eames

Yes, and I've explained why in this thread. Higher wind speeds promote canopy traffic congestion (assuming an upwind opening point), because it takes less time to get to the landing area going downwind. Ground speed comes into play after opening, when everyone is aiming for the same spot on the ground. -Jason

My bad on that point, I got a little too excited.... I hope you understand. I'm very cynical. Not necessarily. The program might be shifting the visual so that the trajectories can be seen more easily. True, the reference points don't move, but it may have been easier to program it with them fixed in place. Maybe we should ask kallend. -Jason

Yeah, I'll admit: You can still collide with someone if you try really hard. -Jason

What? But, I have no merit Colonel!

I've made a diagram. I just want you to see what I see, even if you don't agree. On the diagram, just for argument's sake, the thick black line represents a freefall trajectory and canopy flight. Just say for instance that it's a cylinder instead... and you can roll a marble down it. No tricks, only one tube, not to be moved. Can you see that if you roll one marble down it, then another in 10 seconds, they will still never be at the same place at the same time? -Jason

Have you considered that maybe you were just barely upwind of the dropzone and the people that went in front of you were holding into the wind under canopy for 30 seconds of your skydive? -Jason

The space they occupied only a few seconds earlier? I don't understand... why don't you explain it to me with your bridge analogy. So you throw a wind drift indicator off a bridge, wait 45 seconds, then throw another one, the second one will land within a few seconds of the first? That's quite a riddle.... -Jason

Indicated Airspeed (kts) 80 Upper winds (kts) 80 Headwind Lower winds (kts) 15 Headwind Altitude of wind change 6000 Exit delay (sec) 15 Both slow fallers Separation between deployment points: 484 ft. BUT WAIT A MINUTE, THERE'S NO GROUND SPEED!!! -Jason

I've jumped out of several Cessnas with zero groundspeed before and I definitely didn't have to wait infinitely to exit after the person in front of me. No tracking, no high pulling, and we weren't even close at pull time. How would you explain that? Was it a miracle? -Jason

Go check your math on kallend's website... your second example yields more separation than the first... and the third even more than the second... just as it should. Check out the website, put your examples in there yourself, and then tell me you still disagree. -Jason

Really, how is that exactly? Relative to the moving airmass, the separation will be exactly the same. Of course it won't be the same across the ground, you're going a different speed with respect to the ground! Why is that hard to understand? -Jason

There would be more separation, but your examples are useless anyway because you're not specifying what's happening with wind velocity between altitude and 3000 ft.... as I said above. In your examples, is the change between jumprun wind speed and 15 knots at 3000 ft linear, or what? Put numbers in there and show me that there would be no more separation.... -Jason

Right, in that case you'd need to take more time between exits.... because there would be less (or no) airspeed for separation. Unless you have a function that describes the significant decrease in horizontal airspeed between the winds at altitude and 3000 ft for your scenarios, your scenarios are not objective. But I'll comment anyway: Yes, if the airspeed is greater, less time is required between groups to achieve the same separation. Okay, we disagree. One more try: Let's say there's is NO wind. You throw a toy paratrooper (nearly strait-down trajectory) out of an otter flying at 80 knots (airspeed), and you keep flying strait until the paratrooper hits the ground. Once the paratrooper hits the ground you've covered a certain distance across the ground. Now perform the same experiment at 90 knots (airspeed)... let the paratrooper go at the exact same spot. When the paratrooper hits the ground this time the plane has traveled farther along the ground away from the paratrooper, right? Was this due to a change in groundspeed or airspeed? I know, groundspeed.... Then how would you explain the fact that the difference in distance that the plane traveled away from the impact point of the paratrooper would remain constant regardless of the direction of flight, windspeed, or wind direction? -Jason

Name any jumpship with an airspeed of only 17.36 knots (29.3 fps).... -Jason

I'm not jumping any canopy in 88.6 mph (130 fps, 77.07 knot) winds. I used that number because it's a reasonable airspeed for a plane on jumprun. For the 15 seconds you wait on the plane, you are moving away from the person who got out before you at approximately the airspeed of the plane. Not some arbitrary groundspeed. Separation is a function of airspeed and time between exits. Period. -Jason

Right, I agree. I should have said "Just to supplement what you said...." -Jason

The distance between the two canopies would be the same no matter what the groundspeed is, assuming a constant airspeed. The closing time for the two canopies would be the same as well (yes, regardless of groundspeed). I give up. -Jason

Just to clarify, the ground is irrelevant only until after opening. Exit separation must be increased in higher winds due to the increased speed and reduced time it takes under canopy to get to the target, i.e. due to congestion (assuming an upwind opening point). -Jason

Good observation, but we don't hit the ground when we open our parachutes, do we? We continue moving in the airmass with a relatively slow descent. Think about it. -Jason

Are you serious? If jumper 1 opens at 3000 ft at time t, and continues downwind, then jumper 2 opens (yes, in the same exact position in space as jumper 1 did) at time (t + 15) then they'll have the velocity of the wind multiplied by the change in time (t + 15) between them. Distance = Velocity * Time Distance = 130 ft/sec * 15 sec = 1950 ft -Jason

Uh.... okay.... so if you drop a ball out of a plane with an airspeed of 130 ft/sec and 0 ft/sec groundspeed, the ball will land on the ground directly underneath the plane? This stuff is in the ISP.... -Jason

I'm afraid you're wrong, Hook. Here's an example for you.... A jumpship is moving 130 ft/sec and the the winds aloft (3 thru 12+) are moving 130 ft/sec in the opposite direction. After a jumper exits, she'll accelerate horizontally up to approximately 130 ft/sec (after the forward throw of the jumpship and in conjunction with the vertical acceleration, or "hill") in the opposite direction of the plane (with the winds aloft). So if another jumper exits 15 sec later, the two will have a horizontal separation of about 1950 ft, regarless of groundspeed. This was all figured out many, many years ago.... -Jason

No, they're [more] right. From exit until opening, separation is a function of "airspeed" (the culmination of the initial horizontal airspeed of the aircraft, velocity of winds aloft, and vertical speed). Your analogies are flawed because you failed to account for freefall drift, the velocity of which is approximately equal to the velocity of the winds aloft. As soon as each canopy is open, the term "separation" begins to take on a different meaning because everyone is headed to the same destination. Assuming the duration of descent time is approximately equal in low and in high winds, and that the majority of the load opens upwind of the landing area, canopy traffic around the landing area will be worse in high winds for a given amount of separation time. -Jason

How were they exiting? -Jason

The reason student static line is prone to line twists is that students are trying to learn to skydive (and of course we learn to bellyfly first). The bag is peeled off the student's back strait up over the reserve container, knocks 'em in the head, gets routed around an arm, etc.... All you'd have to do is leave facing the other way, almost in a sit, with your back to the relative wind. I've never seen anything but clean deployments this way. But don't try it 'cause I said so. As my friend Will would say "Do not try this at home, batteries not included, void where prohibited, may be illegal in some states." -Jason