. The aviation pocket-book for 1918; a compendium of modern practice and a collection of useful notes, formulae, rules, tables and data relating to aeronautics . efficiencies determined. The efficiency is approxi-mately proportional to the pitch diameter ratio. For purposes of approximate calculations, when the propellerefficiency is not definitely known, it is usual to assume an efficiencyof 70 to^ 75 per cent., though in practice the efficiency may be aslow as 50, or as high as 80 per cent. BLADE SHAPE FACTORS In designing an airscrew for a particular aeroplane, the followingdata are normall
. The aviation pocket-book for 1918; a compendium of modern practice and a collection of useful notes, formulae, rules, tables and data relating to aeronautics . efficiencies determined. The efficiency is approxi-mately proportional to the pitch diameter ratio. For purposes of approximate calculations, when the propellerefficiency is not definitely known, it is usual to assume an efficiencyof 70 to^ 75 per cent., though in practice the efficiency may be aslow as 50, or as high as 80 per cent. BLADE SHAPE FACTORS In designing an airscrew for a particular aeroplane, the followingdata are normally available—(a) horizontal velocity, (6) rotationalvelocity of engine, (c) horse-power of engine, and {d) the permis-sible diameter. The blade shape is influenced by three considerations :— 1. The limits of structural convenience. 2. The interfering or disturbing action which one blade exertsupon the air dealt with by another. 3. The relative efficiencies of the various parts of the blade asdetermined by their position. The general form of the efficiency expression is— tan A [I -tan K-^jx)]tan A + \j/{x) lOO so ■ 80 ■70 eoso 40302010 Scale iV„. 17 25-5 34 Radius (OC) A Characteristic Efficiency Curve. Div. IV EFFICIENCY AT VARIOUS SPEEDS 105 For a propeller having average blade sections this reaches amaximum when A is in the neighbourhood of 42. An examinationof characteristic curves of the efficiencies of elements plotted againstradii indicates that the most efficient portion of an airscrew bladeis towards the centre of the propeller boss. Constructional reasons,however, seldom allow the blade to be widest at this point.
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectaeronautics, bookyear
