Part 9 (1/2)

STABILITY is a condition whereby an object disturbed has a natural tendency to return to its first and norht suspended by a cord

INSTABILITY is a condition whereby an object disturbed has a natural tendency to move as far as possible away from its first position, with no tendency to return Exaer

NEUTRAL INSTABILITY is a condition whereby an object disturbed has no tendency to move farther than displaced by the force of the disturbance, and no tendency to return to its first position

In order that an aeroplane may be reasonably controllable, it is necessary for it to possess soitudinally, laterally, and directionally

LONGITUDINAL STABILITY in an aeroplane is its stability about an axis transverse to the direction of norht, and without which it would pitch and toss

LATERAL STABILITY is its stability about its longitudinal axis, and without which it would roll sideways

DIRECTIONAL STABILITY is its stability about its vertical axis, and without which it would have no tendency to keep its course

For such directional stability to exist there must be, in effect,[16]

more ”keel-surface” behind the vertical axis than there is in front of it By keel-surface Iat an aeroplane froe, struts, wires, etc The sa applies to a weathercock You knoould happen if there was insufficient keel-surface behind the vertical axis upon which it is pivoted It would turn off its proper course, which is opposite to the direction of the wind It is very much the same in the case of an aeroplane

[Illustration]

The above illustration represents an aeroplane (directionally stable) flying along the course B A gust striking it as indicated acts upon the greater proportion of keel-surface behind the turning axis and throws it into the new course It does not, however, travel along the new course, owing to itsas such momentum lasts, in a direction which is the resultant of the two forces Thrust and Mo in the direction of the new course Therefore its attitude, relative to the direction of motion, is more or less sideways, and it consequently receives an air pressure in the direction C Such pressure, acting upon the keel-surface, presses the tail back towards its first position in which the aeroplane is upon its course B

What I have described is continually going on during flight, but in a well-designed aeroplane such stabilizing ht as to be imperceptible to the pilot

If an aeroplane was not stabilized in this way, it would not only be continually trying to leave its course, but it would also possess a dangerous tendency to ”nose away” froust shown in the above illustration would turn the aeroplane round the opposite way a very considerable distance; and the right wing, being on the outside of the turn, would travel with greater velocity than the left wing Increased velocity , the aeroplane would turn over sideways very quickly

LONGITUDINAL STABILITY--Flat surfaces are longitudinally stable owing to the fact that with decreasing angles of incidence the centre line of pressure (CP) moves forward

The CP is a line taken across the surface, transverse to the direction of motion, and about which all the air forces h which they ine A to be a flat surface, attitude vertical, travelling through the air in the direction ofthe exact centre line of the surface as illustrated In B, C, and D the surfaces are shoith angles of incidence decreasing to nothing, and you will note that the CP le[17]

Now, should sole, _ie_, dive, then the CP moves forward and pushes the front of the surface up Should the surface tend to assule, then the reverse happens--the CP moves back and pushes the rear of the surface up Flat surfaces are, then, theoretically stable longitudinally They are not, however, used, on account of their poor lift-drift ratio

As already explained, caitudinally unstable at those angles of incidence producing a reasonable lift-drift ratio, _ie_, at angles below about 12

A is a ca through the air in the direction M Obviously the CP coincides with the transverse centre line of the surface

With decreasing angles, down to angles of about 30, the CP les above 30 do not interest us, since they produce a very low ratio of lift to drift

[Illustration]

Below angles of about 30 (see C) the dipping front part of the surface assu in the _doard_ air pressure D, and the ative angle and its resultant pressure D Since the CP is the resultant of all the air forces, its position is naturally affected by D, which causes it to ust or eddy tend to le of incidence, _ie_, dive, then the CPup the rear of the surface, causes it to dive the le, then the reverse happens; the pressure D decreases, with the result that CP moves forward and pushes up the front of the surface, thus increasing the angle still further, the final result being a ”tail-slide”

It is therefore necessary to find athe naturally unstable cambered surface This is usually secured bysurface fixed some distance in the rear of the main surface, and it is a necessary condition that the neutral lift lines of the two surfaces, when projected to le In other words, the rear stabilizing surface le of incidence than the main surface--certainly not more than one-third of that of the itudinal dihedral