cords are wound without crossing the strands, thus reducing cutting and wear between the cord turns. Fig. 13 is a variation of Fig. 12, the cord being wound directly around spools on the axle and the lower stationary cross tube. The axle is guided by a slot in the guide plate at the right, while end motion is controlled by a radius link. Fig. 14 is the double wheel arrangement of a large "Twin" bombing plane. Two wheels are placed directly under each of the motor units so that a portion of the load is communicated to the chassis by tubes. Diagonal tubes transmit the body load to the chassis.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Folding Chassis. Owing to the great relative resistance of the chassis it has been suggested by many designers to provide a folding frame which will automatically fold up into the body after the machine has left the ground. This would be a decided advantage but the gear is complicated and probably not altogether reliable.Height of Chassis. The height of the chassis is made as small as possible with a sufficient clearance for the propeller tips. It is usual to have the tips of the propeller blades clear the ground by from 10 to 12 inches when the aeroplane is standing with the body in a horizontal position. Any smaller clearance is almost certain to result in broken blades when landing at a sharp angle or when running through high grass. If the chassis is excessively high the resistance will be high and the machine is also likely to be top heavy.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Location of Wheels. The exact location of the wheels, in a fore and aft direction, is of the greatest importance. If they are too far ahead of the center of gravity, too much weight will be placed on the tail skid and excessive running will be required to get the tail off the ground. If the wheels are too far back, the machine will be likely to nose over when landing or running over the ground. In any case, the wheels must be well ahead of the center of gravity so that the weight will resist a forward overturning moment. In the majority of orthogonal biplanes, in which the leading edges of the upper and lower wings are on the same vertical line, the center of the wheel is from 3 to 6 inches back of the leading edges. In staggered biplanes the wheel center is from 6 inches to one foot in front of the lower leading edge. This difference is caused by the fact that the center of gravity is nearer the leading edge of a staggered wing than with the Orthogonal type, and hence the wheels must be further forward.Fig 15 (upper diagram) shows the conditions when the machine is running over the ground with the body horizontal. The vertical line a-a passing through the center of gravity C G is a distance N from the center of the wheel. The weight acting down has a tendency to pull the tail down, this moment being equal to the weight of the machine multiplied by the distance N, or W x N. The elevator flap M exerts a lifting force Ky which acts through the lever arm L, and opposes the moment due to the weight. The force K must be equal to K = WN/L. The distance I is the distance of the wheel center line from the entering edge of the wing. The weight on the tail skid S when the machine is resting on the ground will be equal to S = WN/M, and this may range anywhere from 40 to 200 pounds, according to the size of the aeroplane.Fig. 16 illustrates a principle of wheel location advanced by Capt. Byron Q. Jones, and published in "Aviation and Aeronautical Engineers," Nov. 16, 1916. The body is shown in a horizontal position with the propeller axis X-X horizontal. The center of gravity is at G on X-X, the weight acting down as at P with the line prolonged meeting the ground line at B. A line E-E is a line drawn tangent to the wheels and the tail skid at D, the angle of this line with the ground determining the maximum angle of incidence. E-E is the ground line when the machine is at rest. For the best conditions, Capt. Jones finds that the line connecting the point of tangency C, and the center of gravity at G, should make an angle of 13 degrees and 10 minutes with the vertical GB dropped through the center of gravity. With the line GA drawn perpendicular to the resting line E-E, the angle BGA should be 10 degrees as nearly as possible. This is for a two-wheel Vee chassis, but with a third front wheel as with the training of type the angle CGB can be made less. Capt. Jones has found that with the wheels in the above location there will be no tendency to nose over even with very poor landings, and this method has been applied to the training machines at the San Diego Signal Corps aviation school. If the angle BGA is greater than 10 degrees it is difficult to "taxi" the machine on the ground, this tending to make the machine spin or turn into the wind. Capt. Jones claims that a two-wheel chassis arranged according to these rules is superior to the three-wheel type for training purposes since the tendency toward spinning is less.The location of the tail skid S should be such that the elevator and rudder surfaces are well off the ground with the skid fully deflected, and yet the skids must be low enough to permit of the maximum angle of incidence or an angle of EXX = 10 degrees. To a certain extent, the maximum angle of incidence determines the chassis height. If the angle EXX is made greater than the greatest angle of incidence, the wings can be used as air brakes in bringing the machine to a quick stop after landing.The track, or the distance between the centers of the wheels measured along the axle, must be about 1/7 or 0.15 of the span of the lower wing. This makes the track vary from 5 to 7 feet on the usual types, and as high as 15 feet on the large bombing planes. The track must be great enough to prevent overturning when making a landing on soft ground or with a cross wind. If the track is excessive, there will be a heavy spinning moment in cases where one wheel strikes a depression or soft spot in the ground.Shock Absorbers. The axle movement allowed by the elastic shock absorbers and guiding appliances averages from 5 to 6 inches. The greater the movement, the less will be the stresses induced by a given drop, but in practice the movement is generally limited by considerations of chassis height and propeller clearance. It can be proved that a movement of 5 inches will produce a maximum stress equal to 8.6 times the weight of the machine under conditions of a one-foot drop, while with an absorber movement of 6 inches the stress is reduced to 7.5 times the weight. This calculation takes the tire deflection into consideration. With the absorber movement limited to one inch, the stress may be as high as 35 times the weight of the machine.F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress produced by the fall, and x = the absorber movement in inches.Landing Gear Wheels. The wheels are generally of the tangent laced wire spoke type, and are enclosed with discs to reduce the resistance. They must have very wide hubs to resist the heavy end stresses caused by landing sidewise. The length of the hub should be at least twice the diameter of the tire and a greater width, say three times the tire diameter, is preferable. The narrow hubs used on motorcycle wheels are not safe against side blows, although they may be capable of withstanding the vertical load. The wheels are rated according to the outside diameter over the tire, and by the diameter of the tire casing. A 26" x 4" wheel signifies that the outside diameter is 26 inches with a casing diameter of 4 inches.Wheel TableThe 26 x 4 tires are used on the majority of training machines of the two-wheel type, while a 20 x 4 wheel is used for the front wheel of the three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been used to some extent, particularly on the Ackerman spring wheels.
CHAPTER XIII. CHASSIS CONSTRUCTION.General Notes. The chassis or landing gear carries the weight of the aeroplane when resting on or running over the ground, and is subjected to very heavy shocks, especially when landing. It is provided with pneumatic tired wheels, an elastic shock absorbing device, and the structural members that connect the axle with the fuselage. In some forms of landing gear, the wheels are supplemented by long horizontal skids which serve to support the machine after the elastic shock absorbers are fully extended or when the wheels collapse. The skids also protect the aeroplane in cases where the wheels run into a ditch and also prevent the machine from nosing over in a bad landing. Since the skids and their structural members cause a high resistance they are now seldom used except on the larger and slower machines. In running over the ground, or in making a hard landing, part of the shock is taken up by the deflection of the tires and part by the deflection of the shock absorber. The greater the movement of the tires and absorber, the less will be the stress in the frame.In the majority of cases, the shock absorbers consist of rubber bands or cords, these being wound over the axle and under a stationary part of the chassis members. Since rubber is capable of absorbing and dissipating a greater amount of energy per pound of weight than steel, it is the most commonly used material. Rubber causes much less rebound or "kick" than steel springs. The principal objection to rubber is its rotting under the influence of sunlight, or when in contact with lubricating oil. The movement of the axle tube is generally constrained by a slotted guide or by a short radius rod.The design of a suitable chassis is quite a complicated problem, for the stresses are severe, and yet the weight and resistance must be kept at a minimum. In running over rough or soft ground for the "Get off," the shocks and vibration must be absorbed without excessive stress in the framework, and without disturbing the balance or poise of the machine. There must be little tendency toward nosing over, and the machine must be balanced about the tread so that side gusts have little tendency in throwing the machine out of its path. It must be simple and easily repaired, and the wheels must be large enough to roll easily over moderately rough ground.Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Types of Chassis. The simplest and most extensively used landing gear is the "Vee" type shown by Fig. 1, and is equally applicable to monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists of two wheels, an axle, a rubber shock absorber, and two sets of Vee form struts. The chassis shown by Fig. 1-a is that of the Hansa-Brandenburg and is typical of biplane chassis. The winding of the rubber cord and the arrangement of the chassis struts are clearly shown. The two struts are connected at the bottom by a metal fitting, and the rubber is wound over the axle and under this fitting. No guiding device is used for the axle, the machine being freely suspended by the chord. The struts are made as nearly streamline form as possible.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view of the same device, the same reference letters being used in each view. The vertical struts C run from the fuselage at F to the connecting axle guide plate G. The wheels W-W are connected with the steel tube axle A, and the struts are braced against side thrust by the cross-tube D and the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with the guiding slot S for the axle A. The elastic rubber cord absorber passes over the axle and is fastened to the plate G by the studs I. Fig. 4 is a side view of the chassis of the Lawson trainer, which like many other primary training machines, uses a front pilot wheel to guard against nosing over. The rear two wheels (W) are elastically supported between the Vee struts C and F, while the front wheel X is attached to the fuselage by the vertical strut E, and to the rear wheel frame by the tube G. It will be noted that the front wheel is smaller than the rear main wheels, as this wheel carries but little load. The tail skid T is hinged to the fuselage and is provided with elastic cord at the upper end so that the shock is reduced when the tail strikes the ground. Fig. 5 shown directly above the Lawson trainer, is the complete assembly of the Hansa-Brandenburg already described. The tail skid of the Hansa-Brandenburg is indicated by T.Figs. 2-3. Typical "V" Chassis With Axle Guide.Figs. 2-3. Typical "V" Chassis With Axle Guide.The metal shod ash skid stick is hinged to the lower face of the fuselage, and at the upper end is attached to a stationary fuselage member through four turns of elastic cord. When the skid strikes an obstacle the rubber gives and allows the tail to move in relation to the ground. By this arrangement the greater part of the device is enclosed within the fuselage and, hence, produces little head resistance.Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig. 7 is the skid chassis of the Farman biplane which shows clearly the arrangement of the skids and the shock absorbing suspension. A metal bridge is attached to the axle, and a series of short rubber bands are used in connecting the axle bridge, and the bridge on the skid. A triangular tubular radius rod is attached to the axle and hinged to the skid. This restrains the travel of the axle in a fore and aft direction. Another form of skid shock absorber is given by Fig. 8, in which the rubber rings pass over a spool on the axle. The guiding links or radius rods on the inside of the skids regulate the axle travel. In general, the use of a radius rod is not desirable as it transmits a percentage of the shock to the machine.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 9. Chassis Details of the Nieuport Monoplane. Fig. 10 Is a Detail of the Nieuport Spring.Fig. 9. Chassis Details of the Nieuport Monoplane. This Has a Central Skid and Uses an Automobile Type Steel Spring Instead of Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)Fig. 9 is an older form of Nieuport monoplane chassis, a steel cross spring being used in place of the usual rubber bands. This is simple, but comparatively heavy, and is subject to frequent spring breakage. To guard against spring failure, a long ash skid is placed under the axle. The spring system is connected with the body by three sets of oval steel struts. An old type of Curtiss chassis is given by Fig. 11. This has been widely used by amateurs and exhibition flyers, but requires a fairly smooth landing ground as there are no shock absorbers. The only shock absorption is that due to the deflection of the tires. The extreme forward position of the front wheel effectually prevents any tendency toward nosing over when landing.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.A Standard H-3 shock absorbing system is given by Fig. 12. This has a bracket or hanger attached to the axle over which the elastic cord is wrapped. The cord is wrapped in continuous turns between the axle hanger and the bottom of the Vee support members. As shown, the upper streamlined bar is the axle, while the lower is the cross bar brace which serves to hold the lower ends of the U's. I am indebted to "Aerial Age" for this cut. In order to guide the axle in a straight line in its up and down movement, two radius links are attached between the axle and the front vertical strut. One decided advantage of the "Standard construction" is that the cords are wound without crossing the strands, thus reducing cutting and wear between the cord turns. Fig. 13 is a variation of Fig. 12, the cord being wound directly around spools on the axle and the lower stationary cross tube. The axle is guided by a slot in the guide plate at the right, while end motion is controlled by a radius link. Fig. 14 is the double wheel arrangement of a large "Twin" bombing plane. Two wheels are placed directly under each of the motor units so that a portion of the load is communicated to the chassis by tubes. Diagonal tubes transmit the body load to the chassis.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Folding Chassis. Owing to the great relative resistance of the chassis it has been suggested by many designers to provide a folding frame which will automatically fold up into the body after the machine has left the ground. This would be a decided advantage but the gear is complicated and probably not altogether reliable.Height of Chassis. The height of the chassis is made as small as possible with a sufficient clearance for the propeller tips. It is usual to have the tips of the propeller blades clear the ground by from 10 to 12 inches when the aeroplane is standing with the body in a horizontal position. Any smaller clearance is almost certain to result in broken blades when landing at a sharp angle or when running through high grass. If the chassis is excessively high the resistance will be high and the machine is also likely to be top heavy.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Location of Wheels. The exact location of the wheels, in a fore and aft direction, is of the greatest importance. If they are too far ahead of the center of gravity, too much weight will be placed on the tail skid and excessive running will be required to get the tail off the ground. If the wheels are too far back, the machine will be likely to nose over when landing or running over the ground. In any case, the wheels must be well ahead of the center of gravity so that the weight will resist a forward overturning moment. In the majority of orthogonal biplanes, in which the leading edges of the upper and lower wings are on the same vertical line, the center of the wheel is from 3 to 6 inches back of the leading edges. In staggered biplanes the wheel center is from 6 inches to one foot in front of the lower leading edge. This difference is caused by the fact that the center of gravity is nearer the leading edge of a staggered wing than with the Orthogonal type, and hence the wheels must be further forward.Fig 15 (upper diagram) shows the conditions when the machine is running over the ground with the body horizontal. The vertical line a-a passing through the center of gravity C G is a distance N from the center of the wheel. The weight acting down has a tendency to pull the tail down, this moment being equal to the weight of the machine multiplied by the distance N, or W x N. The elevator flap M exerts a lifting force Ky which acts through the lever arm L, and opposes the moment due to the weight. The force K must be equal to K = WN/L. The distance I is the distance of the wheel center line from the entering edge of the wing. The weight on the tail skid S when the machine is resting on the ground will be equal to S = WN/M, and this may range anywhere from 40 to 200 pounds, according to the size of the aeroplane.Fig. 16 illustrates a principle of wheel location advanced by Capt. Byron Q. Jones, and published in "Aviation and Aeronautical Engineers," Nov. 16, 1916. The body is shown in a horizontal position with the propeller axis X-X horizontal. The center of gravity is at G on X-X, the weight acting down as at P with the line prolonged meeting the ground line at B. A line E-E is a line drawn tangent to the wheels and the tail skid at D, the angle of this line with the ground determining the maximum angle of incidence. E-E is the ground line when the machine is at rest. For the best conditions, Capt. Jones finds that the line connecting the point of tangency C, and the center of gravity at G, should make an angle of 13 degrees and 10 minutes with the vertical GB dropped through the center of gravity. With the line GA drawn perpendicular to the resting line E-E, the angle BGA should be 10 degrees as nearly as possible. This is for a two-wheel Vee chassis, but with a third front wheel as with the training of type the angle CGB can be made less. Capt. Jones has found that with the wheels in the above location there will be no tendency to nose over even with very poor landings, and this method has been applied to the training machines at the San Diego Signal Corps aviation school. If the angle BGA is greater than 10 degrees it is difficult to "taxi" the machine on the ground, this tending to make the machine spin or turn into the wind. Capt. Jones claims that a two-wheel chassis arranged according to these rules is superior to the three-wheel type for training purposes since the tendency toward spinning is less.The location of the tail skid S should be such that the elevator and rudder surfaces are well off the ground with the skid fully deflected, and yet the skids must be low enough to permit of the maximum angle of incidence or an angle of EXX = 10 degrees. To a certain extent, the maximum angle of incidence determines the chassis height. If the angle EXX is made greater than the greatest angle of incidence, the wings can be used as air brakes in bringing the machine to a quick stop after landing.The track, or the distance between the centers of the wheels measured along the axle, must be about 1/7 or 0.15 of the span of the lower wing. This makes the track vary from 5 to 7 feet on the usual types, and as high as 15 feet on the large bombing planes. The track must be great enough to prevent overturning when making a landing on soft ground or with a cross wind. If the track is excessive, there will be a heavy spinning moment in cases where one wheel strikes a depression or soft spot in the ground.Shock Absorbers. The axle movement allowed by the elastic shock absorbers and guiding appliances averages from 5 to 6 inches. The greater the movement, the less will be the stresses induced by a given drop, but in practice the movement is generally limited by considerations of chassis height and propeller clearance. It can be proved that a movement of 5 inches will produce a maximum stress equal to 8.6 times the weight of the machine under conditions of a one-foot drop, while with an absorber movement of 6 inches the stress is reduced to 7.5 times the weight. This calculation takes the tire deflection into consideration. With the absorber movement limited to one inch, the stress may be as high as 35 times the weight of the machine.F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress produced by the fall, and x = the absorber movement in inches.Landing Gear Wheels. The wheels are generally of the tangent laced wire spoke type, and are enclosed with discs to reduce the resistance. They must have very wide hubs to resist the heavy end stresses caused by landing sidewise. The length of the hub should be at least twice the diameter of the tire and a greater width, say three times the tire diameter, is preferable. The narrow hubs used on motorcycle wheels are not safe against side blows, although they may be capable of withstanding the vertical load. The wheels are rated according to the outside diameter over the tire, and by the diameter of the tire casing. A 26" x 4" wheel signifies that the outside diameter is 26 inches with a casing diameter of 4 inches.Wheel TableThe 26 x 4 tires are used on the majority of training machines of the two-wheel type, while a 20 x 4 wheel is used for the front wheel of the three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been used to some extent, particularly on the Ackerman spring wheels.
General Notes. The chassis or landing gear carries the weight of the aeroplane when resting on or running over the ground, and is subjected to very heavy shocks, especially when landing. It is provided with pneumatic tired wheels, an elastic shock absorbing device, and the structural members that connect the axle with the fuselage. In some forms of landing gear, the wheels are supplemented by long horizontal skids which serve to support the machine after the elastic shock absorbers are fully extended or when the wheels collapse. The skids also protect the aeroplane in cases where the wheels run into a ditch and also prevent the machine from nosing over in a bad landing. Since the skids and their structural members cause a high resistance they are now seldom used except on the larger and slower machines. In running over the ground, or in making a hard landing, part of the shock is taken up by the deflection of the tires and part by the deflection of the shock absorber. The greater the movement of the tires and absorber, the less will be the stress in the frame.
In the majority of cases, the shock absorbers consist of rubber bands or cords, these being wound over the axle and under a stationary part of the chassis members. Since rubber is capable of absorbing and dissipating a greater amount of energy per pound of weight than steel, it is the most commonly used material. Rubber causes much less rebound or "kick" than steel springs. The principal objection to rubber is its rotting under the influence of sunlight, or when in contact with lubricating oil. The movement of the axle tube is generally constrained by a slotted guide or by a short radius rod.
The design of a suitable chassis is quite a complicated problem, for the stresses are severe, and yet the weight and resistance must be kept at a minimum. In running over rough or soft ground for the "Get off," the shocks and vibration must be absorbed without excessive stress in the framework, and without disturbing the balance or poise of the machine. There must be little tendency toward nosing over, and the machine must be balanced about the tread so that side gusts have little tendency in throwing the machine out of its path. It must be simple and easily repaired, and the wheels must be large enough to roll easily over moderately rough ground.
Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."
Fig 1. "V" Type Chassis as Applied to "Zens" Monoplane. Courtesy "Flight."
Types of Chassis. The simplest and most extensively used landing gear is the "Vee" type shown by Fig. 1, and is equally applicable to monoplanes, biplanes or triplanes. Primarily, the Vee chassis consists of two wheels, an axle, a rubber shock absorber, and two sets of Vee form struts. The chassis shown by Fig. 1-a is that of the Hansa-Brandenburg and is typical of biplane chassis. The winding of the rubber cord and the arrangement of the chassis struts are clearly shown. The two struts are connected at the bottom by a metal fitting, and the rubber is wound over the axle and under this fitting. No guiding device is used for the axle, the machine being freely suspended by the chord. The struts are made as nearly streamline form as possible.
Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.
Fig 1a. "V" Type Chassis Used on Hansa-Brandenberg Biplane.
Fig. 2 is a front view of a typical Vee chassis, and Fig. 3 is side view of the same device, the same reference letters being used in each view. The vertical struts C run from the fuselage at F to the connecting axle guide plate G. The wheels W-W are connected with the steel tube axle A, and the struts are braced against side thrust by the cross-tube D and the stay wire braces B-B. In Fig. 3 the metal fitting G is provided with the guiding slot S for the axle A. The elastic rubber cord absorber passes over the axle and is fastened to the plate G by the studs I. Fig. 4 is a side view of the chassis of the Lawson trainer, which like many other primary training machines, uses a front pilot wheel to guard against nosing over. The rear two wheels (W) are elastically supported between the Vee struts C and F, while the front wheel X is attached to the fuselage by the vertical strut E, and to the rear wheel frame by the tube G. It will be noted that the front wheel is smaller than the rear main wheels, as this wheel carries but little load. The tail skid T is hinged to the fuselage and is provided with elastic cord at the upper end so that the shock is reduced when the tail strikes the ground. Fig. 5 shown directly above the Lawson trainer, is the complete assembly of the Hansa-Brandenburg already described. The tail skid of the Hansa-Brandenburg is indicated by T.
Figs. 2-3. Typical "V" Chassis With Axle Guide.Figs. 2-3. Typical "V" Chassis With Axle Guide.
Figs. 2-3. Typical "V" Chassis With Axle Guide.
The metal shod ash skid stick is hinged to the lower face of the fuselage, and at the upper end is attached to a stationary fuselage member through four turns of elastic cord. When the skid strikes an obstacle the rubber gives and allows the tail to move in relation to the ground. By this arrangement the greater part of the device is enclosed within the fuselage and, hence, produces little head resistance.
Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).
Fig 4. (Below). Lawson Training Tractor Biplane. Fig. 5 (Above). Hansa-Brandenburg Fighting Biplane Showing Chassis and Tail Skid (t).
Fig. 7 is the skid chassis of the Farman biplane which shows clearly the arrangement of the skids and the shock absorbing suspension. A metal bridge is attached to the axle, and a series of short rubber bands are used in connecting the axle bridge, and the bridge on the skid. A triangular tubular radius rod is attached to the axle and hinged to the skid. This restrains the travel of the axle in a fore and aft direction. Another form of skid shock absorber is given by Fig. 8, in which the rubber rings pass over a spool on the axle. The guiding links or radius rods on the inside of the skids regulate the axle travel. In general, the use of a radius rod is not desirable as it transmits a percentage of the shock to the machine.
Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.
Fig. 7. (Left). Farman Skid Type Chassis. Fig. 8. Another Type of Skid Chassis in Which the Axle Is Guided by a Radius Rod or Lever.
Fig. 9. Chassis Details of the Nieuport Monoplane. Fig. 10 Is a Detail of the Nieuport Spring.Fig. 9. Chassis Details of the Nieuport Monoplane. This Has a Central Skid and Uses an Automobile Type Steel Spring Instead of Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)
Fig. 9. Chassis Details of the Nieuport Monoplane. This Has a Central Skid and Uses an Automobile Type Steel Spring Instead of Rubber Cord. Fig. 10 Is a Detail of the Nieuport Spring. (At Right.)
Fig. 9 is an older form of Nieuport monoplane chassis, a steel cross spring being used in place of the usual rubber bands. This is simple, but comparatively heavy, and is subject to frequent spring breakage. To guard against spring failure, a long ash skid is placed under the axle. The spring system is connected with the body by three sets of oval steel struts. An old type of Curtiss chassis is given by Fig. 11. This has been widely used by amateurs and exhibition flyers, but requires a fairly smooth landing ground as there are no shock absorbers. The only shock absorption is that due to the deflection of the tires. The extreme forward position of the front wheel effectually prevents any tendency toward nosing over when landing.
Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.
Fig. 11. An Old Type of Curtiss Exhibition Chassis With Three Wheels.
Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.
Fig. 12 (Left). Standard H-3 Shock Absorber. Fig. 13. (Right). Rubber Cord on Axle.
A Standard H-3 shock absorbing system is given by Fig. 12. This has a bracket or hanger attached to the axle over which the elastic cord is wrapped. The cord is wrapped in continuous turns between the axle hanger and the bottom of the Vee support members. As shown, the upper streamlined bar is the axle, while the lower is the cross bar brace which serves to hold the lower ends of the U's. I am indebted to "Aerial Age" for this cut. In order to guide the axle in a straight line in its up and down movement, two radius links are attached between the axle and the front vertical strut. One decided advantage of the "Standard construction" is that the cords are wound without crossing the strands, thus reducing cutting and wear between the cord turns. Fig. 13 is a variation of Fig. 12, the cord being wound directly around spools on the axle and the lower stationary cross tube. The axle is guided by a slot in the guide plate at the right, while end motion is controlled by a radius link. Fig. 14 is the double wheel arrangement of a large "Twin" bombing plane. Two wheels are placed directly under each of the motor units so that a portion of the load is communicated to the chassis by tubes. Diagonal tubes transmit the body load to the chassis.
Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.
Fig. 14. Chassis for Twin Motored Biplane of Bombing Type.
Folding Chassis. Owing to the great relative resistance of the chassis it has been suggested by many designers to provide a folding frame which will automatically fold up into the body after the machine has left the ground. This would be a decided advantage but the gear is complicated and probably not altogether reliable.
Height of Chassis. The height of the chassis is made as small as possible with a sufficient clearance for the propeller tips. It is usual to have the tips of the propeller blades clear the ground by from 10 to 12 inches when the aeroplane is standing with the body in a horizontal position. Any smaller clearance is almost certain to result in broken blades when landing at a sharp angle or when running through high grass. If the chassis is excessively high the resistance will be high and the machine is also likely to be top heavy.
Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.
Figs. 15-16. Methods of Calculating Wheel Position on Two Wheel Chassis. This Is an Important Item in the Design of an Aeroplane.
Location of Wheels. The exact location of the wheels, in a fore and aft direction, is of the greatest importance. If they are too far ahead of the center of gravity, too much weight will be placed on the tail skid and excessive running will be required to get the tail off the ground. If the wheels are too far back, the machine will be likely to nose over when landing or running over the ground. In any case, the wheels must be well ahead of the center of gravity so that the weight will resist a forward overturning moment. In the majority of orthogonal biplanes, in which the leading edges of the upper and lower wings are on the same vertical line, the center of the wheel is from 3 to 6 inches back of the leading edges. In staggered biplanes the wheel center is from 6 inches to one foot in front of the lower leading edge. This difference is caused by the fact that the center of gravity is nearer the leading edge of a staggered wing than with the Orthogonal type, and hence the wheels must be further forward.
Fig 15 (upper diagram) shows the conditions when the machine is running over the ground with the body horizontal. The vertical line a-a passing through the center of gravity C G is a distance N from the center of the wheel. The weight acting down has a tendency to pull the tail down, this moment being equal to the weight of the machine multiplied by the distance N, or W x N. The elevator flap M exerts a lifting force Ky which acts through the lever arm L, and opposes the moment due to the weight. The force K must be equal to K = WN/L. The distance I is the distance of the wheel center line from the entering edge of the wing. The weight on the tail skid S when the machine is resting on the ground will be equal to S = WN/M, and this may range anywhere from 40 to 200 pounds, according to the size of the aeroplane.
Fig. 16 illustrates a principle of wheel location advanced by Capt. Byron Q. Jones, and published in "Aviation and Aeronautical Engineers," Nov. 16, 1916. The body is shown in a horizontal position with the propeller axis X-X horizontal. The center of gravity is at G on X-X, the weight acting down as at P with the line prolonged meeting the ground line at B. A line E-E is a line drawn tangent to the wheels and the tail skid at D, the angle of this line with the ground determining the maximum angle of incidence. E-E is the ground line when the machine is at rest. For the best conditions, Capt. Jones finds that the line connecting the point of tangency C, and the center of gravity at G, should make an angle of 13 degrees and 10 minutes with the vertical GB dropped through the center of gravity. With the line GA drawn perpendicular to the resting line E-E, the angle BGA should be 10 degrees as nearly as possible. This is for a two-wheel Vee chassis, but with a third front wheel as with the training of type the angle CGB can be made less. Capt. Jones has found that with the wheels in the above location there will be no tendency to nose over even with very poor landings, and this method has been applied to the training machines at the San Diego Signal Corps aviation school. If the angle BGA is greater than 10 degrees it is difficult to "taxi" the machine on the ground, this tending to make the machine spin or turn into the wind. Capt. Jones claims that a two-wheel chassis arranged according to these rules is superior to the three-wheel type for training purposes since the tendency toward spinning is less.
The location of the tail skid S should be such that the elevator and rudder surfaces are well off the ground with the skid fully deflected, and yet the skids must be low enough to permit of the maximum angle of incidence or an angle of EXX = 10 degrees. To a certain extent, the maximum angle of incidence determines the chassis height. If the angle EXX is made greater than the greatest angle of incidence, the wings can be used as air brakes in bringing the machine to a quick stop after landing.
The track, or the distance between the centers of the wheels measured along the axle, must be about 1/7 or 0.15 of the span of the lower wing. This makes the track vary from 5 to 7 feet on the usual types, and as high as 15 feet on the large bombing planes. The track must be great enough to prevent overturning when making a landing on soft ground or with a cross wind. If the track is excessive, there will be a heavy spinning moment in cases where one wheel strikes a depression or soft spot in the ground.
Shock Absorbers. The axle movement allowed by the elastic shock absorbers and guiding appliances averages from 5 to 6 inches. The greater the movement, the less will be the stresses induced by a given drop, but in practice the movement is generally limited by considerations of chassis height and propeller clearance. It can be proved that a movement of 5 inches will produce a maximum stress equal to 8.6 times the weight of the machine under conditions of a one-foot drop, while with an absorber movement of 6 inches the stress is reduced to 7.5 times the weight. This calculation takes the tire deflection into consideration. With the absorber movement limited to one inch, the stress may be as high as 35 times the weight of the machine.
F=W (2 + 2.77/x) where W = weight of machine in pounds, F = the stress produced by the fall, and x = the absorber movement in inches.
Landing Gear Wheels. The wheels are generally of the tangent laced wire spoke type, and are enclosed with discs to reduce the resistance. They must have very wide hubs to resist the heavy end stresses caused by landing sidewise. The length of the hub should be at least twice the diameter of the tire and a greater width, say three times the tire diameter, is preferable. The narrow hubs used on motorcycle wheels are not safe against side blows, although they may be capable of withstanding the vertical load. The wheels are rated according to the outside diameter over the tire, and by the diameter of the tire casing. A 26" x 4" wheel signifies that the outside diameter is 26 inches with a casing diameter of 4 inches.
Wheel Table
The 26 x 4 tires are used on the majority of training machines of the two-wheel type, while a 20 x 4 wheel is used for the front wheel of the three-wheel trainer. Two larger sizes, 30 x 4 and 34 x 4, have also been used to some extent, particularly on the Ackerman spring wheels.