Approximately three-fifths of the reaction is due to the decrease of density (and consequent decrease of downward pressure) on the top of the surface; and only some two-fifths is due to the upward reaction secured by the action of the bottom surface upon the air. A practical point in respect of this is that, in the event of the fabric covering the surface getting into bad condition, it is more likely to strip off the top than off the bottom.

[Ill.u.s.tration]

The direction of the reaction is, at efficient angles of incidence, approximately at right-angles to the neutral lift line of the surface, as ill.u.s.trated above; and it is, in considering flight, convenient to divide it into two component parts or values, thus:

1. The vertical component of the reaction, _i.e._, Lift, which is opposed to Gravity, _i.e._, the weight of the aeroplane.

2. The horizontal component, _i.e._, Drift (sometimes called Resistance), to which is opposed the thrust of the propeller.

The direction of the reaction is, of course, the resultant of the forces Lift and Drift. The Lift is the useful part of the reaction, for it lifts the weight of the aeroplane.

The Drift is the villain of the piece, and must be overcome by the Thrust in order to secure the necessary velocity to produce the requisite lift for flight.

DRIFT.--The drift of the whole aeroplane (we have considered only the lifting surface heretofore) may be conveniently divided into three parts, as follows:

_Active Drift_, which, is the drift produced by the lifting surfaces.

_Pa.s.sive Drift_, which is the drift produced by all the rest of the aeroplane--the struts, wires, fuselage, under-carriage, etc., all of which is known as "detrimental surface."

_Skin Friction_, which is the drift produced by the friction of the air with roughness of surface. The latter is practically negligible having regard to the smooth surface of the modern aeroplane, and its comparatively slow velocity compared with, for instance, the velocity of a propeller blade.

LIFT-DRIFT RATIO.--The proportion of lift to drift is known as the lift-drift ratio, and is of paramount importance, for it expresses _the efficiency of the aeroplane_ (as distinct from engine and propeller).

A knowledge of the factors governing the lift-drift ratio is, as will be seen later, _an absolute necessity_ to anyone responsible for the rigging of an aeroplane, and the maintenance of it in an efficient and safe condition.

Those factors are as follows:

1. _Velocity_.--The greater the velocity the greater the proportion of drift to lift, and consequently the less the efficiency. Considering the lifting surfaces alone, both the lift and the (active) drift, being component parts of the reaction, increase as the square of the velocity, and the efficiency remains the same at all speeds. But, considering the whole aeroplane, we must remember the pa.s.sive drift. It also increases as the square of the velocity (with no attendant lift), and, adding itself to the active drift, results in increasing the proportion of total drift (active + pa.s.sive) to lift.

But for the increase in pa.s.sive drift the efficiency of the aeroplane would not fall with increasing velocity, and it would be possible, by doubling the thrust, to approximately double the speed or lift--a happy state of affairs which can never be, but which we may, in a measure, approach by doing everything possible to diminish the pa.s.sive drift.

Every effort is then made to decrease it by "stream-lining," _i.e._, by giving all "detrimental" parts of the aeroplane a form by which they will pa.s.s through the air with the least possible drift. Even the wires bracing the aeroplane together are, in many cases, stream-lined, and with a markedly good effect upon the lift-drift ratio. In the case of a certain well-known type of aeroplane the replacing of the ordinary wires by stream-lined wires added over five miles an hour to the flight speed.

[Ill.u.s.tration]

_Head-resistance_ is a term often applied to pa.s.sive drift, but it is apt to convey a wrong impression, as the drift is not nearly so much the result of the head or forward part of struts, wires, etc., as it is of the rarefied area behind.

Above is ill.u.s.trated the flow of air round two objects moving in the direction of the arrow M.

In the case of A, you will note that the rarefied area DD is of very considerable extent; whereas in the case of B, the air flows round it in such a way as to meet very closely to the rear of the object, thus _decreasing_ DD.

The greater the rarefied area DD, then, the less the density, and, consequently, the less the pressure of air upon the rear of the object.

The less such pressure, then, the better is head-resistance D able to get its work in, and the more thrust will be required to overcome it.

The "fineness" of the stream-line shape, _i.e._, the proportion of length to width, is determined by the velocity--the greater the velocity, the greater the fineness. The best degree of fineness for any given velocity is found by means of wind-tunnel research.

The practical application of all this is, from a rigging point of view, the importance of adjusting all stream-line parts to be dead-on in the line of flight, but more of that later on.

2. _Angle of Incidence_.--The most efficient angle of incidence varies with the thrust at the disposal of the designer, the weight to be carried, and the climb-velocity ratio desired.

The best angles of incidence for these varying factors are found by means of wind-tunnel research and practical trial and error. 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 over the top of the surface 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. Also, too great an angle for the velocity will result in the underside of the surface tending to compress the air against which it is driven rather than accelerate it _downwards_, and that will tend to produce drift rather than the _upwards_ reaction, or lift.

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.

3. _Camber_.--(Refer to the second ill.u.s.tration in this chapter.) The lifting surfaces are cambered, _i.e._, curved, in order to decrease the horizontal component of the reaction, _i.e._, the drift.

_The bottom camber_: If the bottom of the surface was flat, every particle of air meeting it would do so with a shock, and such shock would produce a very considerable horizontal reaction or drift. By curving it such shock is diminished, and the curve should be such as to produce a uniform (not necessarily constant) acceleration and compression of the air from the leading edge to the trailing edge.

Any unevenness in the acceleration and compression of the air produces drift.

_The top camber_: If this was flat it would produce a rarefied area of irregular shape. I have already explained the bad effect this has upon the lift-drift ratio. The top surface is then curved to produce a rarefied area the shape of which shall be as stream-line and free from attendant eddies as possible.

The camber varies with the angle of incidence, the velocity, and the thickness of the surface. Generally speaking, the greater the velocity, the less the camber and angle of incidence. With infinite velocity the surface would be set at no angle of incidence (the neutral lift line coincident with the direction of motion relative to the air), and would be, top and bottom, of pure stream-line form--_i.e._, of infinite fineness. This is, of course, carrying theory to absurdity as the surface would then cease to exist.

The best cambers for varying velocities, angles of incidence, and thickness of surface, are found by means of wind-tunnel research. The practical application of all this is in taking the greatest care to prevent the surface from becoming distorted and thus spoiling the camber and consequently the lift-drift ratio.

4. _Aspect Ratio_.--This is the proportion of span to chord. Thus, if the span is, for instance, 50 feet and the chord 5 feet, the surface would be said to have an aspect ratio of 10 to 1.

For _a given velocity_ and _a given area_ of surface, the higher the aspect ratio, the greater the reaction. It is obvious, I think, that the greater the span, the greater the ma.s.s of undisturbed air engaged, and, as already explained, the reaction is partly the result of the ma.s.s of air engaged. I say "undisturbed" advisedly, for otherwise it might be argued that, whatever the shape of the surface, the same ma.s.s of air would be engaged. The word "undisturbed" makes all the difference, for it must be remembered that the rear part of the underside of the surface engages air most of which has been deflected downwards by the surface in front of it. That being so, the rear part of the surface has not the same opportunity of forcing; the air downwards (since it is already flowing downwards) and securing there from an upwards, reaction as has the surface in front of it. It is therefore of less value for its area than the front part of the surface, since it does less work and secures less reaction--_i.e._, lift. Again, the rarefied area over the top of the surface is most rare towards the front of it, as, owing to eddies, the rear of such area tends to become denser.

[Ill.u.s.tration]

Thus, you see, the front part of the surface is the most valuable from the point of view of securing an upwards reaction from the air; and so, by increasing the proportion of front, or "span," to chord, we increase the amount of reaction for a given velocity and area of surface. That means a better proportion of reaction to weight of surface, though the designer must not forget the drift of struts and wires necessary to brace up a surface of high aspect ratio.

Not only that, but, _provided_ the chord is not decreased to an extent making it impossible to secure the best camber owing to the thickness of the surface, the higher the aspect ratio, the better the lift-drift ratio. The reason of this is rather obscure. It is sometimes advanced that it is owing to the "spill" of air from under the wing-tips. With a high aspect ratio the chord is less than would otherwise be the case.

Less chord results in smaller wing-tips and consequently less "spill."

This, however, appears to be a rather inadequate reason for the high aspect ratio producing the high lift-drift ratio. Other reasons are also advanced, but they are of such a contentious nature I do not think it well to go into them here. They are of interest to designers, but this is written for the practical pilot and rigger.

5. _Stagger_.--This is the advancement of the top surface relative to the bottom surface, and is not, of course, applicable to a single surface, _i.e._, a monoplane. In the case of a biplane having no stagger, there will be "interference" and consequent loss of efficiency unless the gap between the top and bottom surfaces is equal to not less than about 1-1/2 times the chord. If less than that, the air engaged by the bottom of the top surface will have a tendency to be drawn into the rarefied area over the top of the bottom surface, with the result that the surfaces will not secure as good a reaction as would otherwise be the case.

It is not practicable to have a gap of much more than a distance equal to the chord, owing to the drift produced by the great length of struts and wires such a large gap would necessitate. By staggering the top surface forward, however, it is removed from the action of the lower surface and engages undisturbed air, with the result that the efficiency can in this way be increased by about 5 per cent. Theoretically the top plane should be staggered forward for a distance equal to about 30 per cent. of the chord, the exact distance depending upon the velocity and angle of incidence; but this is not always possible to arrange in designing an aeroplane, owing to difficulties of balance, desired position, and view of pilot, observer, etc.

[Ill.u.s.tration: H.E., Horizontal equivalent. D., Dihedral angle.]

6. _Horizontal Equivalent._-The vertical component of the reaction, _i.e._, lift, varies as the horizontal equivalent (H.E.) of the surface, but the drift remains the same. Then it follows that if H.E. grows less, the ratio of lift to drift must do the same.

A, B, and C are front views of three surfaces.

A has its full H.E., and therefore, from the point of view from which we are at the moment considering efficiency, it has its best lift-drift ratio.

B and C both possess the same surface as A, but one is inclined upwards from its centre and the other is straight but tilted. For these reasons their H.E.'s are, as ill.u.s.trated, less than in the case of A, That means less vertical lift, and, the drift remaining the same (for there is the same amount of surface as in A to produce it), the lift-drift ratio falls.

THE MARGIN OF POWER is the power available above that necessary to maintain horizontal flight.

THE MARGIN OF LIFT is the height an aeroplane can gain in a given time and starting from a given alt.i.tude. As an example, thus: 1,000 feet the first minute, and starting from an alt.i.tude of 500 feet above sea-level.

The margin of lift decreases with alt.i.tude, owing to the decrease in the density of the air, which adversely affects the engine. Provided the engine maintained its impulse with alt.i.tude, 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 alt.i.tude, owing to the decreased density of air to be overcome. After reading that you may like to light your pipe and indulge in dreams of the wonderful possibilities which may become realities if some brilliant genius shows us some day how to secure a constant power with increasing alt.i.tude. I am afraid, however, that will always remain impossible; but it is probable that some very interesting steps may be taken in that direction.

THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.

THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for a given power, surface (including detrimental surface), and weight, horizontal flight can be maintained.