Lift (aerodynamics)

“ The air flows over the arched wings in such a way as to produce a suction above them which helps the push from below. The actual amount of lift for a given speed has been determined by experiments for wings of various shapes and sizes and set at various angles to the line of motion. If, when the machine is in the air, the lift is just sufficient to counterbalance the weight of the aeroplane, the latter flies horizontally. An increase in lift causes the machine to rise; a decrease in lift permits gravity to pull it down.
Now suppose the aviator is flying horizontally and wishes to climb. ”

“ The margin of lift decreases with altitude, owing to the decrease in the density of the air, which adversely affects the engine. Provided the engine maintained its impulse with altitude, then, if we ignore the problem of the propeller, which I will go into later on, the margin of lift would not disappear. Moreover, greater velocity for a given power would be secured at a greater altitude, owing to the decreased density of air to be overcome. ”

“ This is best explained by actual figures: assuming that a propeller 15 ft. in diameter is used, almost 50 horse-power would be required to get an upward lift of 1,000 pounds; this amount of horse-power would be continually absorbed in maintaining the machine in the air at any given level; for actual lift from one level to another at a speed of eleven feet per second a further 20 horse-power would be required, which means that 70 horse-power must be constantly provided for; this absorption of power in the mere maintenance of aerodynamic support is a permanent drawback. ”

“ Upon starting the engine, in a machine of this kind, a very strong downward draught of air would be produced, and the whole power of the engines would be used in maintaining this downward blast, but if the machine should at the same time be given a rapid forward motion sufficiently great to bring the blades into contact with new air, the inertia of which had not been disturbed, and which was not moving downwards, the lifting effect would be increased sufficiently to lift the machine off the ground. ”

“ Generally speaking, the greater the velocity the smaller should be the angle of incidence, in order to preserve a clean, stream-line shape of rarefied area and freedom from eddies. Should the angle be too great for the velocity, then the rarefied area becomes of irregular shape with attendant turbulent eddies. Such eddies possess no lift value, and since it has taken power to produce them, they represent drift and adversely affect the lift-drift ratio. ”

“ When in motion, therefore, while the lift, so far as its mass is concerned, does not change, the drift does decrease, or the forward pull is less than when at 45 degrees, and the decrease is less and less until the plane assumes a horizontal position, where it is absolutely nil, if we do not consider head resistance.
TABLES OF LIFT AND DRIFT.—All tables of Lift and Drift consider only the air pressures. They do not take into account the fact that momentum takes an important part in the translation of an object, like a flying machine. ”

“ The greater angle has now produced so much drift as to lessen the velocity to a point where the combined lifts from the surface and from the thrust are only just able to maintain horizontal flight. Any greater angle will result in a still lower lift-drift ratio. The lift will then become less than the weight and the aeroplane will consequently fall. ”

“ That the lifting power of a large machine, held stationary in a wind at a small distance from the earth, is much less than the Lilienthal table and our own laboratory experiments would lead us to expect. When the machine is moved through the air, as in gliding, the discrepancy seems much less marked. That the ratio of drift to lift in well-balanced surfaces is less at angles of incidence of five degrees to 12 degrees than at an angle of three degrees. ”

“ The more Lift we can get for a certain area of Surface, the greater the Weight the latter can carry; and the less the Drift, then the less Thrust and Power required to overcome it. Now it is a fact that, if the Surface is shaped to have the greatest possible span, i.e., distance from wing-tip to wing-tip, it then engages more air and produces both a maximum Reaction and a better proportion of Lift to Drift. ”

“ From a rigging point of view, one must presume that every standard aeroplane has its lifting surface set at the most efficient angle, and the practical application of all this is in taking the greatest possible care to rig the surface at the correct angle and to maintain it at such angle. Any deviation will adversely affect the lift-drift ratio, i.e., the efficiency. ”

“ Therefore every pound of weight added to an aeroplane decreases its speed proportionately and requires an equivalent increase in horse-power to force it through the air. Of course, an increase of speed gives an increase in lift, so by doubling the speed of a plane you increase the lift just four times.
There are, however, a number of factors which tend to decrease the progress of a machine through the air: the head resistance of the fuselage, the motor, the struts, the wires, the landing-gear, etc. ”

“ 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. ”

“ If it is given a deep curve or camber from front to back this may, while exercising a powerful lift, offer too high a resistance as it passes through the air, and thus waste the energy needed to propel the craft; or, if its front edge is dipped too sharply, this may cause the air to act upon its upper surface, and send a machine diving headlong to the ground. The planning of a successful wing becomes a compromise, [Pg 40] having for its object a surface which shall give the greatest lifting influence with the least resistance. ”

“ But it is not always easy to bring the line of thrust and the line of total resistance into coincidence, because the line of thrust is the line of the propeller shaft and when this is high up as in the case of some pushers it may be several inches above the line of resistance. And as the thrust is above the resistance there is a tendency to nose the machine down; to balance which the designer deliberately locates the center of gravity sufficiently far behind the center of lift so that there is an equal tendency to tip the nose upward ”

“ If we are to ascend as quickly as possible the Aeroplane must be SLOW in order to secure the best possible Lift-Drift Ratio, for the drift of the struts wires, body, etc., increases approximately as the square of the speed, but it carries with it no lift as it does in the case of the Surface. ”

“ In a plane the pressure is always perpendicular to the surface, and the ratio of lift to drift is therefore the same as that of the cosine to the sine of the angle of incidence. But in curved surfaces a very remarkable situation is found. The pressure, instead of being uniformly normal to the chord of the arc, is usually inclined considerably in front of the perpendicular. The result is that the lift is greater and the drift less than if the pressure were normal. ”

“ DIFFERENCE BETWEEN LIFT AND DRIFT IN MOTION.—The first observation is directed to the differences that exist between the lift and drift, when the plane is placed at an angle of less than 45 degrees. A machine weighing 1000 pounds has always the same lift. Its mass does not change. Remember, now, we allude to its mass, or density. ”

“ If we wish to make a machine more efficient, we must resort to a multitude of very narrow superposed planes, or sustainers, as Mr. Philipps calls them, or we must increase the speed. If an aeroplane will lift 2·5 lbs. per square foot placed at an angle of 1 in 10, and driven at a velocity of 40 miles an hour, the same aeroplane will lift 1·25 lbs. if placed at an angle of 1 in 20, and as the lifting effect varies as the square of the velocity, the same plane will lift as much more at 60 miles per hour, as 602 is greater than 402—that is, 2·81 lbs. ”

“ The upward draught is not usually dangerous, for it simply lifts the machine; but the down draught will cause it to drop. A swift downward glide under the full power of the motor must then be made, to increase the forward speed and consequently the lift. This explains why it is dangerous to fly near the ground in a wind; likewise why the beginner should never attempt flying at first in anything but a dead calm. ”

“ If the plane is horizontal the head resistance is at a minimum, but the lift is also very slight. If the wing planes were perfectly flat, there would be no lift at all, but because of the curve of the wings there is a certain amount of lift when they are horizontal, and even when they are tipped slightly downward, provided [237] the machine is traveling at high speed. The angle of an aeroplane’s wings is therefore much flatter than that of a kite. ”

“ To illustrate, in a wagon drawn uphill by horses the wheels which support the wagon do not exert any more support than on the level, and the entire force to make the wagon ascend is supplied through extra hard pulling by the horses. Thus in a climbing airplane the propeller furnishes all the climbing force and lift is no greater than in horizontal flight. In fact, the actual lift force may be even less, as the weight of the airplane is partly supported by the propeller thrust which is now inclined upward slightly. ”

“ Because in making a turn, the momentum of the plane sets up a centrifugal force, acting horizontally outward. To counteract this, the force of lift must be inclined until it has a horizontal component equal to the centrifugal force. The machine is therefore tilted to one side, or banked, thus maintaining a state of equilibrium in which it will turn steadily. ”

“ The primary conditions of flight are that lift made by the planes shall be equal to the force of gravity, and that the forward thrust must be equal to the drift. At that point a machine will sustain flight—a fairly simple thing on paper. But the times that machines have stalled in the air, with their motors full on because their pilots have failed to sustain flight, have let the force of gravity overcome lift, are too numerous to mention. ”

“ The velocity of the lift [Pg 113] is ever-increasing—accelerated. Can we extend to accelerated frames our principle that Nature is indifferent to frames of space and time, so that no one frame is superior to any other? I think we can. The only doubt that arises is whether we should not regard the frame of the man in the lift as superior to, instead of being merely coequal with, our usual frame.
When we stand on the ground the molecules of the ground support us by hammering on the soles of our boots with force equivalent to some ten stone weight. ”

“ Most air lift propositions are deep-well cases—that is, the water is lifted a distance greater than 25 feet; and just in proportion as the lift is increased do we get away from the aerated form idea, and so when the air pressure is greater than the head of water, a certain volume of compressed air is received into the eduction-pipe, the water in this pipe is at that time moving rapidly upward; that is, its momentum has been established. Hence the air takes up this velocity and goes upward with the water from the energy received from the elasticity of the air due to its compressor. ”

“ Neutral Lift Line—A line taken through a surface in a forward direction relative to its direction of motion, and starting from its trailing edge. If the attitude of the surface is such as to make the said line coincident with the direction of motion, it results in no lift, the reaction then consisting solely of drift. The position of the neutral lift line, i.e., the angle it makes with the chord, varies with differences of camber, and it is found by means of wind-tunnel research.
Newton's Laws of Motion—1. If a body be at rest, it will remain at rest ”

“ The vertical stabilizer should not be too large, however, as then any side pressure due to deviation from a rectilinear course will cause the machine to swerve violently; the wing which is outermost in the turn will have preponderance of lift due to its higher speed; that is, the airplane will get into a turn where there is too much bank and a spiral dive may result. ”

“ In direct single-acting lift machines, in which the stroke is fixed, and in rotative machines at constant speed it is obvious that the cylinder must be filled at each stroke irrespective of the amount of work to be done. The same amount of water is used whether much or little work is done, or whether great or small weights are lifted. Hence while pressure engines are very efficient at full load, their efficiency decreases as the load decreases. ”

“ To secure maximum climbing ability, we must determine at what velocity the margin of motor power is the greatest. In the above-mentioned machine we know that the horsepower required for [Pg 64] support is least for a speed of near 55 miles per hour, and it is near speed where therefore the excess margin of power is greatest and at which climbing is best done. An airplane designed chiefly for climbing must have low values of motor power necessary for support, namely, must have small resistance, therefore small size, therefore small weight. ”