Project Sign Progress Report, 1948

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MARCH S, 1948
THE AEROPLANE
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Fig. 11.-Willoughby's "Delta 8" experimental
monoplane of 1938.
Thus, an observant student of laboratory tests could have
noticed the extraordinary capacity of circular or square shapes
10 give a very gradual, innocent stall, an~ that at v!!rY high
incidences. Yet, apart from a few broad-minded experimenters,
no designer drew the conclusion that wmgs of such shapes
promised safety in flight, t~oug_h it w~s known f~om ..~rly
practical experience that fly1Dg 10 the
second regime,
1.e.,
at the stall, was positively dangerous.
It is true that spinning- Parker's "Spiral Dive "-was
attributed. in those days, to high i~cideoces, and that most_of
the lightly loaded rectangular wmgs used at that period
rendered the stall relatively. innocuous.
Howeve_r, :iccidental
~tailing was then, as now. the cause of the maJonty of all
crashes. And the nose dive following inadvertent stalls was well
known to be of the most serious consequences.
.
Eiffel, Riabouchinsky, D ines, . Prandtl, etc., began the_1r
laboratory experiments on aerofoils of very small aspect rauo
and the results were generously published (how closely secret
they would be kept to-day!).
Eiffel showed that the ratio of
the resultant forces reached maxima for small aspect ratios_an~
that slender wings gave the greatest drag at ~ degrees 10c1­
. <lence, whilst disc wings had then t_he leas) res\stanc_e of _all.
Riabouchinsky proved that the maximum 11ft with d15:C Wl1'!8S
wa~ reached at incidences of the order of 40 degrees. whilst with
normal aspect ratios (exceeding a value of 4) the maximum lift
took place at incidences of 12 to 14 degrees only.
Beyond
their critical incidence disc wings gave a gradual decrease of
the lift force whereas wings of normal aspect ratio gave a
very abrupt ; nd unsteady one.
1 he tests by Dines on flat
plates in natural wind confirmed this info_rmauon and that the
observation applied 10 cambered aerofoils as well was also
proved (by Riabouchiosky). 0. Foeppl s_howed frof!l system­
atic wind-tunnel tests that a square aerofoil behaves, ID respect
of its lift curve in a remarkably different manner from one
having an aspec't ratio of 1.5, although wall interference and
Reynolds Number may have somewhat affected the results of
the tests.
.
Later, Prandtl was eager to point o~t that his aer~fo1l theory
did not hold for very small aspect ratios, and that, ID fact, the
induced drag of disc wings was less than the theory suggested.
In sP.ite of this, however, nobody seems to have he~ded the
possibilities implied and the " ~attle of the Aspect Ratios" was
decided in favour of slender wings.
.
To be fair, it must be pointed out that there was one senou~
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Fig. 12.-Experimental Hayden glider, 1925. Span, 19.7 rt. :
length, 13·85 rt.; wing area, 173 sq. rt.
All-up_ weight,
310 lb.
No lateral control.
argument against disc wings. The best aerofoil sections in use
prior 10 about 1925 had a plain camber which implies travel of
the centre of pressure when the incidence varies; the length of
such travel is linked up with the wing chord. Hence the change
in trim or stabiluy is. with such aerofoil sections, greater with
large-chord wings. On this consideration of fore-and-aft
stability and control, designers bad some justification for thcir
decision against experimenting with disc wings.
Yet quite
useful aerofoil secuons had already been used in flight which
had practically no travel of the centre of pressure and hence
did not suffer from thi~ disadvantage of the disc wing.
The whole argument, however, lost its importance imme­
diately M. M. Munk proved that very efficient aerofoil sections
could be designed with a completely (or nearly so) stationary
centre of pressure.
It is. therefore, right to say that from that
time all conditions existed for a practical evolution of disc-wing
aeroplanes.
F. Handley Page converted an aerofoil, leaving an aspect
ratio of 6.25, into six square-aerofoil portions by five slots.
each parallel to the chord of the wing.
By so doing he hoped
to have the low drag of a oormal wing combined with the high
stalling angle of the disc-type wmg.
Although a very
slight improvement was claim~d, the principle was that of
eating the cake and having it too: the induced drag is respon­
sible for the stalling properties of the disc wing and you cannot
have the benefits of the high drag without suffering its disad­
vantages.
Moreover, in order to have the effect of the disc
wing, the provision of mere slots is insufficient.
Marginal
vortices need room to deploy.
On the whole, however, the idea proves that at least one
practical aircraft designer had realized that there was some­
thing in wings of abnormally small aspect ratio.
There were other. although not quue as well thought out,
antecedents of the disc-wing aeroplane.
In the tint soaring­
flight competition on the Rhocn ( 1920), Friedrich Richter, a
burly naval pilot of 20 stone or so, performed on a triplane
glider with wings having an individual aspect ratio of far kss
than three. H. Hayden secured, in 1922, a patent for a rhom­
boidal wing with an aspect ratio of nearly unity, claiming for
such a shape high lift and good flying qualities.
In 192S he constructed a glider with a win, having an aspect
ratio of only 2.25. The wing was nearly tnangular in shape,
with its apex leading, and a pronounced wash-out towards the
tips (Fig. 12); no lateral controls were fitted.
The tailplane
with the elevator was fitted underneath the trailing edge at the
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Fig. 13.-A French project of G. Abrial, 1929.
stern of a f11selage.
Flyinf experiments in Styria were said 10
have proved satisfactory flying qualities.
Some early tailless aeroplanes, such as certain experimental
types of Rene Arnoux, had rather stub wings and heavily
reflexed (i.e., positively stable) aerofoil sections. A 19?9 design
of G. Ab rial showed an aspect ratio of 2.88, with, however,
substantial tip discs (which have the effect of increasing the
aerodynamic aspect ratio) and wind-tunnel experiments indi­
cated a creditable performance (Fig. 13).
Russian attempts,
in particular the parabola type of Tscheranowsky, too, were
experiments with aspect ratios of three and even less (Figs. 14
and IS).
Their resemblance in shape to the latest designs of
super-sonic aeroplanes is remarkable.
.
A Modttn Pioneer
Further intcre~t in the aerodynamics of disc wings was
displayed by research workers with th.e arrival of Juan de la
Cicrv:i's Autogiro.
This was indeed s<>mcthing like a circular
wing, and performance estimates were based on the properties
of such wings.
Yet the question of stalling stability did not
arise, bccau~e of the rotor properties.
The real pioneer of the disc wing was a very able American
research engineer of the National Advisory Committee for
Aeronautics (N.A.C.A.), who proved capable of following
independent lines of development.
In about 1930 Charles
H. Zimmermann subjected the properties of disc wings to
NND 927545