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pitch up when thrust is applied.
A higher hang point will also better stabilize the PPC
cart in turbulence because the moment of the weight
and the larger “d” arm creates a larger stabilizing moment.
[Figure 2-22]
For ground operations, the moment arm distance “c”
cannot be too great or the front wheel would lift off
the ground prematurely, trying to inflate the wing (see
right side of Figure 2-22).
During flight, the advantage of a high attachment
point arm “c” is less swinging around of the cart under
the wing, and less cart “pitch up” when throttle is
applied to climb. This high attachment point creates
thrust that is now pointed slightly down to the relative
wind, which has two significant effects. First, it
creates a negative P-factor, counteracting increased
torque. The second effect is a disadvantage: less climb
rate or more thrust required for the same climb rate.
This is due to the increased load on the wing from the
thrust, requiring more speed and/or angle of attack to
lift the total load. [Figure 2-23]
Wing Attachment to Cart
If flying straight and level over a perfectly flat landing
strip, then with the rear wheels 1 inch above the run2-
11
Figure 2-22. Gravity stabilizes thrust moment.
Figure 2-21. PPC designs can have different moments caused by thrust; the propeller thrust may be above the CG (left)
or go through the CG (right). A PPC can have wing attachment thrust moments as shown, or no wing attachment thrust
moment if the thrust line goes through the wing attachment point (not shown).
way, the nose wheel should be between 7 and 11 inches
above the pavement. The POH specifies the wing
fore and aft wing attachment points to the cart. [Figure
2-24] Attaching the wing too far forward would
cause the nose wheel to be higher than it should. Attaching
the wing too far back would place the nose
wheel too low, where it would hit first for landings.
Balancing the cart properly per the POH is important
to make sure the cart is hanging properly under the
wing and ensure thrust is properly aligned as designed
by the manufacturer. Nose wheel low means thrust
pointing down, increasing loads, and airspeed. Nose
wheel high has the opposite effect. Thrust aligned too
high or too low results in reduced thrust and unwanted
P-factor.
2-12
Manufacturers must balance the cart stability for takeoff
and flight, along with the thrust moment to achieve
the best design for the specific application.
Stability
A stable aircraft is one that will routinely return to its
original attitude after it has been disturbed from this
condition; usually this means returning to straightand-
level flight after encountering turbulence that disrupts
a normal flightpath. The more stable the aircraft,
the easier it is to return to a straight and level position.
The natural tendency of the pendulum — the PPC cart
hanging under the wing — is to return to its original
centered position under the wing. The pendulum design
gives the PPC airborne positive dynamic stability
and positive static stability for roll and pitch because
the weight of the pendulum wants to return the PPC
to level stabilized flight. No matter what maneuver
within the POH limitations the PPC is put through
(regardless of whether it is pilot induced or turbulence
created), as soon as the disruptive force stops, the aircraft
is designed to return to a stabilized flight condition,
with virtually no pilot input. Figure 2-25 shows
the movements of the PPC as it auto-corrects from a
side gust of wind.
Figure 2-24. Straight and level flight with the nose wheel 7
to 11 inches above the horizontal flight path.
PPC Angle of Attack Characteristics
Normal Flying Conditions
For all practical purposes, the wing’s lift in a steady
state normal climb is the same as it is in a steady level
flight at the same airspeed. Though the flightpath has
changed when the climb has been established, the angle
of attack of the wing with respect to the inclined
flightpath reverts to practically the same value, as
does the lift. The angle of attack remains relatively
constant for constant weights during stabilized flight
for glide, level cruise or climb. However, wind gusts,
flying in turbulence, quick uncoordinated flight (as
covered later), or aerobatic maneuvers can change the
PPC angle of attack. PPC limitations in the POH are
specifically written to avoid any maneuver that would
temporarily get the PPC into a situation of too high
or too low an angle of attack. The PPC is specifically
　
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