Powered Parachute Stall Primer, by Bill Gargano, Quantum Parachutes, Inc.

Bill Gargano, Quantum Parachutes, Inc.
Powered Parachute Stalls

Nancy (Quantum Parachutes, Inc.) has been monitoring the group discussion
relating to powered parachute stalls and suggested that I provide some
insight from personal experience. For those of you that don't know me, my
work with powered parachutes began in late 1982 with the first Buckeye
powered parachute wing built by GQ Security for the vehicle designed by Jack
McCornack and my work with powered parachutes has continued non-stop
throughout the years. Over those years, we (Quantum Parachutes, Inc.) have
tested powered parachute designs to their extremes. That testing has shown
us many things.

My knowledge and experience has shown me that there are two general
statements that can be made about powered parachute stalls. (1) Given proper
assembly, training, care, preflight, and use (including weather and terrain
conditions being within the vehicle and pilots ability), it is almost
impossible to stall a Buckeye powered parachute. (note that this statement
is likely true for other powered parachutes, but I do not have design
information on other powered parachute wings to be able to make that
statement) (2) All powered parachute wings are capable of stalling given the
correct conditions.

To better understand what causes a stall, we must look at what a powered
parachute wing is. It is obviously not rigid. These parachute based wings
cannot retain their complete inflated shape when not pressurized and are
therefore capable of changing shape while in-flight when aggravated to do
so. The fact that there is nothing more than air maintaining the wings
rigidity tells us that any change in air pressure, no matter how it is
caused, affects the performance of a powered parachute wing. The internal
air pressure must always be greater than the external pressure, or the wing
will stop flying. In other words, stall.

Powered parachute wings have a multitude of cells. When pressurized, each
cells three dimensional inflated shape changes based upon the total amount
of weight that is being carried (wing loading). All powered parachute wings
change shape with changes in wing loading, and therefore perform differently
when flying solo or tandem. Increases in weight lower the effective, or
flying, aspect ratio and increase leading edge drag due to changes in mouth
opening shape.

Take a look at a picture of any powered parachute wing in flight. Notice
that the wing arcs (some more than others) spanwise. This provides a large
component of the systems excellent stability. Notice that the vehicle is
well below the wing tips. This places the center-of-gravity far below the
aerodynamic center of the wing, providing a neutral, hands off, flight mode
that makes the vehicle very easy to fly and similar to a flying a parachute.
Look at the profile of the wing and you can see that it is permanently set
at one angle. This angle defines the flight envelope of the wing. Notice
that the steering system or brakes are attached to the trailing edge and
when pulled, induce drag. This is the quickest way to perform a controlled
turn or to cause dynamic changes to the system, such as a landing flare. If
you pull in both sets of brake lines far enough, the wing will stall.

By definition, a powered parachute wing has stalled when the wings internal
air pressure is equal to or less than the external pressure, and the airflow
around the wing has separated. The wing collapses, and the rate-of-descent
increases rapidly, until the wing is able to re-pressurize. Control
authority, while severely weakened in a full stall, is maintained via the
steering system. This definition describes both steady-state and dynamic

There is one other type of stall, often called a metastable stall, that can
occur with some powered parachute wing suspension line trim settings. A
powered parachute wing is in a metastable stall when the wing has been
dynamically pushed to a very high angle-of-attack relative to the center-of
gravity, and all trailing edge control inputs have been locked out. This
high angle-of-attack sets the wing slightly behind the vehicle instead of
overhead. The wing is stuck in this position resulting in a high
rate-of-descent with no steering control.

The easiest way to stall any powered parachute is to drop the engine to idle
(lighter steering line pressure); push both steering controls as far as they
will go; reach out for the lower steering lines and pull them in until the
wing stalls. Done quickly, these actions will result in a dynamic stall,
where the wing rapidly drops behind the vehicle, the upper surface of the
wing collapses, the vehicle swings back under the wing, and the
rate-of-descent rapidly increases. Pulling the steering lines in further
will cause the lower surface to collapse as well. Pulling the steering lines
in slowly will cause a steady-state stall. At the onset of a steady-state
stall you can feel the wing rock slightly aft, then forward. If at this
point you were to gently let out some of the steering line, the wing would
not go into a stall. However, if you continue to hold in the steering, or
pull in more, the wing will fall off, aft, and stall.

A powered parachute wing is affected by weather and terrain. For example, a
wind shear, or severe turbulence, can cause anything from minor disjointed
movement of the system, to a complete collapse of the wing. The severity of
the disturbance is related to your wing loading and piloting. The key to
avoiding unexpected weather and terrain induced stalls, is for the
pilot-in-command to understand the vehicle, the wing, and micro-meteorology.
If you always fly in good conditions, you are not likely to ever be pushed
into an unexpected stall. If you choose to fly in questionable conditions,
or areas, you are placing yourself (and your passenger) at risk.

Powered parachute wings want to inflate and stay inflated. When pushed into
a stall, the wing doesn't want to stay there. It wants air to re-pressurize.
To get it re-pressurized, you need to let out just enough steering line to
allow it to re-inflate. For example, if you push both steering lines to full
stroke and a stall occurs, you would then change your steering to three
quarters to one half stroke. This will allow the wing some forward velocity
to re-inflate, without giving it the dynamic ability to fly so far forward
that you would momentarily be able to see over the trailing edge. This
method also significantly reduces the altitude required for recovery and
maximizes system stability during stall recovery.

All powered parachute wings are capable of stalling. The pilot-in-command
must pay attention to wing loading, weather and terrain conditions to help
avoid entering a stall. The pilot-in-command must understand the powered
parachute system. The pilot-in-command must know and respect their own
limitations and the limitations of the powered parachute.
Bill Gargano
Quantum Parachutes, Inc.
Woodland, California
voice: (916) 661-0524
fax: (916) 661-0528
internet: bill@parachutes.com
Man can fly without motors, but not without knowledge and skill." - Wilbur
Wright, 1900