4i6 



NATURE 



[February 4, 1909 



the trajectory and the horizontal when the trajectory is 

 such as to give the greatest travel. 

 This condition is satisfied when 



[sie(7)] 



Ian 7 = 



V 2/;S 



(14) 



If from any cause the machine loses velocitv, it will 

 drop and gain kinetic energy by loss of potential, until its 

 velocity is that required. On the other hand, if accident- 

 ally its velocity increases, it will rise, to lose kinetic energy 

 by gaining potential energy. It is this exchange in the 

 form of energy which causes the oscillations in an un- 

 stable glider. Pi-of. Bryan and Mr. Williams have photo- 

 graphed gliders bearing flash-lights, and demonstrated the 

 reality of the long- and short-period oscillation, but the 

 theory needs considerable amplification so as to apply to 

 comple.x cases of combined planes, and simplification so 

 as to be readily applicable to design. 



Mr. Lanchester (" Aerial Flight," vol. il., and British 

 Association Trans., Dublin, igoS) gives new formukne for 

 the stability, and finds that the oscillations are trochoidal.' 



Practice (Aeroplanes). 

 Time will not permit an exhaustive account of the theo- 

 retical principles involved to be given, but the more 

 essential points have been touched upon, and it will bo 

 useful to indicate how these principles will be applied. 



In designing an aeroplane the weight is perhaps the 

 first consideration, and next the minimum velocity required. 

 From formula (8) we can proceed to find S, the area. 



W = 2/.-S\- sin 7 cos 7 . . . . . (8) 



;ince 7 is small, and 



Let cos 7= I, 

 2/.- = o-oo4," 'hen 



and 



sm 7 = 



W = oooi2 SV-, 

 c_ W 



(IS) 



0'00I2 V'^ ' ' * 



Thus, if V is 30 feet per second (say thirty miles. per 

 liour), .S = W, i.e. the area in square feet is the same as 

 ihc weight in lb. Less area will necessitate more speed, 

 and vice versA. 



A useful rule connecting the area and weight (based on 

 bird flight in spite of dimensional theory) is that 



S«WS (16) 



Next, to find the thrust required, we take formukc (7) 

 and (8), and get 



T R + CV= , , C 



--=-tan7+ — p^ . . (17) 



WW ' ' 2/S sin 7 cos 7 



the ratio between the thrust and the load. 



Neglecting 

 a higher 



the second term, which is small (or rather, tak 

 value for the first, so as to include the second), wc write 

 tan 7 = sin 7 = ^ or 5, 



so that 



T = 



W 



3' 



(18 



Since the thrust per B.H.P. with a good projiellc 



about 30 lb. or 40 lb. 



so that 



may write 



40!! = — , 

 4 



W=i6oII .... . (19 



where H is now in B.H.P. 



This may be regarded as a high value, and probably 

 only half this can be safely employed, so that i horse- 

 power will carry, say, 80 lb. Great improvemei.ls should 

 eventually be made in this direction. 



The light motors (such as the .Antoinette, Dufaux, and 

 Esnault-Pclterie types) now made produce about i B.H.P. 

 per 3 lb. of weight, or allowing for transmission gearing 

 and friction losses, sav i B.H.P. per 5 lb. of mechanism, 

 so that the weight of this will be =5 H lb., and hence 

 from (ip) (modified as suggested) we get the available 

 weight of the surfaces, framing, and aeronaut =75 H lb., 



1 .See the Engineer, September 18, 190S. 



- Twice 0-0017 (see p. 414) plus au addilion of o'ood6 for the lateral spread 

 generally employed. 



\'0. 2049, VOL. 7()| 



(2>) 



or for framing and surfaces alone (reckoning aeronaut's 

 weight at 150 lb.) 



W = (75H-IS0) lb (20) 



Employing the rule obtainable from (15) that S = W. 

 we find the weight of surfaces and framing per square 

 foot is 



W_75_ i?o 



"S^to ^^ ■ ■ ■ ' 



Care must be taken to prevent surfaces interfering with 

 one another, and this is generally attained by superposing 

 them at a distance apart equal to their width, or placing 

 them behind one another at the same minimum distance. 



The positions of the aviator and the engines are very 

 important. Generally the first is in front. The Wright 

 machine has them side by side. In any case the position 

 of the common centre of gravity must answer to the rules 

 given in the theory of stability. Lateral balance is assured 

 by the use of a dihedral angle between the wing planes 

 or by a keel plane. Captain Ferber has discovered the 

 laws controlling tlie size and position of the latter, which 

 are to be found in the paper previously referred to. Steer- 

 ing is accomplished in several ways, as will presently be 

 described. 



Constructive Features. 



Several types of machine may be distinguished, but 

 three especially are noteworthy, and are named after their 

 inventors : — (a) Chanute ; (b) Langley ; (c) Wright. The 

 Phillips machine is a fourth type, but is analogous to the 

 first. The Chanute machine is the type adopted by Far- 

 man, Delagrange, and Captain Ferber. It consists of two 

 superposed, narrow surfaces mounted on a transverse 

 girder. A central longitudinal girder connects this front 

 frame with a rear one of similar form, but smaller, some- 

 times divided by partitions into cells after the pattern of 

 the Hargrave kite. The aviator and motor are placed 

 centrally at the rear of the front surfaces, where the e.g. 

 must be, so as to be ahead of the mean centre of area 

 of all the surfaces. The trimming planes arc generally in 

 front, and the steering planes at the rear. This differs, 

 however, and will be discussed presently. One propeller 

 is used between the sets. 



The Langley type, generally termed monoplanar, consists 

 of two pairs of wing surfaces, inclined 67^° from the 

 vertical, .so as to include a dihedral angle of 135°. ■•^ 

 central shaft, or framed girder, supports the cantilever 

 ribs which stay the wings. The engine is between the 

 pairs of wings, and the two propellers are paired along- 

 side. 



Wright tvpr. — Consists simply of two superposed 

 surfaces as in the Chanute type, with no tail. Front 

 trimming planes similar to the main wings, and rear 

 vertical planes for steering. Catapult initial propulsion. 

 Two propellers behind the wings. 



Trimming and Steering. 



Guide planes of various forms are used for trimming 

 and steering. .\ cruciform set of planes for both purposes 

 has been used on the Langley and Ludlow machines. 

 Superposed pairs for trimming, placed in front, have been 

 used by Farman, Delagrange, and the Wrights. Santos 

 Dumont (xiv., his) employed a cellular kite for both pur- 

 poses, and M. Bleriot has used trimming planes, turning 

 on axes, at the tips of the wing planes. .\ sliding weight 

 is used in the Weiss gliders, and the author has suggested 

 a weight on a coarse-pitched leading screw as useful. 

 For steering laterally, vertical surfaces are generally 

 employed at the rear. By slightly canting the machine a 

 lateral thrust is produced which will turn the machine, 

 although the consequent diminution in lift tends to make 

 it lose elevation.' The Wrights also employ torsion nf 

 the main surfaces. 



Starting and .Wghting. 



In starting an aeroplane there are numerous difficulties. 

 The essential is that the soaring velocity shall be reached 

 before the machine leaves the ground. If a machine be 

 simply propelled along a track, so soon as the soaring 

 velocity is approached the friction on the ground becomes 

 negligible, and the propulsive effort is uncertain. L'sually 

 1 See paper by M. Kenard in Com/te^ rendvs, 1908. 



