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Aviation History
1942
1942 - 0474.PDF
184 FLIG.HT FEBRUARY 26TH, 1942 WORLD PRESS SUMMARY RESUME OF TECHNICAL ARTICLES DEALING WITH AIRCRAFT AND ASSOCIATED SUBJECTS For the mmwiiii if and translations from aircraft and technical journals of the world, ire arc indebted to the ... I Directorate of Scientific Research and Technical Development, Ministry of Aircraft Production. Multi-bladed Airscrews THE experiments were carried out in the 2 m. wind tunnel on model propeller having 0.94 m diameter, and covered assemblies of 2, 3, 4, 5 and 6 blades of identical design. Thrust, torque and efficiency were determined over a range of blade settings and V/nD values for each assembly, and the results are recorded in a series of tables and curves. Of special interest is the very large range of V/nD covered, amounting to 9.4, 7.4, 3.7, 3.7, 3-7 lr>r the 2, 3, 4, 5 and 6 blade assemblies respectively. For most of the tests the air screw r.p.m. varied only slightly (700-900 r.p.m.), and as a consequence the Reynolds number of the 0.7R section increased from about 1 x io' to 3 x 10s over the experimental range. The following are some of the principal conclusions: J . With the screws turning at a fixed point (V =0) the thrust coefficient remains practically constant over the range of blade setting angles 0 = 30° to 50°. The thrust coefficient increases almost linearly with 0 over the range /3 = o to 200. 2. Under the same conditions (V = o) the torque coefficient increases continually with R. 3. The V/nD value corresponding to zero thrust or torque are functions of j8 onlv and independent of the number of 1.lades. 4. The envelope of the various efficiency peaks is practically horizontal over the range V/nD=i to 2, and falls off relatively .-.lowly for larger values of V/nD. In case of the 2-bladed airscrew, an efficiency of 59 per cent, obtained at V/nD = 4.5 (5 = 72°). 5. The maximum efficiency diminishes slowly with increasing number of blades (82 per cent. 2 blades to 78 per cent. 6 blades). 6. Maximum possible efficiency occurs at 0~4o° and V 'nD=i.5, irrespective of number of blades. 7. At maximum possible efficiency, increasing the blade number n from 2 to 6, increases the thrust and torque co efficients by multiples of 2.5 and 2.7 respectively. 8. It is suggested that 4-bladed propellers are likely to prove of benefit for high-speed and high-power operation at high altitudes. Experiments with a Family of Multi-bladed Airscrews.—(A. Fula. Atti di Guidonia.) (Italy.) Aircraft Performance A METHOD is presented for rapidly determining the effect of small modifications in the various parameters influencing performance. The only requirements for use of this method are performance values for some basic condition and the corre sponding aircraft polar curve. No recourse is necessary to analytical methods of performance calculation, but only the fundamental concepts of power available and power required. The resulting expressions lead to some interesting corrolaries dealing with critical values of the parameters. Thus, the lift coefficient on the aircraft polar diagram corre sponding to minimum power required satisfies the simple relation L/D of the wing.) Finally at the altitude for minimum drag, d C„ i C, 3 ^D At optimum wing loading we have- d Cn CDW when Cmv = drag coefficient of wing alone. (This means that the slope of the aircraft polar is equal to dCL = cjc» This corresponds to the best L/D point on the polar w and, therefore, to minimum drag for a given weight. Below* the lift coefficient corresponding to this point, an increase in altitude at constant power available results in an increase in level flight speed or rate of climb, while above it, the reverse takes place. This critical point thvls determines the intersec tion of power required curves for adjacent altitudes. It also locates the altitude beyond which supercharging is ineffective. Beyond this there is no increase in speed at constant power. A Simplified Method for Predicting the Change in Aeroplane Performance Due to a Change m Parameter. — (E. C. Posner, /. of Aer. Sci., Vol. 8, No. 11, Sept., 1941, op. 419/25. (U.S.A.)) Elliptic Wings 'TPHE correction commonly applied is that introduced by -L Prandtl and known as the lifting-line theory. The down- flow induced by the wake is considered, in this theory, to u-duce the relative normal velocity and hence the edge velocity of the wing. It is assumed, however, that once the true angle of attack is determined for any section of the wing its effect in producing circulation and lift is the same as in two- dimensional flow. This assumption is expressed by the equation CL = -2.it (a —a,) .. .. .. (1) where 2ir is the slope of the lift curve for the thin wing of infinite aspect ratio, a is the angle of attack of dewnflow. Eq. (1) takes into account the effect of the wake in diminish ing the relative normal velocity of the wing. A further cor rection is indicated by the fact, established in hvdrodynamic theory,"that the surface velocities induced by a given relative motion of a body in three-dimensional flow are generally smaller than those in two. In the case of an elliptic disc the velocity at every point is reduced by the factor i/E, where E is the ratio of the semi- perimeter of the ellipse to the span. The corrected formula for the lift is then q, = (27T/E) («-«,) Since the velocity of the non-lifting potential flow is coil stant all around the edge of the elliptic plate, the circulatii required will be proportional to the chord at each section. 1 circulation is thus elliptically distributed spanvvise. Such a distribution, with the chord-wise distribution assumed earlier, leads, as in the lifting-line theory, to the relation a, = CL/JTA (3) where A is the aspect ratio. Substitution of this value into Eq. (2) gives CL = 27raA/(EA + 2) .. .. (4) Since the chordwise distribution of the circulation in three- dimensional flow' is assumed similar to that in two, and since the similarity is only proved for the non-circulatory flow, E<| (4) must be considered a correction of the wing-section theon rather than a solution of the three-dimensional problem. The assumption of similarity, although its validity is subject to somewhat the same limitations in general, appears to be a more- justifiable one than the assumption of equality made in the lifting-line theory. It is found that the additional correction to the win" section theory accounts for an appreciable fraction of the lo's in lift that is usually attributed to viscosity. It has be*" difficult to reconcile the magnitude of the inefficiency witk-i'1' observed dimensions of the wake, which, in the case of smooth wings, is extremely narrow at the trailing edge. The '<irP' gtiing correction accounts for as much as half of this dis crepancy in cases of wings with sharp trailing edges. . Theoretical Correction for the Lift of Elliptic Wings. (K. *• Jones, /. Aeton. Sci. (U.S.A.)) \
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