Aerodynamic stability

“ Therefore the correct area has been given to the aerofoil when its aerodynamic resistance is equal to its direct resistance.
Thus, if for the given conditions of weight and velocity the aerofoil be made too large, the skin friction will be in excess of the aerodynamic resistance; if insufficient surface is provided, the aerodynamic resistance will be in excess; in neither case will the energy required for a given distance be the least possible. ”

“ The centres of gravity, of pressure and of head resistance should be at the same point. The centre of thrust of the propeller should also pass through this point. In this will be found the secret of the successful model aeroplane. It is only arrived at by careful experiment and calculation.
Head resistance increases stability while weight and speed lessen it. When an aeroplane is gliding (traveling downwards) its stability is greater than when it is rising or flying horizontally. It is the least stable when rising. ”

“ This feature of aeronautics is particularly important, because any device which will give automatic stability when turning corners will go far toward making aviation a safe amusement. Inequalities of velocity exist not only on curves, but also when the wind is blowing at anything but uniform velocity across the whole front of the machine. [Pg 42] The slightest “flaw” in the wind means an at least temporary variation in lifting force of the two arms. Here is a pregnant source of danger, and one which cannot be left for the aviator to meet by conscious thought and action. ”

“ The more stable is a machine, the less delicately is it controlled, and the present consensus of opinion among pilots is that a 50-50 [Pg 68] compromise between stability and controllability is the best thing.
In questions of airplane equilibrium the starting point is the center of gravity; obviously, if the center of gravity were back at the tail or up at the nose there would be no balance; the proper place for it is the same spot where all the other forces such as thrust, lift and resistance act; there it is easy to balance them all up. ”

“ When the work expended in traction is entirely devoted to diverting the stream, as in the theoretical case of the curved pipe system, then there is no spurting forward of the fluid, and no discontinuity in the system of flow; and on the other hand, the operating surfaces are fully exposed to frictional resistance. "When the stream is brusquely diverted by an aeroplane there is an aerodynamic resistance involved in excess of that necessary to divert the flow, and this, by giving rise to a form of flow of the discontinuous type, diminishes the frictional resistance. ”

“ The third point to be considered in the problem of speed is the resistance caused by the multitude of struts and wires, the body of the pilot, the tanks, engine, and all the other impedimenta projecting in all directions from the body of the aeroplane. It has occurred to our builders that if the whole of these things could be collected together and enclosed in a light covered-in car of a proper shape, the skin friction of such a car would be much less than the total head resistance offered by the different obstructions so covered. ”

“ Let an aviator suddenly run into a hot or a cold strait of air and the pressure upon his planes will instantly change. The effect of temperature must be taken into consideration by any one flying model aeroplanes. It may happen that a draft of cool or of hot air, by changing the pressure on the planes, will throw a model aeroplane out of balance and mar an otherwise promising flight. A difference of a few degrees of temperature will often affect a very sensitive model. The only way to combat this is of course to build your model with the greatest possible stability. ”

“ It may be stated at once that no such provision is necessary, and that a properly designed rigid structure is capable of maintaining its own equilibrium, and possesses complete stability within prearranged limits; and further, that such a rigid structure (or aerodone) may be designed to automatically simulate many of the apparently life-like movements of birds in flight or at will glide steadily at its natural velocity at a constant angle of flight path. ”

“ The deficiency in the present experiments is chiefly that of apparatus and opportunity. The launching of free flight models requires a suitable apparatus to be designed, by which the initial velocity shall be placed under definite control; beyond this it must be considered quite essential, if reliable results are required, that experiments should be conducted inside a building; the absolute calm necessary for aerodynamic determinations is so rare a phenomenon as to render outdoor experiment almost impossible. ”

“ Directional stability is its stability about its vertical axis, and without which it would have no tendency to keep its course.
Whenever an airplane does not fly properly, aside from conditions arising from engine or propeller trouble, either its longitudinal, lateral, or directional stability is affected. ”

“ The stability of a model is greater when its course is downward than when in a horizontal flight. In this position, the center of pressure is much less likely to shift, and the pole of gravity has rather a steadying effect. For this reason a glider thrown from an elevation travels much more evenly than a power-driven machine. Try reversing your planes.
Why is it that my aeroplane sails all right, but when I increase the power of my motor it falls off and darts about until it reaches the ground? ”

“ In practice, it will be shown later in the work, the question of wing-form, especially with regard to the extremities, is not decided by aerodynamic considerations alone, and that the question of equilibrium is involved. ”

“ In the lift-off configuration, the vehicle is aerodynamically stable. A proportional control system featuring a combination of jet vanes and aerodynamic tip control surfaces operated by hydraulic servo actuators is used to control the vehicle throughout the entire first-stage burning period. These controls operate in pairs for pitch and yaw control. The jet vanes provide the majority of the control force during the thrusting phase and the aerodynamic tip controls provide all the control force during the coasting phase following burnout of the first stage. ”

“ Since engines could be constructed weighing less than twenty pounds to the horse-power, theoretically such an engine should support ten times its own weight in horizontal flight in an absolute calm. As a matter of fact there is no such thing as an absolute calm in nature, air-currents being constantly stirring even on the calmest day, and this introduces another element in attaining aerial flight that is an all-important one. Indeed it has long been recognized that the mechanical power for flight is not the only requisite for flying—there is, besides, the art of handling that power. ”

“ Flight tests have shown that the DC-8 speed stability in the climb configuration approaches neutral at speeds above 300 knots when the PTC fails to extend the programmed amount. The FAA Witness who testified on maneuvering stability also stated in regard to speed stability, "When I trimmed the airplane at 300 knots, I found that the static stick-free stability was positive when I departed to 85 percent of trim, but when I increased to 115 per-cent of trim, the static stability was within the friction band and, for all practical purposes, would be called neutral." ”

“ Nowadays the high-powered airplane is likely to come to grief in landing more than at any other time. The question of stability in flight has of recent years been treated mathematically and experimentally, using of course the fundamental system of “three axes control” first applied by the Wrights. It has been found that by properly proportioning the tail surfaces and properly arranging the wings and center of gravity, any desired degree of stability may be obtained, such that a machine may be made almost self-flying or, if preferred, may be made very sensitive. ”

“ If the pilots are unaware that this speed increase is caused by windshear, they are likely to react by reducing engine power. However, as the plane passes through the shear, the wind quickly becomes a downdraft and then a tailwind. This reduces the speed of air over the wings, and the extra lift and speed vanish. Because the plane is now flying on reduced power, it is vulnerable to sudden loss of airspeed and altitude. The pilots may be able to escape the microburst by adding power to the engines. But if the shear is strong enough, they may be forced to crash. ”

“ The aerodynamic balance, Fig. 153, consists of a horizontal arm or beam pivoted about a vertical axis the amplitude of motion permitted being regulated by the screws which also form electrical contacts.
For the determination of a normal plane is attached to one end of the beam and a friction plane to the other, the areas of the two being adjusted until they exactly balance. The instrument is used either by being exposed to the wind and held stationary, or fitted in front of an automobile vehicle in still air. ”

“ In practice some prefer to put the centre of gravity in models slightly above the centre of head resistance, the reason being that, generally speaking, wind gusts have a [16] "lifting" action on the machine. It must be carefully borne in mind, however, that if the centre of wind pressure on the aerofoil surface and the centre of gravity do not coincide, no matter at what point propulsive action be applied, it can be proved by quite elementary mechanics that such an arrangement, known as "acentric," produces a couple tending to upset the machine. ”

“ The resistance offered by the air to the passage of an air ship of any type depends upon its density. The air is obviously an exceedingly variable medium, as capricious as quicksilver, as both the sky pilot and the flyer of model aeroplanes have learned. The density and therefore the resistance depends upon the temperature, the pressure and the state of equilibrium. We are in the habit of thinking of density and pressure as affecting enormous volumes or areas miles in extent, such as are reported in the weather forecasts from day to day. ”

“ If, for example, we suppose a plane of infinite lateral breadth gliding edgewise through the fluid to have a force applied at right angles to the direction of motion, this force is borne immediately by the fluid, and the conditions necessary to the development of the cyclic system are fulfilled. In a perfect fluid, however, a plane can move without resistance in any aspect, and thus it is not possible to generate a difference of pressure between its two sides except for the period whilst the normal component of its velocity is undergoing acceleration. ”

“ Both sustaining power and resistance vary with the inclination of the planes, as indicated by the chart on page 24. It is entirely possible to start with no such inclination, so that the direct wind resistance is eliminated. The motor must then overcome only air friction, in addition to providing an accelerating force. The machine runs along the ground, its velocity rapidly increasing. As soon as the necessary speed (or one somewhat greater) is attained, the planes are tilted and the aeroplane rises from the ground. ”

“ Therefore, as long as the aeroplane rushes through the air at that or greater speed the thousands of cubic feet of air forced down beneath the wings deliver up a reaction that results in complete support. When an aircraft fails to move at that velocity it loses “flying speed” and falls to the earth. The net result of this reaction is called “lift,” and as long as the machine sweeps forward at that momentum it has lift. The engine, of course, must supply this forward movement, and when it stalls, the heavier-than-air machine must glide to a landing-place or fall perpendicular to the ground. ”

“ As we rise, the air becomes thinner. Finally a point is reached at which no propeller will ‘bite.’ The ceiling has been attained.
“With jet propulsion, exactly the opposite holds good. The less air there is, the more efficient is the motor. If the ejected gas has an expansion efficiency at rest of 40 feet in one-hundredth of a second—a rate of 4,000 feet a second—the same force exerted in motion would increase the speed up to a point where the maximum efficiency is reached at something like 10,000 miles an hour. Jet propulsion gets better and better as speed and height increase. ”

“ When, however, the principle is applied to bodies aerodynamically supported in the free atmosphere the matter is not so self-evident; here, for example, we find that the weight of a parachutist is borne by the earth's surface almost from the moment he leaves the car, and his presence overhead, or the presence of a passing flight of birds, could be detected barometrically if we possessed an instrument of sufficient delicacy. ”

“ If he is well aloft, with plenty of air space beneath him, and particularly if he has a machine that is inherently stable, he has little to fear from the wind; save, perhaps, should his engine fail him, or should he find—as has been the case in war flying—that the force of the wind, blowing heavily against him, and reducing the speed of his machine, has prevented him from regaining his own lines before his petrol has become exhausted. The modern aeroplane, when its engine-power is ample, and it is at a suitable altitude, can wage battle successfully even with a gale. ”

“ The leaping mode is, in fact, a means of adjustment, by means of which the conditions of least resistance can be approximated under considerable variations of velocity. If one of the larger birds, with its limited relative area, were to force its velocity up to the point at which leaping flight would pay, it would require an amount of energy per second far beyond its actual horse-power capacity. ”