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Aviation History
1926
1926 - 0294.PDF
SUPPLEMENT TO FLIGHT APRIL 29, 1926 THE AIRCRAFT ENGINEER Stability in Pitching. The air screw, body and wings of an aeroplane are gener- ally " instable" in pitching and to attain stability the requisite area of tail-surface must be provided. It is necessary in the first place that, at any attitude of pitch, a small alteration of angle of pitch causes an increment of " stable " pitching moment due to the tail surface greater than the sum of the increments of " instable" pitching- moments due to air-screw, body and wings ; it is necessary axis), and Z-Z (the pitching axis) are both at right angles to axis X-X, but axis Y-Y lies in the central plane-of-symmetry of the aeroplane whilst axis Z-Z is at right angles to this plane. All the values for forces are expressed in the form of co- efficients, §Y being a coefficient of lateral force (i.e., force in direction of axis Z-Z), §Z being a coefficient of normal force 11— " T - f1 J i •"1— •- I U l W* c * Its --- >- i > .._ o rs f ** ^ ^)YP = -OOIOS OLp «tN MRSClteW •.•.!& LiC» ABOVE ^,-X . new «ITSCKCW i5ftnc^D or Y-Y, (_T - DISTANCE , F*A*A LUL TO X~ X , TEjCG. AT •T SZT --OI5B AT OLT TAKErJ A.SXERO. Fig. 1.—Values for lateral force and moments due to airscrew. as well that this excess of " stable "' moment be sufficiently great to ensure reasonably rapid damping of the pitching (or '" phugoidal ") oscillations. In Fig. 1 is given an empirical equation for determining §M,,, the coefficient of alteration of pitching moment due to airscrew per 1° alteration of angle of pitch. In Fig. 2 is given an empirical equation for determining o'Mm the coefficient of alteration of pitching moment due to body per 1° alteration of angle of pitch. Fig. 5.—Values for vertical force and moments due to tail surface. (i.e., force in direction of axis Y-Y). The coefficients of lateral force are values of :— (Lateral force in lbs.) -£- |J p (:-' The coefficients of normal force are values of :— (Normal force in lbs.) -f- apv- similarlv all the values for moments are expressed in the form of coefficients. §L being a coefficient of rolling-moment, x^— I.— — — L y_y v^_j——. C>6" k T— VERTICAL. T-OSITIOM OF C6^T«OID OF sioe - A«£A ._ • it — 1 "B » -OO45 AS SLB ™ C^BX^B :B = -OO3AP <5MS= -j^ooo8AP SNB = _&0009A. -—-n T —•—J ~~ —^_ J lx« is +• wv«e»g 3xLB) + 5YB0-a--27L X As = PROJECTED AREA.) IM "SIDE ELEVATION- Ap= PROJECTED AREA ,|K| TOP PLA^- CEMTROiD Lie^ ABOVE X-X.. 1 Fig. 2.—Values for forces and moments due to body. In Fig. 5 an empirical equation for determining SM1; similar coefficient due to tail-surface. In Pig. 6 one for determining SMW, similar coefficient due to wing surface. In explanation of these figures :— All linear dimensions are to be taken in feet, all areas in square feet, all angles in degrees. X-X, Y-Y and Z-Z are assumed to be the three main axes of the aeroplane and are fixed with respect to the aeroplane ; X-X (the rolling axis) is taken as parallel to the airscrew axis, Y-Y (the yawing of yawing-§M a coefficient of pitching-moment and moment. The coefficients of rolling-moment are values of :— (Rolling-moment in foot lbs.) 4- ^p?'2 The coefficients of pitching-moment are values of :— (Pitching-moment in foot lbs.) 4- apv" The coefficients of yawing-moment are values of :— (Yawing-moment in foot lbs.) -=- j3pt'° (i = angle of yaw, of axis X-X to flight-path, in degrees. 2606
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