curl@wjvax.UUCP (Jim Curl) (01/21/86)
I began jumping last year and made 16 jumps before a variety of
circumstances interrupted my training. I really enjoyed skydiving and
I fully intend to return to the sport this year. There is one aspect
of jumping which bothers me though.
Although I *loved* freefalling, once I was under canopy I tended
to get nervous. The old pros at the drop zone would tell me not to worry
about it when I had an end cell closure or when the wind knocked the
canopy around a little, but I couldn't help imagining it collapsing
with 100 feet to go. Once while making a 180 degree turn downwind, the
wind gusted and the canopy appeared to be shrivelling up. I was still
on radio at the time, and the instructor yelled at me to turn back into
the wind. I also ran across someone who, on her second jump, stalled
her canopy on final approach and broke her leg.
Am I crazy for having "gear-fear" *after* I've got a good canopy,
or are there some real malfunctions that can occur on the ride down?
--- Jim Curl
--- San Jose, Californiakas@hp-pcd.UUCP (kas) (01/23/86)
> Although I *loved* freefalling, once I was under canopy I tended >to get nervous. The old pros at the drop zone would tell me not to worry >about it when I had an end cell closure or when the wind knocked the >canopy around a little, but I couldn't help imagining it collapsing >with 100 feet to go. Once while making a 180 degree turn downwind, the >wind gusted and the canopy appeared to be shrivelling up. I was still >on radio at the time, and the instructor yelled at me to turn back into >the wind. I also ran across someone who, on her second jump, stalled >her canopy on final approach and broke her leg. > Am I crazy for having "gear-fear" *after* I've got a good canopy, >or are there some real malfunctions that can occur on the ride down? Calm your fears. As a mechanical engineering student, I had to take several classes which delt with forces and stresses, aerodynamics, physics and so forth. I can assure you that the canopy CANNOT collapse on its' own. In fact, if it were made of rubber, it would try to stretch itself even larger. Remember though, the canopy is an airfoil. Nothing more, nothing less. You, as the pilot, can MAKE it collapse if you want to, by stalling it. But the instant you let up on the toggles, it will begin to reinflate. Guaranteed. It will take a few seconds, however, for the canopy to reinflate and dampen out the resulting oscillations. That is why you NEVER NEVER stall a canopy at less than 500' off the deck. When you "flare" the canopy to land, you are stalling it. The trick is to do this so that the canopy just stalls at the instant your feet hit the ground. The girl you mentioned who broke her leg obviously stalled, or "flared", a little too soon. You're better off to flare a little late than a little early! As far as your canopy shrivelling when you made a 180 deg. turn, that can occur momentarily if you make a FAST turn, because you are essentially stalling one side of the canopy. As soon as you ease up on the toggle, that side of the canopy will reinflate very rapidly and fly normally again. It makes absolutely NO DIFFERENCE whether you do this upwind, downwind, or crosswind. The results are the same. This is because the canopy is an airfoil, and is only affected by the RELATIVE velocity between itself and the air mass it's flying through. The velocity of the air mass relative to the ground has utterly nothing to do with how well the canopy flies. Thus, if a gust of wind hits the canopy from behind (as would be the case if you were flying downwind), the canopy momentarily "sees" a much lower, or even negative, relative velocity. So, it may lose lift and drop a little, but if left alone it will simply speed up to match the new wind speed and will fly just fine again. Needless to say, you don't want to be in a downwind orientation near the ground -- but then, you shouldn't be anyway. That's why you were taught to always land upwind. That way, a gust will hit you head-on, and will actually increase lift momentarily. Because of this business of relative velocities, it is wise to make slow turns if you are in gusty conditions -- especially if you are less than 500' up. If you are making a hard turn, or if you are in deep brakes, a gust from behind will very likely stall all or part of the canopy. So, in these conditions keep your toggles in the upper half of their range. In summary, don't worry about the canopy collapsing for no apparent reason, but DO worry about how you fly it! Get used to it by doing intentional stalls at a safe altitude (over 2000') so that you KNOW what things feel and sound like just before a stall. Try some stall-turns, too. The ride will scare the hell out of you, but the canopy WILL reopen! Blue Skies and Happy Stalls, * / \ |---/---\---| Ken Scofield C-9355 | Gone | Hewlett-Packard PCD | Jumpin' | Corvallis, OR |-----------| {ucbvax!hplabs, harpo, ogcvax}!hp-pcd!kas