Bill Von Novak - Just Figure It Out

From: billvon
Newsgroups: rec.skydiving
Subject: Re: Exit Order Survey
Date: Fri, 09 Jun 2000 16:19:15 GMT

> (reaction to a bunch of previous posts deleted - skr)

> first, and you keep telling people to "use your head."
> Please: tell me how to use my head to make my day safer.
> Please be as specific in your analysis as is Brian Burke.

       how about these options:

       actually figure it out.  it's not hard.  here's how to do it:

       figure out the speeds involved.  the pilot will probably be able
to get an upper-winds report, and the airplane's airspeed indicator is
right there.  after the first jump, the estimate will likely be
adjusted, when the spotter realizes that there is not as much push from
the north as the forecast predicted.

       figure out your ground speed.  when flying into the wind,
subtract the uppers from the plane's airspeed.  a plane's altimeter
reads lower as you climb, so you may want to add a 20% fudge factor to
the plane's speed.  as a more accurate alternative, check the
groundspeed display on the plane's GPS (if so equipped.)  as an example,
if the plane's jump run is 80 kts, and the winds are 20 kts, the
groundspeed is 60 kts.

       figure out what horizontal separation you want.  1000 feet is
pretty good, unless you have a high breaking big way or a novice
freeflyer (they can be all over the place.)

       figure out the time between groups.  1 knot=1.7 fps, so at 60
knots, you're covering about 100 feet per second.  that means you need
10 seconds between groups to get 1000 feet of separation at that speed.

       take opening winds into account if required.  the above example
assumes light winds at opening.  strong headwinds at opening altitude
help separation, strong tailwinds hurt it, crosswinds don't do much
(though they have to be factored into the spot.)

       take fall rate differences into account.  worst case is the same
wind all the way to opening altitude where they suddenly stop, so assume
that.  if you have a fast freefaller who gets to opening altitude 20
seconds faster than a belly flyer, and the winds at altitude are 20 kts,
the belly flyers will get blown downwind (20 secs * 20 kts * 1.7) = 700
feet or so.  if they're following a VRW group, you have to leave
_another_ 8-10 seconds between them and the previous group.  you don't
need to do this if VRW is out after RW.

       take throw into account.  a jumper who launches a perfect
head-down will 'coast' a little longer along line of flight than a belly
flyer who also gets a perfect launch.  this can add a hundred feet or so
to their position.  since both groups may funnel (and thus mess up any
possible benefit or drawback of throw) you probably just want to add
another second of fudge factor.

       take jumper experience into account.  a low time solo RW or a low
time solo or two-way head down may end up tracking or backsliding all
over the sky.  this can be ameliorated by leaving still more time
between groups, or by asking them to go out last and pull higher.  (low
timers are often planning to pull high anyway.)  OTOH lower experienced
groups often take longer in the door, and this helps negate this.

       do i actually do all of this?  sort of.  i remember that
60kts=100fps, and have that be my norm, since that's our typical jump
run speed.  'normal' groups (i.e. 4-ways) get 1000 feet separation,
solos less, bigways more.  for every 10kts slower i add two seconds.  i
don't correct for opening winds unless they are opposite jump run
(unusual.)  i don't usually correct for differences in fall rate since
we put VRW out last.  and if i'm worried about something (like a slidy
freeflyer) i add more time.

       that's one option.  another good one - the one most skydivers use
- is to just ask someone else, usually the spotter.  as always, you have
to make sure that the guy you ask has both the knowledge and experience
to give a good answer.

       the skratch method - waiting until the plane covers 1000 feet
over the ground, then going - works for RW-then-VRW, but not the other
way around.  also, you have to add more time if opening winds are
opposite uppers.

       the ultimate solution is two passes, one for VRW and one for RW.
a 360 works well since most people from the first pass will be near or
on the ground by the time the plane turns.  a 180 also works, with an
offset from the original jump run.  this works well since it's easier to
fly a racetrack turn than a teardrop.

-bill von