FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1916
1916 - 1168.PDF
l/ycHf) DECEMBER 28, 1916. THE INTERNATIONAL AIRCRAFT RIGID DIRIGIBLE. THE following interesting particulars of an American design for a rigid dirigible appeared in a recent issue of Aircraft:— The International Rigid Dirigible, the rights and patents of which were bought out by the American Aircraft Co., emboc ies the most successful engineering features that have been learned through practical experience and observation and study by its engineers in both Europe and America. Many original ideas in regard to their construction and opera tion are employed. Aside from the completed general design of this type of craft, also all technical data that could be collected in regard thereto, empirical tests to ascertain its resistance, stability, steerability and side stresses have been made at the Navy Yard at Washington, D.C., as well as at other technical institutes, tests were made of constructional features. These tests were observed by Mr. Louis L. Kaess, Mr. Vincent J. Buranelli and Mr. Maximilian Jagemann, designers and experts connected with the Company. The designs for four types have been got out, but the accompanying particulars refer to the largest type (" A4 "). The over-all length of the balloon frame is 440 ft., and is oval in shape, having a maximum diameter of 55 ft., and a minimum diameter of 45 feet. The object of employing this oval system of construction is to obtain vertically a high strength to weight ratio, since the major forces acting on a dirigible are vertical. This is due to the fact that the most serious side stresses act vertically, such as when the balloon is climbing, and also the overhung buoyancy, the moments of which due to the length become considerable. The fine ness ratio of the hull or balloon frame is 8* 75. This ratio, which is low compared to the old Zeppelin, greatly reduces the moments of. the overhung gas, for the lift is more concentrated. Also gives a higher cross section, thereby increasing the constructional efficiency of the entire frame work. Another feature brought about due to the oval forma tion transversely, is that the gas containers, being oval in shape, and as their tendency is, when made cylindrical, to pull in this section, there is then a more even distribution of pressure. The hull is a 16-sided polygon, having 15 compartments having an average length of 25 ft. There are 13 straight compartments, and three tapering—one in the front and two in the rear. The total displacement of the hull is 644,400 cubic ft., having a total lift of 51,554 lbs. The average volume and lift per compartment is 42,900 cubic ft. and 3,437 lbs. respectively. The centre of buoyancy of the balloon is 45 per cent. back. Beneath the hull is a square keel, which is 300 ft. in length. This keel is constructed of members similar to those employed for side members in the hull, the top members being integral with the hull. The purpose of this keel is to act as a backbone, and strengthen the balloon vertically as well as to some extent transversely. 30 per cent, of the strains acting on the balloon frame will be taken up by the keel. Numerous stress diagrams and tests made in regard to the construction of this hull show that a factor of safety of 2*5 is possible. This is very high for a rigid dirigible, and is sufficient, considering that, unlike an aeroplane, a rigid dirigible is not subject to the great variation of pressure due to its dynamic support being altered by dives or gusts. This high factor of safety is possible in this case, due to features which have been incorporated in its design, which, in some cases, require the sacrificing of certain efficiencies, as, for instance, in regard to speed. Also, the hull, which is to be constructed of a combination of steel and aluminium alloy girders, will be heavier than that of European rigid ships, thereby reducing the percentage of useful load that can be carried, "but increasing the factor of safety. This hull construction, which will consist in the main girders of members 8 ins. square, will be trussed with double 4,000 lbs. cable at the centre. This trussing will vary, as do the moments of the forces, both dynamic and static apting on the ship from its centre of buoyancy and gravity to the end. Attached to the rear of the balloon frame at its centre is the empennage, which consists of the horizontal and vertical stabilisers and rudders. These surfaces are monoplane, and have a total area of stabiliser of 2,049 sq. ft. The elevator flaps have a total area of 1,174 sq. ft., and the rudder of 600 sq. ft. These surfaces are trussed securely to the hull by means of cables that are anchored to a mast below the balloon, which is attached to the last ring. The depression wires are attached to the vertical stabiliser, which in turn receives its support from them. This empennage is located so that the centre of pressure of the elevator flaps is at a distance of 242 ft. from the centre of gravity. At a distance forward from the centre of gravity of 158 ft. two auxiliary elevators are located, one on each side, having an aspect ratio of 2 to 1, and an area of 182 sq. ft. These elevators, being operated separately, can be put either positive or negative, as com pared to the rear elevators, thereby making possible an increase of lift dynamically without the balloon being in clined from the horizontal. Wind tunnel tests show that with both surfaces positive the lift at 60 m.p.h., when the front elevators are inclined at a greater angle than the rear, so as to counterbalance them, due to its lesser surface, amounts to 3,600 lbs., at an average of 8 degrees, thereby making possible a considerable increase of altitude. When the front surface is positive and the rear negative at 10 degrees, the lift on the underside of the hull, due to its inclination, amounts to 4,200 lbs. at 60 m.p.h. Also, at the above-men tioned speed the controls, when used to their fullest extent, have six times the effect necessary to elevate or direct the balloon. Due to this, it is possible to still navigate or direct the balloon at a speed of 20 m.p.h. These large elevating surfaces are equipped with tillers 5 ft. in height, to which are attached the f-in. cables which run to the gondola through the inside of the hull. In order to make possible operating these controls by one man, a mechanical advantage has to be employed at the gondola controls of 1 to 12. This is brought about by the use of lever and ratchet mechanical devices. The front elevators are operated by a lever directly opposite the lever operating the rear elevators, and with a reduction of 1 to 3 for sim plicity and ease of operation. The rudder is operated by means of a wheel and sprocket arrangement. Ballast of the water variety is used and is released by means of a valve, operated at the will of the pilot. The gas is released through three valves, one located at the bottom, used for inflation, commonly termed the "tail" valve; and a concentric safety and manoeuvring valve, located at the top of the gas bag, and operated by means of a cord which passes through the tail valve at the bottom. The safety valve works automatically. There are two gondolas, 32 'ft. in length, with a beam of 6 ft. and a depth of 5 ft., directly attached to the keel, and streamlined as much as possible. The keel covering at the gondolas is removed to provide a view for the occupants. The front gondola contains all the controlling apparatus, and the rear one most of the crew. The space in the keel amid ships can be used for passenger accommodation, it being square and quite adaptable for this purpose. The distance between the two gondolas is about 150 ft., and the gondolas are counterbalanced about the centre of buoyancy. Their location is such as to eliminate as much as possible uneven buoyant forces. The petrol and ballast tanks are located at the bottom of the gondolas, and equally divided between each gondola. The consumption or release of ballast must be such as to cause no unequal weights to any great extent in either gondola. There are four motors, which total 400 h.p. The motors are of the 6-cylinder, 4-cycle, vertical type, and are mounted side by side at the rear of each boat. These motors run at a speed of 1,400 r.p.m., and are easily accessible. A gear box located at the driving end of each motor contains gearing mechanism, which, by means of shafting, transmits the power to the propellers, which are overhung at the side of the balloon frame. These propeller brackets are covered in to act somewhat like a bilge keel. No clutches are to be employed, nor are they necessary, considering that the modern aeronautical motor can be throttled at an ineffective propeller speed. The gear ratio is 1*4 to 1, the propeller turning at 1,000 r.p.m. The propellers have a diameter of 9 ft., 7-foot pitch, and are made of laminated wood. The air pumps for the air ballonet are operated by one of the engines in each car. Wireless apparatus is located in forward gondola, the current being supplied from a dynamo driven by one of the motors. 1140
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
