User Guide

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103
“THE BACK OF THE CURVE”
Now for a rather interesting exercise. Start out with the air-
plane in trimmed level flight, then gradually reduce power to
about 1750 RPM and re-trim until you’re flying just below the top
of the white arc. Next, extend the flaps to the second notch, wait
for the “balloon” to run its course, and trim once again for level
flight. The airspeed should settle around 70 to 80 knots.
Notice what your tachometer is indicating - due to the fixed-
pitch prop, it’ll have changed a bit as we slowed. Now, using ele-
vator and trim, slow the airplane by ten knots; then, holding it at
that airspeed, adjust the throttle until we’re neither climbing or
descending. Look at the tach again, and you’ll see that we’ve
reduced power a bit more. Stands to reason, doesn’t it? To go
slower, use less power…
Okay. Now reduce the airspeed another ten knots - careful-
ly, we’re pretty close to the stall, and you may hear the horn inter-
mittently - and, once again, adjust power to maintain altitude.
Guess what? It takes more power this time! We’re flying slower,
but it takes more power to do it.
We’ve entered what’s known as the “region of reversed com-
mand,” also called “the back side of the power curve.” Down to a cer-
tain speed, the airplane seems to be following the rules - more power,
more speed. Below that, however, everything seems backward.
What’s happening here is that as we approach the critical
(stalling) angle of attack, any small increase in angle of attack
causes drag to build up even faster than lift. This is why we have
to be so cautious as we approach a stall - as we get close, the air-
plane has a tendency to “dig in,” if we don’t pay attention to angle
of attack, and slow itself even further.
Okay - as long as we’re down here, somewhere below the
bottom of the green arc, let’s try some gentle turns. This is called
“maneuvering at minimum controllable airspeed,” or simply
“slow flight,” and it’s an excellent exercise. Remember, stall speed
goes up with increasing bank angles (we’ll explore that more in a
bit), so make all your turns very gentle.
Finally, let’s reduce the power to or near idle, and let the air-
plane start descending so that it maintains airspeed. After we have
things stabilized, extend the final notch of flaps, continuing the
descent. To make things even more interesting, start a gentle turn in
either direction.
At this point, we’ve added more lift than drag. Many pilots will
blithely say “the 172 has a big nose-up trim change when you
extend the flaps,” and in a sense, they’re right: it does, at least at first.
However, until your use of trim is completely instinctive, rather than
trimming madly nose-down, then having to trim back nose-up as the
airspeed dissipates, just sit tight for a moment and you’ll find the trim
change wasn’t nearly so large as you thought.
With all these ruminations, we’re probably down below 110
knots now, so run out the second notch of flaps. Again, there’s a
nose-up trim change and a bit of ballooning, but less than the first
time. This is due partly because we have less airspeed, and thus
less energy, starting out, and partly because the flaps are now tran-
sitioning from “pure lift” to a more balanced “lift and drag”
regime. Again, wait until the airplane has settled down. Again,
we’ve shed some airspeed; the nose is down further yet; and
we’re descending a bit faster.
Finally, since we’re now well within the white arc, extend
the flaps all the way. The “balloon” will be very slight, but even
more speed will dissipate, the nose will go down further yet, and
the sink rate will increase. Throughout this evolution, we’ve
touched neither the yoke, the throttle, nor the trim.
All right - now, in one swell foop, bring the flaps all the way
up. The airplane will drop its nose and sink like a rock, at least for
a moment - but as it accelerates, the nose will come back up, and
if you’ve really been honest about not touching pitch, power, or
trim, sooner or later (after a few mild “dipsy doodles”) you’ll be
back at trimmed level speed.
What you’ve experienced here is that the flaps can be used,
not only to configure the airplane for slow flight, but for control.
Particularly as you start flying on instruments, when you’ll have to
keep track of a whole lot of things at once, you’ll find it a mark of
professionalism to control the airplane not only with pitch and
power, but also (and in some cases primarily) with configuration
changes. This will become even more important as you move into
higher-performance aircraft.
Flight Instruction
Flight Instruction