User Guide

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THE PROPELLER
You’ll also have noticed, in the 172, that any time you
changed your airspeed, the engine would speed up or slow down
without your touching the throttle. This is because the 172 has a
simple fixed-pitch propeller. It’s like driving a car with a manual
transmission that’s locked in one gear: the engine speed has a
direct relationship with how fast you’re going down the road.
The Mirage, however, like other high-performance air-
planes, has a variable-pitch constant-speed propeller, which is
much more like an automatic transmission. It allows the engine to
turn at the most efficient or appropriate speed for a given flight
condition, regardless of the airplane’s airspeed at the time. For
example, for takeoff, it’s desirable to run the engine at as high an
RPM as possible. This allows the maximum amount of air and fuel
to run through it over time, so maximum power is available.
For climb, a somewhat lower RPM is appropriate. Once lev-
eled off in cruise, the lowest possible RPM that allows the engine
to produce the required level of power is desirable--primarily
because engines and propellers are most efficient (in terms of
miles per gallon, rather than maximum power) at lower RPMs,
and secondarily to reduce both inside and outside noise.
Thus, the Mirage has two main power controls, and two main
power instruments: the throttle, which controls how much fuel/air
mixture gets into the cylinders (and which is set by reference to the
Manifold Pressure Gauge, of which more in a moment), and the
propeller control, which controls the RPM at which the engine
operates, and which is set by reference to the tachometer.
It does this by varying the pitch, the angle at which the pro-
peller blades meet the oncoming air. They’re like the threads on a
screw: in the low pitch, or “high RPM” position, each turn of the
propeller moves the airplane only a little way forward, as if the
“screw” had very fine threads. In high pitch, the “low RPM” posi-
tion of the propeller control, the blades take a bigger “bite” of air
with each turn, and move the airplane forward faster; the “screw”
is a very coarse-threaded one. I know that the relationship between
high pitch/low RPM and vice versa is confusing at first; the Brits
describe it much more rationally, as “coarse” and “fine” pitch.
Each turbo consists of a turbine and a centrifugal air com-
pressor, linked on a common shaft. The turbine is driven by
exhaust gases, thus powering the compressor to compress the
engine’s induction air, the air supply that will be mixed with fuel
and burned in the cylinders. It’s almost “something for nothing,”
which is why the first turbochargers, in World War II, were some-
times called “bootstrap turbines,” after the legendary Baron
Munchhausen, who claimed to be able to fly simply by pulling
himself up by his own bootstraps.
One reason turbos didn’t appear until World War II is that
they had to wait for the development of sufficiently advanced
alloys. If you could see under the Mirage’s cowling at cruise
power and altitude, you’d find the whole exhaust system, and
both turbos, glowing anywhere from cherry red to a cheerful
orange. Even the compressor side gets pretty warm, which is why
a large intercooler is installed to reduce the temperature of the
induction air before it’s ducted to the cylinders.
Why go to all this trouble? Because, as we gain altitude, the
air gets thinner and thinner; by 18,000 feet, the atmospheric pres-
sure is only half of what it is at sea level. This is both good and
bad: if the air is thinner, airplanes can slip through it with much
less resistance; but there’s also a lot less air for engines to
“breathe,” so they lose power.
With a turbo, however, we can feed the engine “thick” sea-
level air, by way of compression, while the airplane slips rapid-
ly through “thin” high-altitude air. A side benefit, in the case of
the Mirage, is that the turbos give us a supply of sea-level air for
cabin pressurization.
You may have noticed flying the 172 that it took more and
more throttle to maintain a desired RPM and airspeed as altitude
increased. I say “may” because most pilots climb the 172 at full
throttle. This is the case with any non-turbocharged, or normally-
aspirated, aircraft. The Mirage, however, has an automatic con-
troller that regulates how much exhaust flows through the turbos
to spin them; thus, once you’ve set the throttle for the desired
power setting, there’s no need for further adjustment as you climb
or descend.
Flight Instruction
Flight Instruction