User Guide

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While it’s doing this, let’s take a moment to look at why it
can remain so stable on its own. (If it’s not quite doing that, go
ahead and pause the simulation).
Any certificated civil airplane has a fairly high level of pitch
stability. That is, when it’s trimmed for a certain speed (as we did
just now), it’ll tend to hold that speed even if displaced from it.
Let’s take a look at how this works.
You’ll see that the airplane is like a teeter-totter, balanced on
the point at which the wing exerts its lift (called, appropriately
enough, the center of lift). While a good deal of the airplane’s use-
ful load (people, luggage, and fuel) is arranged near the center of lift
(either in front of it or behind it), there’s a significant chunk of iron
stashed away just about as far forward as you can get: the engine.
This means that the airplane’s natural
tendency would be to drop its nose. To
counteract this, the horizontal tail has an
airfoil similar to that of the wing - but
upside down! Thus, it’s actually pushing
downward, and thus balancing the air-
plane and keeping the nose up where it
belongs.
Now, you’ll recall from the introduction - you did read it,
didn’t you?—that the amount of lift an airfoil produces is in pro-
portion to its airspeed. Let’s say, for example, that we hit a gust
that drops the nose of the airplane a bit. Since it’s now going
downhill, it’ll speed up - and as it does, the downforce generated
by the tail increases, thus bringing the airplane’s nose back up
toward level flight. Similarly, if something displaces the airplane’s
nose upward, it loses speed; the downforce created by the tail
decreases, allowing the weight of the engine in the nose to bring
the nose back down.
The process isn’t instantaneous. Let’s get back into the cock-
pit for a demonstration. Once you have the airplane trimmed out
for level flight, pull the nose up until the airspeed has dropped to
85 or 90 knots, then let go of the controls. (You can continue to
nudge them from side to side to keep the wings level, but don’t
make any pitch inputs or corrections. Or, since the 172 autopilot
doesn’t control any pitch functions, just turn it on and it’ll keep
the wings level for you.)
As soon as you turn the controls loose, the airplane will try
to return to its trim speed. In fact, since it’s now flying so slowly,
it doesn’t even have enough “tail power” to keep the nose up to
the normal level flight attitude; the nose will gently drop to some-
where below level flight attitude, and the airplane will speed up.
As it approaches its trim speed, the nose will start to come up
again…and, since we’ve now exceeded our trim speed in a gen-
tle dive, it’ll rise a bit above level flight once again, then go back
down, come back up, etc.—a little less each time , until it’s set-
tled back down at its trim speed.
Basically, then, the trim speed, at which the airplane is stable,
could be considered a “zero point.” All the trim control does is to set
at what airspeed that zero point occurs, so you can fly the airplane
at any speed you want without constantly having to hold pressure
against the controls.
Before we leave the trim control, let’s look at the other major
factor that affects airplane trim: power. With the airplane trimmed
up straight and level once again, and without touching the controls
(except, as before, use the autopilot or little sideways nudges to
keep the wings level), pull the throttle back to around 1900 RPM.
You’d expect the airplane to slow down, wouldn’t you?
Surprise! Its initial reaction is to drop its nose and even speed up
a bit!
Why? Because the horizontal tail is right behind the pro-
peller - so the airspeed it “sees” is a combination of the airplane’s
actual forward speed and the thrust produced by the engine.
Reduce power, and there’s less air passing over the tail; thus, it
produces less downforce, and the nose comes down.
Flight Instruction
Flight Instruction
Tail Airfoil
Stable Pitch Damping
Time
Slower
Faster
Speed
Pull Up and Release