The Aeroplane Speaks. H. L. Barber
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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, although set at the same angle on the machine. It may then be said to have IN EFFECT a less Angle of Incidence. I'll just show you on the Blackboard.”
“And now,” said Efficiency, “I have only to meet the Ailerons and the Rudder, haven't I?”
“Here we are,” replied the Ailerons, or little wings. “Please hinge us on to the back of the Main Surfaces, one of us at each Wing-tip, and join us up to the Pilot's joystick by means of the control cables. When the Pilot wishes to tilt the Aeroplane sideways, he will move the stick and depress us upon one side, thus giving us a larger Angle of Incidence and so creating more Lift on that side of the Aeroplane; and, by means of a cable connecting us with the Ailerons on the other side of the Aeroplane, we shall, as we are depressed, pull them up and give them a reverse or negative Angle of Incidence, and that side will then get a reverse Lift or downward thrust, and so we are able to tilt the Aeroplane sideways.
“And we work best when the Angle of Incidence of the Surface in front of us is very small, for which reason it is sometimes decreased or washed-out towards the Wing-tips. The reason of that is that by the time the air reaches us it has been deflected downwards—the greater the Angle of Incidence the more it is driven downwards—and in order for us to secure a Reaction from it, we have to take such a large Angle of Incidence that we produce a poor proportion of Lift to Drift; but the smaller the Angle of the Surface in front of us the less the air is deflected downwards, and consequently the less Angle is required of us, and the better our proportion of Lift to Drift, which, of course, makes us much more effective Controls.”
“Yes,” said the Lateral and Directional Stabilities in one voice, “that's so, and the wash-out helps us also, for then the Surfaces towards their Wing-tips have less Drift or 'Head-Resistance,' and consequently the gusts will affect them and us less; but such decreased Angle of Incidence means decreased Lift as well as Drift, and the Designer does not always care to pay the price.”
“Well,” said the Ailerons, “if it's not done it will mean more work for the Rudder, and that won't please the Pilot.”
“Whatever do you mean?” asked Efficiency. “What can the Rudder have to do with you?”
“It's like this,” they replied: “when we are deflected downwards we gain a larger Angle of Incidence and also enter an area of compressed air, and so produce more Drift than those of us on the other side of the Aeroplane, which are deflected upwards into an area of rarefied air due to the SUCTION effect (though that term is not academically correct) on the top of the Surface. If there is more Drift, i.e., Resistance, on one side of the Aeroplane than on the other side, then of course it will turn off its course, and if that difference in Drift is serious, as it will very likely be if there is no wash-out, then it will mean a good deal of work for the Rudder in keeping the Aeroplane on its course, besides creating extra Drift in doing so.”
“I think, then,” said Efficiency, “I should prefer to have that wash-out,7 and my friend the Designer is so clever at producing strength of construction for light weight, I'm pretty sure he won't mind paying the price in Lift. And now let me see if I can sketch the completed Aeroplane.”
“Well, I hope that's all as it should be,” she concluded, “for to-morrow the Great Test in the air is due.”
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