The Aeroplane Speaks [8]
was to give me a larger Angle. That dropped the bottom of the V lower down, and as that's the centre of the machine, where all the Weight is, of course that put the Centre of Gravity in its right place. But now there is too much Keel Surface above, and the whole thing's a Bad Compromise, not at all like Our Efficiency.''
And Efficiency, blushing very prettily at the compliment, then asked, ``And how does the Centre of Gravity affect matters?''
``That's easy,'' said Grandfather Gravity. ``I'm so heavy that if I am too low down I act like a pendulum and cause the Aeroplane to roll about sideways, and if I am too high I'm like a stick balanced on your finger, and then if I'm disturbed, over I go and the Aeroplane with me; and, in addition to that, there are the tricks I play with the Aeroplane when it's banked up,[[6]] i.e., tilted sideways for a turn, and Centrifugal Force sets me going the way I'm not wanted to go. No; I get on best with Lateral Stability when my Centre is right on the centre of Drift, or, at any rate, not much below it.'' And with that he settled back into the Lecturer's Chair and went sound asleep again, for he was so very, very old, in fact the father of all the Principles.
[[6]] Banking: When an aeroplane is turned to the left or the right the centrifugal force of its momentum causes it to skid sideways and outwards away from the centre of the turn. To minimize such action the pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the underside of the planes to the air. The aeroplane will not then skid outwards beyond the slight skid necessary to secure a sufficient pressure of air to balance the centrifugal force.
And the Blackboard had been busy, and now showed them a picture of the Aeroplane as far as they knew it, and you will see that there is a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel Surface or fin, as is very often the case in order to ensure the greater effect of such surface being behind the vertical turning axis.
But Efficiency, growing rather critical with her newly gained knowledge, cried out: ``But where's the horizontal Tail Surface? It doesn't look right like that!''
``This is when I have the pleasure of meeting you, my dear,'' said Longitudinal Stability. ``Here's the Tail Surface,'' he said, ``and in order to help me it must be set IN EFFECT at a much less Angle of Incidence than the Main Surface. To explain we must trouble the Blackboard again,'' and this was his effort:
``I have tried to make that as clear as possible,'' he said. ``It may appear a bit complicated at first, but if you will take the trouble to look at it for a minute you will find it quite simple. A is the normal and proper direction of motion of the Aeroplane, but, owing to a gust of air, it takes up the new nose-down position. Owing to Momentum, however, it does not fly straight along in that direction, but moves more or less in the direction B, which is the resultant of the two forces, Momentum and Thrust. And so you will note that the Angle of Incidence, which is the inclination of the Surfaces to the Direction of Motion, has decreased, and of course the Lift decreases with it. You will also see, and this is the point, that the Tail Surface has lost a higher proportion of its Angle, and consequently its Lift, than has the Main Surface. Then, such being the case, the Tail must fall and the Aeroplane assume its normal position again, though probably at a slightly lower altitude.''
``I'm afraid I'm very stupid,'' said Efficiency, ``but please tell me why you lay stress upon the words `IN EFFECT.' ''
``Ah! I was wondering if you would spot that,'' he replied. ``And there is a very good reason for it. You see, in some Aeroplanes the Tail Surface may be actually set at the same Angle on the machine as the Main Surface, but owing to the air being deflected downwards by the front Main Surface it meets the Tail Surface at a lesser angle, and indeed in some cases at no angle at all. The Tail is then for its surface getting less Lift than the Main Surface,
And Efficiency, blushing very prettily at the compliment, then asked, ``And how does the Centre of Gravity affect matters?''
``That's easy,'' said Grandfather Gravity. ``I'm so heavy that if I am too low down I act like a pendulum and cause the Aeroplane to roll about sideways, and if I am too high I'm like a stick balanced on your finger, and then if I'm disturbed, over I go and the Aeroplane with me; and, in addition to that, there are the tricks I play with the Aeroplane when it's banked up,[[6]] i.e., tilted sideways for a turn, and Centrifugal Force sets me going the way I'm not wanted to go. No; I get on best with Lateral Stability when my Centre is right on the centre of Drift, or, at any rate, not much below it.'' And with that he settled back into the Lecturer's Chair and went sound asleep again, for he was so very, very old, in fact the father of all the Principles.
[[6]] Banking: When an aeroplane is turned to the left or the right the centrifugal force of its momentum causes it to skid sideways and outwards away from the centre of the turn. To minimize such action the pilot banks, i.e., tilts, the aeroplane sideways in order to oppose the underside of the planes to the air. The aeroplane will not then skid outwards beyond the slight skid necessary to secure a sufficient pressure of air to balance the centrifugal force.
And the Blackboard had been busy, and now showed them a picture of the Aeroplane as far as they knew it, and you will see that there is a slight Dihedral Angle, and also, fixed to the tail, a vertical Keel Surface or fin, as is very often the case in order to ensure the greater effect of such surface being behind the vertical turning axis.
But Efficiency, growing rather critical with her newly gained knowledge, cried out: ``But where's the horizontal Tail Surface? It doesn't look right like that!''
``This is when I have the pleasure of meeting you, my dear,'' said Longitudinal Stability. ``Here's the Tail Surface,'' he said, ``and in order to help me it must be set IN EFFECT at a much less Angle of Incidence than the Main Surface. To explain we must trouble the Blackboard again,'' and this was his effort:
``I have tried to make that as clear as possible,'' he said. ``It may appear a bit complicated at first, but if you will take the trouble to look at it for a minute you will find it quite simple. A is the normal and proper direction of motion of the Aeroplane, but, owing to a gust of air, it takes up the new nose-down position. Owing to Momentum, however, it does not fly straight along in that direction, but moves more or less in the direction B, which is the resultant of the two forces, Momentum and Thrust. And so you will note that the Angle of Incidence, which is the inclination of the Surfaces to the Direction of Motion, has decreased, and of course the Lift decreases with it. You will also see, and this is the point, that the Tail Surface has lost a higher proportion of its Angle, and consequently its Lift, than has the Main Surface. Then, such being the case, the Tail must fall and the Aeroplane assume its normal position again, though probably at a slightly lower altitude.''
``I'm afraid I'm very stupid,'' said Efficiency, ``but please tell me why you lay stress upon the words `IN EFFECT.' ''
``Ah! I was wondering if you would spot that,'' he replied. ``And there is a very good reason for it. You see, in some Aeroplanes the Tail Surface may be actually set at the same Angle on the machine as the Main Surface, but owing to the air being deflected downwards by the front Main Surface it meets the Tail Surface at a lesser angle, and indeed in some cases at no angle at all. The Tail is then for its surface getting less Lift than the Main Surface,