Axes of Rotation
Motion about the lateral axis or pitch is primarily
controlled by the thrust of the propeller moving the
PPC pitch up (nose up) to climb and pitch down (nose
down) at reduced throttle.
Turning happens about the longitudinal axis and is the
result of the rolling motion similar to an airplane with
aileron and rudder control. To turn, pull down the wing
trailing edge on the side you want to turn to with the
steering controls. This creates drag on the correspond-
ing trailing edge of the wing, thus dropping the inside
wing, and rolling the PPC into a banked turn. [Figure
2-18]
There is not significant turning about the vertical axis
because the PPC wing is designed to fly directly into
the relative wind just like an airplane. Any sideways
skidding or yaw is automatically corrected to fly
straight with the wing design. An airplane uses the
vertical tail to fly directly into the relative wind like a
dart. The unique design of the PPC performs the same
function through the combination of wing profile/
taper, the arch or curvature from tip to tip, washout
built into the wing and/or tip stabilizer design. These
factors make the PPC track directly into the relative
wind and eliminate the need for a vertical tail surface
and rudder to make coordinated turns. Designs and
methods vary with manufacturer and wing type, but
all PPCs are designed to track directly into the rela -
tive wind.
Ground Effect
Ground effect is the interference of the ground with
the airflow and turbulence patterns created by the
wing. The most apparent indication from ground ef -
fect is the unexpected lift given to an aircraft as it flies
close to the ground — normally during takeoffs and
landings.
Figure 2-18. PPC axes of rotation.
Ground effect is usually felt when the wing is at al -
titudes of less than half of the wingspan. The typical
PPC wingspan is approximately 38 feet with an aver-
age wing height of about 20 feet. Therefore, ground
effect is negligible for PPCs and is typically not a factor.
Moments
A body that rotates freely will turn about its cen -
ter of gravity. In aerodynamic terms for a PPC, the
mathematical value of a moment is the product of the
force times the distance from the CG (moment arm)
at which the force is applied.
Wings generally want to pitch nose down or roll for -
ward and follow the curvature of the airfoil creating a
negative pitching moment. This is one of the reasons
airplanes have tails. The powered parachute does not
need a tail because the airfoil is locked into a specific
position relative to the cart by the suspension lines.
[Figure 2-19]. Any pitching moment for the wing is
counteracted by the strong pendulum effect (weight
of the cart hanging directly under the center of lift).
Any swinging of the weight creates moments that
act to stabilize the swing. The wing aids to dampen
swinging. This pendulum effect is unique to the PPC
because the cart has the ability to rotate around the
PPC pendulum axis of rotation in addition to rotating
about the CG.
To understand the pendulum effect, attach a small
weight (a pencil or paper clip works) to a 24-inch
string. Note the weight always wants to hang directly
under where you hold it. If you hold the string still
and move the weight to the side, the weight swings
and stabilizes under where you hold it. Gravity, pull-
ing down on the weight to stabilize it directly un -
der where it is hanging, is PPC pendulum stability.
[Figure 2-20]
Figure 2-20. Pendulum effect is like a weight on a string,
stabilizing weight and line vertically over time.
Pendulum stability is the result of a number of PPC
design characteristics: there is no downward force
from a horizontal tail that must be counteracted by the
wing producing more lift; there is no weight of the tail
that must be lifted; and there is no tail to impose extra
drag on the aircraft. Gravity is the primary force that
stabilizes the aircraft using pendulum stability.
The “dynamic pendulum effect” can be demonstrated
by swinging the weight around and then stopping the
swinging to notice that the weight keeps swinging
from the momentum. The swinging weight is known
as the “dynamic pendulum effect” which will be dis -
cussed in detail later.
Thrust Line Moments
PPC designs can have different moments caused
by thrust; the propeller thrust may be above the
CG (Figure 2-21 left) or go through the CG (Figure
2-21 right). Since the cart swings free in pitch, this
CG thrust moment can slightly affect the pitch of the
cart in relation to the wing.
PPCs with an attachment point above the thrust line
have a pitching nose up moment as shown in Figure
2-21 (both diagrams), with a thrust line, wing attach
point, and arm “b.”
Gravity Moment
There is a moment arm from the CG of the cart,
to the cart/wing attachment (see “arms” in Figure
2-22). The longer the distance from the CG to the
Figure 2-19. Powered parachute airfoil is locked into
position by the lines and weight of the cart.
wing attachment point, the more stable the cart pitch
is from thrust moments. The higher the wing attach -
ment point above the CG, the more stable the cart is
from swinging underneath and the less the cart will
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 mo-
ment. [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 un -
der 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 rela -
tive 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 run -
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 inch-
es above the pavement. The POH specifies the wing
fore and aft wing attachment points to the cart. [Fig -
ure 2-24] Attaching the wing too far forward would
cause the nose wheel to be higher than it should. At -
taching 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.
Manufacturers must balance the cart stability for take-
off 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 straight-
and-level flight after encountering turbulence that dis-
rupts 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 de -
sign 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 air-
craft is designed to return to a stabilized flight condi-
tion, 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 an-
gle 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
designed to fly at an angle of attack to avoid stalls
(resulting from too high an angle of attack), and avoid
wing collapses (resulting from too low an angle of at-
tack). Each manufacturer specifically determines the
limitations so a proper angle of attack is maintained
throughout the flight operation range.
The basic design of the powered parachute is to fly
at a relatively constant speed which results in a con -
stant angle of attack. However, angle of attack can
change just as with any aircraft as when a gust of wind
Figure 2-23. High wing attachment point effects in flight.
dulum (the cart and occupants) moves forward of the
wing, the angle of attack increases, generating more
lift and more drag. The pendulum is the weight of the
CG under the wing which swings forward for this
transient situation due to pendulum effect.
Note that flare provides a temporary large increase in
the angle of attack (AOA) until the pendulum swings
back underneath the wing. This action thus returns the
wing to the normal stable flight configuration — the
cart (the total weight of the pendulum) under the cen-
ter of the wing. Therefore, a flare will only temporari-
ly add an increase to lift and drag. Once the pendulum
swings back down, the drag of the wing, and therefore
the reduced airspeed, will continue until the flare is
released.
Figure 2-26. AOA changes as wind hits the airfoil.
changes the direction the air is hitting the airfoil. [Fig-
ure 2-26]
The pilot can add weight or increase loads which may
also increase the angle of attack slightly.
Flaring Increases Angle of Attack
The flare (pulling down the trailing edges of the
wing—and thus lowering the trailing edge) increases
the angle of attack. [Figure 2-27] In a flare, the trail -
ing edges of the wing are pulled down (usually, as
both foot steering controls are pushed forward). This
is similar to lowering the flaps on an airplane: lift is
increased, drag is increased, and for a PPC, the angle
of attack is increased. The result is that the higher drag
wing slows down and thus the wing moves backward
relative to the cart. So as the total weight of the pen -
Figure 2-25. PPC stability after a side wind gust.
Porpoising Creates Variations in AOA
Another slight variation in the angle of attack is the
swinging pendulum action of the PPC when high
thrust engines provide strong and immediate full
thrust of the propeller. This extra thrust swings the
cart through the pendulum arc relative to its position
under the wing. This is why many times you will see
the PPC take off and porpoise until it stabilizes. This
is a good example of the dynamic pendulum effect.
As the propeller thrust swings the cart out front, the
cart peaks then swings back to center. The cart suc -
cessively swings back and forth, continuing to reduce
oscillations until it stabilizes in a climb. This porpois-
ing is most common with a high power engine. This
can be eliminated by using gradual throttle increases
so as not to create a dynamic pendulum effect enter -
ing a climb.
Stalls: Exceeding the Critical Angle
of Attack
The critical angle of attack is the angle of attack at
which a wing stalls regardless of airspeed, flight at -
titude, or weight. The drawings in Figure 2-28 show
airflow over a typical rectangular PPC wing. The first
shows a laminar, smooth, lift-generating airflow—one
that is typical when the angle of attack is within the
flight range. The second depicts an exceeded angle
of attack, turbulence and loss of the lifting force.
[Figure 2-28]
Figure 2-28. Wings stall due to an excessive angle of attack.
Figure 2-27. Flare on landing typically increases the angle
of attack.
Unlike a fixed-wing aircraft that takes constant aware-
ness of angle of attack to prevent a stall, the powered
parachute wing is designed by the manufacturers to
maintain a specified range of angle of attack and air -
speeds. It is resistant to stalls because for all practical
purposes, it is designed to fly at a constant normal op-
erating range. This range is maintained if the operator
flies within the operating limitations specified in the
POH. Flying the PPC within the limitations specified
in the POH and avoiding turbulence means you will
not exceed the critical angle of attack and stall the
wing.
However, situations that could contribute to a stall are:
• A large increase in wing drag (full-flare)
— which the PPC pilot controls by pulling the
wing back, thus increasing the AOA. (Note: A
full-flare is normally used and recommended
only for landings.)
• A quick full RPM throttle input, creating a
climbing dynamic pendulum effect loading the
wing.
• A quick reduction of throttle during a high pitch
angle climb. This quickly turns a high pitch
climb into a high angle of attack. The wing is
initially pitched high, climbing the inclined
plane under full power, then quickly changes
to a gliding flight path when the throttle is
reduced, just like an airplane.
• A wind gust from flying in turbulent air.
To prevent a stall, do not go to full-throttle while hold-
ing a full-flare, or as specified in the POH. Note: For
explanation of a stall recovery, see Chapter 12: Night,
Abnormal, and Emergency Procedures.
through improper CG balance of the PPC. Rotating
propeller gyroscopic action can also produce turning
tendencies if a force is applied which would deflect
the propeller from its existing plane of rotation.
There is no corkscrew effect of the slipstream on a
PPC because it does not have a tail in the propeller
prop blast.
Weight, Load and Speed Changes
Greater load factor creates increases in speed. For un-
accelerated and stabilized flight there are only slight
variations in speed for different flight conditions.
[Figure 2-29] For an 800-pound PPC in gliding flight,
both the lift and drag components support the weight
of the PPC (lift is 759 pounds, and drag is 252 pounds,
with the resultant 800 pounds vertical force).
In level flight, the PPC does not have the vertical
component of total drag to support the weight so ad -
ditional lift must be generated through speed or angle
of attack increases.
Figure 2-29. Forces differ between gliding and level flight.
Turning Effect
Torque is a reaction to the mass of the turning propel-
ler. If the propeller is turning to the right, the reaction
is for the cart to want to turn to the left. Therefore, a
right turn is sometimes designed into the PPC system
to counteract the torque.
PPC manufacturers compensate for this with various
designs:
1. Dual (counter-spinning) propellers. This is an
ideal way to counter prop torque, but counter-
rotating gearboxes are complicated, more
expensive, and weigh more.
2. Different riser lengths. On a clockwise-turning
propeller, the left riser is longer than the right.
3. Swivel the wing attachment on a tilt bar above
the cart.
4. Adjust the PPC frame (longer on the left, shorter
on the right) to compensate for the engine torque
(for a clockwise spinning prop).
Additionally, P-factor, as discussed in the Pilot’ s
Handbook of Aeronautical Knowledge can be a factor,
producing a left turn if the nose of the cart is too high
The numbers are small and difficult to measure or
feel so the industry rule of thumb is to assume the
PPC flies at a constant speed and angle of attack with
changes in throttle. However, increased load (weight)
and load factor has a significant effect on speed. [Fig-
ure 2-30]
As discussed in Chapter 2 of the Pilot’ s Handbook of
Aeronautical Knowledge the following points apply
to powered parachute operations:
• During straight and level flight, the lift opposes
the weight.
• In a banked turn, the lift is no longer directly
opposite the weight; it is losing some of the
vertical component of lift. More lift is needed
so the vertical component of lift will equal the
weight.
• In a banked turn, lift must be increased and load
factor goes up. [Figure 2-30]
In a 45-degree turn, the 800 pounds weight would
equal 800 times 1.4 or 1,120 pounds total load. This
45-degree turn, 1.4 G loading is the same as adding
320 pounds of weight to the 800 pounds. This new
1,120 pounds of lift must be produced by the wing by
increased speed and/or angle of attack. If all the addi-
tional lift is produced from airspeed, airspeed would
increase about 5 MPH, a noticeable difference. Refer-
ence your Pilot’ s Operating Handbook to understand
the specific design considerations for the aircraft you
will be flying.
PPC Aerodynamics Summary
The following summarizes PPC aerodynamics:
• Increasing throttle causes the PPC to climb,
decreasing the throttle allows the PPC to
descend.
• The PPC flies at a relatively constant airspeed
and angle of attack in normal flying conditions.
• The weight of the PPC controls steady state
airspeed under similar conditions of wing size
and type, trim, steering line settings, and pilot
flare. Increased weight or load factor increases
speed and/or angle of attack.
• The lighter the weight, the slower the airspeed,
the less control pressure on the wing, and the
slower your descent.
• The heavier the cart (total weight under the
wing), the faster the airspeed, the more effort
required for maneuvers (foot steering pressure),
and the faster your descent.
• The wing set (rectangular or elliptical), size,
and cart weight have a strong influence on PPC
performance and maneuverability.
Figure 2-30. Loads and turning forces.