Flight, March 1940

DYNAMIC SIMILARITY
Saunders-Roe Produce a " Flying Scale Model"

  AIRCRAFT production has become an expensive business. Time was when a new aeroplane could be (in the early days of flying it had to be) produced for a few hundred pounds. But size has increased, materials have become expensive, equipment has multiplied a hundredfold, and modifications have become almost impossible once a type has gone into production. One result of all this is that designing "by eye" or "by guess" no longer meets the case. Not only so, but the most elaborate calculations, the most careful tests of scale models in wind tunnels, and even tests at high Reynolds Number in compressed-air tunnels and with airscrews running are apt to leave unknown, or at least doubtful, features which may assume important dimensions in the finished machine. Even the so-called "dynamic model," that is to say, the model in which not only the external shape but the c.g. position and the moments of inertia are truly represented, cannot be relied upon to provide all the information sought in the early stages of a new design.
  This, of course, is particularly so in the case of a design which departs considerably from any previous ones of which the firm in question has had experience. During recent years there has been a tendency among aircraft manufacturers to go to the trouble and expense of building actual scaled-down models of any important new type proposed. In this way all features which might be doubtful can be thoroughly examined and tested, and modifications are nothing like so expensive to make as would be the case if the full-size machine had gone into production.
  To be truly comparable with the large aircraft which it
represents, the flying model must be in every way a replica
flight ' photograph.
The somewhat unusual shape of the steps is clearly visible in this view of the SARO
. . Flying Scale Model. . ,
• ';. • . • • "•-.. ' - . K •.., - • • . '*'-• ' .--•• • •-••. - - . ,-. , » .
o'f the full-size machine. In the airframe itself this "does
not present very great difficulties. It is different with the
engines, however. Strictly speaking, these should be exact
scale models of the "real " engines which
it is proposed to use, and the airscrews
should be to scale also. That is an ideal
which obviously cannot always be attained.
It so happens, however, that the Pobjoy
engine of some go b.h.p. comes very close
to being a scale model of modern large
radials. "What helps is that, in spite of its
relatively low power, the Pobjoy engine has
reduction gearing for its airscrew.
The latest firm to build a flying scale
model is Saunders-Roe, Ltd.; Of Cowes. The
new machine was demonstrated last week
by Fit. Lt. Ash, and created a very favourable impression, not only being fast and very
manoeuvrable in the air, but showing good
sea qualities, clean running and absence
of porpoising.
That the actual Saro flying model is not a
very tiny machine is shown by the fact that
the overall length is 42ft. 8|in. and the wing
span 50ft. If it is assumed that the large flying boat of which this is a model is to be two
and a half times as large, we arrive at a
four-engined flying boat of 125ft. wing span.
There is, of course, no reason why the full-sized machine
should not be much larger than that. The similarity, except
possibly for percentage structure weight, holds good.