Headwinds and spotting one more view
Jim RASMUSSEN
cleared it up for me. So, here is the results of my attempt at
figuring out the effect that a headwind has on separation:
To simplify the problem, I removed the airspeed of the jumpship, as
well as the horizontal speed and vertical descent of the open canopy.
I wanted to focus solely on the effect of a headwind, so I conducted
three thought experiments involving a hot air balloon, two courageous
jumpers and the nifty graphics previously supplied.
First, have two jumpers exit a balloon on a no wind day. Each jumper
will follow the same trajectory straight down and there will be no
horizontal separation, regardless of how long the delay between jumps.
(Ignoring the forward speed of the open canopy and it's descent rate.)
| exit pt.
| _ winds
exit altitude ( )
| U 0 mph
13000 : 0 mph
12000 : 0 mph
11000 : 0 mph
10000 : 0 mph
9000 : 0 mph
8000 : 0 mph
6000 : 0 mph
5000 : 0 mph
4000 : 0 mph
3000 : 0 mph
2000 v Opening pt. Open altitude
1000
----------------------------------------------------------------------
Next, the same two jumpers exit the balloon with a constant wind of 10
mph at all altitudes. Now, both the jumpers and the balloon drift in
the wind at the same rate, so that both jumpers open directly under
the balloon. Also, jumper A continues to drift at the same rate,
under canopy, so jumper B will open right next to him. So again,
there is no horizontal separation between jumpers no matter how long
the delay between exits.
| exit exit
| pt.A pt.B
| _ _ _ wind
exit altitude ( ) drift> ( ) drift> ( ) -->
| U U U 10 mph
13000 . . 10 mph
12000 . . 10 mph
11000 . . 10 mph
10000 . . 10 mph
9000 . . 10 mph
8000 . . 10 mph
6000 . . 10 mph
5000 . . 10 mph
4000 . . 10 mph
3000 . . 10 mph
2000 A opens v - and > v B opens Open altitude
1000 drifts next to A
----------------------------------------------------------------------
Lastly, have them jump on a day when there is a wind at jump altitude
which steadily decreases with altitude until there is no wind at
opening altitude. The balloon drifts with the wind just as before,
and jumper A drifts in freefall, but not as fast as the balloon.
Jumper A is not directly beneath the balloon when he opens, and once
under canopy he does not drift at all, because there is no wind at
opening altitude. Next jumper B exits the balloon and drifts in
freefall, he follows and identical trajectory as jumper A, but will
open down wind from jumper A.
This separation is do solely to the wind gradient and has two
components. First is the separation due to the fact that jumper A was
falling through slower moving air, while the jumper B remained in the
faster moving air at jump altitude. The second component of separation
is due to the fact that jumper A was hanging under his canopy in still
air while jumper B was freefalling through moving air. These two
components added together place jumper B a certain distance down wind
of jumper A. This distance is due solely to the wind gradient
present, and the delay between jumps. The greater the wind gradient
between jump altitude and opening altitude the greater the distance
jumper B will open downwind of jumper A.
| exit exit
| pt.A pt.B
| _ _ _ wind
exit altitude ( ) drift> ( ) drift> ( ) -->
| U U U 10 mph
13000 . . 9 mph
12000 . . 8 mph
11000 7 mph
10000 . . 6 mph
9000 5 mph
8000 . . 4 mph
6000 3 mph
5000 . . 2 mph
4000 1 mph
3000 . . 0 mph
2000 note no A opens v v B opens Open altitude
1000 drift |separation|
----------------------------------------------------------------------
What does this mean for a typical jump run with the airplane flying
into the wind? I can't see how adding airspeed to the jumpship will
affect this phenomenon. So, unless someone can explain to me, the
error in my reasoning:
It seems that, if there is no wind or the wind is constant at all
altitudes, the horizontal separation is not affected. It is simply
the airspeed of the plane x the delay between exits.
Even if the wind is blowing at the same velocity as the plane's
airspeed (i.e. the plane is not moving across the ground) the
separation between groups will not be affected. This also means that
the distance the plane travels across the ground does not determine
the separation at opening altitude.
If there is a wind gradient present, the separation between groups is
decreased (remember in a gradient jumper B drifts down wind relative
to A). The greater the wind gradient the greater the separation
needed at jump altitude to allow for adequate separation at opening
altitude. So if we know the wind at opening altitude and the wind at
jump altitude, there should be a simple rule such as add X seconds for
each 10mph wind speed difference between these altitudes. Who wants
to do the math for this?
This explains the common misconception, expressed several times in
this forum, that if the plane is not moving across the ground
(airspeed = headwind), the jumpers will stack up on opening,
regardless of the delay. This situation will only occur if there is
no wind at opening altitude, and that would be one helluva wind
gradient. :)
If anyone wants to spring for several 13,000ft balloon jumps, I will
be more than happy to conduct verifying experiments. :)
I my reasoning has gone astray somewhere, please enlighten me.
Jim Rasmussen,
UW-Madison
Robert Bonitz
> This explains the common misconception, expressed several times in
> this forum, that if the plane is not moving across the ground
> (airspeed = headwind), the jumpers will stack up on opening,
> regardless of the delay. This situation will only occur if there is
> no wind at opening altitude, and that would be one helluva wind
> gradient. :)
It is not a misconception, but it is based on the assumption that the
open canopy will not continue to drift downwind. This is a practical
assumption because parachutes have forward speed and can nullify the
effect of the wind drift. Most of the viewpoints that have been presented
have been correct, but based upon different assumptions. If ground speed
is zero and the jumpers don't slide around, the opening points will be the
same if the opening altitudes are.
So Skratch's approach is a valid one. You can get separation between the
opening points by looking at your ground separation. Now how much you need
depends on how much the jumpers track towards each other plus how much
the canopies fly towards each other after opening plus your safety margin.
--
Question with boldness even the existence of God; because if there be one,
He must more approve of the homage of reason, than that of blindfold fear.
Thomas Jefferson (1743-1826)
Bill Von Novak
>It is not a misconception, but it is based on the assumption that the
>open canopy will not continue to drift downwind. This is a practical
>assumption because parachutes have forward speed and can nullify the
>effect of the wind drift.
suppose he's way upwind of the LZ?
you have to assume that (worst case) there will be one canopy
flying directly downwind from the spot and another one flying upwind.
this is what we're 'supposed' to do after we open - fly straight away
from the center until everyone opens and has their canopies under
control. effectively the 'danger area' spreads out in a big circle as
the group flies away from the center. notice that behavior under canopy
doesn't affect the size of this circle, but faster canopies make it
larger.
that's a potential problem with leaving a long time between
group - the previous group has more time to spread out, and murphy says
at least one of them is going to be spreading out towards the next
group. of course if the delay gets absurd (30 seconds) the group is
going to have time to get well below 2000 feet before the next group
shows up.
-bill von novak D16479 AFF/SL JM95
Pete Guisasola
> This explains the common misconception, expressed several times
in this forum, that if the plane is not moving across the ground
Ground speed is the only way to seperate freefall paths. For those who
are still having trouble with this concept, try this:
Get a 20" box fan.
Make a paper airplane (be sure to draw a door on it).
Attach paper airplane to front of box fan using an old wire coat
hanger. (this will be used to simulate zero ground speed)
Make little skydivers using raisins or marshmallow bits wrapped in tin
foil. (you can paint vector of javelin designs on their backs if you
want to, neon ok).
Get a pair of tweezers to hold little skydivers near airplane door.
You are now ready to conduct a series of experiments.
SIMULATION #1 - Zero ground, Zero wind speed
Do not turn on fan. Drop little skydivers from airplane. Did they
land at same spot on floor? (If not, check for bad box man position
and try again).
SIMULATION #2 - Zero ground speed, slight wind speed
Turn fan on to low. Repeat step #1. What happened? (Fall rate
adjustments may be necessary at this point).
SIMULATION #3 - Zero ground speed, big time winds
Turn fan on extra high. Repeat Steps. (I'll bet their all at the same
spot on the floor).
SIMULATION #4 - Forward ground speed, big time winds
Leave first group of skydivers on the floor and more fan five feet
forward. (this will simulate ground speed delay). Drop second group
of little skydivers. Is there seperation? How come?
If you let me know how things worked out, I'll report the results back
to the group. Maybe the USPA Board can help us find some grant money
to conduct a series of full scale tests to solve this physics mystery?
I hear theres a first class test facility at Quincy.
Blue skies,
Pete