672 
NATURE 
[May 19, 1923 
The Low-power Aeroplane or Aviette. 
By Prof. L. Bairstow, F.R.S. 
M UCH attention is being paid in the general and 
technical press of this and other countries to 
flight with low-powered engines. The addition of small 
engines follows quickly on the gliding successes of the 
past year, but no marked connexion is discernible 
between the new features of the two types of flying 
machine. The change from aeroplanes having two or 
three hundred horse-power, and carrying a single 
individual, to aeroplanes of five to ten horse-power 
is now so striking as to have stirred public imagination. 
Many of those now interested are probably unaware 
of the flights made by A. V. Roe more than ten years 
ago with a nine horse-power J.A.P. engine. 
“There is little in the new applications which would 
warrant the use of the word discovery, and the change 
is probably due in large measure to an emancipation 
from the fetters of war ideas. There is reason to 
suppose that aeroplanes designed for official use are 
subject to so many restrictions that development. is 
difficult. The advent of the low-power aeroplane is 
therefore welcomed as a new outlet for ideas, one which 
gives scope to individual initiative, and one which it is 
hoped will make a popular appeal. 
The unofficial character of the British development 
of the aviette is emphasised by the offering of a prize 
of 5ool. by the Duke of Sutherland for a light aeroplane 
of British manufacture and design ; for the Duke is 
Under Secretary of State for Air, though he is not acting 
in his official capacity. A separate prize of 1oool., open 
to the world, is offered by the Dazly Mail. The most 
striking conditions of the competition are: “The 
power unit must not exceed 750 cc. total cylinder 
capacity, and the prize will be awarded to the machine 
which flies the longest distance on one gallon of petrol, 
with a minimum distance of fifty miles to qualify.” 
Many other conditions apply which relate to ease 
of handling on the ground; there is also the usual 
vague reference to stability. It is in accordance with 
our present state of knowledge and application that, 
while the requirements for performance are always 
precise and well-defined, those for stability and control 
are valueless. In the competition rules for the low- 
power aeroplane there is neither definition nor means 
of checking aircraft as delivered in order to ensure a 
due measure of stability and control. 
The French are, as is usual in matters relating to 
aviation, taking an active interest in the aviette, and 
in the Times of April 6 we find an article based on a 
flight by M. Barbot from Francazal to Toulouse in a 
seven horse-power aviette. The ideas of the flyer 
appear in the following quotation : 
““M. Barbot, who is in Paris, expresses the belief 
that his aviette is the forerunner of the aero-taxi 
of the immediate future. He contemplates the pro- 
duction of a machine which will cost about five 
thousand francs (nominally 2o0/.), which can rise 
from almost anywhere and, furnished with a ten or 
fifteen horse-power motor, can land slowly within 
thirty yards of its objective and without any risk. 
The cost will be only that of a gallon of petrol per 
hundred miles.’ 
NO. 2794, VOL. 111 | 


In the same issue of the Times is a reference to a 
British “ power-driven glider” : 
“ The test began yesterday at Ashton Park, Preston, 
of the first British auxiliary-engined glider, the 
Wren, designed and built by the English Electric 
Company at their Preston works. It is expected 
that the machine will provide valuable data on which 
to base the development of the economical aeroplane 
of the future. It has been constructed for the Air 
Ministry. It is remarkably light, being about 
3 cwt., and the dimensions are: span 37 ft., length 
23 ft., height 5 ft. The engine is a three horse- 
power A.B.C. motor-cycle engine, developed to seven 
horse-power, and gives a maximum speed to the aviette 
of forty-eight miles an hour. . . .” 
These two extracts exhibit the features which define 
the low-power aeroplane, and in the remainder of this 
article it is proposed to analyse the projects and 
achievements in relation to the scientific knowledge 
involved. 
100 miles per gallon of petrol in an aero-taxi carrying 
pilot and passenger is optimistic but not wildly in 
excess of what can be immediately foreseen, using 
arguments based on well-established data. On the 
other hand, risk is only incidentally reduced, and 
neither the extracts nor current technical literature 
show any attempt on the part of the designers of light 
aeroplanes to pay special attention to the fundamental 
problems of stability and control. 
For the past eight or ten years it has been realised 
that all good aeroplane design tends to produce a 
result in which, for the most economical speed of flight, 
the weight carried is about nine times the resistance 
experienced. All high-power aeroplanes are able to 
exceed this most economical speed very greatly, with 
the result that at their maximum speed near the ground 
the resistance is more than one-quarter of the weight. 
If there be no great reserve of power, considerable 
height above the ground is unattainable, and the most 
economical speed is fixed by the design of the aeroplane, 
in particular by the magnitude of the load carried per 
unit area of wing surface.— A common loading has been 
7-8 lb. per sq. ft. corresponding with an economical 
speed of about 60 miles per hour ; a reduction of loading 
2 lb. per sq. ft. would bring the best speed to about 
30 m.p.h. 
If the making of a specified journey be the basis of 
comparison, then speed in itself has no influence on 
petrol consumption per unit of weight carried, but the 
horse-power required is directly proportional to the 
speed. Indirectly, low speed is advantageous, since a 
smaller and light engine suffices for transport. Taking 
figures given in the Press as a basis, it appears that the 
aviette with a single occupant would weigh about 
550 lb. all told, whereas the single-seater fighting craft 
weighs from three to four times this amount. The 
maximum speed claimed for the aviette is only about — 
one-third of that achieved by the scout. 
Further data lead to an estimation of a probable 
mileage per gallon of petrol. 
in which the petrol consumption does not exceed o+5 Ib. 
It will be found that the expectation of 
Many aero-engines exist 
—_— ee 
