曝光台 注意防骗
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from the tail boom:
Having the wind from the rear helps
you keep an eye on the low blade at
the front, but this means landing
downwind in the first place.
Do not use collective pitch to slow the
blades down –droop stops depend on
friction for proper operation, and all
you will be doing is lightening the
load where it ought not to be.
Some Questions
1. What factors affect the amount of
lift produced by an aerofoil?
2. What are the 3 axes that an
aircraft moves around, and
associated stability?
3. Why does an increase in all-upweight
lower speed in a helicopter?
4. What is the boundary layer?
Some Answers
1. The angle of attack, air density,
velocity of airflow, surface area.
2. Longitudinal (roll) – lateral
stability, lateral (pitch), longitudinal
stability, yawing axis – directional
stability.
3. More collective is required and the
retreating blade will stall earlier.
4. The layer of retarded air
immediately in contact with the
aircraft skin.
Engines & Systems
Engines, turbine or piston, all work
on more or less the same principle –
a quantity of air is sucked in and
mixed with fuel, compressed, set on
fire and slung out of the back (suck,
push, bang, blow in other words or,
more technically, induction, compression,
power and exhaust). The difference is
that the power comes from the
ignition stage in the piston, and the
exhaust stage in the turbine, which is
always ignited, whereas the piston
only does so when the spark plugs
operate. The jet is also a whole lot
lighter, and spins a lot faster. In
short, engines convert heat energy
into mechanical energy, and not very
efficiently at that (if they were,
exhausts would be cold – the
thermal efficiency of a piston engine
is only 30%, although it does
increase with altitude). The
mechanical energy may be used to
drive electrical, hydraulic and
pneumatic systems as well, which is
why engines are also called
powerplants.
For short range aeroplanes, the
propeller is the most economical
method of propulsion, which can be
driven by a piston or turbine engine
(piston-driven ones are generally
found on aircraft below 5700 kg).
Because of inefficiencies in design,
there is a loss of energy in the
process, and the thrust horse power is
about 80% of the shaft or brake horse
power actually coming out of the
engine. Sometimes, engine output is
too great for the transmission, so the
manufacturer will derate it to make
sure it doesn't damage anything.
Reciprocating Engines
A typical piston engine consists of a
series of identical cylinders which can
be arranged in many ways, according
to what the engine is going to be
used for. The Beaver, for example
had a Pratt & Whitney R985, which
was a radial engine that had the
cylinders in a circle, attached to the
crankshaft in the centre (with the
propeller bolted to it). In this case,
the cylinders stayed still, and the
crankshaft moved, but earlier
engines made the crankshaft stay still
while the cylinders moved. As you
42 JAR Private Pilot Studies
can imagine, maintenance was
difficult, to say the least.
Most modern light aircraft, including
helicopters, have their cylinders
opposite each other (horizontally or
vertically opposed), to cancel some
forces out, but many have them in
line and even in a V formation.
Some engines are even upside down,
as found with the Gypsy Major in
the Chipmunk (since the pistons in
this case are at the bottom, there is a
danger of oil leaking into the
combustion chambers, which is why
the engines are turned through one
complete cycle before being started,
to make sure the insides are free to
move. You will break the starter if
the cylinders are blocked with oil).
One big difference the average car
driver will find is that an aero engine
has two spark plugs per cylinder,
which are powered by independent
magnetos (see below). When doing
power checks before takeoff, they
are checked against each other for
power and whether they actually are
independent. Another is that car
engines are a lot smoother (there are
some diesel engines that won't upset
a coin standing on its edge). As
cylinders in an engine are all the
same, we will look at just one in a
moment to see how they work (see
The 4 Stroke Cycle, below).
The cylinder is just that, but it is
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