50
GA
/ Vol. 5 / No. 2 / FEBRUARY 2013
There's a long held tradition in
aviation that we aim to follow every
flight with a landing. Those who
have observed this tradition for many
years, also maintain that this landing
should wherever possible permit
the aircraft to be used for another
flight later. To achieve this, designers
decided that it was a good idea to
fit something to the bottom of the
aircraft which would fit between the
aircraft and the surface and take the
loads ideally, that something, which
came to be known as the "landing
gear" or "undercarriage" should be
useable multiple times as well.
But, the gear is often one of the
toughest bits of aeroplane for the
designer to get right. Whilst a wing is
pretty straightforward flight loads
are fairly predictable and consistent,
gear loads cover a much wider
spectrum, and also can affect many
other aspects of the aircraft.
Think, for example, the loads on
the retractable gear of a large single
such as a PC12 operating from a
dirt or grass strip as many do. The
day starts easy, with only the empty
weight of the aeroplane to be held
up, together with whatever buffeting
the morning's winds decide to bring
along: from any quarter, and at any
strength that the wind might throw
at the aeroplane. The loads go up
a bit as the aeroplane is loaded up,
but then can do all sorts of things as
the aeroplane taxies to the runway.
The occasional hole or step will put
sudden peaks on the whole structure,
and a hole under the nosewheel,
particularly whilst turning, will
put really nasty sideways peaks
onto it as well. A rough surface can
easily increase peak loads on an
undercarriage to five times the loads
by Dr Guy Gratton
Aircraft Technical
on smooth tarmac, and even exceed
landing loads.
Check the engine power against
the brakes of-course, and this puts
large bending loads on the gear,
and shear loads on both the brake
attachments and the gear attachments.
Then the aeroplane lines up and starts
the take-off roll.
Aerodynamic loads start to
build up, but also the weight of the
aeroplane reduces until as it lifts off.
The undercarriage, which was bent
backwards a bit like a spring, springs
forward putting a sudden peak load in
the opposite direction: known as the
"spring back load".
The aerodynamic loads might
eventually overstress the gear, but
that's why there's a maximum gear
extension speed and why the gear
will be raised as quickly as possible
after take-off, and certainly before
V
GE
is reached during the post take-
off acceleration. The gear doors as
they transit will be stuck out into
the airflow, and see their own peak
loads, before tucking back into their
recesses and steady loads although
in many aeroplanes the hydraulics
will be under load for the whole
flight, keeping those up and the gear
in place they're designed that way
so that a hydraulic failure drops the
gear. That however, this does vary a
lot from one aeroplane to another.
At the end of the flight, the
gear will normally be put down
somewhere reasonably early in the
approach so the gear doors will
see peak loads again briefly, then the
gear will see continuous aerodynamic
loads as soon as it's lowered. Many
aeroplanes with retractable gear will
be designed so that these drag loads
are high in effect the undercarriage
is also an air brake. This is one of the
big differences between the design
of fixed and retractable gear fixed
gear wants to be as low drag as
possible as it's there throughout the
cruise, and also to keep the loads
on it down anywhere up to V
NE
,
whilst retractable gear can be, and
often wants to be, much draggier.
Also the fixed gear needs to be able
to take aerodynamic loads beyond
V
NE
, whilst a retractable gear has an
aerodynamically easier life.
That drag will also affect the
handling of the aeroplane most
times the aeroplane will pitch nose-
down as the gear is lowered, so the
pilots are likely to have to trim nose-
up, but also there might be changes
to the lateral and directional stability,
possibly changing other aspects of
the handling often for example
more rudder is needed in turns with
the gear down. Many pilots won't
even notice this, because they're so
used to just adapting to changing
handling in different modes of flight.
But it's there nonetheless, and when
aeroplanes are certified it's important
that the flight test team have ensured
the forces are high enough to allow
pilots to detect changes in airspeed
or flight path, and that they're low
enough to hold until the aeroplane
has been trimmed.
So, on approach the undercarriage
sees aerodynamic loads which are
at a maximum as it's lowered, then
decrease as the aeroplane slows
towards landing then it has its
big moment: touchdown! Every
aeroplane has downward velocity
as it reaches the ground, it also has
weight, and it has forward speed.
The undercarriage has to deal with
all of that during what engineers call
the "spin-up" and in a crosswind
landing, that might all be on one
wheel, with an added side load as
the aeroplane's likely to be wing
down and if the pilot didn't handle
the crosswind landing quite right,
possibly moving sideways. You'd
think that what matters is the forces
on the undercarriage, but in fact this
is only part of the picture: what most
matters when getting the design of
an undercarriage right is the amount
of energy that the undercarriage will
need to absorb, as well as the angles
from which that energy will come.
Which is the big point the
real job of an undercarriage is as an
energy absorber. Engineers when they
design and certify the undercarriage
need to understand that, and most of
the clever maths goes into working
out where all that energy is coming
from, and how to get rid of it. All
sorts of things become important,
including the tyre and oleo pressures,
the amount of lift still acting during
the landing, the touchdown speed, and
the all-important geometry. Which
is why so much of this becomes
really important during the pre-flight
inspection tyre pressures and oleo
extensions really do matter, a lot. Too
high a pressure and the loads on the
whole undercarriage structure go up,
too low a pressure and tyres or oleos
may bottom, causing a massive peak
in load. Neither is good.
And of-course after all this effort,
loads of maths, thousands of hours of
design, analysis and testing - the pilot
will turn up, give it a cursory glance,
then strap in and just assume it'll all
be fine. Which, is at it should be. ·
Be nice to your undercarriage