February 17, 1916.' 
LAND AND WATER 
BRITISH AEROPLANE POLICY. 
By F. W. Lanchester. 
ONK-oI the most common reasons put forward 
for the justification of an increase in the size 
and weight of the miUtary aeroplane, is the 
need for increased petrol capacity in order to 
command a greater range or radius of action. 
Whatever the nature of the military duty happens 
to be, there is always some definite number of men (some- 
times a pilot alone, at other times also an observer or 
kinematograph operator, or one or more gunners) which 
is appropriate and necessary, and the dead weight this 
represents has to be deducted from the total freight- ' 
carrying capacity of the machine, and correspondingly 
limits the amount of fuel which can be carried. In an 
extreme case, if this dead weight, or militarv load as we 
may call it, be equal to the total freight capacity, there 
will be no margin for carrying petrol, and the design 
must be considered useless. If we consider the total 
freight-carrying capacity as one definite percentage of 
the gross weight of the machine, then the bigger the 
machine, the greater will be the relative pcti^ol capacity 
and range or radius of action. 
If it be assumed in the present state of the construc- 
tors' art that 30 per cent, of the gross weight can be carried 
as freight, the freight capacity for a machine 2,000 lbs. 
gross is 600 lbs. and supposing the military load to be 
400 lbs., the difference, 200 lbs., is the petrol capacity, 
equal to 10 per cent, of the gross weight of the machine. 
The range of flight will then be about .,00 miles. If we 
take the machine as of twice the above wjight, namely, 
4,000 lbs., the total freight^at 30 per cent., becomes 1,200 
lbs. ; deducting the military load 400 lbs. (as before)there 
remains 800 lbs. or 20 per cent, of the gross weight for 
petrol, with a range or radius of action of about 600 miles 
and so the calculation may be made for any other size of 
machine. 
Such is the argument in its most bald simplicity. 
When, however, the conditions are examined critically, 
it is found that the story has not half been told. There 
are factors of vital and commanding importance which 
have yet to be taken into consideration. 
Weight of Wing Structure. 
The weight of the wing structure in a flying machine 
is itself not constant in relation to tlie gross weight. When 
discussing the weight of the wing structure and its influ- 
ence, we have to be careful to avoid being misled by 
appearances ; figures given by different constructors 
are not properly comparable. The factors of safety 
used at the present time by aeronautical constructors 
differ mdely ; in my James Forrest Lecture*, it is given 
that the factor of safety varies from three in certain ma- 
chines (which shall be nameless), to as high as seven or 
eight in the case of certain of the machines constructed 
at the Royal Aircraft Factory. The recommendation of 
the Advisory Committee for Aeronautics, formulated after 
duly considering the conditions with which military 
aeroplanes have to comply, is that the factor .should not 
be less than five or six. Now with this great variation of 
nearly three to one in existing practice in the matter of 
wing strength, it is necessary to be careful in comparing 
different designs of machine, for it is evident that in some 
cases the petrol capacity might be nearly doubled merely 
by cnttin.g down the weight of wing structure without 
going outside the limits of existing practice. 
In dealing with the question of wing structure 
weight, therefore, the comparison between one aeroplane 
and another must be based on the assumption of a con- 
stant or uniform factor of safety. On this basis for a given 
type and class of construction the wing-structure weight 
will vary as the cube of the span. Also for a given flight 
velocity, which is the proper criterion to take, the gross 
load supported varies as the square of the span, and con- 
sequently, the weight of the wing structure, expressed 
as a percentage of the gross weight, increases in proportion 
as the span in increased. 
The following are not actuaj figures, but they arc 
sufficiently near actual figures for the purpose of illus- 
•rroc.Inst.CE., Vol. OXCVIU, 
tration. We will assume the gross weight of a ma- 
chine as 2,000 lbs., and its span as 40 feet, and we will 
take the \reight of the aerofoil or wing structure for this 
machine to be 20 per cent, of the gross total, that is to 
say, it will be 400 Ibs.f We will take it that in this 
machine the total freight capacity is 30 per cent, as in the 
earlier example. 
The Big Machine. 
Now let us take a machine of twice the span, 
namely, 80 feet. The gross weight will be 4x2,000 lbs. 
=8,000 lbs., and the weight of the wing structure, on the 
law just given, will be eight times as great as previously, 
that is to say, 3,200 lbs. or 40 per cent, of the gross 
weight ; this will encroach, to the extent of 20 per cent., 
on the freight capacity which is now reduced to 10 per 
cent, as compared with the previous 30 per cent. Hence, 
although a gain may be made by an increase in the size 
of a machine, from the point of view of the relatively 
less military load, this gain will be partially, wholly 
or more than wholly, discounted by the increase in the 
weight of the wing structure ; we have only to go a little 
further than in the above example and the machine will 
have no surplus or freight-carrying cay>acity at all. Clearly 
an advantage in flight range can only be secured by in- 
creasing the size up to a certain point, after which, any 
further increase is detrimental. 
The forcgomg figures are only given for the purpose 
of illustration, if I were to give real figures the results 
would not be greatly different, though it may be said in 
fairness to the big machine that the foregoing figures 
have been based on a high factor of safety, and the result 
given may therefore be looked upon as in the direction of 
t)eing an exaggeration. 
By a simple mathematical demonstration (which it is 
not necessary to enter into here) it may be shown that the 
size of machine of greatest range or duration of flight (as 
determined by its petrol capacity) may be defined 
for any given type of wing structure (monoplane 
or biplane, for example), and tor some stated factor of 
safety, as that at which the weight of the win9 structure 
is equal to twice the weight represented by the military 
load. 
This is an important result ; it is not altogether exact 
lor many reasons— it may, from a mathematical stand- 
point, be regarded as a " first approximation " ; but it 
can never be far from the truth, and it is a result which 
should be " writ large " wherever the question of aero- 
plane size is debated, to prevent those with whom the 
decision rests from being carried away by. the grandiose 
suggestions of the charlatan. 
Factors of Safety. 
It is so easy to juggle with factors of safety, and 
questions of strength and scantling, so as to show an 
imaginary advantage in the design of a large machine, 
that the present warning requires the maximum pubUcity 
possible. In some cases, and I think we may be generou5 
enough to say that in most cases, the designer who thinks 
he can get everything he wants out of the bigger machim 
actually deceives himself ; also the fact that not infre- 
quently he may be comparing a construction based or 
all the latest knowledge and improvements in material 
with something inferior, may-be a design dating from 
two or three years back. This may lead to a false con- 
clusion. Thus any improved material or methods of 
construction which may be applied to a large machine 
may, with unimportant exceptions, be likewise applied 
to a small machine. 
The matters which tend in some slight degree to modify 
the result above given as defining the design of greatest 
range do not lend themselves to discussion in a non- 
technical article ; I propose to deal with these elsewhere. 
It may be said here, however, that the influence of these 
disturbing factors is not serious ; I am convinced that some 
of the large machines which have recently been projected, 
and in some cases actually constructed, will not in the 
t Incli.ding other flight organs whose weight varies in like ratio. 
