User Guide

This time, while the spiraling propeller slipstream continues
to play a role, there’s another force: the notorious “P-factor.” This
is not to be confused with the distress pilots feel when they’ve
been flying for several hours after drinking too much coffee before
takeoff.
Take a look at this sketch to see what’s happening. When the
nose of the airplane is pointed up, the propeller isn’t just spinning
in a vertical plane. The downgoing blade (the right one for
American engines) is also moving forward, in the direction of
flight, while the upgoing (left) one is moving backward. Thus, the
right blade “sees” a higher airspeed, and takes a bigger “bite” out
of the air, than the left one.
This, in turn, means that the right side of the propeller is
doing more work, pulling harder, so its center of effort - the appar-
ent point at which it’s pulling - is no longer in line with the pro-
peller hub. Instead, it’s displaced some distance to the right (typi-
cally, up to half the blade length), thus pulling the nose of the air-
plane off to the left. In a steep, low-speed, high-power climb,
you’ll have to hold a good deal of right rudder pressure to keep
the airplane straight.
By the way, as you might expect, all Russian and many other
European engines turn the other way - and, sure enough, you need
a heavy left foot in their airplanes.
92
93
Now shove the throttle wide open. The nose comes up - and
while the airplane will ultimately settle down near its formerly
trimmed speed, it’ll first go a bit below it, for the same reason.
What if the tail weren’t right behind the propeller(s)? Sure
enough: airplanes with T-tails have much less trim response to
power changes.
CLIMBS AND DESCENTS
These little trim exercises lead logically into the next two
fundamentals: climbs and descents.
There are two ways to make the airplane go up and down,
and they can be used together or separately. Changing the pitch
attitude (within reason, of course), simply makes the airplane go
“uphill” or “downhill.” If you leave the power alone during such
changes, you’ll see the same reaction as if you were to drive a car
on a hilly road with the gas pedal locked in one position: it’ll slow
up going uphill, and go faster on the downhill stretches. Try it!
You can also adjust the power, as we did in the last trim
exercise. If you leave the pitch trim alone, and don’t use any con-
trol pressures, the results are also predictable: reduce power, and
the airplane will go downhill (possibly speeding up a bit as well);
add power, and it’ll climb, possibly slowing.
In the real world, of course, you use both controls at once. To
climb, raise the nose to get the airplane to an efficient climb speed
(70 to 80 knots works well in the 172) and adjust the power to get
the rate of climb you want. In a real-world 172, particularly if you
have a couple of buddies along on a warm day, you’ll typically use
full throttle and accept whatever rate of climb you can achieve—
“She’s givin’ ye all she’s got, cap’n!” To descend, set your pitch atti-
tude for the desired airspeed (typically cruise speed or a bit more),
then set power to achieve the desired rate of descent without
exceeding the redline RPM.
Here’s a fine point: just as on takeoff, you’ll notice that as
you add power and pull the nose up for a climb, the airplane
wants to veer to the left.
Flight Instruction
Flight Instruction
Downgoing blade takes
bigger bite.
Center of effort is displaced
to the right.
P-Factor
Top View
Side View