CHAPTER XIV. ESTIMATION OF WEIGHT.Effect of Weight. Weight is an all important consideration and is most difficult to estimate unless one has accurate data on existing machines of the same type. The total weight in flying order depends upon the useful load to be carried, and upon the weight of the power plant. The weight of the latter varies both with the useful load and with the speed, climb, and duration of flight. The type of aeroplane determines the relative head resistance which again reflects back to the weight of the power plant.The only reason for the existence of an aeroplane is to carry a certain useful load for a given distance, and this useful load is the basis of our weight calculations. The basic useful load consists of the passengers and cargo, although in some specifications the live load may be construed as including the weight of the fuel, oil and instruments, and in the case of military aeroplanes, the weight of the armament, armor, ammunition, wireless and cameras. For comparison, the elements constituting the live load should always be specified.For a given horsepower, speed and climb, it is obvious that the dead or structural weight should be at a minimum for a maximum live load capacity. The dead load carried in present aeroplanes will be undoubtedly reduced in the future by the adoption of lighter and stronger materials, better methods of bracing, and by reductions in the weight of the power plant. Just as the automobile industry developed light and powerful materials of construction, so will the aeroplane designer develop more suitable materials for the aircraft. While the present power plant has been refined to a remarkable extent when compared with the older types, it is still far from the lowest possible limit. At present the complete power unit—the motor, radiator, propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.With a given aeroplane, the performance is determined by the total weight and power. The duration and flight range can be increased by increasing the fuel weight at the expense of the passenger or cargo weight. The power available for climbing is the excess of the total power of the motor over the power required for horizontal flight. Since the power for horizontal flight depends principally upon the weight, it is at once evident that the weight is a regulating factor in the climbing speed. In fact the climbing speed may be almost directly determined from the weight carried per horsepower at normal flight speed. A fast climbing scout may weigh from 8 to 12 pounds per horsepower, while the large low climbing machine will weigh from 16 to 20 pounds per horsepower, the respective climbing speeds being approximately 1,200 and 350 feet per minute.Fuel Efficiency and Weight. The efficiency of the motor, or its fuel consumption for a given output, has a very marked effect upon the total weight of the aeroplane. Under certain conditions a very light motor with a high fuel consumption will often contribute more to the total weight than a heavier but more economical motor. In short flights, up to 3 hours, the very light rotating cylinder motor with its high fuel consumption probably gives the least total weight, but for longer flights the more efficient and heavier water-cooled type is preferable. For flights of over three hours the fuel weight is a considerable percentage of the total weight. The proper motor for any machine must be selected by computing the weight of the fuel and oil required for a given duration and then adding this to the total weight of the engine and its cooling system.Distribution of Weight. Practically the only way to predict the weight of a proposed machine is to compare it with a similar existing type. After the ratio of the useful load to the total load has been determined, the useful load of the proposed machine can be divided by the ratio factor to obtain the total weight. It should be noted in this regard that if the proposed machine is much larger than the nearest existing example, a liberal allowance must be made to compensate for the increase in the proportional weight of the structural members. There have been many mathematical formulas advanced for predicting the weight, but these are very inaccurate in the majority of cases.As a rough estimate, based on a number of successful machines, the weight of the actual aeroplane structure without power-plant, live load, fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total weight. The remaining 68 per cent is divided up among the power-plant, fuel and live load. Thus, the aeroplane structure proper of a machine weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the weight of the power plant, tanks and piping at 28 per cent, the total dead load of the bare machine without fuel or oil will be 60 per cent of the total. With a training aeroplane built for a 6-hour flight, the fuel and oil will approximate 16 per cent, so that the total percentage possible for the crew and cargo will be 24 per cent. With a given live load, the total load can now be calculated by dividing the live load by its percentage. Using the above value, for example, the total weight in order of flight with a live load of 720 pounds becomes: W = 720/0.24 = 3000 pounds.In government specifications the total weight of the pilot and passenger are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for a 4-hour flight. A safer average figure will be 170 pounds per man, and a fuel allowance of 6 hours. The floats of a seaplane or flying boat bring the percentage of the dead load much higher than with the land type of chassis.The following table will give an idea as to the weight distribution expressed both in pounds, and as a percentage of the total weight. It covers a wide range of types, varying from the training types Curtiss JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the Nieuport speed scout. The average values found by Hunsaker for a number of machines weighing in the neighborhood of 2500 pounds is given in the fourth column. Under each heading are the actual weights and the percentages of the total weight for each item. Items marked (*) include both gasoline and oil. Mark (C) is the power plant complete, and (@) includes radiator.Weight Per Horsepower. As already explained, the weight carried per horsepower varies with the type of machine. When the total weight is determined for any aeroplane, the power requirements can be calculated by dividing the total weight by the weight per horsepower ratio. A fair value for a training or exhibition machine is from 18 to 20 pounds per horsepower, while for a very high speed machine, such as a chaser, the weight will be taken at 10 pounds per horsepower. For two-seater fighters 16 to 18 pounds is fair practice. For a comparison of the horsepower-weight ratios used on different well-known machines see tables in Chapter II. Thus, if our total weight is found to be 2400 pounds as determined from the above table, and if this is a training machine, the horsepower will be: 2400/20 = 120 horsepower. Using the same total weight, but powered for two-seater fighter conditions, the power will be increased to 2400/16= 150 horsepower. As a scout the power will be increased still further to 2400/10=240 horsepower.Aeroplane Weight Distribution TableAs a problem in solving the weight and horsepower from the data, we will assume that we are to design a two-seater fighter with a total useful load of 1200 pounds. This load consists of the following items: Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and ammunition = 370 pounds. The nearest example that we have to this live load is that of the Standard H-3, which carries 744 pounds and in which the percentage of live load is 28.1 per cent. As our machine will be somewhat larger, we will not be far from the truth if we take the percentage as 0.27 instead of 0.281. The total weight, in flying order, will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the motor will be: 4440/16=277 horsepower.An empirical formula for a high-speed scout was set forth in "Aviation and Aeronautical Engineering" by D. W. Douglas. This is based on the horsepower unit. A unit wing loading of 8.45 pounds per square foot, and a low speed of 55 miles per hour was assumed. The wing section chosen was the U.S.A.-1. In the formula, H = horsepower:Power plant weight = 3 H.Chassis weight = 0.7 H.Tail weight = 0.25 H.Fuel for 2.25 hours = 1.4 H.Military load = 250 pounds.Tanks and piping = 0.42 H.Fuselage weight = 1.84 H.Wing weight = 1 lb. sq. ft.Propeller = 2.8/H.(Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully loaded in the order of flight.Weight of Wings. The weight of the wings depends upon the span, very small machines having wings that weigh only 0.38 pounds per square foot, while the wings of very large machines may run as high as 1.1 pounds per square foot. For average size biplanes from 0.75 to 0.80 pounds per square foot would probably be safe—that is, for areas ranging from 450 to 550 square feet. The weight of the upper wing of the Nieuport is 0.815 pounds per square foot, while the lower wing (short chord) is 0.646 pounds per square foot. The wings of the Standard H-3 trainer will average 0.77 pounds per square foot, the lower wing and center section being heavier than the upper wing. The wings of the Curtiss JN-4B will average 0.75 pounds per square foot. These weights do not include the interplane wires or struts, nor the fittings. The total weight of the interplane struts of the JN-4B, the Aviatic, and machines of similar size will average from 28 to 30 pounds. The ailerons will weigh about 12 pounds each.Weight of Motors. There is a considerable difference in the weight of air-cooled and water-cooled motors. The water, water piping, radiators and jackets of the water-cooled motors adds considerably to the weight of the complete power plant. The mountings are heavier for the water-cooled motors, and because of the tandem arrangement of the cylinders, the crankshaft and crankcase weigh more. In taking the bare weight of the power plant all of the accessories must be included. In the following table, the "bare engine" includes the carbureter, magneto, and necessary integral accessories, but does not include the jacket water, mounting, radiator, oil in base, water piping, nor controls. Water-cooled motors are marked by (W) and air-cooled by (A). Rotary air-cooled are (RA), and gallons (G).WEIGHTS OF AERONAUTICAL MOTORS.The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.Weights of Motors and Accessories TableControls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.Example. We will take the case of an aeroplane carrying a personnel load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting to 190 pounds, instruments 100 pounds. Total live load will be 1000 pounds. Taking the live load percentage as 0.30, the total load will be 1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the maximum Ky of the chosen wing is 0.003 at this speed, the area will be A = W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a biplane with a correction factor of 0.85, the corrected area will be: 444/0.85 = 523 square feet. The unit loading, or weight per square foot will be: 3333/523 = 6.36 pounds. The corrected area includes the ailerons and the part of the lower wing occupied by the body.Empirical Formula for Loading. After investigating a large number of practical biplanes, the author has developed an expression for determining the approximate unit loading. When this is found, the approximate area can be found by dividing the total weight by the unit loading. This gives an idea as to the area used in practice.It was found that the unit loading increased with the velocity at nearly a uniform rate. This gave an average straight line formula that agreed very closely with 128 examples. If V = Maximum velocity in miles per hour, and w = weight per square foot, then the unit loading becomes:w = 0.065V - 0.25 for the average case. For high speed scouts this gives a result that is a trifle low, the formula for a fast machine being more nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.A two-seat machine of average size weighs 2500 pounds, and has a maximum speed of 90 miles per hour. Find the approximate unit loading and area. The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6 pounds per square foot. The approximate area will be: 2500/5.60 = 446 square feet.If the above machine had a speed of 110 miles per hour, the formula would be changed for the high-speed type machine, and the loading would become:w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot. The required area will be: 2500/7.0 = 372 square feet. When the unit load is also determined in this way it is a very simple matter to choose the wing section from Ky = w/V².Area From Live Load and Speed. By a combination of empirical formula we can approximate the area directly. For the average size machine, w = 0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V - 0.08.Thus if an aeroplane travels at 90 miles per hour and has carried a useful load of 800 pounds (including gas and oil), the approximate area is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet. This assumes that the useful load is 0.32 of the total load and that the speed is less than 100 miles per hour.
CHAPTER XIV. ESTIMATION OF WEIGHT.Effect of Weight. Weight is an all important consideration and is most difficult to estimate unless one has accurate data on existing machines of the same type. The total weight in flying order depends upon the useful load to be carried, and upon the weight of the power plant. The weight of the latter varies both with the useful load and with the speed, climb, and duration of flight. The type of aeroplane determines the relative head resistance which again reflects back to the weight of the power plant.The only reason for the existence of an aeroplane is to carry a certain useful load for a given distance, and this useful load is the basis of our weight calculations. The basic useful load consists of the passengers and cargo, although in some specifications the live load may be construed as including the weight of the fuel, oil and instruments, and in the case of military aeroplanes, the weight of the armament, armor, ammunition, wireless and cameras. For comparison, the elements constituting the live load should always be specified.For a given horsepower, speed and climb, it is obvious that the dead or structural weight should be at a minimum for a maximum live load capacity. The dead load carried in present aeroplanes will be undoubtedly reduced in the future by the adoption of lighter and stronger materials, better methods of bracing, and by reductions in the weight of the power plant. Just as the automobile industry developed light and powerful materials of construction, so will the aeroplane designer develop more suitable materials for the aircraft. While the present power plant has been refined to a remarkable extent when compared with the older types, it is still far from the lowest possible limit. At present the complete power unit—the motor, radiator, propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.With a given aeroplane, the performance is determined by the total weight and power. The duration and flight range can be increased by increasing the fuel weight at the expense of the passenger or cargo weight. The power available for climbing is the excess of the total power of the motor over the power required for horizontal flight. Since the power for horizontal flight depends principally upon the weight, it is at once evident that the weight is a regulating factor in the climbing speed. In fact the climbing speed may be almost directly determined from the weight carried per horsepower at normal flight speed. A fast climbing scout may weigh from 8 to 12 pounds per horsepower, while the large low climbing machine will weigh from 16 to 20 pounds per horsepower, the respective climbing speeds being approximately 1,200 and 350 feet per minute.Fuel Efficiency and Weight. The efficiency of the motor, or its fuel consumption for a given output, has a very marked effect upon the total weight of the aeroplane. Under certain conditions a very light motor with a high fuel consumption will often contribute more to the total weight than a heavier but more economical motor. In short flights, up to 3 hours, the very light rotating cylinder motor with its high fuel consumption probably gives the least total weight, but for longer flights the more efficient and heavier water-cooled type is preferable. For flights of over three hours the fuel weight is a considerable percentage of the total weight. The proper motor for any machine must be selected by computing the weight of the fuel and oil required for a given duration and then adding this to the total weight of the engine and its cooling system.Distribution of Weight. Practically the only way to predict the weight of a proposed machine is to compare it with a similar existing type. After the ratio of the useful load to the total load has been determined, the useful load of the proposed machine can be divided by the ratio factor to obtain the total weight. It should be noted in this regard that if the proposed machine is much larger than the nearest existing example, a liberal allowance must be made to compensate for the increase in the proportional weight of the structural members. There have been many mathematical formulas advanced for predicting the weight, but these are very inaccurate in the majority of cases.As a rough estimate, based on a number of successful machines, the weight of the actual aeroplane structure without power-plant, live load, fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total weight. The remaining 68 per cent is divided up among the power-plant, fuel and live load. Thus, the aeroplane structure proper of a machine weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the weight of the power plant, tanks and piping at 28 per cent, the total dead load of the bare machine without fuel or oil will be 60 per cent of the total. With a training aeroplane built for a 6-hour flight, the fuel and oil will approximate 16 per cent, so that the total percentage possible for the crew and cargo will be 24 per cent. With a given live load, the total load can now be calculated by dividing the live load by its percentage. Using the above value, for example, the total weight in order of flight with a live load of 720 pounds becomes: W = 720/0.24 = 3000 pounds.In government specifications the total weight of the pilot and passenger are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for a 4-hour flight. A safer average figure will be 170 pounds per man, and a fuel allowance of 6 hours. The floats of a seaplane or flying boat bring the percentage of the dead load much higher than with the land type of chassis.The following table will give an idea as to the weight distribution expressed both in pounds, and as a percentage of the total weight. It covers a wide range of types, varying from the training types Curtiss JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the Nieuport speed scout. The average values found by Hunsaker for a number of machines weighing in the neighborhood of 2500 pounds is given in the fourth column. Under each heading are the actual weights and the percentages of the total weight for each item. Items marked (*) include both gasoline and oil. Mark (C) is the power plant complete, and (@) includes radiator.Weight Per Horsepower. As already explained, the weight carried per horsepower varies with the type of machine. When the total weight is determined for any aeroplane, the power requirements can be calculated by dividing the total weight by the weight per horsepower ratio. A fair value for a training or exhibition machine is from 18 to 20 pounds per horsepower, while for a very high speed machine, such as a chaser, the weight will be taken at 10 pounds per horsepower. For two-seater fighters 16 to 18 pounds is fair practice. For a comparison of the horsepower-weight ratios used on different well-known machines see tables in Chapter II. Thus, if our total weight is found to be 2400 pounds as determined from the above table, and if this is a training machine, the horsepower will be: 2400/20 = 120 horsepower. Using the same total weight, but powered for two-seater fighter conditions, the power will be increased to 2400/16= 150 horsepower. As a scout the power will be increased still further to 2400/10=240 horsepower.Aeroplane Weight Distribution TableAs a problem in solving the weight and horsepower from the data, we will assume that we are to design a two-seater fighter with a total useful load of 1200 pounds. This load consists of the following items: Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and ammunition = 370 pounds. The nearest example that we have to this live load is that of the Standard H-3, which carries 744 pounds and in which the percentage of live load is 28.1 per cent. As our machine will be somewhat larger, we will not be far from the truth if we take the percentage as 0.27 instead of 0.281. The total weight, in flying order, will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the motor will be: 4440/16=277 horsepower.An empirical formula for a high-speed scout was set forth in "Aviation and Aeronautical Engineering" by D. W. Douglas. This is based on the horsepower unit. A unit wing loading of 8.45 pounds per square foot, and a low speed of 55 miles per hour was assumed. The wing section chosen was the U.S.A.-1. In the formula, H = horsepower:Power plant weight = 3 H.Chassis weight = 0.7 H.Tail weight = 0.25 H.Fuel for 2.25 hours = 1.4 H.Military load = 250 pounds.Tanks and piping = 0.42 H.Fuselage weight = 1.84 H.Wing weight = 1 lb. sq. ft.Propeller = 2.8/H.(Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully loaded in the order of flight.Weight of Wings. The weight of the wings depends upon the span, very small machines having wings that weigh only 0.38 pounds per square foot, while the wings of very large machines may run as high as 1.1 pounds per square foot. For average size biplanes from 0.75 to 0.80 pounds per square foot would probably be safe—that is, for areas ranging from 450 to 550 square feet. The weight of the upper wing of the Nieuport is 0.815 pounds per square foot, while the lower wing (short chord) is 0.646 pounds per square foot. The wings of the Standard H-3 trainer will average 0.77 pounds per square foot, the lower wing and center section being heavier than the upper wing. The wings of the Curtiss JN-4B will average 0.75 pounds per square foot. These weights do not include the interplane wires or struts, nor the fittings. The total weight of the interplane struts of the JN-4B, the Aviatic, and machines of similar size will average from 28 to 30 pounds. The ailerons will weigh about 12 pounds each.Weight of Motors. There is a considerable difference in the weight of air-cooled and water-cooled motors. The water, water piping, radiators and jackets of the water-cooled motors adds considerably to the weight of the complete power plant. The mountings are heavier for the water-cooled motors, and because of the tandem arrangement of the cylinders, the crankshaft and crankcase weigh more. In taking the bare weight of the power plant all of the accessories must be included. In the following table, the "bare engine" includes the carbureter, magneto, and necessary integral accessories, but does not include the jacket water, mounting, radiator, oil in base, water piping, nor controls. Water-cooled motors are marked by (W) and air-cooled by (A). Rotary air-cooled are (RA), and gallons (G).WEIGHTS OF AERONAUTICAL MOTORS.The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.Weights of Motors and Accessories TableControls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.Example. We will take the case of an aeroplane carrying a personnel load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting to 190 pounds, instruments 100 pounds. Total live load will be 1000 pounds. Taking the live load percentage as 0.30, the total load will be 1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the maximum Ky of the chosen wing is 0.003 at this speed, the area will be A = W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a biplane with a correction factor of 0.85, the corrected area will be: 444/0.85 = 523 square feet. The unit loading, or weight per square foot will be: 3333/523 = 6.36 pounds. The corrected area includes the ailerons and the part of the lower wing occupied by the body.Empirical Formula for Loading. After investigating a large number of practical biplanes, the author has developed an expression for determining the approximate unit loading. When this is found, the approximate area can be found by dividing the total weight by the unit loading. This gives an idea as to the area used in practice.It was found that the unit loading increased with the velocity at nearly a uniform rate. This gave an average straight line formula that agreed very closely with 128 examples. If V = Maximum velocity in miles per hour, and w = weight per square foot, then the unit loading becomes:w = 0.065V - 0.25 for the average case. For high speed scouts this gives a result that is a trifle low, the formula for a fast machine being more nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.A two-seat machine of average size weighs 2500 pounds, and has a maximum speed of 90 miles per hour. Find the approximate unit loading and area. The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6 pounds per square foot. The approximate area will be: 2500/5.60 = 446 square feet.If the above machine had a speed of 110 miles per hour, the formula would be changed for the high-speed type machine, and the loading would become:w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot. The required area will be: 2500/7.0 = 372 square feet. When the unit load is also determined in this way it is a very simple matter to choose the wing section from Ky = w/V².Area From Live Load and Speed. By a combination of empirical formula we can approximate the area directly. For the average size machine, w = 0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V - 0.08.Thus if an aeroplane travels at 90 miles per hour and has carried a useful load of 800 pounds (including gas and oil), the approximate area is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet. This assumes that the useful load is 0.32 of the total load and that the speed is less than 100 miles per hour.
CHAPTER XIV. ESTIMATION OF WEIGHT.Effect of Weight. Weight is an all important consideration and is most difficult to estimate unless one has accurate data on existing machines of the same type. The total weight in flying order depends upon the useful load to be carried, and upon the weight of the power plant. The weight of the latter varies both with the useful load and with the speed, climb, and duration of flight. The type of aeroplane determines the relative head resistance which again reflects back to the weight of the power plant.The only reason for the existence of an aeroplane is to carry a certain useful load for a given distance, and this useful load is the basis of our weight calculations. The basic useful load consists of the passengers and cargo, although in some specifications the live load may be construed as including the weight of the fuel, oil and instruments, and in the case of military aeroplanes, the weight of the armament, armor, ammunition, wireless and cameras. For comparison, the elements constituting the live load should always be specified.For a given horsepower, speed and climb, it is obvious that the dead or structural weight should be at a minimum for a maximum live load capacity. The dead load carried in present aeroplanes will be undoubtedly reduced in the future by the adoption of lighter and stronger materials, better methods of bracing, and by reductions in the weight of the power plant. Just as the automobile industry developed light and powerful materials of construction, so will the aeroplane designer develop more suitable materials for the aircraft. While the present power plant has been refined to a remarkable extent when compared with the older types, it is still far from the lowest possible limit. At present the complete power unit—the motor, radiator, propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.With a given aeroplane, the performance is determined by the total weight and power. The duration and flight range can be increased by increasing the fuel weight at the expense of the passenger or cargo weight. The power available for climbing is the excess of the total power of the motor over the power required for horizontal flight. Since the power for horizontal flight depends principally upon the weight, it is at once evident that the weight is a regulating factor in the climbing speed. In fact the climbing speed may be almost directly determined from the weight carried per horsepower at normal flight speed. A fast climbing scout may weigh from 8 to 12 pounds per horsepower, while the large low climbing machine will weigh from 16 to 20 pounds per horsepower, the respective climbing speeds being approximately 1,200 and 350 feet per minute.Fuel Efficiency and Weight. The efficiency of the motor, or its fuel consumption for a given output, has a very marked effect upon the total weight of the aeroplane. Under certain conditions a very light motor with a high fuel consumption will often contribute more to the total weight than a heavier but more economical motor. In short flights, up to 3 hours, the very light rotating cylinder motor with its high fuel consumption probably gives the least total weight, but for longer flights the more efficient and heavier water-cooled type is preferable. For flights of over three hours the fuel weight is a considerable percentage of the total weight. The proper motor for any machine must be selected by computing the weight of the fuel and oil required for a given duration and then adding this to the total weight of the engine and its cooling system.Distribution of Weight. Practically the only way to predict the weight of a proposed machine is to compare it with a similar existing type. After the ratio of the useful load to the total load has been determined, the useful load of the proposed machine can be divided by the ratio factor to obtain the total weight. It should be noted in this regard that if the proposed machine is much larger than the nearest existing example, a liberal allowance must be made to compensate for the increase in the proportional weight of the structural members. There have been many mathematical formulas advanced for predicting the weight, but these are very inaccurate in the majority of cases.As a rough estimate, based on a number of successful machines, the weight of the actual aeroplane structure without power-plant, live load, fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total weight. The remaining 68 per cent is divided up among the power-plant, fuel and live load. Thus, the aeroplane structure proper of a machine weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the weight of the power plant, tanks and piping at 28 per cent, the total dead load of the bare machine without fuel or oil will be 60 per cent of the total. With a training aeroplane built for a 6-hour flight, the fuel and oil will approximate 16 per cent, so that the total percentage possible for the crew and cargo will be 24 per cent. With a given live load, the total load can now be calculated by dividing the live load by its percentage. Using the above value, for example, the total weight in order of flight with a live load of 720 pounds becomes: W = 720/0.24 = 3000 pounds.In government specifications the total weight of the pilot and passenger are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for a 4-hour flight. A safer average figure will be 170 pounds per man, and a fuel allowance of 6 hours. The floats of a seaplane or flying boat bring the percentage of the dead load much higher than with the land type of chassis.The following table will give an idea as to the weight distribution expressed both in pounds, and as a percentage of the total weight. It covers a wide range of types, varying from the training types Curtiss JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the Nieuport speed scout. The average values found by Hunsaker for a number of machines weighing in the neighborhood of 2500 pounds is given in the fourth column. Under each heading are the actual weights and the percentages of the total weight for each item. Items marked (*) include both gasoline and oil. Mark (C) is the power plant complete, and (@) includes radiator.Weight Per Horsepower. As already explained, the weight carried per horsepower varies with the type of machine. When the total weight is determined for any aeroplane, the power requirements can be calculated by dividing the total weight by the weight per horsepower ratio. A fair value for a training or exhibition machine is from 18 to 20 pounds per horsepower, while for a very high speed machine, such as a chaser, the weight will be taken at 10 pounds per horsepower. For two-seater fighters 16 to 18 pounds is fair practice. For a comparison of the horsepower-weight ratios used on different well-known machines see tables in Chapter II. Thus, if our total weight is found to be 2400 pounds as determined from the above table, and if this is a training machine, the horsepower will be: 2400/20 = 120 horsepower. Using the same total weight, but powered for two-seater fighter conditions, the power will be increased to 2400/16= 150 horsepower. As a scout the power will be increased still further to 2400/10=240 horsepower.Aeroplane Weight Distribution TableAs a problem in solving the weight and horsepower from the data, we will assume that we are to design a two-seater fighter with a total useful load of 1200 pounds. This load consists of the following items: Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and ammunition = 370 pounds. The nearest example that we have to this live load is that of the Standard H-3, which carries 744 pounds and in which the percentage of live load is 28.1 per cent. As our machine will be somewhat larger, we will not be far from the truth if we take the percentage as 0.27 instead of 0.281. The total weight, in flying order, will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the motor will be: 4440/16=277 horsepower.An empirical formula for a high-speed scout was set forth in "Aviation and Aeronautical Engineering" by D. W. Douglas. This is based on the horsepower unit. A unit wing loading of 8.45 pounds per square foot, and a low speed of 55 miles per hour was assumed. The wing section chosen was the U.S.A.-1. In the formula, H = horsepower:Power plant weight = 3 H.Chassis weight = 0.7 H.Tail weight = 0.25 H.Fuel for 2.25 hours = 1.4 H.Military load = 250 pounds.Tanks and piping = 0.42 H.Fuselage weight = 1.84 H.Wing weight = 1 lb. sq. ft.Propeller = 2.8/H.(Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully loaded in the order of flight.Weight of Wings. The weight of the wings depends upon the span, very small machines having wings that weigh only 0.38 pounds per square foot, while the wings of very large machines may run as high as 1.1 pounds per square foot. For average size biplanes from 0.75 to 0.80 pounds per square foot would probably be safe—that is, for areas ranging from 450 to 550 square feet. The weight of the upper wing of the Nieuport is 0.815 pounds per square foot, while the lower wing (short chord) is 0.646 pounds per square foot. The wings of the Standard H-3 trainer will average 0.77 pounds per square foot, the lower wing and center section being heavier than the upper wing. The wings of the Curtiss JN-4B will average 0.75 pounds per square foot. These weights do not include the interplane wires or struts, nor the fittings. The total weight of the interplane struts of the JN-4B, the Aviatic, and machines of similar size will average from 28 to 30 pounds. The ailerons will weigh about 12 pounds each.Weight of Motors. There is a considerable difference in the weight of air-cooled and water-cooled motors. The water, water piping, radiators and jackets of the water-cooled motors adds considerably to the weight of the complete power plant. The mountings are heavier for the water-cooled motors, and because of the tandem arrangement of the cylinders, the crankshaft and crankcase weigh more. In taking the bare weight of the power plant all of the accessories must be included. In the following table, the "bare engine" includes the carbureter, magneto, and necessary integral accessories, but does not include the jacket water, mounting, radiator, oil in base, water piping, nor controls. Water-cooled motors are marked by (W) and air-cooled by (A). Rotary air-cooled are (RA), and gallons (G).WEIGHTS OF AERONAUTICAL MOTORS.The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.Weights of Motors and Accessories TableControls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.Example. We will take the case of an aeroplane carrying a personnel load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting to 190 pounds, instruments 100 pounds. Total live load will be 1000 pounds. Taking the live load percentage as 0.30, the total load will be 1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the maximum Ky of the chosen wing is 0.003 at this speed, the area will be A = W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a biplane with a correction factor of 0.85, the corrected area will be: 444/0.85 = 523 square feet. The unit loading, or weight per square foot will be: 3333/523 = 6.36 pounds. The corrected area includes the ailerons and the part of the lower wing occupied by the body.Empirical Formula for Loading. After investigating a large number of practical biplanes, the author has developed an expression for determining the approximate unit loading. When this is found, the approximate area can be found by dividing the total weight by the unit loading. This gives an idea as to the area used in practice.It was found that the unit loading increased with the velocity at nearly a uniform rate. This gave an average straight line formula that agreed very closely with 128 examples. If V = Maximum velocity in miles per hour, and w = weight per square foot, then the unit loading becomes:w = 0.065V - 0.25 for the average case. For high speed scouts this gives a result that is a trifle low, the formula for a fast machine being more nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.A two-seat machine of average size weighs 2500 pounds, and has a maximum speed of 90 miles per hour. Find the approximate unit loading and area. The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6 pounds per square foot. The approximate area will be: 2500/5.60 = 446 square feet.If the above machine had a speed of 110 miles per hour, the formula would be changed for the high-speed type machine, and the loading would become:w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot. The required area will be: 2500/7.0 = 372 square feet. When the unit load is also determined in this way it is a very simple matter to choose the wing section from Ky = w/V².Area From Live Load and Speed. By a combination of empirical formula we can approximate the area directly. For the average size machine, w = 0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V - 0.08.Thus if an aeroplane travels at 90 miles per hour and has carried a useful load of 800 pounds (including gas and oil), the approximate area is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet. This assumes that the useful load is 0.32 of the total load and that the speed is less than 100 miles per hour.
Effect of Weight. Weight is an all important consideration and is most difficult to estimate unless one has accurate data on existing machines of the same type. The total weight in flying order depends upon the useful load to be carried, and upon the weight of the power plant. The weight of the latter varies both with the useful load and with the speed, climb, and duration of flight. The type of aeroplane determines the relative head resistance which again reflects back to the weight of the power plant.
The only reason for the existence of an aeroplane is to carry a certain useful load for a given distance, and this useful load is the basis of our weight calculations. The basic useful load consists of the passengers and cargo, although in some specifications the live load may be construed as including the weight of the fuel, oil and instruments, and in the case of military aeroplanes, the weight of the armament, armor, ammunition, wireless and cameras. For comparison, the elements constituting the live load should always be specified.
For a given horsepower, speed and climb, it is obvious that the dead or structural weight should be at a minimum for a maximum live load capacity. The dead load carried in present aeroplanes will be undoubtedly reduced in the future by the adoption of lighter and stronger materials, better methods of bracing, and by reductions in the weight of the power plant. Just as the automobile industry developed light and powerful materials of construction, so will the aeroplane designer develop more suitable materials for the aircraft. While the present power plant has been refined to a remarkable extent when compared with the older types, it is still far from the lowest possible limit. At present the complete power unit—the motor, radiator, propeller, water, etc.—will weigh from 2 to 5 pounds per horsepower.
With a given aeroplane, the performance is determined by the total weight and power. The duration and flight range can be increased by increasing the fuel weight at the expense of the passenger or cargo weight. The power available for climbing is the excess of the total power of the motor over the power required for horizontal flight. Since the power for horizontal flight depends principally upon the weight, it is at once evident that the weight is a regulating factor in the climbing speed. In fact the climbing speed may be almost directly determined from the weight carried per horsepower at normal flight speed. A fast climbing scout may weigh from 8 to 12 pounds per horsepower, while the large low climbing machine will weigh from 16 to 20 pounds per horsepower, the respective climbing speeds being approximately 1,200 and 350 feet per minute.
Fuel Efficiency and Weight. The efficiency of the motor, or its fuel consumption for a given output, has a very marked effect upon the total weight of the aeroplane. Under certain conditions a very light motor with a high fuel consumption will often contribute more to the total weight than a heavier but more economical motor. In short flights, up to 3 hours, the very light rotating cylinder motor with its high fuel consumption probably gives the least total weight, but for longer flights the more efficient and heavier water-cooled type is preferable. For flights of over three hours the fuel weight is a considerable percentage of the total weight. The proper motor for any machine must be selected by computing the weight of the fuel and oil required for a given duration and then adding this to the total weight of the engine and its cooling system.
Distribution of Weight. Practically the only way to predict the weight of a proposed machine is to compare it with a similar existing type. After the ratio of the useful load to the total load has been determined, the useful load of the proposed machine can be divided by the ratio factor to obtain the total weight. It should be noted in this regard that if the proposed machine is much larger than the nearest existing example, a liberal allowance must be made to compensate for the increase in the proportional weight of the structural members. There have been many mathematical formulas advanced for predicting the weight, but these are very inaccurate in the majority of cases.
As a rough estimate, based on a number of successful machines, the weight of the actual aeroplane structure without power-plant, live load, fuel, oil, or tanks, is very nearly 32 per cent (0.32) of the total weight. The remaining 68 per cent is divided up among the power-plant, fuel and live load. Thus, the aeroplane structure proper of a machine weighing 2000 pounds total will be 2000 x 0.32 = 640 pounds. Taking the weight of the power plant, tanks and piping at 28 per cent, the total dead load of the bare machine without fuel or oil will be 60 per cent of the total. With a training aeroplane built for a 6-hour flight, the fuel and oil will approximate 16 per cent, so that the total percentage possible for the crew and cargo will be 24 per cent. With a given live load, the total load can now be calculated by dividing the live load by its percentage. Using the above value, for example, the total weight in order of flight with a live load of 720 pounds becomes: W = 720/0.24 = 3000 pounds.
In government specifications the total weight of the pilot and passenger are taken at 330 pounds, or 165 pounds per man. Gasoline and oil are for a 4-hour flight. A safer average figure will be 170 pounds per man, and a fuel allowance of 6 hours. The floats of a seaplane or flying boat bring the percentage of the dead load much higher than with the land type of chassis.
The following table will give an idea as to the weight distribution expressed both in pounds, and as a percentage of the total weight. It covers a wide range of types, varying from the training types Curtiss JN-4B and the Standard H-3, to the Handley-Page Giant bomber and the Nieuport speed scout. The average values found by Hunsaker for a number of machines weighing in the neighborhood of 2500 pounds is given in the fourth column. Under each heading are the actual weights and the percentages of the total weight for each item. Items marked (*) include both gasoline and oil. Mark (C) is the power plant complete, and (@) includes radiator.
Weight Per Horsepower. As already explained, the weight carried per horsepower varies with the type of machine. When the total weight is determined for any aeroplane, the power requirements can be calculated by dividing the total weight by the weight per horsepower ratio. A fair value for a training or exhibition machine is from 18 to 20 pounds per horsepower, while for a very high speed machine, such as a chaser, the weight will be taken at 10 pounds per horsepower. For two-seater fighters 16 to 18 pounds is fair practice. For a comparison of the horsepower-weight ratios used on different well-known machines see tables in Chapter II. Thus, if our total weight is found to be 2400 pounds as determined from the above table, and if this is a training machine, the horsepower will be: 2400/20 = 120 horsepower. Using the same total weight, but powered for two-seater fighter conditions, the power will be increased to 2400/16= 150 horsepower. As a scout the power will be increased still further to 2400/10=240 horsepower.
Aeroplane Weight Distribution Table
As a problem in solving the weight and horsepower from the data, we will assume that we are to design a two-seater fighter with a total useful load of 1200 pounds. This load consists of the following items: Personnel (2) = 330 pounds; gas and oil = 500 pounds; guns and ammunition = 370 pounds. The nearest example that we have to this live load is that of the Standard H-3, which carries 744 pounds and in which the percentage of live load is 28.1 per cent. As our machine will be somewhat larger, we will not be far from the truth if we take the percentage as 0.27 instead of 0.281. The total weight, in flying order, will now be 1200/0.27 = 4440 pounds. At 16 pounds per horsepower the motor will be: 4440/16=277 horsepower.
An empirical formula for a high-speed scout was set forth in "Aviation and Aeronautical Engineering" by D. W. Douglas. This is based on the horsepower unit. A unit wing loading of 8.45 pounds per square foot, and a low speed of 55 miles per hour was assumed. The wing section chosen was the U.S.A.-1. In the formula, H = horsepower:
Power plant weight = 3 H.
Chassis weight = 0.7 H.
Tail weight = 0.25 H.
Fuel for 2.25 hours = 1.4 H.
Military load = 250 pounds.
Tanks and piping = 0.42 H.
Fuselage weight = 1.84 H.
Wing weight = 1 lb. sq. ft.
Propeller = 2.8/H.
(Total) = (7.61 H + 2.5/H + 250)/7.45 = Weight of aeroplane fully loaded in the order of flight.
Weight of Wings. The weight of the wings depends upon the span, very small machines having wings that weigh only 0.38 pounds per square foot, while the wings of very large machines may run as high as 1.1 pounds per square foot. For average size biplanes from 0.75 to 0.80 pounds per square foot would probably be safe—that is, for areas ranging from 450 to 550 square feet. The weight of the upper wing of the Nieuport is 0.815 pounds per square foot, while the lower wing (short chord) is 0.646 pounds per square foot. The wings of the Standard H-3 trainer will average 0.77 pounds per square foot, the lower wing and center section being heavier than the upper wing. The wings of the Curtiss JN-4B will average 0.75 pounds per square foot. These weights do not include the interplane wires or struts, nor the fittings. The total weight of the interplane struts of the JN-4B, the Aviatic, and machines of similar size will average from 28 to 30 pounds. The ailerons will weigh about 12 pounds each.
Weight of Motors. There is a considerable difference in the weight of air-cooled and water-cooled motors. The water, water piping, radiators and jackets of the water-cooled motors adds considerably to the weight of the complete power plant. The mountings are heavier for the water-cooled motors, and because of the tandem arrangement of the cylinders, the crankshaft and crankcase weigh more. In taking the bare weight of the power plant all of the accessories must be included. In the following table, the "bare engine" includes the carbureter, magneto, and necessary integral accessories, but does not include the jacket water, mounting, radiator, oil in base, water piping, nor controls. Water-cooled motors are marked by (W) and air-cooled by (A). Rotary air-cooled are (RA), and gallons (G).
WEIGHTS OF AERONAUTICAL MOTORS.The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.Weights of Motors and Accessories TableControls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.Example. We will take the case of an aeroplane carrying a personnel load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting to 190 pounds, instruments 100 pounds. Total live load will be 1000 pounds. Taking the live load percentage as 0.30, the total load will be 1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the maximum Ky of the chosen wing is 0.003 at this speed, the area will be A = W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a biplane with a correction factor of 0.85, the corrected area will be: 444/0.85 = 523 square feet. The unit loading, or weight per square foot will be: 3333/523 = 6.36 pounds. The corrected area includes the ailerons and the part of the lower wing occupied by the body.Empirical Formula for Loading. After investigating a large number of practical biplanes, the author has developed an expression for determining the approximate unit loading. When this is found, the approximate area can be found by dividing the total weight by the unit loading. This gives an idea as to the area used in practice.It was found that the unit loading increased with the velocity at nearly a uniform rate. This gave an average straight line formula that agreed very closely with 128 examples. If V = Maximum velocity in miles per hour, and w = weight per square foot, then the unit loading becomes:w = 0.065V - 0.25 for the average case. For high speed scouts this gives a result that is a trifle low, the formula for a fast machine being more nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.A two-seat machine of average size weighs 2500 pounds, and has a maximum speed of 90 miles per hour. Find the approximate unit loading and area. The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6 pounds per square foot. The approximate area will be: 2500/5.60 = 446 square feet.If the above machine had a speed of 110 miles per hour, the formula would be changed for the high-speed type machine, and the loading would become:w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot. The required area will be: 2500/7.0 = 372 square feet. When the unit load is also determined in this way it is a very simple matter to choose the wing section from Ky = w/V².Area From Live Load and Speed. By a combination of empirical formula we can approximate the area directly. For the average size machine, w = 0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V - 0.08.Thus if an aeroplane travels at 90 miles per hour and has carried a useful load of 800 pounds (including gas and oil), the approximate area is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet. This assumes that the useful load is 0.32 of the total load and that the speed is less than 100 miles per hour.
The bare radiator will weigh from 0.48 to 0.56 pounds per horsepower, the average being safe at 0.52. The water contained in the radiator will average 0.35 pounds per horsepower. The weights of the piping and the water contained therein will be computed separately. The circular sheet metal cowl used over the rotary cylinder air-cooled motor is equal to twice the square root of the motor weight, according to Barnwell. Propeller weight varies considerably with the diameter, pitch, etc., but a safe rule will give the weight as 2.8 √H where H = horsepower. The tanks will weigh from 0.75 to 1.2 pounds per gallon of contents, or approximately 1/5 the weight of the contents when completely filled.
Chassis and Wheel Weight. The chassis of a two-wheel trainer will weigh about 90 pounds complete, although there are chassis of training machines that weigh as much as 140 pounds. The chassis of speed scouts will be from 22 to 40 pounds complete. Tail skids can be taken at from 6 to 8 pounds.
Tangent wire wheels complete with tires are about as follows: 26 x 4 = 21 pounds; 26 x 5 = 28 pounds; 26 x 3 = 14 pounds. Ackerman spring spoke wheels are estimated as follows: 20 x 4 = 17.5 pounds; 26 x 3 = 22 pounds; 26 x 4 = 32 pounds; 30 x 4 = 35 pounds; 34 x 4 = 45 pounds.
Military Loads. A 20-mile wireless outfit devised by Capt. Culver weighed 40 pounds with storage batteries, while the 120-mile outfit weighed 60 pounds with a 180-watt generator. The 140-mile U.S.A. mule-back wireless of 1912 weighs 45 pounds. The "Blimp" specifications allow 250 pounds.
The Lewis gun as mounted on the "11" Nieuport weighs 110 pounds, including mount, gun and ammunition. Lewis gun bare is 26 pounds. The Davis 6-pounder, Mark IV, weighs 103 pounds with mounting but without ammunition, while the same make of 3-inch 12-pounder weighs 238 pounds under the same conditions.
Weights of Motors and Accessories Table
Controls and Instruments. The Deperdussin type controls used on the Curtiss JN-4B weigh 16 pounds per control, while those installed in the Standard H-3 weigh about 13 pounds. An average of 15 pounds per control is safe. An instrument board for the aviators' cock-pit, fully equipped, weighs from 20 to 24 pounds. The front, or students' instrument board will average 10 pounds. Pyrene extinguisher and brackets = 7 pounds; Speaking tube = 3 pounds; Oil pressure line and gage = 3 pounds; Side pockets = 3 pounds; Tool kit = 10 pounds.
Control Surfaces. The rudder, stabilizer, fin, and elevator can be made so that the weight will not exceed 0.60 to 0.65 pounds per square foot.
General Notes on Weight. Before starting on the weight estimates of the machine the reader should carefully examine the tables in Chapter II which give the weights, and general characteristics of a number of modern machines.
Weights and Wing Area. When the weight of the machine is once determined, the next step will be to determine the wing area. For speed scouts or very large heavy duty machines the choice of a wing section must be very carefully considered. For the speed scout several wings giving a minimum high speed resistance should be examined, such as the Eiffel 37 or the U. S. A-1 or U.S.A.-6. For the low-speed aeroplane to be designed for great lift, a number of sections such as the U.S.A.-4 or the R.A.F.-3 should be tried for a number of speeds and angles. For training machines a wing of the "All around" type such as the R.A.F.-6 should be adopted, the structural characteristics in the case of a trainer having an important bearing on the subject. If W = weight of the machine in pounds, V = low speed in miles per hour, A = total area in square feet, and Ky=lift coefficient, then the area becomes A=W/KyV². Compensation must be made for biplane interference for aspect ratio, and stagger as previously explained. For an ordinary training machine with the usual gap/chord ratio, and aspect ratio, the correction factor of 0.85 may be safely employed.
Example. We will take the case of an aeroplane carrying a personnel load of 340 pounds, oil and gasoline 370 pounds, and baggage amounting to 190 pounds, instruments 100 pounds. Total live load will be 1000 pounds. Taking the live load percentage as 0.30, the total load will be 1000/0.30 = 3333 pounds. If the low speed is 50 miles per hour, and the maximum Ky of the chosen wing is 0.003 at this speed, the area will be A = W/KyV² = 3333/0.003 x (50 x 50) = 444 square feet. Since this is a biplane with a correction factor of 0.85, the corrected area will be: 444/0.85 = 523 square feet. The unit loading, or weight per square foot will be: 3333/523 = 6.36 pounds. The corrected area includes the ailerons and the part of the lower wing occupied by the body.
Empirical Formula for Loading. After investigating a large number of practical biplanes, the author has developed an expression for determining the approximate unit loading. When this is found, the approximate area can be found by dividing the total weight by the unit loading. This gives an idea as to the area used in practice.
It was found that the unit loading increased with the velocity at nearly a uniform rate. This gave an average straight line formula that agreed very closely with 128 examples. If V = Maximum velocity in miles per hour, and w = weight per square foot, then the unit loading becomes:
w = 0.065V - 0.25 for the average case. For high speed scouts this gives a result that is a trifle low, the formula for a fast machine being more nearly w = 0.65V - 0.15, for speeds over 100 miles per hour.
A two-seat machine of average size weighs 2500 pounds, and has a maximum speed of 90 miles per hour. Find the approximate unit loading and area. The loading becomes: w = 0.065V - 0.25 = (0.065 x 90) - 0.25 = 5.6 pounds per square foot. The approximate area will be: 2500/5.60 = 446 square feet.
If the above machine had a speed of 110 miles per hour, the formula would be changed for the high-speed type machine, and the loading would become:
w = 0.065V - 0.15 = (0.065 x 110) - 0.15 = 7.00 pounds per square foot. The required area will be: 2500/7.0 = 372 square feet. When the unit load is also determined in this way it is a very simple matter to choose the wing section from Ky = w/V².
Area From Live Load and Speed. By a combination of empirical formula we can approximate the area directly. For the average size machine, w = 0.065V - 0.25. And the total weight W = U/0.32 where U is the useful or live load. Since A = W/w, then A = U/(0.65V - 0.25) x 0.32 = U/0.021V - 0.08.
Thus if an aeroplane travels at 90 miles per hour and has carried a useful load of 800 pounds (including gas and oil), the approximate area is: A = U/0.021.V = 0.08 = 800/(0.021 x 90) - 0.08 = 442 square feet. This assumes that the useful load is 0.32 of the total load and that the speed is less than 100 miles per hour.