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Aviation History
1914
1914 - 0082.PDF
JANUARY 24, 1914. (pig GRAVITY CONTROLLED VELOMETERS Two main kinds of instrument are used for indicating the of the ^Jj^^^^J^^Si s^rf'a'n^pk^^rough £ ST ioth^epend on smallness <rf the speed Such «o«Mll^^n»l the dynamic pressure of the air acting on some part of to result, whatever the speed, if only the angle of incidence the instrument, but they differ in the nature of the opposing force. When this control is by springs or their equivalent we may call the instrument an air speed indicator, and when gravity is the controlling force the term velometer may be used. These appliances are distinct and are useful for different purposes. The spring controlled air speed indicator will tell the true air speed at any moment, and when it is desired to know this, as for instance for pur poses of scientific investigation, it should be used. It also has the advantage of not depending on a liquid gauge, so that it is perhaps easier to make fool proof. The velometer, on the other hand, does not tell the true air speed at any moment, but it does give definite in formation as to safety, warning the pilot if he is in danger of over-stepping the limits of control of his machine. It is suggested that some confusion between the two objects exists in the article in FLIGHT of January 3rd, entitled " A Warning to Pilots," since in that article pilots are warned against the very peculiarity which makes for safety in the velometer type of instrument. This would have been clear if Mr. Booth had carried his argument a little further in the above article. Mr. Booth showed, quite rightly, that since the apparent weight of objects on an aeroplane is affected by any up or down acceleration of the aeroplane, therefore the apparent weight of the liquid in the gauge is so affected. This was, I think, pointed out by Mr. Horace Darwin last year in his paper before the Aeronautical Society. The result is that the velometer does not always read the true air speed. Here Mr. Booth's argument stops. Let us carry it a little further and consider, firstly, what it is that a practical flyer should be informed of so that he can fly safely, and, secondly, what it is that the velometer tells him. It will then be plain that the two are the same, and that the velometer tells him exactly what he wants to know. The argument following may be summarized by saying that gravity is our enemy on an aeroplane, and air speed is our friend, and the velometer, by balancing gravity against air speed, shows us whether friend or foe is the stronger. In Mr. Booth's first paragraph he mentions that the velometer is used as a safeguard against stalling. The term " stalling " denotes the condition of affairs when the wings have reached a certain large angle of incidence. Beyond this angle, which is different for different wing shapes, the form of the air-flow round the wing changes, and a dead air region forms above the wings. The results of this are :— (1) The lift falls suddenly, and a further increase of angle of incidence reduces the lift still more. (2) The drift of the wing is increased considerably. These cause the machine to be uncontrollable, (1) causing the ordinary warping motion to have an effect in the opposite direction to that expected, and (2) causing the aeroplane to further lose speed, so that an unusually prolonged dive is necessary before the ordinary speed is regained. The above effects must be familiar to all pilots and students of flight, and I merely recapitulate them to point out that the cause of the trouble is in the largeness is made large enough. It is, therefore, of excessive angle of incidence that flyers require to be warned. Let us now further consider what are the exact con ditions under which excessive angle of incidence occurs. The lift produced by the wings is, in flight, always equal to the apparent weight of the aeroplane. The latter is the sum of the components perpendicular to the wings of (a) the actual weight plus (b) the force necessary for any acceleration that the machine is performing, as, for instance, the centrifugal force during a banked turn, When the angle of incidence is nearly as great as the critical angle, the planes have reached their maximum lift coefficient. The lift which occurs then cannot be increased without increasing the speed, and it (i.e., the lift at the critical angle) is equal to a quantity which varies as the square of the speed. As long as the apparent weight of the aeroplane is less than this quantity, so long will the lift coefficient be less than the maximum, and the angle of incidence less than its critical value. To put it more exactly, the maximum lift is equal to : (maximum lift coefficient) x (area of planes) x (p, the density of air) x (speed)2. The first two terms are constants, so we see that for safety it is necessary that: (apparent weight of aero plane) < (a constant) x p x (speed)2. Turn now to the velometer and consider what it shows. This instrument balances a pressure due to the speed through the air against the apparent weight of a column of liquid. The pressure received from the pressure head is equal to: (a constant) x p x (speed)2. The apparent weight of the column of liquid is pro portional to the apparent weight of everything else in the aeroplane, and therefore, of course, to the apparent weight of the aeroplane itself. The velometer, therefore, shows us the relation of (apparent weight of aeroplane) to (a constant) x p x (speed)2. And this is what the flyer wants to know, as has been shown above. The two constants must, in practice of course, be calcu lated, and so the minimum reading for safety determined. This will be equal to the minimum safe speed reading, which one knows for any given aeroplane in terms of miles per hour for straight flight. It has now been shown that the velometer may be relied on as a safety indicator whether flying straight or doing banked turns, sudden dives or flattening out. Even with the velometer, however, there are times when full protection is not afforded, and it is for these times that a " warning to flyers " may be useful. Thus a reading which is only just safe when flying straight may not be quite safe when turning. This is because it may be necessary to warp or bring the dihedral angle into effect, and both these increase the angle of incidence of part of the wing, and the flyer is not safe if any part of the wing exceeds the critical angle. It will, therefore, be advisable always to have a good margin, and especially when turning. Again, in the matter of turns, it must be remembered that the velometer is only a guide which asserts that the
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