The crowd was larger than ever, and among the newcomers some of the most important were the three Stabilities, named Directional, Longitudinal, and Lateral, with their a.s.sistants, the Rudder, Elevator, and Ailerons.

There was Centrifugal Force, too, who would not sit still and created a most unfavourable impression, and Keel-Surface, the Dihedral Angle, and several other lesser fry.

"Well," said Centrifugal Force, "I wish this Efficiency I've heard so much about would get a move on. Sitting still doesn't agree with me at all. Motion I believe in. There's nothing like motion--the more the better."

"We are entirely opposed to that," objected the three Stabilities, all in a breath. "Unless it's in a perfectly straight line or a perfect circle. Nothing but perfectly straight lines or, upon occasion, perfect circles satisfy us, and we are strongly suspicious of your tendencies."

"Well, we shall see what we shall see," said the Force darkly. "But who in the name of blue sky is this?"

And in tripped Efficiency, in a beautifully "doped" dress of the latest fashionable shade of khaki-coloured fabric, a perfectly stream-lined bonnet, and a bewitching little Morane parasol,[4] smiling as usual, and airily exclaiming, "I'm so sorry I'm late, but you see the Designer's such a funny man. He objects to skin friction,[5] and insisted upon me changing my fabric for one of a smoother surface, and that delayed me.

Dear me, there are a lot more of us to-day, aren't there? I think I had better meet one at a time." And turning to Directional Stability, she politely asked him what he preferred to do.

"My purpose in life, miss," said he, "is to keep the Aeroplane on its course, and to achieve that there must be, in effect, more Keel-Surface behind the Vertical Turning Axis than there is in front of it."

[Ill.u.s.tration]

Efficiency looking a little puzzled, he added: "Just like a weatherc.o.c.k, and by Keel-Surface I mean everything you can see when you view the Aeroplane from the side of it--the sides of the body, struts, wires, etc."

"Oh, now I begin to see light," said she; "but just exactly how does it work?"

"I'll answer that," said Momentum. "When perhaps by a gust of air the Aeroplane is blown out of its course and points in another direction, it doesn't immediately fly off on that new course. I'm so strong I pull it off the new course to a certain extent, and towards the direction of the old course. And so it travels, as long as my strength lasts, in a more or less sideways position."

"Then," said the Keel-Surface, "I get a pressure of air all on one side, and as there is, in effect, most of me towards the tail, the latter gets pressed sideways, and the Aeroplane thus tends to a.s.sume its first position and course."

"I see," said Efficiency, and, daintily holding the Chalk, she approached the Blackboard. "Is this what you mean?"

"Yes, that's right enough," said the Keel-Surface, "and you might remember, too, that I always make the Aeroplane nose into the gusts rather than away from them."

"If that was not the case," broke in Lateral Stability, and affecting the fashionable Flying Corps stammer, "it would be a h-h-h-o-r-rible affair! If there were too much Keel-Surface in front, then that gust would blow the Aeroplane round the other way a very considerable distance. And the right-hand Surface being on the outside of the turn would have more speed, and consequently more Lift, than the Surface on the other side. That means a greater proportion of the Lift on that side, and before you could say Warp to the Ailerons over the Aeroplane would go--probable result a bad side-slip" (see ill.u.s.tration A, over-leaf).

"And what can the Pilot do to save such a situation as that?" said Efficiency.

"Well," replied Lateral Stability, "he will try to turn the Aeroplane sideways and back to an even keel by means of warping the Ailerons or little wings which are hinged on to the Wing-tips, and about which you will hear more later on; but if the side-slip is very bad he may not be able to right the Aeroplane by means of the Ailerons, and then the only thing for him to do is to use the Rudder and to turn the nose of the Aeroplane down and head-on to the direction of motion. The Aeroplane will then be meeting the air in the direction it is designed to do so, and the Surfaces and also the controls (the Rudder, Ailerons, and Elevator) will be working efficiently; but its att.i.tude relative to the earth will probably be more or less upside-down, for the action of turning the Aeroplane's nose down results, as you will see by the ill.u.s.tration B, in the right wing, which is on the outside of the circle, travelling through the air with greater speed than the left-hand wing. More Speed means more Lift, so that results in overturning the Aeroplane still more; but now it is, at any rate, meeting the air as it is designed to meet it, and everything is working properly. It is then only necessary to warp the Elevator, as shown in ill.u.s.tration C, in order to bring the Aeroplane into a proper att.i.tude relative to the earth."

[Ill.u.s.tration]

"Ah!" said the Rudder, looking wise, "it's in a case like that when I become the Elevator and the Elevator becomes me."

"That's absurd nonsense," said the Blackboard, "due to looseness of thought and expression."

"Well," replied the Rudder, "when the Aeroplane is in position A and I am used, then I depress or _elevate_ the nose of the machine; and, if the Elevator is used, then it turns the Aeroplane to right or left, which is normally my function. Surely our _roles_ have changed one with the other, and I'm then the Elevator and the Elevator is me!"

[Ill.u.s.tration]

Said Lateral Stability to the Rudder, "That's altogether the wrong way of looking at it, though I admit"--and this rather sarcastically--"that the way you put it sounds rather fine when you are talking of your experiences in the air to those 'interested in aviation' but knowing little about it; but it won't go down here! You are a Controlling Surface designed to turn the Aeroplane about a certain axis of the machine, and the Elevator is a Controlling Surface designed to turn the Aeroplane about another axis. Those are your respective jobs, and you can't possibly change them about. Such talk only leads to confusion, and I hope we shall hear no more of it."

"Thanks," said Efficiency to Lateral Stability. "And now, please, will you explain your duties?"

"My duty is to keep the Aeroplane horizontal from Wing-tip to Wing-tip.

First of all, I sometimes arrange with the Rigger to _wash-out_, that is decrease, the Angle of Incidence on one side of the Aeroplane, and to effect the reverse condition, if it is not too much trouble, on the other side."

"But," objected Efficiency, "the Lift varies with the Angle of Incidence, and surely such a condition will result in one side of the Aeroplane lifting more than the other side?"

"That's all right," said the Propeller, "it's meant to off-set the tendency of the Aeroplane to turn over sideways in the opposite direction to which I revolve."

"That's quite clear, though rather unexpected; but how do you counteract the effect of the gusts when they try to overturn the Aeroplane sideways?" said she, turning to Lateral Stability again.

"Well," he replied, rather miserably, "I'm not nearly so perfect as the Longitudinal and Directional Stabilities. The Dihedral Angle--that is, the upward inclination of the Surfaces towards their wing-tips--does what it can for me, but, in my opinion, it's a more or less futile effort. The Blackboard will show you the argument." And he at once showed them two Surfaces, each set at a Dihedral Angle like this:

[Ill.u.s.tration: H.E., Horizontal equivalent.]

"Please imagine," said the Blackboard, "that the top =V= is the front view of a Surface flying towards you. Now if a gust blows it into the position of the lower =V= you see that the horizontal equivalent of the Surface on one side becomes larger, and on the other side it becomes smaller. That results in more lift on the lower side and less on the higher side, and if the =V= is large enough it should produce such a difference in the lift of one side to the other as to quickly turn the Aeroplane back to its former and normal position."

"Yes," said the Dihedral Angle, "that's what would happen if they would only make me large enough; but they won't do it because it would too greatly decrease the total horizontal equivalent, and therefore the Lift, and incidentally it would, as Aeroplanes are built to-day, produce an excess of Keel Surface above the turning axis, and that in itself would spoil the Lateral Stability. The Keel Surface should be equally divided above and below the longitudinal turning axis (upon which the Aeroplane rolls sideways), or the side upon which there is an excess will get blown over by the gusts. It strikes me that my future isn't very promising, and about my only chance is when the Junior Draughtsman makes a mistake, as he did the other day. And just think of it, they call him a Designer now that he's got a job at the Factory! What did he do? Why, he calculated the weights wrong and got the Centre of Gravity too high, and they didn't discover it until the machine was built. Then all they could do 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.

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.

[Ill.u.s.tration]

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:

[Ill.u.s.tration]

"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 a.s.sume its normal position again, though probably at a slightly lower alt.i.tude."

"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."

[Ill.u.s.tration]

"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."

[Ill.u.s.tration: "Wash out" on both sides.]

"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."