User Guide

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ENGINE OPERATING TECHNIQUE, PART 2:
Mixture Control
There’s a third handle on the power quadrant, with a red
knob. (The 172 has a similar control.) This is the fuel mixture con-
trol, which sets the ratio of fuel and air flowing into the cylinders.
How come cars don’t have one? Three reasons: one is that
unless you’re driving in the Andes or the Himalayas, cars operate
over a fairly limited altitude range. Even then, you’ll notice a sig-
nificant loss of performance driving in the mountains; and, if
you’re going to confine all your driving to higher altitudes, you
can have your car’s carburetor set for a leaner mixture by chang-
ing fuel jets.
Another is that modern cars have electronic fuel injection
systems. Somewhere in the bowels of such systems are hundreds
of angels, dancing on the head of a pin to set the fuel mixture
exactly right for the right altitude. But those angels need electric-
ity, and sometimes they get tired, or confused, and you have to
pull over to the side of the road. That’s harder in an airplane. The
manually-controlled Bendix continuous-flow fuel injection sys-
tem used even on the Mirage’s sophisticated engine is crude--but,
barring contaminated fuel (or the problem, common to all air-
planes, of their inability to manufacture more fuel inflight when
needed for longer-than-planned flights), there’s almost nothing
that’ll make it quit working.
Finally, most light aircraft engines are called “air-cooled,”
and, indeed, they are--at cruise power. If, however, their cowlings
and cooling fins were big enough to handle their cooling needs at
takeoff and cruise power, the hapless pilot would have a hard time
seeing past them. Not that it would be much of a problem, since
there’d be so much drag the airplane couldn’t fly, anyway.
Instead, at high power settings, aircraft engines are run at
much richer fuel mixtures than optimum, allowing the excess
unburned fuel to carry away the additional heat. (Pollution? Don’t
even ask...) At high power, they’re not just air-cooled; they’re fuel-
cooled as well. Car engines, by contrast, can run at much higher
internal temperatures, because they have heavy water-cooling
systems to carry off the excess heat.
Let’s stick with the image of a wood screw a moment longer.
Imagine you’re driving two screws, a coarse one and a fine one,
into the same seasoned block of oak. It’ll take a lot more force to
twist the screwdriver when you’re driving the coarse one; the fine
one will drive a lot more easily, although it will take many more
turns to get it screwed in the same distance.
It’s the same in the air. When you set the prop control (the
blue handle on the power quadrant) for a desired RPM, you’re actu-
ally setting a hydraulic governor on the engine that, in turn, meters
oil to the propeller hub to set the blades at the correct angle. If you
increase either airspeed or engine power, the propeller will try to
speed up; the governor will automatically adjust the blades to a
coarser pitch, making the propeller “more difficult to turn,” to
maintain RPM. Similarly, if you slow up or reduce power, the gov-
ernor will sense the RPM beginning to decrease and will “fine off”
the blades to maintain the correct value. The governor also has
minimum and maximum set points. With the prop control all the
way forward, the engine will run at its 2500 RPM redline if there’s
enough power available; if not (for example, at low power on the
ground), the blades will go to the fully-fine position and will act as
a fixed-pitch propeller. The minimum set point corresponds with
the bottom of the green arc on the tachometer.
ENGINE OPERATING TECHNIQUE, PART 1:
Power Settings and Changes
Power setting for high-performance piston airplanes are
almost always expressed in terms of a pair of numbers: the mani-
fold pressure, or throttle setting, and the RPM, or propeller setting-
-for example, “35 in. Hg./2500 RPM.” What’s an “in. Hg.?” It’s an
inch of mercury, an ancient measure of air pressure dating from
the days when pressure gauges were vertical glass tubes full of
quicksilver. (Does the measurement seem familiar? It’s the same
unit, at least in the USA, that you’ll find for altimeter settings; nor-
mal sea-level pressure is around 30 in. Hg.)
The rule of thumb to avoid overstressing an engine (rather like
the “lugging” you feel if you try to drive up a steep hill by flooring
your car in too high a gear) is that when making a power increase,
you increase the RPM first, then the manifold pressure. Power
decreases go exactly the other way: manifold pressure first, then
throttle. As a reminder, you can use the mental image of “Propping
something UP” and “Throttling something DOWN.” (For small
power changes within the cruise regime, you may often find your-
self changing only one control without moving the other at all.)
Flight Instruction
Flight Instruction