The Theory and Practice of Model Aeroplaning Part 2

[8] I.e., to express it as a decimal fraction of the resistance, encountered by the same plane when moving "face" instead of "edge" on.

CHAPTER III.

THE QUESTION OF BALANCE.

-- 1. It is perfectly obvious for successful flight that any model flying machine (in the absence of a pilot) must possess a high degree of automatic stability. The model must be so constructed as to be naturally stable, _in the medium through which it is proposed to drive it_. The last remark is of the greatest importance, as we shall see.

-- 2. In connexion with this same question of automatic stability, the question must be considered from the theoretical as well as from the practical side, and the labours and researches of such men as Professors Brian and Chatley, F.W. Lanchester, Captain Ferber, Mouillard and others must receive due weight. Their work cannot yet be fully a.s.sessed, but already results have been arrived at far more important than are generally supposed.

The following are a few of the results arrived at from theoretical considerations; they cannot be too widely known.

(A) Surfaces concave on the under side are not stable unless some form of balancing device (such as a tail, etc.) is used.

(B) If an aeroplane is in equilibrium and moving uniformly, it is necessary for stability that it shall tend towards a condition of equilibrium.

(C) In the case of "oscillations" it is absolutely necessary for stability that these oscillations shall decrease in amplitude, in other words, be damped out.

(D) In aeroplanes in which the dihedral angle is excessive or the centre of gravity very low down, a dangerous pitching motion is quite likely to be set up. [a.n.a.logy in shipbuilding--an increase in the metacentre height while increasing the stability in a statical sense causes the ship to do the same.]

(E) The propeller shaft should pa.s.s through the centre of gravity of the machine.

(F) The front planes should be at a greater angle of inclination than the rear ones.

(G) The longitudinal stability of an aeroplane grows much less when the aeroplane commences to rise, a monoplane becoming unstable when the angle of ascent is greater than the inclination of the main aerofoil to the horizon.

(H) Head resistance increases stability.

(I) Three planes are more stable than two. [Elevator--main aerofoil--horizontal rudder behind.]

(J) When an aeroplane is gliding (downwards) stability is greater than in horizontal flight.

(K) A large moment of inertia is inimical (opposed) to stability.

(M) Aeroplanes (naturally) stable up to a certain velocity (speed) may become unstable when moving beyond that speed. [Possible explanation.

The motion of the air over the edges of the aerofoil becomes turbulent, and the form of the stream lines suddenly changes.

Aeroplane also probably becomes deformed.]

(N) In a balanced glider for stability a separate surface at a negative angle to the line of flight is essential. [Compare F.]

(O) A keel surface should be situated well above and behind the centre of gravity.

(P) An aeroplane is a conservative system, and stability is greatest when the kinetic energy is a maximum. [Ill.u.s.tration, the pendulum.]

-- 3. Referring to A. Models with a plane or flat surface are not unstable, and will fly well without a tail; such a machine is called a simple monoplane.

[Ill.u.s.tration: FIG. 4.--ONE OF MR. BURGE WEBB'S SIMPLE MONOPLANES.

Showing balance weight A (movable), and also his winding-up gear--a very handy device.]

-- 4. Referring to D. Many model builders make this mistake, i.e., the mistake of getting as low a centre of gravity as possible under the quite erroneous idea that they are thereby increasing the stability of the machine. Theoretically the _centre of gravity should be the centre of head resistance, as also the centre of pressure_.

In practice some prefer to put the centre of gravity in models _slightly_ above the centre of head resistance, the reason being that, generally speaking, wind gusts have a "lifting" action on the machine.

It must be carefully borne in mind, however, that if the centre of wind pressure on the aerofoil surface and the centre of gravity do not coincide, no matter at what point propulsive action be applied, it can be proved by quite elementary mechanics that such an arrangement, known as "acentric," produces a couple tending to upset the machine.

This action is the probable cause of many failures.

[Ill.u.s.tration: FIG. 5.--THE STRINGFELLOW MODEL MONOPLANE OF 1848.]

-- 5. Referring to E. If the propulsive action does not pa.s.s through the centre of gravity the system again becomes "acentric." Even supposing condition D fulfilled, and we arrive at the following most important result, viz., that for stability:--

THE CENTRES OF GRAVITY, OF PRESSURE, OF HEAD RESISTANCE, SHOULD BE COINCIDENT, AND THE PROPULSIVE ACTION OF THE PROPELLER Pa.s.s THROUGH THIS SAME POINT.

[Ill.u.s.tration: FIG. 6.--THE STRINGFELLOW MODEL TRIPLANE OF 1868.]

-- 6. Referring to F and N--the problem of longitudinal stability.

There is one absolutely essential feature not mentioned in F or N, and that is for automatic longitudinal stability _the two surfaces, the aerofoil proper and the balancer_ (elevator or tail, or both), _must be separated by some considerable distance, a distance not less than four times the width of the main aerofoil_.[9] More is better.

[Ill.u.s.tration: FIG. 7. _PeNAUD 1871_]

-- 7. With one exception (Penaud) early experimenters with model aeroplanes had not grasped this all-important fact, and their models would not fly, only make a series of jumps, because they failed to balance longitudinally. In Stringfellow's and Tatin's models the main aerofoil and balancer (tail) are practically contiguous.

Penaud in his rubber-motored models appears to have fully realised this (_vide_ Fig. 7), and also the necessity for using long strands of rubber. Some of his models flew 150 ft., and showed considerable stability.

[Ill.u.s.tration: FIG. 8.--TATIN'S AEROPLANE (1879).

Surface 07 sq. metres, total weight 175 kilogrammes, velocity of sustentation 8 metres a second. Motor, compressed air (for description see -- 23, ch. iv). Revolved round and round a track tethered to a post at the centre. In one of its jumps it cleared the head of a spectator.]

With three surfaces one would set the elevator at a slight plus angle, main aerofoil horizontal (neither positive nor negative), and the tail at a corresponding negative angle to the positive one of the elevator.

Referring to O.[10] One would naturally be inclined to put a keel surface--or, in other words, vertical fins--beneath the centre of gravity, but D shows us this may have the opposite effect to what we might expect.

In full-sized machines, those in which the distance between the main aerofoil and balancers is considerable (like the Farman) show considerable automatic longitudinal stability, and those in which it is short (like the Wright) are purposely made so with the idea of doing away with it, and rendering the machine quicker and more sensitive to personal control. In the case of the Stringfellow and Tatin models we have the extreme case--practically the bird entirely volitional and personal--which is the opposite in every way to what we desire on a model under no personal or volitional control at all.

[Ill.u.s.tration: FIG. 9.--CLARK'S MODEL FLYER.

Main aerofoil set at a slight negative angle. Dihedral angles on both aerofoils.]

The theoretical conditions stated in F and N are fully borne out in practice.

And since a curved aerofoil even when set at a _slight_ negative angle has still considerable powers of sustentation, it is possible to give the main aerofoil a slight negative angle and the elevator a slight positive one. This fact is of the greatest importance, since it enables us to counteract the effect of the travel of the "centre of pressure."[11]

[Ill.u.s.tration: FIG. 10.--LARGE MODEL MONOPLANE.

Designed and constructed by the author, with vertical fin (no dihedral angle). With a larger and more efficient propeller than the one here shown some excellent flights were obtained. Constructed of bamboo and nainsook. Stayed with steel wire.]