FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1921
1921 - 0233.PDF
MARCH 31, 10.21 P7TII1) r 11 rn 11 mi \\) 111mm j|ii jMODEL AEROPLANES NOTE.—All communications should be addressed to the Model Editor. Some Observations on Stability and What it MeansTHE centre of pressure of any wing is the resultant pressure of all the air pressures acting on it. It is well establishedthat the location of the centre of pressure varies with different speeds or air pressures and also at different inclinations to theline of motion. The first thing, therefore, to be decided on is the size of planes required to lift a given weight, and alsotheir shape and curvature (all points which may be fairly easily determined) in consonance with the speed required.Consider now the centre of gravity, and resolve it into its two component forces, upward lift and horizontal motion,the upward lift acting through the centre of pressure. For purposes of experiment fix the plane so that the e.g. is eitherforward or aft of the c.p. (see Fig. 1) ; in the former position it will dive and in the latter it will stall. This fact is trueof either monoplanes or biplanes, unless the centre of pressure of one of the superposed foils is on one side of the verticalline passing through the e.g. and the c.p. of the other plane //of or"motion Pig. 2. \ Fig. 4. is on the other side, in which case the instability of one isneutralised by what may be termed the inverse instability of the other (see Fig. 2). It has been stated that the c.p. changes, and in order for themachine to remain stable the position of the e.g. must be made to move in synchronism with it. Now take into considerationthe line along which the force acts ; this line must be through the c.p. of the whole machine. To prove this, plaee.it below(set Fig. 3), and it will be found that thrust forms a couple with the drift and the machine tends to stall; place it aboveand the machine tends to dive. At high speeds, owing to increase of drag, this instability is very pronounced, whilst atlow speeds it renders tuning-up difficult (if the loading is light, almost impossible). It is therefore regarded as proved thatthe line of force or thrust must act through the c.p. Now, as the c.p. moves with various pressures and anglesof incidence, the line of force should move as near as possible A stamp should be enclosed Jor a postal reply. to the locus of the c.p. In a biplane it should be disposedalong the resultant c.p. of the two planes (Fig. 4). If twin screws are used the resultant line of force shouldpass through the resultant c.p. Now, since the motive power has to move the weight ofthe machine, which may be taken as at the e.g., it is obvious that the thrust would be best applied at this spot. Since, therefore, it is best to have the thrust at the e.g.,although it has been previously proved that it must also pass through the c.p., for a machine to be stable the line ofthrust must pass through the centre of pressure and the centre of gravity of the machine. A Mechanical Paradox TWBNTY-SIX readers sent in solutions to the mechanicalparadox, but not one gave a correct analysis of the pheno- menon, which is as follows :— Taking the general formula e case of the stationary wheel, m =60 with 61 teeth, e = ^- and is less than unity. n — a 60 • ' ' — a ~ 61 .•. 6IM — 61a =» — 60a. Whence t» = ^— and is positive. ———, we have in theK — at in the case of the wheel 60For the planetary wheel with 60 teeth, e = T^ -» 1 .•. w — a = — a, orn = o. 60For the wheel with 59 teeth, e = --, which is greater than unity. n — a 60 59 Whence n — and is negative. Only one reader saw that it is really theoretically impossibleto cut 60 teeth, 59 teeth and 61 teeth, all of the same pitch on blanks of the same diameter, although it is prac-ticably possible to do so by either altering the pressure angle of the tooth or cutting thick and thin teeth in the 59- and61-tooth gear. Some readers went out of their way to show me that it wouldn't do it really—only on paper. Tothis may I state that I have a working model in my possession which I am prepared to lend to any sceptic. After carefulchecking, I consider the solution sent in by Mr. Cecil D. Holland, 12, Fordwych Road, Cricklewood, N.W.2, to bethe clearest and most succinct, and the book is awarded to him accordingly. The following readers sent in meritorioussolutions: J. W. Jones, 8, Oxberry Avenue, S.W.6; B. Chandler, 62, Regent Street, Kingswood, Bristol; andD. F. Anderson, Ovoca, Farnborough, Kent. Replies to Correspondents D. F. A. (Farnborough) and R. S. (Dublin).—I replied direct. A. S. H. (Halifax).—Many thanks for the photos. Please let me have larger and clearer ones of the model when com- pleted. C. F. (Gossnorgh).—An idler is not a ne'er-do-wheel! Rates Ton the Berlin-Brunswick-Dortmund Service FARES per passenger (including conveyance by motor-carto and from the aerodrome, Berlin, and the Hapag office):— Berlin-Dortmund (or vice versa), 500 mks. Berlin-Brunswick (or vice versa), 300 mks. Brunswick-Dortmund— (or vice versa), 300 -inks. Each passenger is allowed 15 kg. luggage free of charge. Excess 10 marks for every kg. or part of a kg., in so far asthere is accommodation. Air Mail Fees.—In addition to the usual fees for ordinaryregistered or express- mails, the following fees (when possible in air mail stamps) must be paid :—Postcards, 20 pfennigs ; letters up to 20 grammes, 20 pfennigs ; 50 grammes, 80 pfennigs ; 100 grammes, 160 pfennings ; 250 grammes, 240 pfennigs ; printed matter up to 50 grammes, 80 pfennigs*; 100 grammes, 160 pfennigs; 250 grammes, 240 pfennigs'; 500 grammes, 480 pfennigs ; 1,000 grammes, 960 pfennigs. Fees, Etc., for Packages.—In Germnay : In addition to the fee for express packages, 10 marks surcharge for every kg. or part .of a kg. The packages must be securely fastened and the dimensions must not exceed 60 x 60 x 60 cm. Letters and packages for conveyance by air mail will be accepted at all post offices. Mails between Dortmund, Hoerde, Witten, Hagen and Bochum are conveyed by motor-cycle. 233
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
