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Aviation History
1918
1918 - 0540.PDF
MAY 16, 1918. THE MODERN AEROPLANE. , By F. S. BARNWELL, Captain, R.F.C. (Concluded from page 514.) Now for the longitudinal stability. We saw that for practically the whole range of value of i, or angle of attack useful for flight, that is from about minus i£° to about + 130, an aerofoil alone is instable; because the centre of pressure moves forwards as * increases, backwards as * decreases. We attain stability by adding a tail plane behind the aerofoil and at a smaller virtual angle of attack. As to why this does so :— • Consider an aeroplane flying at say 70 value for », with its centre of gravity so placed that it lies on the line of total reaction on the aerofoils for this particular i value (Fig. 13) ; also that its tail plane is set so that it is neutral in this condi- tion, meaning that it is edge on to its relative motion to the air. The whole aeroplane is now balanced in this condition, assum- FLIGHT F/a/3. CONDITION 3.NOSE TIPPED OP L MOVES AHEAP OFC.' fWARD RE5T0SIW6 FORCE ON TAIL PkAnf. ing, of course, that thrust = total drag and that (for simplicity) there is no thrust-drag couple. Now if by some momentary external force the nose of the aeroplane is tipped down, the reaction on the aerofoils moves backwards and introduces a lift-weight couple which tends to turn the nose still further down ; but simultaneously a downward reaction is brought into play on the tail plane, because its leading edge is now canted down relative to the air flow over it, and. this introduces a restoring couple. Precisely similarly for what happens if the nose is tipped up. So, roughly speaking, for any aeroplane, longitudinal stability is a question of a iarg« enough tail plane set at what is termed a fore and aft dihedral angle to the aerofoils. . . Two points are worth noting, first that the tail plane acts in the wash of the aerofoils and this wash has a downward tread compared to the flight path. So the required size and setting for the tail plane is affected by this wash. Secondly, as the movement of centre of pressure on the aerofoils is smaller at large values of i than at small, it follows that, between limits, the greater the negative setting of the tail plane compared to the aerofoils, the smaller the tail plane required for stability. This really means that to obtain longitudinal stability with a tail plane of reasonable size, it is necessary that the centre of gravity of the whole aeroplane be at the correct fore and aft position relative to the aero- foils. Probably the best position is from .33 to .38 of the chord length behind the leading edge of the mean chord of the aerofoils. By " mean chord " is meant the chord of an imaginary single aerofoil, so placed that the reaction on it would be the same as the combined reactions on the separate aerofoils of a multiplane form. If the C.G. lie outside these fore and aft limits, we shall need a larger tail plane, to prevent tendency to nose dive if the C.G. is farther ahead, or to rear or " stall " if the C.G. is farther behind^ As the tail plane, to maintain stability, must act both up- wards and downwards, and is most advantageously placed when approximately equal upward and downward lift co- efficient values are required from it, modern practice is to fit a tail plane of so-called " non-lifting " type. This is a most dangerous and misleading name, but as, unfortunately, it is current practice by now, one is fain to use it. It really means a tail plane of symmetrical fore and aft section, either flat, or of a more or less torpedo shape. • The last point to note about the tail plane is that its setting determines the normal speed of the aeroplane. If it be large enough for stability it tends to maintain the machine at one particular attitude to its flight path, which means one particular speed for level flight. If the thrust necessary to maintain this speed be increased, the result will be to make the machine climb, and thereby use up the surplus power, it will not increase the speed. If the thrust be decreased, the machine will take up a downward flight path such that the component of gravitational force along the line of thrust, makes up the total pull, necessary for this particular, or " normal gliding," speed. Of course, this is assuming that the pilot does not move the elevator flaps. These flaps being hinged at their forward edges to the rear edge of the tail plane, allow of the pilot's altering the form of the whole tail into an approxi- mately cambered form ; so he can vary the attitude of the aeroplane, and, therefore, its speed and vertical direction by their use. The tendency on modern aeroplanes, particularly large ones, is to fit an adjusting device to the tail plane. The commonest form is one in which the rear spar may be raised or lowered by a vertical screw which the pilot can rotate by a small hand wheel at his side. It saves fatigue on long flights as the tail plane can be set for any desired speed, but it adds weight and complication. Directional, or " weather cock," stability, is attained by the correct sizes and positions of its equivalent vertical fin surfaces. The body, the landing gear, the.aerofoils (especially if they are set at a dihedral .angle), the struts,. &c, all have vertical fin surface values, which are difficult to estimate at all accurately, so the safest •proceeding, for a novel design, is to determine what additional rear fixed fin and rudder is necessary for directional stability, by experiment on a complete model in' a wind tunnel. Figures estimated by analogy from other machines are reliable if no great type variation is in question. Finally, for lateral stability (see Fig. 12). The first point to note is that, in normal flight, there is no motion relative to the air at right angles to the fore and aft axis, the axis about which, approximately, an aeroplane rolls. So if an aeroplane flying steadily in a straight line were canted over, and no other motion took place, no force would be created tending to right it. But when it cants, the line of total lift force on it cants with it—out of line with the force of gravity. Hence the resultant of the lift and gravity forces is no longer zero, but is a sideways and downwards force. The aeroplane, therefore, begins to move sideways and downwards, and if the vertical fin surfaces are suitable, a righting couple will be Jormed thereby. Obviously, therefore, the main factor for Attaining lateral stability is the dihedral angle of the aerofoils. This value also is most safely decided upon by model experi- ments, or by analogy from other machines of approximately the same type. In modern machines .lateral control is attained almost universally by double-acting ailerons, or wing-flaps. This method is more efficient at large angles of attack than warping the aerofoils, because pulling down a wing-flap is equivalent to increasing the camber of the aerofoil as well as increasing its angle of attack, whilst warping increases only the angle of attack, hence at an angle of attack near the *' critical angle," pulling down a wing-rflap will increase the lift whilst warping will decrease it. At small angles of attack, however, warping is quite as efficient, and a wing without flaps is slightly lighter and more efficient than one with them. The main reasons for the preference for wing flaps is that this method of lateral control is lighter work for the pilot, and permits of a completely braced and therefore safer aerofoil structure. . . Considering the aeroplane. now, as a whole, the points required in modern war machines are :— , ; :; ; , (1) Great climbing power. (2) Great speed especially at great heights.. (3) Good view in every direction, any blind spot being a source of danger. (4) Extreme ease and quickness in manoeuvring. (5) Greatest possible field of fire. (6) Slow landing speed. 538
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