A snippet of C, deployment speeds.

[remko 0]

When the weather sucks there is time to play with some numbers.

At the WR I figured why not drop in a descent, since the formation is sinking with about 1300'/min. Hard openings was the answer. I couldn't believe a little descent (1000'/min = 5 kts) would make that much of a difference but the answer is not as easy as I thought. Horizontally the jumper decelerates due to drag. Vertically the jumper accelerates due to gravity minus the drag. Drag depends on speed. So I wrote this little snippet of C to figure out deployment speeds. Depending a little bit on the numbers you feed it (jumpers mass, dragcoefficient and altitude) I came up with the following:

Plane flying at 13.000 feet with 78 kts, standard atmosphere.

Level:
At 3.0 sec delay the deployment speed is 75.9 kst.
The lowest speed is found at 1.5 sec: 70.3 kts.

500'/min:
At 3.0 sec delay the deployment speed is 78.4 kts. This equals to a delay of 3.4 sec flying level.
The lowest speed is found at 1.3 sec: 72.0 kts.

1000'/min:
At 3.0 sec delay the deployment speed is 80.8 This equals to a delay of 3.8 sec flying level.
he lowest speed is found at 1.1 sec: 73.5 kts.

500'/min climb:
At 3.0 sec delay the deployment speed is 73. kts.
The lowest speed is found at 1.6 sec: 68.4 kts.

1000'/min climb:
At 3.0 sec delay the deployment speed is 70.5
The lowest speed is found at 1.7 sec: 66.4 kts.

#include 

#include 


main()

{

const double    Na, R, g, mmt, p0, T0, y, kts, ft;

double          Cd, S, m, h, a, ay, vx, vy, v, t, dt,\

W, M, T, p, rho, dx, dy;


Na = 6.0221E+23;        /* constante van Avogadro       */

R = 8.3143;             /* gasconstante                 */

g = -9.8;               /* gravitatie constante         */

mmt = 4.80793E-26;      /* molecuulmassa troposfeer     */

p0 = 101325.0;          /* standaard druk               */

T0 = 273.15 + 15.0;     /* standaard temperetuur        */

y = -.0065;             /* temperatuur gradient         */

kts = 3.6 / 1.852;      /* meter/seconde => knopen      */

ft = 1 / .3048;         /* meters => voeten             */


Cd = 1.2;               /* weerstands coefficient       */

S = .6;                 /* frontaal oppervlak           */

m = 85.0;               /* massa lichaam                */

h = 4000.0;             /* afpringhoogte in meters      */

dt = .1;                /* delta tijd                   */

v = 40.0;               /* totale snelheid              */

vy = -0.0;              /* vertikale snelheid           */

vx = sqrt( v * v - vy * vy );   /* horizontale snelheid */


W = (Cd * .5 * S) / m;  /* weerstands variabele         */

M = Na * mmt;           /* molaire massa troposfeer     */


for( t = 0.0; t <= 5.0; t += dt ) {

T = T0 + y * h;

p = p0 * exp( M * g * h / R / T);

rho = p * M / R / T;

dx = W * rho * vx * vx;

dy = W * rho * vy * vy;

ay = dy + g;

a = sqrt( dx * dx + ay * ay );

printf( "%9.1f %9.1f %9.1f %9.1f %9.0f %9.1f %9.1f %9.1f\n",\

t, vx*kts, vy*kts, v*kts, (4000-h)*ft, dx, ay, a );

vx += -dx * dt;

vy += ay * dt;

v = sqrt( vx * vx + vy * vy );

h += vy * dt;

}

}

[scotts 0]

Huh???

My mind is now screwed for the day.

[dragon2 2]

Remko I'm not at work I don't wanna see code go away!

I'll try this when I'm back at work, luckily I don't have a C compiler at home! (Hmmm I do actually have Cobol lying around here somewhere heh heh)

But the point was, 'ouch'? Could've told ya that!

ciel bleu,
Saskia

[CRWCheryl 0]

THank you Remko! We had been having a conversation about opening shock.

I had complained of hard openings, even seeing stars, jumping out of the otter at the record at Florida. In fact, I was surprised that my openings hurt more jumping out of an otter than a casa.

He had mentioned that he had good openings when the plane was in a dive. I hate openings when the plane is in a dive, OUCH! And I do believe that the dive will have more of an impact if you are last out versus first out.

I know I for one was trying to make a choice on every exit. Do I go for a quick opening, not as hard, and then work into a crazy dizzying spiral to get to down to the formation already built? I generally was taking 4-5 seconds and spiraling some.

It seems that one of the things we need is to train jump pilots on exit trims for CRW exits. A good friend has left CRW because of hard openings. It's got to be an issue for more than just the two of us. Establishing the ideal exit trim would promote safety in the sport.

[chuckbrown 0]

The way I read Remko's analysis is the best jump run would be in a 1000' climb. The slower ground speed would mean a slower (and softer) opening. This would permit a longer delay for those coming in later on a bigway without getting slammed. I too have seen stars on some openings. No fun.

[faulknerwn 38]

I was very pleasantly surprised at how not-hard (wouldn't go so far as to say soft!) my openings were out of the Casa. Generally by the time I left we were WAY high so I was trying to take 7-8 second delays, but the openings were still quite acceptable. Not sure how the pilots were flying though.
W

[CRWCheryl 0]

I was also pleasantly surprised by the CASA exits. Whatever the pilot in the trail casa was doing, he was doing it right. I hadn't even been concerned when I switched over to the otter.... until the first opening.

Yes, it does sound as if it would be better to exit when the plane is in a slight climb.

[remko 0]

> He had mentioned that he had good openings when the plane was in a dive.

Hmmm... I don't recall saying that. The plane was never in a descent (dive is a
bit much).

> And I do believe that
> the dive will have more of an impact if you are last out versus first out.

Makes no difference, unless the plane is accelerating.

> I know I for one was trying to make a choice on every exit. Do I go for a
> quick opening, not as hard, and then work into a crazy dizzying spiral to get
> to down to the formation already built? I generally was taking 4-5 seconds
> and spiraling some.

I had the same dilemma and choose for soft openings and more work to get
down. All though you really don't have a choice, you must jump as briefed.
Exiting with 500'/min & 3.0 sec delay seems like a good trade-off.

> It seems that one of the things we need is to train jump pilots on exit trims
> for CRW exits.

There are a few things a pilot can do about exits.

Slow down for one, using flaps. But there is a balance between safety and
comfort. A slow flying plane (close to stall speed) is a dangerous one. Some
pilots choose for comfort and a little less safety some for more safety and a
little less comfort. Jumpers can help A LOT by staying in the front of the plane
as long as possible. If the pilot knows the jumpers are disciplined and do this
consistently he can slow down a bit more without jeopardizing safety.

Climb. As shown above this will decrease deployment speed a bit (like
throwing up a ball). furthermore the increased airflow over the wing by the
prop-wash will allow him to slow down even more, using landing flaps requires
even more power. But I think this is not what we want on a big-way jump-run.

> A good friend has left CRW because of hard openings.

Maybe use a RW triathlon and re-line it for CReW...

> It's
> got to be an issue for more than just the two of us. Establishing the ideal
> exit trim would promote safety in the sport.

So, stress on staying/moving to the front of the plane on exit!

--
Everything you know is wrong. But some of it is a useful first approximation.

[CRWCheryl 0]

Remko, you are right. I used the wrong word. We had talked about the plane descending, not being in a dive.

[crewdog2 0]

I'm not sure the otter pilot was giving us the exit speed requested. I was up front in the aircraft, and due to injuries I sustained in a previous hard opening (a week before the world record jumps), I definitely had a vested interest in slowing the aircraft down as much as possible. I asked the pilot on each jump for the slowest exit speed he could obtain (short of a stall). A quick glance at the airspeed indicator prior to exit suggested that we were still going a bit fast.

As for the hard openings, I had to put a stop to mine or quit and go home. Due to the torn stomach muscles I had I couldn't go on, and was planning to throw in the towel and fly home on Monday. I didn't want to go. I love being a part of these jumps as much as anyone, but I couldn't take the opening shocks. Because I felt the most pain during opening, I had to look for ways to slow the deployment of my canopy. Chris Gay allowed me to install a slider cover on the record jumps which I could rip off after opening. It helped a little bit. But, I still couldn't take that "thump-opening" you get on a Lightning. Me and my Washington buddies tried a number of things to slow my openings - just short of tying the slider off with 550. Roger McClelland suggested I pack the canopy with the brakes at full glide (unstowed). Man, what a difference! The canopy mushed open and I had to pump the slider down. Saved my ass, and allowed me to be on the record jumps. Thanks Roger!

I wouldn't suggest packing your Lightning this way for competition, but for big-ways or fun jumps, I don't see any problem.

Russ

[remko 0]

I borrowed a 143 with deployment bag which opened a lot harder than my 126
with tail-pocket. When I measured lines for the event I discovered that the 126
has a very deep brake setting. I wonder which is the biggest contributing factor, the loadfactor, the tail-pocket or the brake setting.

It almost seems that half brakes on the Lightning are tuned for slammers. Were
these no brake openings consistent and on heading?

[crewdog2 0]

I made around 10 jumps at Lake Wales with the brakes unstowed. All of the openings were soft and on heading. Besides the brake setting, we also rolled the three outside cells on each side tightly and stuffed them into the center cell. The canopy has a tail pocket and I always flat pack it (I know, old school). In between jumps at Lake Wales I tried putting the canopy in a bag, but didn't have the correct bridle. We were getting ready to go up again so I yanked it off real quick and used the tail pocket.