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£1,000,000 Test Bed Bluebird
CN.7/62
Published in AUTOCAR, 13th July
1962, writer unknown
Bluebird is ready for Land Speed Record Attempt
It is seven years since preliminary investigations begun into
the design of CN.7, the car built by Donald Campbell to attack
the World's land speed record. The work was entrusted to Norris
Brothers Ltd., the design consultants of Burgess Hill, Sussex,
who had been responsible for the Bluebird hydroplane with
which Campbell holds the World's water speed record at 260.35
mph.
All previous record breakers had been designed around existing
piston engines and this was the first machine which was to
be powered by a gas turbine unit. A free turbine engine was
necessary to provide the required torque-speed characteristics
and to avoid using a change-speed gearbox. The Bristol Siddeley
Proteus, as used in the Britannia airliner, was very suitable
for the task and the manufacturers made this available.
The designers, K. W. and L. H. Norris, tackled the problem
of designing Bluebird from first principles, without drawing
upon preconceived ideas of what a record-breaker should look
like. Only after they had reached conclusions on the basic
configuration of the car did they make a study of existing
machines, so the hard-won experience could be drawn upon.
 The
shape of the envelope was all important to achieve minimum
aerodynamic drag and to ensure that the car was neither lifted
nor forced downwards by the air passing around it. It might
be thought that a downward force component would be useful
to increase tyre adhesion, but this would overload the tyres.
The optimum shape depends not only on its outline in side
elevation but also on its cross section and these were arrived
at independently in intensive wind tunnel tests on models.
An all up weight of 7,000lbs was aimed at but all calculations
were based on a figure of 9,000lbs. This has since risen to
9,500lbs for the latest, rebuilt version of the car. As this
mass has to be accelerated, without damaging the tyres, in
about five miles to speeds upwards of 450 mph at the beginning
of the timed section, drive to all wheels was essential for
maximum adhesion and the Bristol Siddeley Proteus engine was
modified to have a rearward power take-off from the turbine
shaft. Thus at each end of the car is a spiral bevel final
drive unit and differential, the wheels are driven through
shafts with constant velocity couplings.
Each final drive assembly has to transmit over 2,000 horsepower.
Special lubricants were developed by BP and oil is circulated
through each unit from a large tank to lower the operating
temperature.
Apart from modifications which are outlined later, CN.7/62
- the replacement car for the one destroyed at Bonneville
Salt Flats, Utah in 1960, is identical and much of the original
has been retained.
Bluebird CN.7/62 is constructed on aircraft lines, and that
the hull is a riveted monocoque structure built from light
alloy honeycomb panels and alloy sheeting. Double wishbone
suspension is employed with Girling oleo-pneumatic spring-damper
units, the springing medium being nitrogen. Girling have met
the challenge of stopping the car from 400 mph in 60 seconds
by developing twin-caliper disc brakes. They are operated
by compressed air, as are the aerodynamic brakes at the rear
which provide initial retardation. Dunlop are designing and
developing the enormous tyres and in all 68 companies have
played their part in the design and manufacture of components.
This second car, like the first, was constructed at Motor
Panels (Coventry) Ltd. All concerned have, without hesitation,
again rallied round Campbell, in the construction of CN.7/62,
in anticipation of what everyone hopes will be a successful
onslaught on the record.
Lessons and Changes
As expected, Norris Brothers have taken full advantage of
the structural test offered by the crash during the 1960 record
attempt to find out which are the weaker points in the structure,
suspension and steering components, and to strengthen these
accordingly.
One requirement of the initial specification was that if
the car should roll over the wheels should remain attached.
The gyroscopic forces of these very large wheels were so high,
however that during the roll the steering arms fractured,
allowing the wheels to saw into the main structure. Modifications
have been made to prevent this happening.
Among several structural changes are the double skin construction
for the D-section boxes between front and rear wheels bays.
These provide space in which equipment may be placed strategically
to obtain the required position of the centre of gravity,
which can now be adjusted to any given point up to 10 inches
forward of the mid-wheelbase station. Previously two fuel
tanks having a total capacity of 25 gallons, were mounted
on either side of the car just ahead of the rear wheels. These
have been replaced by a single 16 gallon tank in the forward
end of the starboard D box.
Apart from the effect on weight distribution, the rearrangement
of the ancillary equipment has enabled servicing points to
be grouped so that less time will be spent working on the
car between runs. With record attempts a quick turn round
is essential so that full advantage can be taken of favourable
weather and course conditions. Also the regulations demand
that runs in opposite directions must be made within one hour.
By placing the seat lower, the driver is better protected
by the main beams of the structure. The cockpit canopy has
also been strengthened, being a glass-fibre moulding, and
head cushioning has been provided for the driver. The instrument
panel has been repositioned to increase the space for installing
instruments and to facilitate their removal for maintenance.
A single pointer instrument to indicate power output is fitted
in place of one with two pointers; the combined speedometer-accelerometer
has a new, more easily read face. Redesign of the throttle
control has increased its range of movement and reduced the
effort required to move it.
As the car had been damaged by rolling during a spin, the
question of aerodynamic stability has been reviewed. Had the
car spun without rolling it would have come to no harm and
so the problem is resolved into one of ensuring that the rate
of spin, should this happen, is limited to a value which will
not cause roll-over. To achieve this a tail fin has been installed,
which moves the centre of lateral wind pressure from a position
at the front wheel centres to just behind the centre of gravity
of the car, approximately mid-way between front and rear wheels.
At its highest point the tail fin is 7ft 8.5inches above
the ground bit it can be reduced in size, section by section,
if required, or removed altogether. There is inevitably a
performance penalty with a fin, which imposes extra aerodynamic
drag, but as the area can be altered, a good stability-performance
compromise should be achieved.
Below certain speeds, the first Bluebird CN.7/60 and the
Railton Mobil Special were directionally stable in the aerodynamic
sense but above these speeds they were "oversteerers."
It has been found from wind-tunnel tests on models that the
fully finned Bluebird CN.7/62 is a considerable improvement
in this respect.
In one way however, Bluebird is at a disadvantage compared
with the Railton Mobil and that is in the great size of the
fairings for the 52inch diameter tyres. This tyre size was
fixed by Dunlop who made the covers. By comparison, those
on the Railton Mobil were 44inch diameter and Mickey Thompson's
Challenger 1 used 30inch tyres.
Hence in the fully finned condition, the area presented by
the sides of the car must be considerable and the effect of
side gusts increased. This makes it all the more important
that calm conditions should prevail during a record attempt
as the car could be put into a zero yaw drift by a side wind
and slip angles generated at the tyres could use up enough
of the available adhesion to start wheelspin. At Utah, the
wind could vary in magnitude and direction along the course
so that observations had to be made at frequent stations.
Resisting Wheelspin
For a subsequent attempt on the record the differentials at
the front and rear final drive units will be locked, so as
to provide the maximum resistance to wheelspin. The freewheel,
however, at the front drive unit, will remain in use. The
purpose of this is to allow the front wheels to overrun the
rear wheels during braking when forward weight transfer reduces
the rolling radius of the front tyres and increases that of
the rear tyres. During acceleration, which at Utah had to
be as high as 0.5g, tyre slip, because of rearward weight
transfer, has to be accepted and is balanced between front
and rear tyres by the choice of the position of the centre
of gravity.
The choice of the most suitable overall steering ratio was
the subject of considerable experiment during trials in 1960,
variations between 25 to 1 and 100 to 1 being allowed for.
It is now considered that the higher geared systems should
be tried and the range variants now extends between 16 to
1 and 76 to 1. A ratio of 25 to 1 was used during trials,
but this was thought to be to direct and 32 to 1 is likely
to be employed.
So far no plans have been released regarding a future record
attempt but when the car is shown to the public at the B.A.R.C.
Jubilee festival of motoring at Goodwood shortly, it is expected
that an announcement will be made.
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