Why the 45 degree method does not work (long)

[crazy 0]

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The 45-degree method is by no means 100% accurate. When there are not head winds affecting the aircraft, which is not realistic 100% of the time, the 45-degree does work.

There are the experimental results (with the pictures and videos) showing that even in the improbable case where the angle would initially be greater than 45 degrees, it becomes (and remains) smaller than 45 degrees extermely quickly (less than 1 second).
There are simple and convincing theories demonstrating that the only way to reach 45 degrees (without tracking) is to have an aircraft flying faster than the terminal velocity of the skydivers (then, the main problem is to convince the skydivers to exit faster :-).
Reliable computations show that with an aircraft speed of 90 knots, the maximum angle over the first 20 seconds after exit is 37 degrees for bellyflyers, 28 degrees for freeflyers. Slower aircraft speeds give smaller angles.
The same computations show that anyway the variations of the angle are way too small to be measured without instrumentation. With an aircraft speed of 90 knots, during the first 20 seconds, the angle stays within 35+/- 2 degrees for bellyflyers, within 25 +/- 3 degrees for freeflyers. This is of course an idealised case. Everything else would be random noise anyway and would reinforce the fact that eyeballing the angle is really ineffectual.
The angle depends more on the actual freefall position (belly vs head down for instance) than on the exit separation (cf previous example or the graph provided earlier in this thread). A stable belly-to-wind exit with long legs, short arms, and dearched body would result in a greater angle than a RW4.

This is already a long list of objective arguments against the 45 degrees method.

Of course, you can still claim that people are not clever enough to aim properly with a camera, to understand basic physics, or to compute simple motion. However, doing so without any other backup than unjustified opinions won't really help. On the other hand, if you could show a video of a nice clean 2 way head down exit, where the group would be at 45 degrees after a reasonable time, then i'm sure you would impress many people.

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To illustrate this I need to ask a question. How many time will a group who exit an aircraft, appear to be at the 45 degree angle from vertical during the time they let go and through their fly away? (Once, Twice, or Three time) You might want to take a look out side the door on the next load your on before you answer.

Assuming that the aircraft speed is not faster than 100 knots and that the groups are not tracking:
- if you are standing at the rear of the door, the group exiting the aircraft will never appear to be at the 45 degree angle (always at a smaller angle).
- if you are at the front of the door, crouching low enough, maybe once, as shown by billvon's experiments. This is useless for exit separation anyway because it will happen right after the exit (1 second or less).

--
Come
Skydive Asia

[billvon 2,989]

>but I was curious what people think of freeflyers exiting first
>when you allow double exit time between freefliers and flat flyers.

That can work, but you may need to wait a _long_ time between the last freeflyer and the first flat flier (on the order of 25 seconds.) I've found that it's difficult to convince people to wait that long; most TM's start screaming "GO!" after 10-15 seconds. Given that I don't think there's any advantage over putting flat flyers out first.

[hoym 0]

With a jump run into the wind, I think it is ABSOLUTELY true that freeflyers should not exit first. And the stronger the winds, the more important this is.

But what about a cross wind jump run (winds same perpendicular to jump run direction from exit to opening)? It seems that on a cross wind jump run, that exit order between flat flyers and freeflyers would not matter because the separation at opening altitude would be even greater than the separation between the respective exit points.

[billvon 2,989]

>But what about a cross wind jump run (winds same perpendicular to
> jump run direction from exit to opening)? It seems that on a cross
> wind jump run, that exit order between flat flyers and freeflyers
>would not matter . . .

Definitely true, and if your DZ does downwind jump runs, putting freeflyers out first makes even more sense. The only problem with crosswind jump runs is that they have to be accurate; even 20-30 degrees into the wind and you've got that separation problem again. But if you have a good pilot and up-to-date weather it's doable. We did unintentional crosswind jump runs for years at Brown since we could not fly perpindicular to the border.

[377 22]

GPSs are so cheap now. Saw one for under $70 today. Why not just have one attached to the plane interior near the door reading out groundspeed? Groundspeed tells you how much time to give the previous jumper before you exit. Sooner or later jumpers would get used to using groundspeed numbers to time exits. Some jump pilots are really aware and will caution the jumpers about really low groundspeed and the need for long delays between exits, but they seem to be the exception. A pilot at Monterey once advised 30 sec between exits and he was right. The headwinds were screaming and the Otter was almost standing still relative to the ground on jump run. Many of the jumpers ignored his advice and they ended up in an almost vertical stack even after 10 seconds of exit delay. Forget 45 degree visual angles and intuition, its all about groundspeed. GPS measures it accurately and cheaply. It will even give you CMG, course made good, which is often very different from the aircraft heading.

2018 marks half a century as a skydiver. Trained by the late Perry Stevens D-51 in 1968.

[kallend 2,026]

Safety Day is coming up. I suggest that we all raise this topic for discussion at our DZs. I'm doing a session at SkydiveChicago's Expo too.

Bill, did you manage to post your video anywhere? I'd like to use it.

...

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

[freakydiver 0]

You should make this a sticky at the top of this forum Bill.

-- (N.DG) "If all else fails – at least try and look under control." --

[freakydiver 0]

You wanna bet? I have video of a student leaving no more than 3 seconds after the last experienced 4 way sit. I'm the video for the sit dive, I look up at the plane and sure enough, there someone exiting the plane.

-- (N.DG) "If all else fails – at least try and look under control." --

[bmcd308 0]

>>most TM's start screaming "GO!" after 10-15 seconds<<

That's why they get the girls - they're so cute like that.

A great way to counter this is to pretend you could not hear them and walk back toward them to see what they are saying. You can take as long as you want apologizing, then exit when you would have exited, anyway.

----------------------------------
www.jumpelvis.com

[billvon 2,989]

>Bill, did you manage to post your video anywhere? I'd like to use it.

Compressed video is at:

www.cajunchickens.com/files/45degrees.mp4

[masher 1]

That is a neat video Bill.

I'll have to bring it to the DZ next time I go...

--
Arching is overrated - Marlies

[annna 0]

Regarding: GPSs are so cheap now. Saw one for under $70 today. Why not just have one attached to the plane interior near the door reading out groundspeed? Groundspeed tells you how much time to give the previous jumper before you exit. Sooner or later jumpers would get used to using groundspeed numbers to time exits. Some jump pilots are really aware and will caution the jumpers about really low groundspeed and the need for long delays between exits, but they seem to be the exception.

That sounds like one of the most practical solutions to this problem i've heard so far ..... ?

In lieu of that, does anyone know of some kind of decent table which gives a rough guide to group spacing based on groundspeed, or better yet - you can look up your jump plane and the windspeed that day and look up the advised time to leave between groups. ???
If i had something like that to refer to it would make me a much more confident jump master i'm sure. And in time, i'm sure i would cease to rely on it and would instead have taught myself something damn useful for the rest of my skydiving career.

if i'm talking nonsense please excuse me, haven;t jumped since i broke my ankle before Christmas so feel decidedly rusty !!!!

anna

[kallend 2,026]

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>Bill, did you manage to post your video anywhere? I'd like to use it.

Compressed video is at:

www.cajunchickens.com/files/45degrees.mp4

Thanks. That SHOULD be the stake through the heart (but it probably won't be).

...

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

[markbaur 0]

Probably won't be. The winds were 30kts at altitude, and there are a few folks insisting the method works in no-wind conditions.

Mark

[billvon 2,989]

>The winds were 30kts at altitude, and there are a few folks insisting
>the method works in no-wind conditions.

Fear not; I now have video of jump runs both upwind and downwind, and they show the same thing.

[AndyMan 7]

I'd like to use that in a presentation on safety day.

Can you post a higher resolution version?

_Am

__

You put the fun in "funnel" - craichead.

[flyinchicken 0]

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I was curious what people think of freeflyers exiting first when you allow double exit time between freefliers and flat flyers.

Only problem with this is flat flyers have more freefall drift. a freeflyer heads straight down the tube with faster speeds less drift a flat flyer out after them could possibly drift over top of them, the larger the group of flat flyers the more drift.

Some of the "instructors" at my home DZ decided that they would switch the exit sequence once without informing the DZO while we were in the plane. With just over 100 jumps I listened to my instructors and went out b4 a 2 way belly(I was doing a solo sit). One of the flat flyers went low and another jumper in the air, actually thought he went through my canopy. I watched him sail past me not 20 ft from me. The instructors were instructing everyone as to exit time and sequence. Makes me nervous exiting b4 anyone on their belly now. I know some Dz's have a dif system but after this happened makes me question it.

I pulled this from the LakeElsinore site. One of my fave sites for info. Got my bf to study the AFF cartoon they have on there b4 he did his training. Helped him lots.
***Exit Separation:

With reported upper winds of 0-10 knots, please allow between 5 - 7 seconds between exits (take into account the time it takes to climb out of the aircraft.) For winds of 15 knots or higher please use the following formula: Take Upper winds /2 which will give you the recommended amount of time between exits!

Upper winds:
15 knots - 7 seconds between exit groups
30 knots - 15 seconds between exit groups
40 knots - 20 seconds between exit groups

I was taught the 45 degree thing at my DZ but have never been comfortable about it. When I went to Elsinore and discovered this, I have taken it as my personal law.

"Diligent observation leads to pure abstraction". Lari Pittman

[flyinchicken 0]

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GPSs are so cheap now. Saw one for under $70 today. Why not just have one attached to the plane interior near the door reading out groundspeed? Groundspeed tells you how much time to give the previous jumper before you exit. Sooner or later jumpers would get used to using groundspeed numbers to time exits.

That is an GREAT idea!

"Diligent observation leads to pure abstraction". Lari Pittman

[quade 4]

I could post a 720x480 Mpeg4 file that would be about 15.5mb. The original file is DV25 and about 220mb.

I tried to email it to bill, but his ISP kept bouncing it back (probably because it violated some single email size limit).

Ok, HERE is the file folder with the larger Mpeg4 file.

quade -
The World's Most Boring Skydiver

[tombuch 0]

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In lieu of that, does anyone know of some kind of decent table which gives a rough guide to group spacing based on groundspeed, or better yet - you can look up your jump plane and the windspeed that day and look up the advised time to leave between groups. ???
If i had something like that to refer to it would make me a much more confident jump master i'm sure.

I built one for Twin Otters at my home DZ. See: http://ranchskydive.com/safety/tb_article15.htm. You can make your own charts for your airplanes and the spacing your drop zone wants. It just takes a bit of math...think back to your high school algebra class. Keep in mind an American mile has 5,280 feet, a nautical mile has 6,076 feet.

Umm, please, let's not use this thread to discuss how to do the specific calculations. Check with a local math guru or pilot.

Tom Buchanan
Instructor (AFF, SL, IAD, Tandem)
S&TA
Author JUMP! Skydiving Made Fun and Easy

Tom Buchanan
Instructor Emeritus
Comm Pilot MSEL,G
Author: JUMP! Skydiving Made Fun and Easy

[annna 0]

Perfect; that makes excellent reading - thank you.

I shall agree to JM my next load with renewed enthusiasm !

cheers, anna

[billvon 2,989]

Another picture that shows some stuff:

This is a picture taken 27 seconds after the first group exited. You can just see the first group and the second groups, both below the 45 degree line.

[flyinghuntster 0]

Thanks for doing the leg work to disprove the 45deg rule. I am just starting out and it will be an example that I bring with me on every jump.

Off topic-----Where are the pix from? We don't get Mountains like that in Florida, just the "twin peaks" kind! GREAT pix!!!!

[Speer 0]

If you get your current employer to send you for freefall training, you'll find out where it is first hand...

Those are the mountains surrounding Otay Mountain, southeast of San Diego, at Skydive San Diego. It really is a beautiful location, and has incredibly stable wind conditions.

Russ

Generally, it is your choice; will your life serve as an example... or a warning?

[flyinghigh 0]

Well, it just goes to show you all that there is no perfect method. I happen to use the 45, have been teaching the sport for almost 15 yrs and teach my students the same. So far, there hasn't been any problems. Tom has a good point that I have been trying to get across to jumpers for awhile. Having over 2100 hrs myself flying jumpers, groundspeed and time isn't that bad of a method if you really know how to count accurately and do the basic math. As I said there is no perfect solution, I feel that you should use the method that seems to work best for you and your DZ. You don't have to go to MIT to realize that you really should leave more than a couple of seconds between groups Bill. And I really have a hard time with the fact that your otter pilot was flying in a level attitude!! How much separation do you really need? Where is it most important? I kind of think it is at the opening point. Have a great day Y'all.