On the Early History of Spinning and Spin Research in the UK

73 Pages·2015·4.75 MB·English

by  John Ackroyd

Journal of Aeronautical History Paper No. 2015/03
On the Early History of Spinning and Spin Research in the UK
Part 2 : The period 1930 - 1940

B J Brinkworth
Waterlooville, Hants, UK

Abstract

In Part 1 (Journal of Aeronautical History, Paper 2014/03) a continuous thread of experience
and research since WW1 was traced, which had provided a growing understanding of the
propensity of an aircraft to spin and a standard procedure that gave the pilot a good chance of
being able to recover if one developed. The 1930s were a period of rapid change in air defence
policy and technology, leading to the emergence of the monoplane as the preferred configurat-
ion for fighters. During this time there was an expansion of work on spinning, with particular
reference to progression to the fast flat spin, from which recovery was usually impossible.

A major step forward came with the opening of the RAE vertical Free Spinning Tunnel.
Systematic investigations of spinning behaviour could now be made with models that were
correctly scaled dynamically. From the results of these and the growing body of tests at full
scale, a procedure was devised that distinguished between physical characteristics of aircraft
that could be recovered from a spin and of those that could not. Although empirical, this was
firmly founded in theory, as the work had been from the beginning.

This procedure allowed designers to include for the first time routine checks at all stages of
the design to estimate the likelihood that a new type could be recovered from a spin. The
procedure would be refined and become standardised in later work, but the stage reached
around the beginning of WW2 marks a distinctive end-point for the early part of the history
of spinning in the UK.

1. Introduction

The scientific study of spinning had started in 1917, with a theoretical account of the
dynamics of an aircraft in a steady spin, together with the first reported observations of the
motion in flight (see Part 1, paper 2014/03). Reviewed at the start of the 1930s, it could be
said that work during the intervening years had materially consolidated that position. The
critical influence of the moments of the aerodynamic forces acting on the aircraft had soon
been recognised. These moments could not yet be calculated, but measurements with models,
latterly using the ‘rotating balance’ in the wind tunnel, had begun to give more relevant values
of their magnitudes and a general understanding of their effects. In particular, it was revealed
that any displacement in yaw of the aircraft relative to its helical path was of first importance
in determining the required moments, though that had been neglected in earlier work.

Flight-testing at full scale would always be required, to expose new problems and to validate
conclusions reached from theory and testing with models. A standard instrument package
had been developed for this, which could now record automatically all the measurements
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required to define the behaviour of an aircraft in the spin. It had also been understood that the
outcome of research needed to be communicated more readily to personnel in the industry,
where designers were becoming increasingly receptive and better placed to implement advice
when it became available.

Yet in its Technical Report for 1929-30, the ARC was obliged to state that ‘Existing knowledge
is insufficient to prevent the occasional appearance of an aeroplane upon which it is difficult,
(1)
or impossible, to check a fully developed spin’ . And in the Supplement to that report, it
had gone on to endorse the opinion of its Air Ministry member that ‘the spinning of aircraft
presents one of the most important practical problems yet to be solved’. Accordingly, the
Committee had recommended that research on the subject should ‘take first priority’.

From a purely practical viewpoint, it was necessary that much of the work on spinning would
continue to be done at RAE and NPL with models. It had been realised that the aerodynamic
environment of an aircraft in a spin was unique, and for measurements on models to be useful
the situations in which they had been obtained had to represent that environment closely. This
would be a leading theme in the development of experimental methods in the following years.
There were continuing concerns about recovery from spinning, particularly from the flat spin,
characterised by very high angles of incidence and high rates of rotation. Unexpectedly large
moments were being required from the controls in both the pitch and yaw directions to effect
recovery.

The spinning trials of service aircraft at A&AEE Martlesham Heath and MAEE Felixstowe
would continue. In this Annual Report there was a specific mention of further tests to be
made on the Fairey IIIF aircraft, for which the spin had ‘sometimes been a very unsteady
motion’. This single-engined aircraft was a versatile machine that served in a variety of roles
for both the RN and RAF over a service life of 14 years. Unsteady motion usually consisted
of nutation (a nodding movement) imposed on the rotation, and there was a belief that this
indicated that the aircraft was close to moving on to a flat spin. The Fairey IIIF could be fitted
interchangeably with wheels or floats. There was concern about the possible effects of the
proportionately large contributions made by floats to the moments of inertia and the base and
side areas, which are apparent
in the photograph of Figure 1.
The case shows the complexities
likely to be faced in reaching
reliable judgments about the
spinning characteristics of a
given aircraft type at the time.
It had not yet been possible to
do that fully before the aircraft
had been required to spin during
trials for acceptance into service,
and modifications made to it
afterwards could also be
problematic.

Figure 1 The Fairey IIIF multi-role aircraft, with floats fitted
(National Aerospace Library Collection)
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The wings of the Fairey IIIF had been set at zero stagger, a feature known to be implicated in
progression to the flat spin, but the gap between the two wings was large enough to be a signif-
icant mitigating factor in countering that. To establish a numerical base for an investigation,
initial measurements were made of the rolling and yawing moments due to roll with models
(2)
at 1/15th scale, both with and without floats, in the rotating balance at NPL .

The rolling moment about the spin axis due to the fin and rudder was negative (against the
o
spin) at all incidences tested, except for the highest value of 61 , a value that could be reached
in a flat spin, when the moment became positive. However, the moment due to the addition
of floats was positive at all incidences. The yawing moment about the body axis due to the
floats was also negative over most of the incidence range, but it too became positive at the
highest incidence tested. Measurements without rotation were made of the forces and couples
for all body axes, reflecting the considerable variations with incidence and sideslip that had
been noted regularly in other cases previously.

The opportunity was taken in these tests to build up further basic information on the effects of
changes to the tailplane shape and position and effects of using differential and floating ailerons
and ‘interceptors’ (shallow strips that could be raised from the upper surface of wings, usually
in association with tip slots, although the slots themselves were not represented on the models
tested on this occasion).

When used in calculations, these results gave quite good agreement between the calculated
values of incidence and spin rate and the full-scale spin results already available for the
landplane version of the aircraft. Though the margin of safety was lower for the seaplane
version, it was indicated that there should be no great difficulty in recovering it from the spin.
However, during the trials made at MAEE at Felixstowe the testers had been unable to do so
at one point and had to abandon the aircraft, so in service the crews were advised to do that at
once if a spin developed or continued below 1,500ft. This would be a region in which
seaplanes would commonly be operating, but it appears that no further work was done on
spinning with that version of the type.

More generally, progress in understanding spinning was a result of interaction between
theoretical analysis, experiments in wind tunnels and flight tests. As such, it is an excellent
example of the value of this combined approach to aeronautical problems.

2 Communication with designers

During the 1920s, UK industry had not followed the US in raising its productivity sufficiently
to be fully competitive in world markets. This situation was to continue into the 1930s due to
national monetary policies which over-valued the currency, while at home it was difficult to
obtain the finance necessary to modernise plant and equipment. Stringent economy was
necessary for aircraft firms to remain in business, but some changes did occur that
contributed to an increasing awareness of the importance of what was being found in the
various aspects of aeronautical research and to apply that to design.

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At this time the number of persons in the industry with full professional qualifications was
still quite low. The design team for a new project would continue to be a small group,
affording few opportunities for ambitious newcomers to enter. Although a modest proportion
of senior people were members of the RAeS, many had risen under the pressures of WW 1,
largely on a basis of the hard lessons of experience. There was only a gradual realisation of
the need for higher competence in engineering science as a basis for commercial success in
this field. Where for example a recent graduate was taken on, it would at best be in a lowly
role as an assistant to one of the principals in the design team. Opportunities to show initiative
were few and advancement was slow, but later memoirs would show that some of those who
were able to contain their frustrations at that time were to become the more significant figures
in the industry as they gained seniority.

A lecture on spinning to the Yeovil branch of the Society by S Scott-Hall, then of A&AEE,
was typical of the continuing support given to professionals, despite the contracted state of the
Air Ministry establishments, and provides a convenient illustration of the situation as it stood
at the start of the new decade. The text of this lecture was published in the Journal of the Royal
(3)
Aeronautical Society for 1931, though oddly without its illustrations . Those attending, and
later readers of the paper, were first warned that the spin must still be taken very seriously.
Whatever research had been done and would be done in the future, any spin must be considered
to be hazardous. The acceleration experienced by the pilot in the rotation would not seem
large enough to be troublesome, but even the most experienced could quickly become
disorientated by the motion. If he was in cloud, the pilot might not even realise that a spin
was happening, but in clear air with plenty of visual cues, it had been found that the pilot
could become so confused as to be unable to tell even in which direction he was rotating. If
the spin was prolonged he was likely to become nauseated also. This was made worse when
the motion was unsteady, where substantial periodic oscillations in nutation could be
superimposed on what was otherwise a steady rotation.

Firms engaged only in the production of civil aircraft should not think of spinning as irrelevant
to them. When there was any possibility that aerobatics would be undertaken, the official
responsibility for issuing a Certificate of Airworthiness for the aircraft lay with A&AEE, where
nearly the same investigation of proneness to spin would be included as on aircraft being
evaluated for possible service with the RAF. All aircraft were first required to be stabilised
for eight turns after entering the spin. Then to satisfy the criteria for acceptance, military
types must come out in not more than three further turns after moving the controls to begin
recovery, and civil aircraft in not more than four turns. Spin tests were made in both directions,
as the motion was affected by the angular momentum of the engine and airscrew, even when
at idle. So far, test protocols had been established only for single-engined aircraft, though it
had been conjectured that spinning could be more troublesome in some twin-engined types.
This would perhaps result from the different distribution of mass in these, tending to change
the relative moments of inertia about the three principal axes.

Scott-Hall went on to review recent research activity along lines as reported towards the end
of Part 1 of this paper. Though theory had revealed the forces and moments that were most
strongly involved in the spin, and measurements with the rotating balance had enabled these
to be evaluated to some extent, it was still only when an aircraft was spinning at full scale
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that it was fully free to respond to all of these acting together. The standard set of instruments
developed by RAE for measuring the response in flight now allowed the relevant data to be
recorded automatically. Work at full scale progressed slowly and was expensive, but would
need to continue.

A good part of the lecture was devoted to features of aircraft that were already shown to
increase proneness to spinning and slowness or impossibility of recovery, with detail that
would be expected to be of especial interest to designers. For this audience, Scott-Hall
showed what could be learnt from interpretation of the three Euler equations for the moments
in roll, pitch and yaw. The importance of the moments of inertia was emphasised, with
illustrations showing how calculated values of these properties had been checked by measure-
ment, suspending an aircraft in various ways and measuring the periods of small oscillations.

Factors affecting the aerodynamic moments available from the empennage were outlined.
Designers would now be accustomed to ensuring that moments available from tailplane and
fin/rudder, represented by the volume coefficients, were adequate for longitudinal and lateral

stability in normal flight. But in a deep stall with sideslip the effectiveness of the empennage
was greatly reduced by interference from the wakes from the tailplane and rear fuselage, and
in some orientations, the wings also. The ARC had recommended limiting values for the fin
and rudder volume coefficients, currently not to be less than 0.08. Practices adopted for
reducing the shielding of the fin and rudder by the tailplane and rear fuselage in the spin were
reviewed, including some reference to approaches used in the USA.

This comprehensive presentation was followed a year later by another article along similar
(4)
lines by Scott-Hall in Aircraft Engineering , which would also reach the wider audience of
technicians and draughtsmen mentioned earlier. Some of the highlights in these presentations
will be reported more fully later.

The Society of British Aircraft Constructors Ltd (SBAC; today the Society of British
Aerospace Companies) had been formed in 1916, and from the beginning one of its aims had
been to increase awareness of new developments across all parts of the industry. In 1932 it
approached the ARC to enquire whether it was now possible to formulate ‘a set of simple
rules for the use of aeroplane designers to guide them in making their machines safe from
spinning’. The composition of such a review was remitted to H B Irving of RAE, who was
the principal investigator of spinning there. It would be published as R&M 1535 in March
(18)
1933 , which is reviewed in Section 5 below.

3 New departures
3.1 Approaches in the early 1930s

There would always be uncertainties about the validity of applying results from wind tunnel
tests to the evaluation of aircraft performance, particularly arising from the general influence
of what was known as ‘scale effect’. The work of Prandtl and others had shown that the
airflow in the boundary layer next to the surfaces of the model could not be fully represent-
ative of that on the aircraft. The flow was determined by the Reynolds number, which as well
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as factors such as size and speed includes the effects of the viscosity of the air. On the model
this number would be typically orders of magnitude smaller than at full scale. As yet, there
were few indications about the importance of this. However, in the spin the lifting surfaces
and associated controls were operating in deeply separated and highly turbulent flows, where
it was thought possible that Reynolds number effects might be less significant. Spinning trials
at full scale would continue to be essential, where key quantities could now be measured that
allowed more reliable estimations to be made of the aerodynamic moments experienced. This
was vital, for providing data by which both the success of theory and the validity of model
experiments could be judged, though inevitably it would be a slow process. Meanwhile,
refinements of the theory were being pursued, and thoughts were turning towards new experi-
mental methods with models, in which all the leading factors involved in spin behaviour could
act together, with a greater prospect that the intricacies of that state could be reproduced more
closely.

Hitherto, all theoretical work had been on modelling the steady spin, and this aspect was now
thought to be fairly well represented. The unsteady phases of entry into and recovery from the
spin could also be monitored with automatic recording in flight, but theoretical modelling of
these would involve velocity-dependent derivatives of which nothing was yet known.
Developments in this area were not to be expected shortly. But at this time, staff at NPL were
engaged in a long-term project to investigate the stability of spins, drawing on contemporary
developments in other areas of stability and control, by which they hoped to be able to
identify and estimate the key derivatives required. An early report from this, concerning the
relative effectiveness of the three controls in starting a manoeuvre that would lead to recovery,
would appear in due course.

In the area of model testing, significant developments of the rotating balance were being put
in hand at NPL that would allow moments to be measured with whole aircraft models in
rotation about an offset axis, with the objective of obtaining a closer representation of the full
extent of coupling between the actions around all three body axes. Meanwhile, attempts were
made in a new direction under A V Stephens at RAE, where the behaviour of spinning models
was observed for the first time in free descent. These new approaches were to be exercised
immediately in the continuing study of a further case of serious difficulty in recovery from
the spin.

3.2 Aircraft ‘H’ and the Hawker Hornbill

Spinning tests with models in the rotating balance and at full scale had been made towards
the end of the 1920s concerning a single-seater fighter aircraft identified simply by the
designation ‘H’, as reported in Part 1. This is recognisable as the Mk 1 version of the Gloster
Gamecock, of which 90 had entered service with the RAF. It had proved to be prone to
accidents, in some of which spinning had been implicated. The rotating balance testing had
indicated that spinning could be alleviated by lengthening the rear fuselage of the model and
fitting an enlarged fin with a horn-balanced rudder. Full-scale tests at RAE on a modified
aircraft in which these changes had been approximated showed that the spinning behaviour
was significantly improved.
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In 1931 a revised form of the aircraft was submitted for testing, probably with a view to
preparing a new Mark II of the type that would be acceptable for entry into service. As
suggested from the previous tests, an enlarged fin and rudder had been incorporated, though
without significant changes to the rear fuselage. In renewed spinning tests at full scale at
A&AEE it was found that after a few slow steep turns, the new machine had a tendency to
flick rapidly into a fast flat spin, even when the centre of gravity was moved to a forward
(5)
position . Recovery could be effected from spins to the left, but in a spin to the right this
proved very difficult. With great coolness, the pilot, Fl Lt C E Maitland, had continued to try
various measures to start recovery during an estimated 30 turns at a rate of almost one
o
revolution per second at an incidence of 57 . Having descended 8,000ft while spinning, he

had decided to abandon the aircraft, but his movements in standing up to do so somehow
(1)
started a recovery and he had managed to regain control and land safely . Subsequently, he
reported that he could not be certain that his actions had been entirely rational throughout the
descent, as he was ‘only in full possession of his faculties towards the end’, when he had ‘got
used to the conditions’. Records of rudder position suggested that at times he had been
uncertain about the direction required for recovery. Despite this experience, he went on to
make a further spin, with the centre of gravity further forward, though again the fast flat spin
had developed and he found that applying full rudder against the spin needed a force that was
hard to maintain for more than a few seconds. Rocking the controls and opening and closing
the throttle had no effect, but when the engine stopped the aircraft finally recovered. This was
presumably due to the absence of the additional angular momentum of the engine and airscrew
about the longitudinal axis when these were rotating, perhaps combined with the disappearance
of the slipstream effect on the fin, which together had brought the recovery within reach. But
estimates made by calculation shortly afterwards indicated how marginal the situation had
been - the pedal force required to balance the centrifugal effects alone on the rudder at this
(6)
rate of spin would have been about 100lb .

Following these experiences, further tests were made involving different modifications to the
(7)
original form of Aircraft ‘H’ . Changes reported previously had included lengthening the
rear fuselage and substantially enlarging the fin and rudder, as shown in Figure 2a. For new
model tests in the NPL rotating
balance, the original length was
retained (as in the revised aircraft),
but the after part of the fuselage
was deepened and the tailplane
raised from near the centre-line to
the top of the body. Rolling and
yawing moments due to rolling and
to sideslip were measured over a
range of incidence, on the whole
(1/10th scale) model and on the
wings and body separately. At the
higher incidences, pitching moments

and rudder control while rolling
Figure 2a Aircraft with very different spinning
were also measured. Although
characteristics - RAE Aircraft 'H'
valuable comparative data were
(National Aerospace Library Collection)

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gathered on the moments with a deepened and with a lengthened rear fuselage, it was
o
concluded that at an incidence of around 50 , at which the full-size aircraft had spun, the

differences were not great.

There appears to have been wide discussions of the situation at RAE before resuming full-
scale testing of Aircraft ‘H’, as the modifications then adopted are said to have been suggested
(7)
by William Farren, formerly head of Aerodynamics Department, but then at Cambridge .
The rear fuselage was retained in length and profile, but was deepened in a practical fashion
by extensions to the fin forwards along both the dorsal and ventral surfaces. The large rudder
with horn balance, a distinctive feature of this aircraft, was also retained, but the tailplane,
though unchanged in shape and area, was raised nearly to the top of the fin. The usual
instrumentation fitted for the tests provided records of the rudder position and the normal
acceleration at the centre of gravity. As shown earlier, the incidence in the spin could be
estimated from the recorded normal acceleration alone. However, there was now the novel
addition of a pinhole ciné-camera mounted above the top wing on the starboard side, enabling
the rate of rotation to be obtained from the film record. If the sun came within the field of
view, a further estimate of the incidence could be determined also, by reference to the sun’s
elevation given in astronomical tables. This provided a check on the incidence value obtained
from the accelerometer reading, together with a way of estimating it if the engine was running,
when the usual assumption that the acceleration was normal to the wing chord was no longer
valid. The rate of descent was still obtained by the pilot from the altimeter and a stopwatch.

It was found that this raising of the tailplane and increasing the fuselage lateral area had
entirely eliminated the vicious spinning characteristics of Aeroplane ‘H’. There had been an
appreciable reduction of incidence and rate of turn in the spin, with no creep towards a flat
spin, even when the descent was prolonged to 25 or 30 turns. Moreover, rapid recovery
could be made from all spins. The opportunity was taken to extend the trials to include
studies of the effects of moving the centre of gravity position, varying the engine power
during the spin, and use of the elevators at different times during the descent and recovery.
These indicated that with the engine running during the spin or at the moment of reversing
the controls the machine could be brought out with a smaller loss of height and at a dive
angle that was noticeably less steep.

When the tests were concluded with a programme of aerobatics to see if the excellent
manoeuvrability of the original machine had been impaired by the modifications made to it,
the pilots were of the opinion that its qualities had not just been maintained but enhanced,
both in aerobatics and in normal flight. However, the Mk II Gamecock that incorporated
many of these changes was not ordered for the RAF, though three were delivered to the
Finnish air force and a further 15 were built in Finland under licence, remaining in service
there for nearly 20 years.

Not all such investigations were made because of dangerous behaviour, and not all could yet
produce such conclusive results. An example of the frustrations that continued to arise in
spinning work is provided by the case of the Hawker Hornbill, shown in Figure 2b. When
evaluated at A&AEE, it was considered to be ‘an advanced concept in high-speed design’,
perhaps because of its sleek profile due to the use of an in-line engine. However, it had been
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found unsuitable for service use on
the grounds of being under-powered,
so that its operational ceiling was
below the requirements of the test
programme. Its directional stability
and control in normal flying
conditions were also unsatisfactory.
But pilots who had tested the
prototype had given very
favourable reports about its
stability near the stall and its
spinning behaviour, where no

difficulty had been experienced in
recovering from spins with it in Figure 2b Aircraft with very different spinning
either direction. It was decided to characteristics - Hawker Hornbill
remit it to RAE for further (National Aerospace Library Collection)
investigation.

A substantial programme of work was undertaken in 1931/32, aimed at discovering the
(8)
reasons for its favourable spinning qualities, beginning with model tests . Initially, these
were mainly made with a model having wings and body only, which unusually was sent to
the USA to obtain its lift and drag characteristics in the NACA variable density tunnel, where
possible Reynolds number effects might be revealed. The results showed that there was an
appreciable scale effect on the maximum lift coefficient, but little change in the shape of the
lift curve, which had a considerable region of negative slope at incidences beyond the stall.
This was expected to help in stabilising the post-stall behaviour and, by Glauert’s rule, to
limit the range of incidence in which autorotation could occur.

More extensive tests were carried out with a 1/12th scale model in the rotating balance at
o
RAE, at incidences up to 60 and for the parameter ps/V up to 0.9 (a measure of the angle of

the spiral traced by the wing tip; notation is given in the Appendix after the references), with
wings and body and wings only. These showed that the presence of the body had a marked
effect on the rolling properties of the wings, and the yawing moment in rolling with the
original body compared favourably with that of the lengthened body fitted experimentally to
Aeroplane ‘H’. Spinning calculations made on the basis of these measurements indicated that
flat spins should not be possible with this aircraft and recovery should be easy.

Model tests were followed by further flight tests at RAE. For these, the aircraft was equipped
with extensive instrumentation similar to that used for Aircraft ‘H’. The installation of this
had taken the centre of gravity to an abnormally rearward location, and a slightly forward stick
position was necessary to obtain consistent behaviour in the spin. It was now found that the
o
motion in the spin was uneven, with the incidence fluctuating by up to 7 on either side of the
mean value. The stall-stability tests had also shown variations that had not been reported for
the tests at A&AEE, though these effects were now attributed to differences in the atmospheric
turbulence levels on the two occasions. But as before, recovery was found to be easy and
prompt for spins in both directions.
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It seems that these extensive tests had served mainly to confirm the difficulty of proving a
negative - why something does not happen. While the model tests and calculations had
agreed with the flight tests in that spin recovery should be easy, no particular features of this
aircraft were identified that could account for its good qualities. It was concluded that they
had been due to ‘a happy but fortuitous combination of parts of the design’, but quite what
that had been was still obscure.

Even though they would not always produce the evidence hoped for, the outcome of
comparative tests of this kind was considered by the ARC to be ‘highly satisfactory’.
Further, they had shown the growing maturity of spinning test methods at both NPL and
RAE.

3.3 Free spinning with models

Aircraft ‘H’ featured again in a new technique using models in free descent which was being
explored at RAE. Correctly scaled in physical shape, mass, moments of inertia and centre of
gravity position, these were released indoors from a walkway in the apex of the roof of the
Balloon Shed. A drop of 80 ft in still air was available before the models were caught in a
(9)
net spread close to the floor .

At first, the models were launched in a
stalled attitude from a swinging trapeze,
with the control surfaces set to produce
a spin. For investigating spin recovery,
a simple device using an air bleed was
fitted, that moved the controls to the
required positions after a suitable time
delay. However, the height lost in
establishing the spin state meant that
only a few steady rotations could be
obtained before the model reached the
net. With the stabilised spin limited to
such a short period towards the end of
the descent, there was uncertainty about
whether it could correctly be assumed
to have become steady and gave little
time for recovery actions to act, so a
different method of launching the
model was devised. In this, it was
given a suitable combination of initial
orientation, vertical velocity and

rotation by the apparatus shown in
Figure 3 Rig for launching a dynamic model in
Figure 3. The model was mounted on a
spinning attitude for free descent (Aircraft ‘H’)
carriage that could slide down and round
(Reference 9)
a vertical rod with a thread to produce

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