wierd canopies
Jim Curl
going bad on the ride down. I was assured by response postings that this
was a very unlikely occurance provided I did not intentionally stall my
canopy at a low altitude. I ran across the following incident report in
"Parachutist" which has rekindled my fears somewhat:
Age: 28 Sex: F
Number of jumps: 452
Time in sport: 5 years
System: Annex Mirage
Main: Performance Designs PD-170
Reserve: Pioneer K-XX
Type of injury: concussion, fractured cheek, deep laceration of eye
Description of incident:
The jumper was making her approach to the landing area when at about
30 feet the top skin of her canopy "rippled" and the right side rolled
underneath, pivoting the jumper into the ground. Jumper was unconscious
for 10 minutes with breathing stopped for about two minutes until CPR was
administered. Surface winds were reported at 0-3 mph, and no turbulance
had been experienced by others. The injured jumper had made one previous
jump that day, and had about 100 jumps on this canopy.
Conclusions:
The reporter stated that there was little in the area to create
turbulance that may have contributed to this accident, and that the
jumper was not in a turn when this mishap occurred. There were no
listed canopy defects, and although a statement by the reporter
indicates he felt this to be a factor, a conclusion cannot be drawn
without submission of more data. It should be mentioned here also,
that turbulence may occur without apparent cause; it may have begun
some distance away. Also, more data on microbursts and wind shear
becomes available as more funds are provided for research; eventually
we my all learn more about why our canopies "weird out" from time
to time.
If this can happen so easily (albeit rarely) to an experienced jumper,
familiar with his/her rig, should I not be concerned as I am a beginner
using rented gear (often a slightly different rig for each jump)? Or
maybe that's just life in the fast lane...
-- Jim Curl
-- San Jose
kas
I think the remarks made in the "Conclusions" section of that report said
it best. There wasn't enough data presented to ascertain the cause of that
accident. As I said before, it is physically impossible for the canopy to
"weird out" totally on its own. It MUST be acted on by some external force
for it to do what it did. Downdrafts, wind gusts from behind, too deep in the
brakes, turbulence, following too close behind another canopy, etc., etc., all
could be factors in a situation like that, whether or not the canopy was in
a turn at the time. My point is, it is imperative to understand and recognize
all the things that COULD go wrong, and then make sure you don't get yourself
into a situation that invites disaster. The best hedge against such problems
is AIRSPEED. The faster the canopy is going relative to the air, the less
likely it is to be affected by one of those gremlins. That's why it is so
important to make your final approach at nearly full airspeed, and flare only
when you are 3 or 4 seconds from touchdown. (Canopies vary, so the time may
vary. You should try to use the same type of canopy on each jump, if possible,
and learn how to fly it well.) Whatever you do, DON'T go into deep brakes
when you are below 100 feet or so, especially if you are landing in an area
that has objects large enough to cause turbulence nearby. One of the paradoxes
that we skydivers must deal with is the fact that most turbulence occurs near
the ground, where we want it the least. Cars, buildings, trees, parked planes,
any kind of "bump" on the ground will create turbulence as the breeze passes
over or around it. Plowed fields and concrete runways will likely cause large
updrafts on a sunny day, but an adjacent green field is likely to have a
corresponding downdraft.
Again, the key is airspeed; so stay outta those brakes near the ground,
OK? I usually approach the target at about 1/4 brakes, and flare when I am
3 to 4 seconds from touchdown. It takes practice to judge that well, but you
will get a feel for it as you gain experience. I would suggest you do the
following: On your next jump, when you still have plenty of altitude, go to
zero brakes and let the canopy build up full speed for a few seconds. Then,
pull the toggles all the way down and begin counting "1000, 2000, 3000...",
until the canopy stalls. Recover from the stall and repeat the procedure
from 1/2 brakes, 3/4 brakes, and full brakes, and you'll find how long it
takes for a stall to occur from each brake setting. At 3/4 and full brakes,
you'll find that the stall occurs almost instantly. That is why you don't
want to be that deep in the brakes near the ground!
Happy, and soft, landings!...
*
/ \
|---/---\---| Ken Scofield C-9355
| Gone | Hewlett-Packard PCD
| Jumpin' | Corvallis, OR
|-----------|
{ucbvax!hplabs, harpo, ogcvax}!hp-pcd!kas
Paul Fries
>
> I think the remarks made in the "Conclusions" section of that report said
>it best. There wasn't enough data presented to ascertain the cause of that
>accident. As I said before, it is physically impossible for the canopy to
>"weird out" totally on its own. It MUST be acted on by some external force
>for it to do what it did. Downdrafts, wind gusts from behind, too deep in the
>brakes, turbulence, following too close behind another canopy, etc., etc., all
>could be factors in a situation like that, whether or not the canopy was in
>a turn at the time. My point is, it is imperative to understand and recognize
>all the things that COULD go wrong, and then make sure you don't get yourself
>into a situation that invites disaster.
This sounds fine to me. Recognition of the dangerous possibilities and
dealing with them BEFORE they become disasters is the key. However, I
must take (small) exception to the following:
>The best hedge against such problems is AIRSPEED. The faster the canopy is
>going relative to the air, the less likely it is to be affected by one of
>those gremlins. That's why it is so important to make your final approach
>at nearly full airspeed, and flare only when you are 3 or 4 seconds from
>touchdown. (Canopies vary, so the time may vary.
The flight manuals for my canopies (CRUISLITE, RAVEN IV, RAVEN III) all
call for reduced airspeed (not to exceed 1/2 brakes) through turbulence.
This is so that the canopy is NOT subjected to so radically different
forces over too short period of time. Turbulence, after all, is simply
air that is moving in a different direction, which in turn alters the
relative pressures found within the canopy. When the canopy is subjected
to varying forces too quickly, the airfoil will be disturbed. Slowing
the canopy allows it to adjust and hence, stay more stable. There are
some things we simply have to accept involved here. As the canopy has
no power of its own, running into a downdraft is going to increase the
rate-of-descent. There is nothing we can do about that. The best we
can hope for is to keep the canopy fully inflated to get the most we can
from it.
Ideally, we SHOULD approach the ground with the highest possible airspeed.
The "flare" that we use during landing requires airspeed to happen. What
we are actually doing is increasing the angle of attack of the airfoil to
trade airspeed for lift. As the brakes are applied, you pendulum forward
under the canopy. The canopy pitches up, which is actually a rotation
about the center of lift. The airspeed drops rapidly, but we get that
momentary dramatic increase in lift, hence, the reduced rate-of-descent.
Again, as the canopy has no power of its own, we can only get away with
this once when landing (the only way to get airspeed back is to let it
sink for a while), so the timing is critical (as we well know).
So overall, the "correct" airspeed is one that will keep the canopy fully
pressurized, while still having enough speed for the flare. Of course,
there will be times when keeping the canopy inflated will mean that you
will NOT be able to get a full flare, so we should be ready (and willing)
to do a good PLF.
Anyway, thanks for all the good comments! I am sure that the novice will
be better off for them.
Paul Fries
C-17147