Part 7 (1/2)

Approximately three-fifths of the reaction is due to the decrease of density (and consequent decrease of doard 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

[Illustration]

The direction of the reaction is, at efficient angles of incidence, approxiles to the neutral lift line of the surface, as illustrated 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, _ie_, Lift, which is opposed to Gravity, _ie_, the weight of the aeroplane

2 The horizontal component, _ie_, 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

_Passive 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 frooverning 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 re the whole aeroplane, we must remember the passive 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 + passive) to lift

But for the increase in passive 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 wepossible to diminish the passive drift

Every effort is thenall ”detrimental” parts of the aeroplane a forh the air with the least possible drift Even the wires bracing the aeroplane together are, in ood effect upon the lift-drift ratio In the case of a certain well-known type of aeroplane the replacing of the ordinary wires by streaht speed

[Illustration]

_Head-resistance_ is a term often applied to passive 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 illustrated the flow of air round two objectsin 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 _ 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 streath to width, is deterreater 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, fro all streaht, but le of Incidence_--The le of incidence varies with the thrust at the disposal of the designer, the weight to be carried, and the cliles 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 sle of incidence, in order to preserve a clean, stream-line shape of rarefied area and freedoreat for the velocity, then the rarefied area over the top of the surface becoular 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 coainst which it is driven rather than accelerate it _doards_, and that will tend to produce drift rather than the _upwards_ reaction, or lift