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should a load be jettisoned, usually
from excessive swinging
(commercially, dropping loads is
regarded as a non-macho thing to
do, but it's your backside the
helicopter is strapped to). As a point
of interest, 10 gallons of fuel from
Specialised Tasks 29
500 feet will go straight to the
basement of a 3-storey house. If you
drop anything obnoxious in water,
expect your company to pay for the
clean-up and testing.
Unless in rough air, beeping the
RPM down helps with the lift and
the fuel burn.
If you get an engine failure in the
mountains with a load attached,
consider not jettisoning it. This may
sound daft, but it may stop you from
falling down a crevice or something
– just try to keep the line taut so you
don’t get a nasty jerk at the end.
Load Behaviour
Every load has its own VNE,
unfortunately usually only found by
experiment, which is why you should
always start off slowly and build up
to a point where it starts to give
trouble, then back off, as low as
possible, so there is less height to get
rid of in a hurry. Most helicopters
will carry loads at quite high speeds,
but the load itself might not be able
to handle it—a sudden input of drag
when something falls off could
become quite a problem. Although
customers don’t like to pay for
unnecessary flying, there’s no rush.
Take it easy. Also, remember your
machine’s VNE with the doors off!
External loads increase the frontal
area of the whole aircraft, which
naturally increases drag, so you will
need more power overall. A load
may be easy to lift, but present
enough drag to cause severe
difficulties, particularly where you
reach power limits too quickly to
maintain forward flight, and the load
overtakes you and pulls you along. A
long-line needs more anticipation, so
you need a high degree of coordination
and patience. It's not the
sort of thing that can be learnt in any
other way than with lots of practice.
Unevenly shaped loads will tend to
spin and, if they're slung without
reference to their centre of gravity,
could tip over. A drogue chute can
stabilise them, but use a windsock
type rather than a pure parachute,
which will bounce around trying to
spill the air out (or punch holes in
it). Naturally, these must be kept well
away from the tail rotor. Logs or cut
timber usually fly poorly unless a tail
is installed, which can be made out
of a bough or piece of plywood, so it
sticks out of the back.
Oscillation or excessive vibration
can come from a number of places,
usually a combination of the stability
characteristics of the load and
forward speed. Heavy or dense
loads, such as bags of cement or
drums of kerosene, will not usually
present problems due to their mass,
but large-volume loads of low
density can oscillate at a certain
critical speed, again usually only
found by trial and error.
You can dampen oscillation by
reducing your airspeed to at least
10% below this critical one, going
slower if necessary while increasing
power. Turning could provide
enough centrifugal force to stop it as
well, which is also the usual remedy
when the load starts to swing, but
this will increase its effective weight,
possibly to more than your lifting
capabilities (a good reason for not
being too tight on payload), so
applying centrifugal force in these
cases could make things worse.
30 The Helicopter Pilot’s Handbook
Load swing is proportional to speed
and the length of the sling – the
faster you go, the more it occurs
because of the load’s own lift and
drag. Put simply, the load will always
move further in a swing than the
aircraft does – if you move 4 feet to
the right, the load will travel eight,
after a short pause, as it moves four
feet to the other side of you. As it
takes as long to swing through 15° as
it does through 45, like any
pendulum, if you move the opposite
way, as is natural, you just make the
load swing faster. Going with the
swing, that is, load to the left,
helicopter to the left, will stop it
quickest (do it on your side so you
can see what’s going on, but it works
both ways). This has the effect of
moving the fulcrum point from
under the helicopter to over the
load, which removes the directional
vector. If you want it to in a
particular direction, wait till it is
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(19)