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Aviation History
1910
1910 - 0448.PDF
I/UGHT| reaction (Fig. 6). It is this reaction which constitutes the lift of an aeroplane, and the argument which we have thus advanced step by step is that which shows funda mentally why aeroplane flight is possible. Momentum. It still remains to investigate the quantitative side 01 the question however, and it is this aspect of the case which constitutes our correspondent's real difficulty. At first sight there appears to be no difficulty in the matter at all, because Newton's laws themselves distinctly provide the necessary link to enable real forces to be calculated. The second law states that the rate of change of momentum is proportional to the force causing it, and the third law makes the re action equal and opposite to the force, hence it is a direct deduction that the lift of an aeroplane is pro portional to the rate of change of The momentum MOMENTUM fug Mv Wjr momentum communicated to the air of a moving body is by definition the product of its mass multiplied by the velocity (Fig. 7); and if the air is at rest to begin with, the latter factor will be given by the down ward velocity imparted to the air by the wedge-like passage of the aeroplane through it. Air possesses mass to the extent of ^ lb. per cubic foot, consequently if the volume of air swept downwards per second is also known, the up ward reaction can be immediately stated by multiplying together its mass and acceleration, and dividing by gravity if the pressure is required in lbs. instead of absolute units. It should be observed that the mass of air dealt with per second is like acceleration a function of the flight velocity, whence the air pressure becomes proportional to V2. Taking the case of a normal plane (Fig. 8) as a basis, the application of the Newtonian method results in the JU.VE ii, 1910. well-known formula which gives the pressure in lbs. per square ft. = —^ where the velocity is here expressed in miles per hour. It is equally well known that this formula, which is theoretically accurate, is practically inaccurate, and in order to appreciate why this is so, it is necessary to bear very clearly in mind the exact hypothesis to which it applies. In the case of a normal plane, and for the formula cited above, the hypothesis V V.-O AlR. STKEAm BROUCHT TO REST ( V - V, = V ) V- VELOCITY ft/at, r\*ftR£A Of STREAM XtV» f> • DErtSITX OF MR. - VJ-tB/CU.FT. Mftss/sec -/AV ; CHANGE OF MOMENTUM •/>Av(v-V,) PRESSURE^*,). £AV- g£. *£(*• Vn*h) . Fig a. supposes that the plane constitutes an obstruction of such a nature as to bring the air-stream to rest instan taneously at the rate of its velocity; that is to say, a> wind blowing at V ft. per second upon a plane of A square ft. area, is eaten up, so to speak, at the rate of AV cubic ft. per second. The mass of air striking the plane AV , , . is —• lbs. per second, and its velocity is reduced to zero at the rate of V ft. per second, whence the change in its momentum, or in other words its force in lbs., is AV2 AV2 equal to = which is the same formula as that M I3S" 420 previously mentioned except that the velocity is here stated in ft. per second. To be concluded.) BOOH REVIEWS. (Cr The By Art of Aviation. R. W. A. BREWER. osby, Lockwood and Son. zos. 6d. net.) IT does not exactly meet the case to speak of this book as a text book on aeroplane construction and design for it has a lighter and somewhat more popular side that is intended to give it a wider sphere of interest, even if at the same time it is apt to be irritating to the strict student of technical detail. The chapters in Mr. Brewer's work are seventeen in number, and their titles give a fairly comprehensive idea of the scope of the treatise. The first relates to a comparison between monoplanes and biplanes, and therein will be found a paragraph to which some exponents of flight may take exception. It reads as follows : "Learning to fly with a monoplane is undoubtedly a more simple matter than with a biplane of the Wright type, the former, when of the Bleriot type, is much lighter, and its inertia in flight is therefore less. Successful beginners of the monoplane school vastly out- $ © Appointment Kept by Aeroplane. ON Wednesday afternoon the members of the International Conference on Aerial Navigation now sitting in Paris paid a visit to Issy, at the invitation of the Aero Club of France, and Count Lambert, who had been asked to attend, determined to fly over from Villacoublay, about 9 kiloms. from Issy. Rising quickly to a height of about 300 metres, Count Lambert flew to Issy in about number those who are learning to fly biplanes, and the time taken- during the process is generally much shorter." We hold no brief for either type of machine, and very possibly what Mr. Brewer says is correct, but, on the other hand, we think it only proper to point out that we have had occasion to see and chronicle more actual flying with biplanes than with monoplanes in England up to the present time. Among the other chapters are those relating to " The flight engine and its design," wherein is included an excellently illustrated description of the famous Gnome rotary engine. A couple of chapters are devoted to propellers, and two others to methods of construction and details of manufacture. Gliding and its advantages as a road to flight receive attention, and there are some very interesting illustrations in the chapter devoted to future develop ment of high-angle guns constructed in Germany for attacking dirigibles. The concluding chapter is devoted to a glossary of terms, wherein we are pleased to observe that the majority of those adopted in the first instance in this journal have been used. It will tend to simplify matters considerably in the future if these terms pass current among aviators. © ® 8 mins., and on arriving there circled round the ground twice before- coming to rest, landing eventually right in front of the delegates. At their request he made another trip, flying round the military camp, and on his descent was warmly congratulated by all present. Previous to this the delegates had witnessed demonstrations of the Bleriot machine by' MM. Leblanc, Morane and Mollien, while M. Obre, on his machine, and M. Clement, on his biplane, were also flying. 446
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