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Few issues in parachute design have met with as much controversy as Airlocks.
Simply put, Airlocks are one-way valves in the leading edge of a ram-air canopy
that maintain the internal pressurization. The intent of such valve parachutes
is to prohibit instantaneous deflation of the airfoil, thus increasing the safety
margin when flying in rough conditions. Although the author stresses that most
of the ram-air canopies available today are very safe, the purpose of this article
is to look toward the future of the ram air canopy, and the role that airlocks may
play in this evolution.
A discussion of the empirical and theoretical issues relating to valve parachutes.
By Brian S. Germain
June 3, 2001
Origin of the Idea
When I was spiraling toward the earth with an 80% collapse, there was not
enough inflated nylon over my head to slow my descent, or allow me to recover
prior to impact. The landing put me in a wheelchair for many months, leading me
to take serious interest in the valve idea. As I explored the many variations of
airlock ideas, I soon realized that the game was fairly simple: “Gather as much
pressure as possible, and seal the wing off when the air tries to escape.” Hence,
the “Germain Airlock” allows the air to enter the wing in its normal pathway,
sealing against the top-surface when necessary.
Despite the availability of airlock designs since 1994, more than eighty five
percent of the canopies sold are still devoid of this invention. This may be due
to a lack of information about the advantages of the airlock design. Many jumpers
still don’t know exactly what an airlock is. Others believe that airlocks are just
another unnecessary gimmick. Supporting this counter-airlock philosophy is the
belief that “even cell pressurization” is all that is necessary to maintain a
parachute’s stability. In order to look closer at this perspective, we must first
define what is meant by the term “stability”.
First there is what may be termed “Primary Stability”. This is a ram-air
canopy’s ability to remain in control and under adequate line tension during normal
flying conditions. Primary stability is generally attributed to the airfoil shape
and the line trim configuration. This is the result of painstaking testing and
adaptation, as generalities are often disproved when applied to different designs.
Primary stability includes everything from slow flight to accelerated flight modes
as well as pilot-induced airfoil distortions such a front-riser input.
This being said, the feeling of an airlock parachute in bumpy air is distinctly
different from that of conventional designs. Since the air pressure maintaining the
shape of the wing is relatively constant in an airlock parachute, the wing tends to
distort and move around less than “open-cell” designs. This is very noticeable when
flying in turbulence, during which an airlocked wing generally does not encounter
significant spanwise oscillations, also called the “accordion-effect”.
The bottom line in primary stability is, however, “does the canopy stay at the end
of the lines?” Some parachutes clearly do not, regardless of whether or not they have
airlocks. If you pull on the front risers on a canopy that has sufficiently long
brake lines and it bucks wildly or the leading edge bends under, it has bad primary
stability, at least in that configuration. In other words, you can’t just sew
airlocks into any parachute and expect it to be better in every way. It’s not that
“Secondary Stability” is the term that can be applied to how a parachute responds
to catastrophic situations such as a bent wing or full-frontal collapse. In some
situations, properly designed airlock parachutes can make a difference, keeping the
wing inflated and ready to resume stable flight. If, however, the unstable air is
encountered very close to the ground, airlocks may not be enough. In fact, one must
realize, there are atmospheric circumstances in which a metal wing may not be enough.
The philosophy of the valved parachute design is that a wing that has air in it can
reduce the risks of total surface area loss, but not eliminate the possibility of
serious injury. Sometimes no amount of preparation or technology is enough.
Several limitations are handed to the parachute designer that other aeronautical
engineers need not consider. One such challenge concerns the suspension lines. Your
suspension system is flexible, allowing for line-twists and line-slack. A canopy
locked into an asymmetric configuration will most certainly exhibit an extreme spiral
accompanied by a rapid loss of altitude. Line-slack due to improper pitch-control or
aggressive maneuvering can also create unknown variables that can lead to accidents.
Maintaining line tension on any parachute is essential for minimizing the risks of
catastrophic failure. We minimize the risk of slack lines by executing smooth
control inputs, and smooth release of input as well. Modern ram air canopies require
much more than just pulling toggles, we are required to fly them.
Another limitation of the ram-air canopy is its ability to handle significant
vertical air movement. A strong downdraft can increase your vertical speed beyond
your canopy’s ability to climb. Remember; even a rigid-wing airplane can crash in
severe turbulence. There will always be a time for wise pilots to ground themselves.
The old saying goes: “It’s better to be on the ground wishing you were in the air,
than in the air, wishing you were on the ground”.
All things considered, however, it makes a lot of sense that a parachute capable
of maintaining its internal pressurization and surface area increases your chances
of surviving. The most elegant, common sense ideas need little justification.
Airlocks certainly fall into this category. We at Big Air believe that a ram air
canopy is not complete without airlocks.
“A Canopy is simply Air shaped into a wing...
No Air... No Wing.”
...One must consider the fundamental definition of a “cell”. In biology, a cell
is the smallest component of life as we know it. It is a self-contained entity,
perhaps existing in cooperation with others, which defines itself in terms of its
cell wall or cellular membrane… a little bubble of energy. A living cell defends
itself from outside forces, and therefore perpetuates its own existence. This is
life at its simplest. A dead cell has no complete barrier to the outside world. It
allows its internal volume to ebb and flow, amorphically maintaining equilibrium
with the outside world. By airlocking a parachute, we give it a life of its own,
and it will fight to stay alive.
Do airlocks increase the safety of the ram-air parachute? In most cases, yes.
It’s very similar to wearing a seatbelt on take-off. There are no guarantees
that we will walk away from a crash, but I still always wear one. Many skydivers
will continue to take a “wait and see” approach to airlocks, and this conservatism
clearly has a strong component of sanity. Nevertheless, six years and over 2000
airlock canopies certainly are a compelling amount of evidence...