The Aeroplane Speaks [23]
wonder of the future . . . all these things she holds for him.
``Ah!'' he cries. ``You'll never leave me now, when at last there is no one between us?''
And Efficiency, smiling and blushing, but practical as ever, says:
``And you will never throw those Compromises in my face?''
``My dear, I love you for them! Haven't they been my life ever since I began striving for you ten long years ago?''
And so they walked off very happily, arm-in-arm together; and if this hasn't bored you and you'd like some more of the same sort of thing, I'd just love to tell you some day of the wonderful things they accomplish together, and of what they dream the future holds in store.
And that's the end of the Prologue.
CHAPTER I
FLIGHT
Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum. It therefore obeys Newton's laws[[14]] and resists movement. It is that resistance or reaction which makes flight possible.
[[14]] See Newton's laws in the Glossary at the end of the book.
Flight is secured by driving through the air a surface[[15]] inclined upwards and towards the direction of motion.
[[15]] See ``Aerofoil'' in the Glossary.
S = Side view of surface.
M = Direction of motion.
CHORD.--The Chord is, for practical purposes, taken to be a straight line from the leading edge of the surface to its trailing edge.
N = A line through the surface starting from its trailing edge. The position of this line, which I call the Neutral Lift Line, is found by means of wind-tunnel research, and it varies with differences in the camber (curvature) of surfaces. In order to secure flight, the inclination of the surface must be such that the neutral lift line makes an angle with and ABOVE the line of motion. If it is coincident with M, there is no lift. If it makes an angle with M and BELOW it, then there is a pressure tending to force the surface down.
I = Angle of Incidence. This angle is generally defined as the angle the chord makes with the direction of motion, but that is a bad definition, as it leads to misconception. The angle of incidence is best described as the angle the neutral lift line makes with the direction of motion relative to the air. You will, however, find that in nearly all rigging specifications the angle of incidence is taken to mean the angle the chord makes with a line parallel to the propeller thrust. This is necessary from the point of view of the practical mechanic who has to rig the aeroplane, for he could not find the neutral lift line, whereas he can easily find the chord. Again, he would certainly be in doubt as to ``the direction of motion relative to the air,'' whereas he can easily find a line parallel to the propeller thrust. It is a pity, however, that these practical considerations have resulted in a bad definition of the angle of incidence becoming prevalent, a consequence of which has been the widespread fallacy that flight may be secured with a negative inclination of the surface. Flight may conceivably be secured with a negative angle of chord, but never with a negative inclination of the surface. All this is only applicable to cambered surfaces. In the case of flat surfaces the neutral lift line coincides with the chord and the definition I have criticised adversely is then applicable. Flat lifting surfaces are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses it and accelerates it DOWNWARDS. As a result of this definite action there is, of course, an equal and opposite reaction UPWARDS.
The top surface, in moving forward, tends to leave the air behind it, thus creating a semi-vacuum or rarefied area over the top of the surface. Consequently the pressure of air on the top of the surface is decreased, thus assisting the reaction below to lift the surface UPWARDS.
The reaction increases approximately as the square of the velocity. It is the result of (1) the mass of air engaged, and (2) the velocity and consequent force with which the surface engages
``Ah!'' he cries. ``You'll never leave me now, when at last there is no one between us?''
And Efficiency, smiling and blushing, but practical as ever, says:
``And you will never throw those Compromises in my face?''
``My dear, I love you for them! Haven't they been my life ever since I began striving for you ten long years ago?''
And so they walked off very happily, arm-in-arm together; and if this hasn't bored you and you'd like some more of the same sort of thing, I'd just love to tell you some day of the wonderful things they accomplish together, and of what they dream the future holds in store.
And that's the end of the Prologue.
CHAPTER I
FLIGHT
Air has weight (about 13 cubic feet = 1 lb.), inertia, and momentum. It therefore obeys Newton's laws[[14]] and resists movement. It is that resistance or reaction which makes flight possible.
[[14]] See Newton's laws in the Glossary at the end of the book.
Flight is secured by driving through the air a surface[[15]] inclined upwards and towards the direction of motion.
[[15]] See ``Aerofoil'' in the Glossary.
S = Side view of surface.
M = Direction of motion.
CHORD.--The Chord is, for practical purposes, taken to be a straight line from the leading edge of the surface to its trailing edge.
N = A line through the surface starting from its trailing edge. The position of this line, which I call the Neutral Lift Line, is found by means of wind-tunnel research, and it varies with differences in the camber (curvature) of surfaces. In order to secure flight, the inclination of the surface must be such that the neutral lift line makes an angle with and ABOVE the line of motion. If it is coincident with M, there is no lift. If it makes an angle with M and BELOW it, then there is a pressure tending to force the surface down.
I = Angle of Incidence. This angle is generally defined as the angle the chord makes with the direction of motion, but that is a bad definition, as it leads to misconception. The angle of incidence is best described as the angle the neutral lift line makes with the direction of motion relative to the air. You will, however, find that in nearly all rigging specifications the angle of incidence is taken to mean the angle the chord makes with a line parallel to the propeller thrust. This is necessary from the point of view of the practical mechanic who has to rig the aeroplane, for he could not find the neutral lift line, whereas he can easily find the chord. Again, he would certainly be in doubt as to ``the direction of motion relative to the air,'' whereas he can easily find a line parallel to the propeller thrust. It is a pity, however, that these practical considerations have resulted in a bad definition of the angle of incidence becoming prevalent, a consequence of which has been the widespread fallacy that flight may be secured with a negative inclination of the surface. Flight may conceivably be secured with a negative angle of chord, but never with a negative inclination of the surface. All this is only applicable to cambered surfaces. In the case of flat surfaces the neutral lift line coincides with the chord and the definition I have criticised adversely is then applicable. Flat lifting surfaces are, however, never used.
The surface acts upon the air in the following manner:
As the bottom of the surface meets the air, it compresses it and accelerates it DOWNWARDS. As a result of this definite action there is, of course, an equal and opposite reaction UPWARDS.
The top surface, in moving forward, tends to leave the air behind it, thus creating a semi-vacuum or rarefied area over the top of the surface. Consequently the pressure of air on the top of the surface is decreased, thus assisting the reaction below to lift the surface UPWARDS.
The reaction increases approximately as the square of the velocity. It is the result of (1) the mass of air engaged, and (2) the velocity and consequent force with which the surface engages