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.
STALL PROOF? NOT EXACTLY.
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
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
WHAT CONTROLS THE SHAPE OF A POWERED PARACHUTE?
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
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.
WHAT IS A POWERED PARACHUTE 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
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.
HOW TO MAKE A POWERED PARACHUTE STALL.
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
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
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.
Quantum Parachutes, Inc.
voice: (916) 661-0524
fax: (916) 661-0528
Man can fly without motors, but not without knowledge and skill." - Wilbur