The Aeroplane Speaks. Fifth Edition

entum. It therefore obeys Newton's laws14 and resists movement

the air a surface15 inclined upward

view of

ction of

, taken to be a straight line from the lead

aries with differences in the camber (curvature) of surfaces. In order to secure flight, the inclination of the surface must be such that the neutral lift line makes an angle wit

. This is necessary from the point of view of the practical mechanic who has to rig the aeroplane, for he could not find the neutral lift line, whereas he can easily find the chord. Again, he would certainly be in doubt as to "the direction of motion relative to the air," whereas he can easily find a line parallel to the propeller thrust. It is a pity, however, that these practical considerations have resulted in a bad definition of the angle of incidence becoming prevalent, a c

pon the air in the

nd accelerates it DOWNWARDS. As a result of this definite acti

cuum or rarefied area over the top of the surface. Consequently the pressure of air on the to

e velocity and consequent force with which the surface engages the air. If the reaction was produced by only one of those facto

e; and only some two-fifths is due to the upward reaction secured by the action of the bottom surface upon the air. A practical point in respect of

hord of the surface, as illustrated above; and it is, in considering fl

on, i.e., Lift, which is opposed to Gra

(sometimes called Resistance), to which

is, of course, the resultant

of the reaction, for it lift

vercome by the Thrust in order to secure the necessa

nsidered only the lifting surface heretofore) may b

the drift produced by

of the aeroplane-the struts, wires, fuselage, under-carri

The latter is practically negligible having regard to the smooth surface of the modern aeroplane,

ency of the aeroplane (as distinct from engine and propeller). A knowledge of the factors governing the lift-drift ratio is, as will be seen la

tors are

t, being component parts of the reaction, increase as the square of the velocity, and the efficiency remains the same at all speeds. But, considering the whole aeroplane, we must remember the passive

t would be possible, by doubling the thrust, to approximately double the speed or lift-a happy state of affairs whic

with the least possible drift. Even the wires bracing the aeroplane together are, in many cases, stream-lined, and with a markedly good effect upon the lift-drift ra

ey a wrong impression, as the drift is not nearly so much the result of the hea

f air round two objects moving

derable extent; whereas in the case of B, the air flows round it in such a

e pressure of air upon the rear of the object. The less such pressure, then, the better is h

determined by the velocity-the greater the velocity, the greater the fineness. The be

t of view, the importance of adjusting all stream-line parts

varies with the thrust at the disposal of the designer, the

the angle of incidence, in order to preserve a clean, stream-line shape of rarefied area and freedom from eddies. Should the angle be too great for the velocity, then the rarefied area becomes o

ent angle, and the practical application of all this is in taking the greatest possible care to rig the surface at the corr

The lifting surfaces are cambered, i.e., curved, in order to de

erable horizontal reaction or drift. By curving it such shock is diminished, and the curve should be such as to produce a uniform (not necessarily constant) ac

xplained the bad effect this has upon the lift-drift ratio. The top surface is then curved to produce

of incidence. With infinite velocity the surface would be set at no angle of incidence (the neutral lift line coincident with the direction of motion relative to the air),

ans of wind-tunnel research. The practical application of all this is in taking the greatest care to prev

s, if the span is, for instance, 50 feet and the chord 5 feet

e reaction. It is obvious, I think, that the greater the span, the greater the mass of air eng

vanced that it is owing to the "spill" of air from under the wing-tips. With a high aspect ratio the chord is less than would otherwise be the case. Less chord results in smaller wing-tips and consequently less "spill." This, however, appears to be a rather inadequate reaso

e will be "interference" and consequent loss of Efficiency unless the gap between the top and bottom surfaces is equal to not less than 1 1/2 times the chord. If less than that, the air engaged by the bottom o

ved from the action of the lower surface and engages undisturbed air, with the result that the efficiency can in this way be increased by about 5 per cent. Theoretically the top plane should be staggered forward for a distance equal to about 30 pe

as the horizontal equivalent (H.E.) of the surface, but the drift remains the same.

front views of

t of view from which we are at the moment conside

ted. For these reasons their H.E.'s are, as illustrated, less than in the case of A. That means less vertical lift, and,

r available above that necessar

and starting from a given altitude. As an example, thus: 1,000 feet the

rgin of lift would not disappear. Moreover, greater velocity for a given power would be secured at a greater altitude, owing to the decreased density of air to be overcome. After reading that, you may like to light your pipe and indulge in dreams of the wonderful possibili

hich, for a given power, surface (including detrimental s

hich, for a given power, surface (including detrimental s

atio is highest. In modern aeroplanes it is that angle of incidence p

oximately half-way between the

ontal Velocity. We will compare the essentials for two aeroplanes,

S FOR MAX

order to secure the

ll be necessary in order to engage the neces

with its propeller thrust horizontal, then a large angle relative to the direction of the thru

f, when climbing, the propeller thrust is at such an angle as to tend to haul the aeroplane upwards, then it is, in a measure, acting as a helicopter, and that means inefficiency. The reason of a helicopter being inefficient in practice is due to the fact that, owing to mechanical difficulties, it is impossible to construct within a reasonable weight an air-screw of the requisite dimensions. That being so, it would be necessary, in order to absorb the power of the

ws that for that reason also the angle o

ow velocity, and therefore possess a large angle

maximum velocity for its power, and possessing merely e

tively HIG

ximum climber, a greater mass of air will be engaged for a given surface and tim

opeller thrust, since the latter co

angle of incidence by rea

ber follows as a result of t

MM

Maximum Essenti

. Vel

ocity. Hig

urface. Sm

elative to Small

hrust. prop

ive to Small angle r

of motion.

amber. Sma

of which it would be possible to very the above opposing essentials. Theref

e at low altitude a slight margin of lif

ED AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND POSSESSING A SLIG

MUM

designed to just maintain horizontal flight with a horizontal thrust. Also, in such case, the propeller would not be thrusting downwards, but along a horizontal

LE (Thrust

d angle of incidence of the surface is such as to secure a slight ascent at a low altitude. The camber of t

LIMBIN

locity at A GIVEN ANGLE produces less lift, but the increased angle more or less offsets the loss of

MUM

om the thrust are only just able to maintain horizontal flight. Any greater angle will result in a still lower lift-drift ratio. The l

r exceed the Best Climbing Angle. Always

), and which is, consequently, the altitude where it is just possible to maintain horizontal flight, the aeroplane

t the higher altitude at which the margin of lift is lost is that altitude at which most of th

, the altitude where most of the work is done is that at which both

for the pilot to vary at will the above-mentioned opposing essentials. Then we shall get maximum velocity, or maximum margi