FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1931
1931 - 1164.PDF
SUPPLEMENT TOFLIGHT 80 OCTOBER 30, 1931 THE AIRCRAFT ENGINEER Interior Ballistics of a Powder Catapult. If cx be the cubic capacity of the explosion chamber and w the weight of the charge, then the expansion volume v1 is, by definition, ^^ P= Hadcock's empirical relation for the pressure and expansion volume for cordite or nitro powder in a closed vessel is :— 107-14 \ where P and V represent the initial V 1-28 / pressure and volume. These particulars are taken from the " Text Book of Gunnery." As with the compressed air type the expansion is adiabatic and '. PA the average pressure = cable is as before = 0 • 128PA X A. P and the tension in the The Launch of a Plane by the Compressed Air Catapult. The aircraft is a " Vought " UO two-seater seaplane as used in the U.S. Navy and the leading particulars are :— Weight of plane, 2,500 lb \ wei nt kunched 2 ?00 lb Weight of carriage, 200 lb. J ° Length of run, 40 ft. Total wing area, 276 sq. ft. Lift coefficient at launching attitude K^ = 0-02 L lift L-, i.e., ratio, „ „ - = 4-78 D drag D Minimum flying speed = 80 ft. per sec. Engine thrust TK for 220 h.p. \_220x550 x 0-65 7) = propeller efficiency=0-65j = gQ Lift of wings at launching L — KL pAV2 p = 0-00238 slugs, density at sea level, then L = 835 lb. Air resistance \ L 835 at launching/ = jjjy = 4^78 = la 175 Resistance constant A = = 0-027 1* Friction of slide = y. W = 0-04 x 2,700 = 107 lb. JA = 0-04 for metal surfaces well lubricated. = 984 lb. NowT0 = c -TE — 0027 X 802 1-2 x 40 x 32 x 0-027 ~C 2,700 ~ - 984 + 107 = 5,393 lb. (tension in the cable) this is required to launch the aircraft. The initial pressure in the air container is 400 lb. per aq. in., and its capacity is 30 cu. ft. The expansion cylinder ia 12 in. in diameter and 6 ft. long with a capacity of 4-75 cu. ft. /V'l-25 / 30 \l-25 . The final pressure Px = P —) =400—— = 332 Ib./sq. in. . P + Pi 400+332 n „ , .Average pressure = = = 366 lb./sq. in. Tension in the cable = 0-128 PA x A = 0-128 x 366 x — ........ • . . : 4 = 5,3001b. This agrees fairly well with the tension required and shows that the method of calculation gives a close approximation to actual practice. Launch of a Large Monoplane by Powder Catapult. •» The type of monoplane was known in the U.S. Navy as the " MO1 " observation monoplane. We"fol # Length of run, 50 ft. Wing area, 400 sq. ft. L Attitude of wings give — = 5 and KL = 0-35 for this type, / wso flying speed = V = * / ——- = 120 ft. per sec. V Ki,pA (375 h.p.) x 550 x 0-7Engine thrust TE, 7j = 0-7 =- 120 (engine was 375 Curtiss 12-cylinder). Aeroplane drag = —— = = 960 lb. L/U 5 Friction = To = Aw2/ = 1,2001b. . 1 — e = 004 x 5,300= 212 lb. - \ - TK - W = 0-027 x 12O2, - 2 x 50 x 32 x 0-027 1 —e 5,300 = 23,040 lb., or 10-3 tons. — 1,200 + 212 Now the charge used is the same as that used in a 5-in. naval gun, viz., 15 1b. of cordite. The explosion chamber is a cylinder 6 ft. long and 2 ft. in diameter, of capacity 32,600 cu. in., also, of the cubic capacity of the cartridge case in an adjoining cylinder 24 in. long and 5 in. diameter of capacity 470 cu. in. Total volume of explosion chamber = 33,070 cu. in. The expansion cylinder is 7 ft. long and 9 in. in diameter = 5,300 eu. in. C 33,070 Now the " expansion volume » V =-^1^^ = 27.73 x 15" = 80 units. 107-14so P = 1 -\ —— = 1 • 393 tons per sq. in. yl ' 2.O Final pressure P1=P(_J' = l-393^-^()^V \YV = 1 • 156 tons Average pressure = 1-393+ 1156 = 1 • 275 tons2 2 per sq. in. Area of piston = 64 sq. in, so tension in the cable = To = 0-128 PA x A= 0-128 x 1-275 X 64 = 10-4 tons, which agrees well with the tension actually required. These examples illustrate the procedure of arriving at the necessary particulars for designing. Length of Run and Acceleration. For practical reasons, such as the effect of a large hori- zontal acceleration on the strength of the structure of an aeroplane, and the effect on the physique of the personnel, the maximum acceleration during a launch is limited to about 2-5 g. The simplest type of expression connects acceleration, launching speed and length of run, 1? = 2fx and length of catapult = —-where v is the stalling speed. There are many reasons which make 60 m.p.h. almost :;<H arbitrary fixed value for stalling speed for commercial aircraft and the length of a catapult then becomes about 50 *t. independent of the size or weight or type of aircraft. It is possible to tabulate the initial pressures for compressed a-s, or the weight of powder charge for the launching of the rapt* of aircraft weights within the capacity of the catapult. i'e remainder of the problem of catapult design then becomes one of structurel design and the design of control device • release gear, energy shock absorbers for the ram and *•it carriage, and the mounting details. 1086 h
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
