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Aviation History
1916
1916 - 0115.PDF
FEBRUARY IO, 1916. (TQGHT A ••POPULAR" TYPE AEROPLANE DESIGN. By C. M. POULSEN. Continued from page 96.) IN our last issue we arrived at the result that with the design as got out up till then there would be a thrust- resistance couple of a magnitude of 149-26 ft./lbs., and that this couple would have to be counteracted in some way, either by raising the line of thrust or by placing the wings so that the centre of lift was behind the centre of gravity a distance sufficient to form a weight-lift couple equal and opposite to the thrust-resistance couple. If the latter is chosen, it will mean that when the engine is switched off and a glide commenced the centre of pressure, by travelling back will further increase the weight- obtain a weight-lift couple of 77 01 ft./lbs. or a little over as it is desired to carry a slight negative load on the tail plane. In the accompanying sketch the suggested position of the wings is shown. As the position of the centre of pressure at 2° incidence is 0425 of the chord from the leading edge, we mark off this point on each of the wings and connect these two points by a line. Somewhere on this line is the position of the resultant reaction on the wings. Since, however, the upper wing carries a greater load than does the lower, we mark off the position of the Diagram showing new position of front formers and method of finding rdative positions of centre of gravity and centre of pressure. lift couple, which might reach an undesirable magnitude necessitating a very considerable negative load on the ta'l plane and elevator. It would, therefore, be better to raise the line of thrust, or, what amounts to the same, lower the line of resistance. If the gap be kept the same as before and the two wings lowered so as to bring the lower one below the bottom of the fuselage, the centre of resistance of the wings will be lowered approximately one foot, the thrust-resistance couple thereby being reduced t0 77'or ft./lbs., which is no more than can be easily taken care of by suitably arranging the wings in the fore- and-aft direction, that is to say so placing them as to resultant 4/7 of the connecting line measured from the chord of the lower wing. Through this point are drawn vertical and horizontal lines on which are measured off to some suitable scale the lift and drift respectively of the wings. The resultant of this rectangle is, of course, the resultant reaction on the wings. For the centre of gravity of the wings it will not be far wrong if this is taken as lying on a line connecting the centres between the front and rear spars. As the top plane will be somewhat heavier than the lower the e.g. may be guessed as lying a little above the middle of this connecting line. As it happens this c g. almost IIS D 2
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