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baggage or top up with that bit of
fuel that will save you making a stop
en route, but consider the
implications. Firstly, any Certificate
of Airworthiness (and hence
insurance) will be invalid if you don’t
fly the aircraft within the limits of
the flight manual, and, secondly, you
will be leaving yourself nothing in
hand for turbulence and the like,
which will increase your weight
artificially. The designer will have
allowed for 60-degree turns all the
way up to MAUW, but not heavier
than that. Even worse, engine-out
abilities will be less than expected,
particularly with autorotation.
There are very few light aircraft that
will allow you to fill all the seats and
cargo holds and still take full fuel.
The effects of overloading include
reduced acceleration capabilities
(leading to longer takeoff and
landing distances), decreased climb
capability (watch for those
obstacles), reduced range, ceiling,
manoeuvrability, braking and
margins, to mention but a few.
Here are some of the most common
weights you will encounter:
· The Empty Weight is that in the
Weight and Centre of Gravity
Schedule (in the Flight Manual),
which is established by actual
weighing before the machine is
used. It is the weight of the
empty aircraft, plus unuseable
fluids, and any fixed equipment.
· The Basic Empty Weight is the
empty weight, above, plus
operating fluids (fuel, full oil).
Flight Planning 159
· The Maximum Takeoff Weight,
which is simply the Basic Empty
Weight plus the payload
(passengers, cargo), which will
not necessarily coincide with the
full maximum, for performance
reasons. The conditions at your
destination, for example (it may
be hotter and higher) may mean
taking off lighter so you can
land safely. Any weight less than
the full maximum due to
performance factors is known
as the Restricted (or Regulated)
Takeoff Weight (RTOW) and is
actually the starting point for
calculating payload (see below).
A couple of things to bear in mind
are that the Basic Weight and the
payload will not change during flight,
but the fuel load will. Another is
that, in small aircraft, passengers and
cargo should be weighed separately.
Distribution
Incorrect loading naturally affects
aircraft performance, and will
possibly prevent the thing from even
getting airborne. A Centre of Gravity
too far forward will make it more
difficult to raise the nose on take-off
(or landing), possibly overstress the
nosewheel as a result, and make the
flight less economical by excessive
use of trim tabs, which causes more
drag. There are certain advantages to
having the C of G towards the rear
(by making the tailplane contribute
to total lift, or at least not detract
from it, which also reduces the
power required and hence fuel used),
but too much will make the aircraft
less stable, more fatiguing to fly and
cause similar drag and nosewheel
problems (but in reverse) as
excessive forward C of G. Also, if
you don’t have the elevator
movement to get yourself out of a
stall, you could end up in a flat spin
you can’t get out of.
In a helicopter, if the C of G is too
far aft or forward of its ideal
position, there is a danger of running
out of cyclic control in the opposite
direction – one too far forward, for
example, will mean you will not be
able to pull the cyclic back far
enough to cope with certain stages
of flight (as fuel is consumed, for
example, when the C of G generally
moves forward), as a lot of its range
will be taken up with the unusual
attitude, although a forward position
is needed to counteract flapback.
Not being able to flare in an
autorotation could well ruin your day
(in fact, if your engine fails and you
don't have enough cyclic movement
to counteract the nose down
tendency, the airflow will meet the
disc edge-on and not go up through
it, so you will not enter autorotation,
and the RPM will decrease even
more – ouch!). As well, lateral C of G
may be affected with some loads,
such as when hoisting.
The reference datum is an imaginary
point from which all calculations
start and where some C of G ranges
are expressed (for example, 106" aft
of datum). Mostly, it is at, or slightly
forward of, the nose, but can be at
the rotor mast of a helicopter.
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