FLIGHT,
4 April 1952
425
BOGIE U N D E R C A R R I A G E S
A Necessity for Heavy Aircraft: Current Practice Reviewed in R.Ae.S. Lecture
I
N a Section L e c t u r e to the Royal Aeronautical Society on
M a r c h 4 t h , R. C. Cussons, D . F . C . , M . A . , A . F . R . A e . S . ,
assistant chief designer of D o w t y E q u i p m e n t , L t d . , gave
o n e of t h e first public assessments to b e m a d e of the bogie
undercarriage. H e confined his paper to a discussion of the
use of bogies i n general, together with a n appraisal of a n u m b e r of p a t t e r n s currently in use. A digest of his remarks
follows.
One of the chief advantages cf the bogie undercarriage, said
Mr. Cussons, was that it enabled a given aircraft to impose a
much reduced maximum bending moment upon a concrete slab
runway. With aircraft weights progressively increasing, the only
alternative to the use of bogies appeared to be to increase the concrete thickness—a much more costly undertaking.
T h e lecturer gave examples of aircraft which, originally designed with single main wheels, had later appeared with bogies,
adding that even the 85,000 lb Lincoln was on the verge of
exceeding the permissible runway loading and, if designed from
scratch today, would have at least two main wheels per undercarriage, if not four.
T h e runway loading index, in fact, forced the designer to adopt
the bogie undercarriage for any aircraft of over about forty tons
gross weight. With the adoption of such an undercarriage secondary advantages usually accrued; in particular, though the gear
itself would weigh more, an overall weight-saving was effected by
the lower aggregate weight of wheel, tyre and brake assemblies.
T h e prototype Comet's single-wheel main undercarriages weighed
together, 3,815 l b ; those of the production aircraft, complete with
the eight wheels and brakes, weighed only 3,598 lb.
Further advantages were, in certain installations, the possibility
of easier stowage of a fore aft retracting bogie (with small-diameter
tyres) by rotating the bogie during retraction so as to keep the
wheel axles in a near-horizontal plane, and the benefit of "safety
in numbers" where burst tyres were concerned.
Turning to the debit side of the picture, Mr. Cussons cited the
difficulty of ground manoeuvre as the bogie's greatest drawback.
If a bogie were turned about its vertical centre, the direction of
movement of the tyres was almost parallel to the wheel axles, so
that they scrubbed sideways with practically no rotation (Fig. 1).
Handling notes for bogie-equipped aircraft generally recommended
Fig. 2 (left).
The first
Comet bogie (port unit
with the inner wheels
removed) with
fittings
machined from solid. The
production bogies, with
parts made from dropstampings,
look even
neater.
Fig. 3 (right). Brabazon 2
bogie on the drop-test rig;
again, the inner wheels
(four of them) have been
removed to reveal the
bogie construction.
Fig. 1. Careless taxying can cause damage to bogie tyres; this shows
the effect of turning a Brabazon 2 unit at 12ft radius in a test rig.
a minimum turning radius, together with the corresponding nosewheel steering angle.
Another disadvantage was the difficulty of performing droptests; with a single wheel, drag and side loads could be simulated
by wedges, but space limitations often prevented this with bogies.
T h e drag case could only be obtained by dropping the bogie with
backward-spinning wheels, giving the same effect as occurred at
touch-down
In the bogie an increase in complication had to be accepted
while there had been rumours—from America—of a tendency to
pitch, or porpoise, while taxying. This trouble had not, however,
been a bother in this country.
Dealing first with the "beam-engine" type of bogie, Mr. Cussons
said that a good example was the undercarriage fitted to the
Convair B-36. This was really a double-levered-suspension unit,
with the levers pointing both fore and aft. The main shockabsorber connected the middle of the front lever to the front end
of an upper pivoted beam which then transmitted the deflection
to the middle of the rear lever. This had the effect of translating
the upward movement of the rear wheels (which struck the ground
first) into a downward movement of the front wheels, without
compressing the shock absorber. Thus the front wheels hit the
ground at about twice the aircraft descent-rate.
In the de Havilland Comet, a very similar principle was employed, although in this undercarriage (Fig. 2) brakeloads were
transmitted by links to the main leg instead of to the bogie. These
links were so arranged as to give no tipping moment to the bogie
at the static position; at other positions of the wheel levers there
was a slight out-of-balance, either backwards or forwards, depending upon whether the wheel axles were above or below the normal
static position.
The next unit dealt with by the lecturer [that for the Short