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Aviation History
1909
1909 - 0027.PDF
•JANUARY 9, 1909. THE WRIGHT AND VOISIN TYPES OF FLYING MACHINE,' A COMPARISON BY F. W. LANCHESTER. (Concluded from page 16.) IT would thus appear that in addition to being considerably less efficient in its screw propeller (a tax paid for the constructional advantage of a direct drive), the Voisin machine is also slightly less efficient considered as a glider, that is to say, its gliding ancle is not quite as good as that of the Wright machine ; the machine is aerodynamically less efficient. The reason of this may be due to the fact that it has a less aspect ratio, but it may quite well also be due to many other causes ; the Voisin machine has relatively greater idle suriace subject to skin friction, also the sustaining surfaces of the tail act on air that has already been trodden by the aerofoil. The author is not altogether satisfied that the gliding angle is actually as low as that deduced above ; it is possible that the motors with the machines at the velocity stated in both cases run some- what faster than that declared, and that consequently the pitch of the propellers is proportionately less, since this has been deduced from the revolution speed. An error of this kind, so long as it is much the same for both machines, would not materially affect the results except that in both cases the gliding angle would be proportionately greater, the error may possibly amount to a matter of about 10 per cent. It is also worth while noting that what is termed the mean or actual pitch of the propeller blades will be greater than the effective pitch ; the pitch as measured from the blade angle is probably in the Wright propeller about 15 per cent, more than the effective pitch, and in the Voisin about 25 per cent. On this basis the Voisin 3 "6 ft. becomes 4'5 ft. (— I'37 metres), and the Wright 9/6 ft. becomes II ft. ( - 3*35 metres). Taking the gliding gradient tan y for the Wright machine as •135, and that of the Voisin machine '150, values which the author considers most probably a close approximation to the truth, we may roughly look upon the resistance as accounted lor as follows :— Wright. Voisin. Skin friction, I = '01 ... ... ... ... 40 lbs. 60 lbs. Struts and wires ... ... ... ... ... 30 ,, 20 Aeronaut, motor, &c. ... ... ... ... 20 ,, 10 Radiator and tanks ... ... ... ... ... 5 >> 25 Alighting gear .. ... .. ... ... — ,, 10 Sustentation (power expended aerodynamically)... 60 ,, 100 I5S » 225 „ The above do not correspond exactly with the suggested values of tan y, but they are as near as the author can estimate at present. The addition in the case of the Wright machine is a trifle high, and that of the Voisin is a little low. Possibly the fault is with suggested values themselves, and there is really less difference between the gliding angles than has been supposed. In conversation with the author, Mr. Wilbur Wright has stated that he makes no allowance for skin friction and that he believes it to be negligible. There is evidently considerable scope yet for guess-work. It is quite likely the designers themselves could not give a much better approximate balance-sheet of the resistance account than that here presented. It is possible that the co-efficient of skin friction £ is less than -oi ; for these large surfaces and high velocities it is conceivably no more than half this value. It is equally possible that the other direct resistances, struts, wires, &c, have been underestimated, there may also be faults of as much as 10 or 15 per cent, in the estimate of the energy ex- pended in sustentation, but it is quite certain that skin friction is not negligible but that it is a substantial quantity of the order indicated, it is also quite certain that the gliding angle of the machines is round about the values given 1:6 to 1:8, and is nowhere near 1:12 as has been stated in a recent paper on the subject; it is also improbable that the efficiency of propulsion is in any case as high as 75 per cent, as it has sometimes been represented (in the case of the Wright machine) although it may in both cases be a few per cent, greater than given in the present paper. On the whole the advantage certainly rests with the Wright machine from the aerodynamic standpoint. Stability.—We now pass on to consider the question of stability and control. (A) Longitudinal stability. In the case of the Wright machine it is claimed by Mr. Wright himself that the stability depends entirely on the skill and address of * A paper read before the Aeronautical Society of Great Britain, Decem-ber Sth, 190S. the aeronaut ; in fact, if we are to credit the unchallenged account of Mr. Wright's declaration on the subject, he does not believe in the possibility of safety, under ordinary weather conditions, being achieved by the inherent properties of the machine. He says that sooner or later the fatal puff must come that will end the flight. The author's own observations on the flight of the Wright machine fully confirm the statement that Mr. Wright does depend entirely upon his manipulative skill. It appears that in fligh t the leading plane travels through the air, carrying little or no lo ad ; in the ordinary conditions of straight flight their direction is as nearly as can be estimated parallel to the frame of the main aerofoil, and both seem to move almost exactly edgeways. It follows from this that the machine cannot be automatically stable, for if the plane were fixed for any period of time, and if during that period the machine made the smallest pitching movement either one way or the other, the resulting change of pressure on the leading plane (or planes) would tend to exaggerate the initial movement, and the machine would turn over. The position of the machine with the leading planes fixed is comparable to an arrow travelling feather first, and this condition is one of instability. In brief, not only does Mr. Wright design definitely for hand- controlled equilibrium, but he has no belief in the possibiliy of making a machine safe by its own inherent stability. The sutccess of the VV right method shows that there is at least more than one xvayto fly. In the Voisin machine, on the contrary, it has been the intention of the designer that the machine should be automatically and inherently stable, and unquestionably to a great extent he has suc- ceeded. The author is at present compelled to speak with some reserve as to the degree of success that MM. Voisin have achieved ; they have promised to supply particulars that will enable the point to be investigated, but up to the time of writing this promise has not been redeemed. In the meantime it may be remarked that the disposition of the organs of the Voisin machine is such as will give automatic stability if the following conditions are ful- filled :—(i) If the pressure is less (per sq. ft.) on the tail than on the main aerofoil so that the attitude of the aerodrome to its line of flight is one of stable equilibrium ; (2) if the areas and disposition of the surfaces, the amount of inertia, the velocity of flight, and the natural gliding angle, are related to comply with the n/uatioii oj stability^ so that any oscillation in the vertical plane of flight will not tend to an increase of amplitude. From the behaviour of the machine it is not possible to tell whether these conditions are complied with, because it is fitted with a horizontal rudder in front, by which the aeronaut can correct any departure from the straight line, and this appliance is unquestionably utilised to destroy any oscillation that would otherwise arise ; it is a big rudder, about one-quarter the area of the aerofoil, and skilfully handled it would entirely mask the natural free oscillation period of the machine. From observation of the flight, the author is of opinion that whether or no the machine has inherent stability or not, the actual fact is that its motion (in the sense under discussion) is just as much hand-controlled as the Wright machine. In the hands of a beginner the machine would, however, very likely be able to take care of the aeionaut to some extent, performing oscilla- tions the while, until the aeronaut has learned to take care of the machine ; this view is suggested by the fact that many of the observers who saw Farman and Delagrar.ge early in their career witnessed the phugoid oscillation, whereas the author, who saw Farman only a few weeks back, could not detect any oscillation at all, except for a brief period after he first left the ground, and this in spite of the fact that the day was by no means calm—a very perceptible breeze was blowing. M. Colliex, engineer to MM. Voisin, claims that the flight path of their machine is stable on the following grounds : — (1) A one-tenth scale model showed itself quite stable in gliding flight. (2) A machine mounted by Delagrange made a smooth glide to earth without the intervention of the aeronaut in any way when the ignition was cut off at 8 metres altitude. The first of these tests would be quite satisfactory if due pre- cautions are taken to ensure that the model test is made under the conditions of corresponding speed. As a matter of fact, the velocity of the model was nearly half that of the full-sized machine, instead of slightly less than one-third, as it should have been. In conse- t " Aerial Flight," Vol. II, Aerodonetics, Chap. V and VI.
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