Angled jump run - good idea?

[Telek 0]

Ok, so I have heard several different opinions on this one.

The question is this: "Is doing a jump run angled to the wind line a good idea?"

The idea being that a 30 degree angle to wind-line jump run provides 15% more distance while maintaining the same wind-line distance to the spot. Since moving perpendicular to the wind line isn't much of an issue.

Thoughts?

[Scrumpot 1]

Much more to this, than just that. All depends on several variables, as well as even just relative preference and desires. Also, layout of the DZ, landing area and surrounding terrain, topography, etc. Other VARIABLES such as what uppers, versus mid-levels, versus ground winds are doing too, ALL must be considered, and come into play, if one wants to truly consider the most efficient jump run / flight ops.

No one single pat answer to this exists.

Throw in now too, if you want off-set jumpruns, etc. On & on. Everythng just depends on the particular specific situation and layout, then the outcome it is you are looking for and desire. My advice? - Get with your DZO and pilot at your location directly, and talk about it / figure it out.

No one single "answer" exists for this.

coitus non circum - Moab Stone

[peek 20]

Almost any direction jump run can work in certain conditions if everyone understands it and doesn't second guess the pilot/spotter, or if someone has control over it, like the DZO saying, "We are doing it this way, period."

But skydivers can be stupid about such things, so sometimes it is better just to keep things simple, even if it is not ideal.

[Scrumpot 1]

Also, coming off of the wind-line, affects drift patterns and compresses (potentially) opening separation differences. Meaning, that any perceived gain in "giving more distance availability" is offset by also needing to extend longer, the jumprun (to increase opening separation) as a trade off.

What are you looking for? Economic gains (or balance) or practical ones? Chances are, that when you are tipping the scale towards one, you are equally as well, just taking away (or trading off against) the other.

coitus non circum - Moab Stone

[billvon 2,400]

>"Is doing a jump run angled to the wind line a good idea?"

No reason it's any better or worse than any other angle - PROVIDED people still understand spotting and exit separation. A 100% crosswind jump run works well; timing stays the same no matter what. Eloy has experimented with a curved jump run, which works OK - but the shape must be adjusted based on winds, and that can be difficult to pull off well.

[kallend 1,623]

Quote

>"Is doing a jump run angled to the wind line a good idea?"

No reason it's any better or worse than any other angle - PROVIDED people still understand spotting and exit separation. A 100% crosswind jump run works well; timing stays the same no matter what. Eloy has experimented with a curved jump run, which works OK - but the shape must be adjusted based on winds, and that can be difficult to pull off well.

I'd suggest it's no better PROVIDED people still understand spotting and exit separation, and it's worse if they don't.

...

The only sure way to survive a canopy collision is not to have one.

[pchapman 261]

Doing a jump run angled across the wind line doesn't give any more space in terms of dropping area where jumpers can get back to the DZ.

The shape of the area from which jumpers can get back to the DZ tends to be an elongated oval, with the axis along the wind line. Any drop points off that axis will result in a shorter usable jump run in terms of distance.

One would have to curve the jump run to use more of that usable area.


The messy details if I have to try to prove my assertion:

This is actually handy for clarifying one's thinking about the area from which a jumper can get back to the landing area. Some skydiving instruction mentions the 'wind cone' but isn't really clear on the shapes involved, what exactly the usable area is in which the jumper can deploy and make it back.


Technically the "shorter available jump run off axis" is because the local curvature of the circular arcs that make up the ends of the oval area are necessarily less than the curvature achieved by drawing an arc from the center of the elongated oval area.

Why is the shape a sort of oval?

Let's first look at the simple case where any given parachute has one forward speed and one descent rate. Then a parachute can get back to the DZ from a circular area. I'll call this the usable area. (Freefall drift can be considered separately, doesn't invalidate the concept, and need not be dealt with here.)

If there is no wind, the circular area is centered on the DZ. The point at the center of the circle is the only drop point for an unmodified round canopy that has no forward speed at all. (Fig 1a). For example, if a canopy flies 30 mph horizontally, is open at 2000', and has a 1000 fpm descent rate, the canopy will have 2 minutes in which to fly, and can travel 1 mile. (60 mph = a mile a minute). These numbers are picked to make the math easy to comprehend.

The same circle will also apply to a canopy that has half the speed but half the descent rate, so that the doubled time in the air makes up for the halved speed.

If the canopy has half the speed, the radius of the circle is halved.

So that's all simple enough.

Fig 1b:
When wind is added, the circle shifts upwind by the amount the canopy would drift in the time available until landing. Say the wind is 30 mph. (We're only looking at constant wind here.) Then the 30 mph canopy can be dropped over the DZ and hold into wind all the way down, or be dropped 2 mi upwind and run with the wind all the way.

It can also drop 1 mi off to the sides, but has to be exactly 1 mile upwind if it is to make it back. If crabs and drifts confuse the issue, think if it this way: The 30 mph canopy has 2 minutes to landing. Where ever it drops, it has to fly to the guy who was dropped with the round canopy in the ideal spot, and reach him by the time he touches down.

The guy with the 15 mph canopy has the same ideal drop point, but has a circle of half the diameter in which he can drop and still make it back to the DZ.

Fig 1c:
Now let's be more realistic about canopies, as each has more than one forward and downward speed. For each speed and descent rate combination we can draw a circle for the usable area when flying at that particular speed combination. The circle location upwind depends on the drift time available, and the size of the circle depends on the distance the canopy can travel, a function of its speed and the time available based on its descent rate.

In reality a canopy will have a continuous variation in speeds and descent rates depending on brake, rear riser, and front riser positions. For simplicity say the canopy can fly at any of these three speed combinations:

35 mph, 1500 fpm descent rate front risering (if it can actually be held the whole flight…)
30 mph, 1000 fpm normally
20 mph, 600 fpm in brakes

These give circles that are of the following sizes and locations:

distance upwind (miles), diameter (miles)
.67 , .78
1.0 , 1.0
1.67 , 1.11

Fig 1c shows these all superimposed.

If there is a continuous variation between all of these different flight modes, we can fair the different circles together -- as in 1d, where we end up with a sort of elongated oval. The radii of the circles at the top and bottom end will just depend on the tradeoffs between speed and descent rate.

For example, the circle at the top (upwind) applies to where the jumper is trying to float the canopy in brakes. The circle may be small if the canopy is slowed down a lot while floating, but in this example the radius is relatively large because the canopy floats well without losing a lot of speed. (Lost 33% of its speed but has 67% more time in the air.)

Getting back to the issue of jump run, the longest dimension within the oval shape of the usable area is along the main axis, along the wind line.

If the jump plane started doing curved jump runs, or upwind and downwind runs, or multiple upwind runs, of course one would be able to make more use of the usable area, spreading opening points more, at the cost of increased complexity and fuel use.

When different canopies are on the load, one of course ends up with a mix of usable areas. The usable areas will also vary depending on the opening altitude, so that it makes sense that tandems that open high drop near the end up the jump run well upwind.

[jumpwally 0]

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