The Aeroplane Speaks [24]
the air. If the reaction was produced by only one of those factors it would increase in direct proportion to the velocity, but, since it is the product of both factors, it increases as V<2S>.
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.
The direction of the reaction is approximately at right- angles to the chord 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, 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.
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 roughnesses 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 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 we may, in a measure, approach by doing everything possible to diminish the passive 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 pass 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.
Head-resistance
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.
The direction of the reaction is approximately at right- angles to the chord 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, 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.
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 roughnesses 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 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 we may, in a measure, approach by doing everything possible to diminish the passive 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 pass 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.
Head-resistance