FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1918
1918 - 0174.PDF
FEBRUARY 14, 1918?* DIMENSIONS OF STEEL TUBE STRUTS. By CARLO MAURILIO LERICI, Electrical and Aeronautical Engineer. THE accompanying graphs have been prepared in order to facilitate the calculation of the dimensions of such circular and streamline steel tubes as are employed in aeroplane con- struction. The diagrams give directly the compression per- missible for a maximum load of 10 kg. per sq. mm. (13,750 lbs./sq. in.) ; that is to say, with a factor of safety of six for steel commonly employed having an ultimate strength of 60 kg. per sq. mm. (82,500 lbs./sq. in., or nearly 37 tons per sq. in.). The streamline section tube is that of minimum resistance (Fig. 1), tested by Eiffel and adopted as standard graph shoWs that for each value of W there are' generally" several solutions according to the different values of the thickness adopted. In this connection it should be noticed1 that, in order to attain maximum lightness, one should adopt the minimum possible thickness s. The ratio of s to d should'' not be less than 1/30. It should also be pointed out that the condition of minimum • weight for streamline tubes exposed to a current of air, as the" struts of an aeroplane, is only convenient in machines having; a low flight speed. Generally it is possible to so dimension' these tubes as to render minimum the power absorbed by" their weight and by their head resistance. In the best of? modern high-speed machines the total power P absorbed^ by one meter of streamline tube exposed to the air is given* by the formula :— 7•85yds 10" jo* d in cm. s in cm. ' " v in metres per second. io8 d in inches. 5 in inches. v in ft./in. Fig. 1 Standard section for streamline steel tube. by nearly all the constructors of the world. The finenessratio of this section is about 3. The ratio of the two moments of inertia I,x and lyy is about 5. It will be easily seen thatthere is an almost constant relation between strength in compression of circular and streamline steel tubes. For the standard type (Fig. 1) I have found that thestreamline tube whose section has the external dimensions d x 3d has the same compressive strength as a circular sectiontube of the same length and thickness (gauge), having an external diameter = 1 • 5 d. This factor has formed thebasis for the graph (Fig. 2). The following data are given : IV = normal compressive load, L •= length of tube. The An examination of this formula gives the following results : (1) For machines having a speed of not more than 120 km. per hour (75 m.p.h.) the minimum value of P is obtained by giving the struts a thickness of between o-8 and i*6 mm. (s\ in. and T^ in. respectively). (2) For machines of which the normal speed varies between 120 and 160 km. per hour (75 and 100 m.p.h.) the optimum thickness varies between 1-2 and 2 mm. (^ in. and ^j- in.). (3) For speeds between 160 and 200 km. per hour (100 to 125 m.p.h.) the optimum thickness varies from i*6 to 2-4 mm. (TV in. to •£? in.). (4) For speeds above 200 km. per hour (125 m.p.h.) the struts should be given the maximum possible thickness, weight having no longer any appreciable influence. It sometimes happens that one has to depart from these dimensions giving maximum efficiency in order to avoid during flight every vibration that might diminsh the compressive strength of struts. For instance, this happens when the fre- quency of the struts' proper vibration under normal load \xt. diameter. in. mm. 60 56 52 44 44 40 36 32 Zi thickness. 172 . mm. m, 3 2.1 2.6 24 2.2 j 2 ft Ifi I I* |3 1 | QA 06 0» | 0.1. ^40 «0 Uo l60f 100 MO 2»:3M 560 400 440' 480 520 560 6 5 tf/VCTfi • f' 500 400 500 800 100 MO 900 1000 1100 1100 IWO IW0 Fig. 2.—Graph showing strength of circular steel tube struts of various dimensions, the material beingassumed to be steel of ultimate strength of 37 tons per sq. in. For the section shown in Fig. 1 the values found for circular struts must be divided by VS. Other streamline sections would, of course,have different values. 170
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
