has a natural tendency to return to its first and n

dency to move as far as possible away from its first position, with no t

has no tendency to move farther than displaced by the force of

ble, it is necessary for it to possess some degree of

about an axis transverse to the direction of normal hori

about its longitudinal axis, and wi

ut its vertical axis, and without which it

king at an aeroplane from the side of it-the sides of the body, undercarriage, struts, wires, etc. The same thing applies to a weathercock. You know what would happen if there was insufficient keel

new course, owing to its momentum in the direction B. It travels, as long as such momentum lasts, in a direction which is the resultant of the two forces Thrust and Momentum. But the centre line of the aeroplane is pointing in the direction of the new course. Therefore its

t in a well-designed aeroplane such stabilizing movements are,

f the side gusts. In such case the gust 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

stable owing to the fact that with decreasing angles of i

e direction of motion, and about which all the air forces may

rough the air in the direction of motion M. Its C.P. is then obv

incidence decreasing to nothing, and you will note

.P. moves forward and pushes the front of the surface up. Should the surface tend to assume too l

e longitudinally. They are not, however, use

ngitudinally unstable at those angles of incidence producing a rea

oving through the air in the direction M. Obviously the C.

forward as in the case of flat surfaces (see B), but angles above 30 degree

the resultant of all the air forces, its position is naturally affected by D, which causes it to move backwards. Now, should some gust or eddy tend to make the surface decrease its angle of incidence, i.e., dive, then the C.P. moves backwards, and, pushing up the rear of the surface, caus

r of the main surface, and it is a necessary condition that the neutral lift lines of the two surfaces, when projected to meet each other, make a dihedral angle. In other words, the rear

t the same angle as the main surface, but, in such cases, it attacks ai

air (the angle of incidence) is therefore less

illustration, try to explain how the l

e C.P. of the main surface and the C.P. of the stabilizing surface. For the sake of illustration, the stabilizing surface has been given an angle of incidence, and therefore has a lift

m in the old direction pulls it off that course. M is now the resultant of the Thrust and the Momentum, and you will note that this results in a d

ch decrease applies to both main surface and stabil

egrees angle, has now only 10 de

grees angle, has now only 2 de

y its lift, than has the main surface. It must then fall relative to the main surface. The ta

ence in the same amount, but the angle, and therefore the lift, of the stabilizer increases in greater proportion than does the lift of the

horizontal. It varies with such angle, but not as anything approaching it. Remember that the stabilizing eff

taking place all the time, even

l dihedral. The defect of such design lies in the fact that the main surface must have a certain angle to lift the weight-say 5 degrees. Then, in order to secure a sufficiency of longitudinal stability, it is necessary to set the forward stabilizer at about 15 degrees. Such a large angle of incidence results in a very poor lift-d

bilizing surface distinct from the main surface, but the

creasing angle of incidence and corresponding camber

io, then, the less the lift. This design, then, produces less lift for weight of surface than would the same surface if arranged as a parallelogram. (3) In order to secure the longitudinal dihedral, the angle of incidence has to be very much decreased towards the wing-tips. Then, in order that the lift-drift ratio may be preserved, there must be a corresponding decrease in the camber. That calls for surface ribs of varying cambers, and results in an expensive and lengthy job for the builder. (4) In order to secure directional stability, the surface is, in the centre, arranged to dip down in

of the aeroplane when it is turned to the left, and to elevate it when it is turned to the right. In modern aeroplanes this tendency is not sufficiently important to bother about. In the old days of crudely d

r directional stability. Some degree of lateral stability may be secured by means of the

ntal equivalent (H.E.) of the left wing is the same as that of the right wing. Therefore, the lift of one

ou will note that the H.E. of the left wing increases, and the H.E. of the right wing decreases. The

fect is not proportional to the differ

opposed to the direction of gravity or weight. The two forces R R and gra

nd so the resultant direction of motion of the aeroplane is no longer directly forward, but is along a line the resulta

the latter must be pushed sideways. That causes the aeroplane to turn; and, the highest wing being on the outside of the turn, it has a greater velocity than the lower win

oplane tilts, a difference in the H.E.'s of the two wings, which difference must be sufficient to not only oppose the tilting

t, since its effect is not very great, and since it must be paid for in loss of H.E. and consequently loss of lift, thus decreasing the lift-drift ratio, i.e., the ef

divided by the longitudinal turning axis of the aeroplane. If there is an excess of keel-surface a

eral stability. If too low, it produces a pendulu

to travel to a position as far as possible from its original position. It would then t

, the best position for the centre of gravi

aeroplane tends to turn over sideways in the op

nd it is always advisable, if practical considerations allow it, to also decrease the angle upon the other side. In tha

rm applied to the

erm applied to th

be improved by washing out the angle of i

gusts upon the wing-tips which is just where they have the most ef

re effective, as, in order to operate them, it is not then necessary to

ill note that, while the aileron attached to the surface with washed-out angle is operated to the same extent as

rse be paid for in some loss of lift, as t

described advantages, a combina

e centre of the turn. Its lifting surfaces do not then meet the air in their correct attitude, and the lift may fall to such an extent as to become less than the weight, in which case the aeroplane must fall. This bad effect is minimized by "banking," i.e., tilting the aeroplane sideways. The bottom

en only such as is necessary to secure an air pressure

of the centrifugal force, and therefore the s

ng outward from the centre of the turn, and will cause the aeroplane to bank-perha

he turn. It will tend to make the aeroplane bank the wrong w

little below the centre of drift. It tends to bank the aeroplane the right way for

al axis. An excess of keel-surface above the longitudinal axis will, when banking, receive an air pressure cau

hus decreasing the horizontal equivalent, and therefore the lift, of the surface. An excessive "bank," or sideways tilt, result

then there must be a fall, and the direction of motion will be the resultant of the thrust and the fall as illustrated above in sketch A. The lifting surfaces and the con

in the higher wing, which is on the outside of the turn, travelling with greater velocity, and therefore securing a greater reaction than the lower wing, thus tending to tilt the aeroplane over still more. The aeroplane is now almost up

t which the margin of lift is lost, and, if a sharp turn n

n such an attitude that the air pressure is always directly in the pilot's face. The aeroplane is then always engaging the air as designed to do so, and both lifting and controlling

redicaments the pilot can find himse

ficiency of keel-surface behind the vertical axis, or the jamming of the rudder end or elevato

of the keel-surface or controlling surfaces opposed to it; and, when once such a condition occurs, it is difficult to see what can be don

lize it, maintains the correct attitude of the aeroplane by observing its position relative to the horizon. Flying into a fog or cloud the horizon is lost to view, and he must then rely upon his instruments-(1) the compass for direction; (2) an inclinometer (arched spirit-level)

rectional stability. That is a feat beyond the capacity of the ordinary man. If, however, by the simple movement of throttling down the power and thrust, he can be reli

e is then, as a glider, nose-heavy-and the distance the C.G. is placed in advance of the C.L. should be such as to ensure a gli

ng and descent not required, the centre of thrust is placed a little below the c

re of drift should be such as to produce a force equal and

s the aeroplane to nose-down, and assists the pilot in making a reasonably small loop along the course C and in securing a quick recovery. If the engin

y change the direction of motion so suddenly as to produce dangerous air st

tled down, then the elevator must be operated to secure a course approximately in the direction B. If it is

e the correct angle. Path B should slo