Forums: Skydiving: Safety and Training:
Jump Run Direction

 


ZoneRat  (D 26968)

Feb 19, 2004, 12:42 PM
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Jump Run Direction Can't Post

This is something I probably should already know... but don't.Blush

I understand that technically, jump run can be in any direction relative to the upper winds. Typically it seems to be into the wind or occasionally with the wind.

It seems to me that a jump run that is perpendicular to the upper winds would minimize the dangers of jumper horizontal drift. Jumpers would tend to drift more parallel to one another, rather than possibly over one another. Why isn't a perpendiclar jump run the norm then?

Is it more difficult to orient the jump plane at the cut with winds to the side? Is Pilot spotting with GPS easier/ more accurate when running with/ into the winds?

I'm sure there are very good reasons. I just don't know what they are.

Thanks in advance. I've been rolling this around in my mind for a while...

Robin


Nutz  (D License)

Feb 19, 2004, 1:01 PM
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Into the wind means that the last guy out can make it back (Usually) Tongue


Premier quade  (D 22635)
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Feb 19, 2004, 1:04 PM
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That and it also mean more time over the target for the pilot which equates to more jumpers getting out per pass. If they can get the entire load out in one pass, it's more profitable for everyone concerned.


pccoder  (A 43773)

Feb 19, 2004, 1:06 PM
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I could be wrong, but I would imagine it has more to do with the plane than the jumpers. It's probably for the same reason that the plane takes off in to the wind, to get more lift without as much effort. Since the plane is slowing down it can use all the help it can get to maintain the proper altitude without having to work so hard. ??? Crazy


bmcd308  (D 27472)

Feb 19, 2004, 1:10 PM
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Please do a search on airspeed or groundspeed.

Plane has same airspeed on jump run regardless of into the wind or with the wind. It just covers less ground per unit time into the wind.

Airspeed vs. groundspeed is a really really really critical thing for all skydivers to understand, as it has an impact on every aspect of skydiving.

Brent


pccoder  (A 43773)

Feb 19, 2004, 1:16 PM
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Re: [bmcd308] Jump Run Direction [In reply to] Can't Post

Regardless of speed, doesn't the plane have to worker harder to maintain altitude if going with the wind than against it?


diverdriver  (D 19012)

Feb 19, 2004, 1:20 PM
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In reply to:
I could be wrong, but I would imagine it has more to do with the plane than the jumpers. It's probably for the same reason that the plane takes off in to the wind, to get more lift without as much effort. Since the plane is slowing down it can use all the help it can get to maintain the proper altitude without having to work so hard. ??? Crazy


More lift into the wind? Ok, I will suggest that if you don't really understand the aerodynamics right now you might not want to comment. You can learn about them. I'm not calling anyone stupid. But you just said something that was really wrong in aerodynamics. We don't produce more lift by facing into the wind. We just produce the lift we need (same) using less territory. Since the already moving air imparts an initial airspeed as we sit on the runway we use less runway to get up to flying speed. We produce the same lift. Not more.

K?


diverdriver  (D 19012)

Feb 19, 2004, 1:21 PM
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In reply to:
Regardless of speed, doesn't the plane have to worker harder to maintain altitude if going with the wind than against it?

No.


bmcd308  (D 27472)

Feb 19, 2004, 1:22 PM
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I am not sure what you mean by work harder.

The plane maintains its altitude balancing the lift created by its wings with its weight. Lift is a function of airspeed.

It does not matter how much ground the plane is covering.


diverdriver  (D 19012)

Feb 19, 2004, 1:27 PM
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In reply to:
This is something I probably should already know... but don't.Blush

I understand that technically, jump run can be in any direction relative to the upper winds. Typically it seems to be into the wind or occasionally with the wind.

It seems to me that a jump run that is perpendicular to the upper winds would minimize the dangers of jumper horizontal drift. Jumpers would tend to drift more parallel to one another, rather than possibly over one another. Why isn't a perpendiclar jump run the norm then?

Is it more difficult to orient the jump plane at the cut with winds to the side? Is Pilot spotting with GPS easier/ more accurate when running with/ into the winds?

I'm sure there are very good reasons. I just don't know what they are.

Thanks in advance. I've been rolling this around in my mind for a while...

Robin

Yes, actually, I do like to run crosswind jumpruns when possible. This usually occurs when the winds on the ground are light (zero) to about 10 mph. I can keep my seperation time down and people can exit faster with safe seperation. But running crosswind jumpruns (I fly a Twin Otter) also means that you could have 23 people with 11 groups on board. This can be a very strung out exit line. So, at my DZ, I do hook turn jumpruns. The first part of the jumprun is straight and level. But about half way through I will start to bank (try to make it to the left) and continue to let people jump. This keeps them all in the "cone" (as I call it) where they can still make the landing area.

There is an area (the cone) that canopies can open up in and still make it to the landing area. The size and shape of the cone is predicated on what the winds are doing, how big the landing area is, and what type of canopies are being jumped on that particular load.

I could spend hours explaining it all but my fingers won't last that long. Just to answer your question, yes, some DZs do use the crosswind jumprun quite effectively. But it is NOT always a good jumprun depending on wind conditions.


FrogNog  (C 34484)

Feb 19, 2004, 1:27 PM
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Re: [pccoder] Jump Run Direction [In reply to] Can't Post

In reply to:
Regardless of speed, doesn't the plane have to worker harder to maintain altitude if going with the wind than against it?

The airplane doesn't know whether it's going with the wind or against it; think of the air (not wind) as a 3-dimensional homogeneous medium that is moving steadily in one direction. The plane pulls or pushes itself through this and generates lift by the motion of the medium past the lift surfaces. If the entire medium is holding still, or moving steadily in some direction at some speed, it doesn't matter to the plane because lift is based on the speed of the plane through the medium.

The reason planes take off and land into the wind is mostly because the runway is of a fixed length in a fixed place, and the plane needs to be over that runway for a time while taking off or landing. Taking off and landing into the wind again has no meaning to the relationship of the airplane to the air, but it has a huge effect on how quickly the airplane moves across the ground and thus how long the runway is available for rolling and optionally braking.

Also, a plane at rest on the ground does not have zero airspeed when there is a wind; the plane _is_ moving relative to the air. If the plane is facing into the wind when it begins its takeoff roll, its airspeed is positive before it even begins rolling. This means the airplane has to perform less total acceleration to reach takeoff airspeed. If the plane is facing out of the wind when it begins its takeoff roll, its airspeed is negative when it begins rolling. This is very not good...


ZoneRat  (D 26968)

Feb 19, 2004, 1:29 PM
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Re: [quade] Jump Run Direction [In reply to] Can't Post

In reply to:
That and it also mean more time over the target for the pilot which equates to more jumpers getting out per pass. If they can get the entire load out in one pass, it's more profitable for everyone concerned.

Really? That's the only reason? Money? Bummer.

Hmm. Are you certain it effects how many jumpers can get out in one pass? (Attached).

I'm missing something obvious again. I can sense it...lol


(This post was edited by ZoneRat on Feb 19, 2004, 1:29 PM)
Attachments: jump_run.gif (5.90 KB)


bmcd308  (D 27472)

Feb 19, 2004, 1:32 PM
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Please search on airspeed and groundspeed.

Flying into the wind at a given AIRspeed, you will be over the target area on the GROUND for a longer amount of time than flying crosswind. Flying crosswind, airspeed = groundspeed. Flying into the wind, groundspeed = airspeed - wind.


ZoneRat  (D 26968)

Feb 19, 2004, 1:36 PM
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Re: [diverdriver] Jump Run Direction [In reply to] Can't Post

The size and shape of the cone


That's probably what I'm missing... I'll dwell on that a bit.

Thanks DD. I knew you'd come through.


ZoneRat  (D 26968)

Feb 19, 2004, 1:46 PM
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In reply to:
Please search on airspeed and groundspeed.

Flying into the wind at a given AIRspeed, you will be over the target area on the GROUND for a longer amount of time than flying crosswind. Flying crosswind, airspeed = groundspeed. Flying into the wind, groundspeed = airspeed - wind.

Yeah, but wouldn't you eat that time with additional exit sep time? Looks like a wash to me...


bmcd308  (D 27472)

Feb 19, 2004, 1:52 PM
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Maybe, maybe not.

From the second group on, conventional wisdom is to start the climbout once separation has been achieved, so that if someone falls off, you have the desired separation when the rest of the formation dives out after them. So there is always "wasted" time for climbouts. During a slow climbout on an 80 knot jump run, the plane can cover a ton of real estate.


Premier quade  (D 22635)
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Feb 19, 2004, 2:16 PM
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Re: [ZoneRat] Jump Run Direction [In reply to] Can't Post

The X marks the target.
The circle or ellipse represents how far a canopy can travel under certain conditions.

If there are no winds at all, then it really doesn't make any difference what direction jump run is from a canopy flight point of view.

If there are winds from opening altitude to the surface, then it makes quite a bit of difference.


(This post was edited by quade on Feb 19, 2004, 2:19 PM)
Attachments: Jumprun.jpg (46.8 KB)


diverdriver  (D 19012)

Feb 19, 2004, 2:22 PM
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Re: [quade] Jump Run Direction [In reply to] Can't Post

Excellent diagrams Quade. That's petty much how I "see" my jumprun when I'm flying.


ZoneRat  (D 26968)

Feb 19, 2004, 2:37 PM
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Ok. No. Wait. You're right.
It makes more sense when I run it to extremes.

Upwind JR. Say the uppers are haulin' and the load gurus are recommending a 12 second count. The zooload belly fliers take an extra 4 seconds setting up their meeker. That means an additional 33% more travel is covered over the Happy Cone.

The same run at Cross wind: Recommeded exit sep is 4 seconds because horizontal drift is no longer much of a factor. The meeker takes 4 extra seconds to set up. 50% more travel is covered over the Happy Cone. Possibly more since the plane may have to fly faster at the cut to maintain airspeed over the wing surfaces. (But I'm guessing there).

Wouldn't take many of those before you'd need a second pass.

I'm startin' to get it...


bmcd308  (D 27472)

Feb 19, 2004, 2:38 PM
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>>Possibly more since the plane may have to fly faster at the cut to maintain airspeed over the wing surfaces. <<

Aargh!


ZoneRat  (D 26968)

Feb 19, 2004, 2:55 PM
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In reply to:
Aargh!

Why? Why you do dat?

Bernoulli effect, right? Groundspeed no matter. Right? Need wind over wing surface to create low pressure zone hence lift? Guessing crosswind JR not as fast wind over wing as into-the-wind JR? Right? Maybe?

I *thought* I understood that stuff... layman style at least...

Not convinced I fucked up there.


Hooknswoop  (D License)

Feb 19, 2004, 2:59 PM
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An aircraft flying at 90 knots indicated has the same amount of air going over the wings regardless if it happens to be flying down wind, cross wind, or into the wind.

Derek


bmcd308  (D 27472)

Feb 19, 2004, 3:00 PM
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You were almost there.

See Hook's post.


kallend  (D 23151)

Feb 19, 2004, 5:00 PM
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As best I can tell, going purely by time between groups needed for adequate separation, upwind vs downwind doesn't matter much because the increade time over target upwind is eaten up by the extra time between groups.

Crosswind is intermediate between the two.

In REAL LIFE there's additionl stuff to worry about, like can that 8-way really climb out and get set up in 3 seconds on a downwind run, if 3 seconds gives adequate separation (which it may well for a downwind run). Obviously it can't (IMO).

And additional factors that influence jump run direction are such things as avoiding lakes, oceans, swamps, rivers, etc. These are constraints that vary from DZ to DZ.


diverdriver  (D 19012)

Feb 19, 2004, 5:30 PM
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In reply to:
And additional factors that influence jump run direction are such things as avoiding lakes, oceans, swamps, rivers, etc. These are constraints that vary from DZ to DZ.


Now you REALLY are getting into the "art" of spotting. We could argue the flat open ground all day long. But when you take into account group size and type, topography, aircraft size (total jumpers), canopy types commonly jumped at that DZ, and ATC climb requirements/constraints you have to put it all together.


kallend  (D 23151)

Feb 19, 2004, 5:59 PM
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In reply to:
In reply to:
And additional factors that influence jump run direction are such things as avoiding lakes, oceans, swamps, rivers, etc. These are constraints that vary from DZ to DZ.


Now you REALLY are getting into the "art" of spotting. We could argue the flat open ground all day long. But when you take into account group size and type, topography, aircraft size (total jumpers), canopy types commonly jumped at that DZ, and ATC climb requirements/constraints you have to put it all together.

I spent lot of time studying a master.Wink


ZoneRat  (D 26968)

Feb 19, 2004, 10:26 PM
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Ok. I'm going to do some research on Hooks and 308's statement because I don't want to take another swing at what is likely a very dead and beaten horse... I smell a laymans trap there. I find the phrase 90 knots "Indicated" interesting. Seems like that means speed of wind over wing rather than distance travelled (otherwise, I'd still have a valid point up there... I think)... but I'll try to look it up in a thread somewhere. I respect your time enough to do that before I answer back.

But regarding the art of spotting... (black art?):
I can see where canopy types make a dif. Same with load compliment. And I can see where different aircraft have different capabilities/ requirements regarding how much time you have vs need to exit jumpers so they can deploy in the Happy Cone...

But topography like lakes and rivers? A good spot's a good spot. Right? Just more pressure to make sure it really is right. Because there's more at stake when a jumper lands off. Might get eaten by a croc, or land in a river or somethin'..

Mountains... I can see that making a dif topographically speaking. But a swamp?

The Happy Cone probably doesn't care what's beneath it for the most part.

I'd guess.

(btw: I really do appreciate your helping me in this. I'm not trying to just argue... I do want to understand.)


sundevil777  (D License)

Feb 19, 2004, 10:38 PM
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Iindicated air speed means uncorrected for pressure changes at altitude, or something like that. True air speed - TAS vs IAS.


velo90

Feb 20, 2004, 1:51 AM
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Quote:
The Happy Cone probably doesn't care what's beneath it for the most part.

Quite right, but your happy cone is rather big. An aircraft flies in straight lines (or maybe there is a hook turn in there). This means that in reality the jump run cannot use all of that cone. If the pilot knows there are swamps and crocs to the left of the cone he can fly the aircraft more to the right of the cone.
Quote:
A good spot's a good spot. Right?
A good spot can turn into a bad one if the jumper has a mal.Unsure


pccoder  (A 43773)

Feb 20, 2004, 4:43 AM
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In reply to:
In reply to:
Regardless of speed, doesn't the plane have to worker harder to maintain altitude if going with the wind than against it?

No.

So, just clarify for me. Quite simply, not some huge explanation. If the planes heading is North and wind is from the North, does the plane have to use more fuel to maintain the same altitude as if the wind is from the South? Or is it the other way around?


kallend  (D 23151)

Feb 20, 2004, 7:36 AM
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In reply to:
In reply to:
In reply to:
Regardless of speed, doesn't the plane have to worker harder to maintain altitude if going with the wind than against it?

No.

So, just clarify for me. Quite simply, not some huge explanation. If the planes heading is North and wind is from the North, does the plane have to use more fuel to maintain the same altitude as if the wind is from the South? Or is it the other way around?

The plane doesn't know which way the wind is blowing over the ground unless it's on the ground.


bmcd308  (D 27472)

Feb 20, 2004, 7:37 AM
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Think of an airspeed indicator as a device that measures how hard the air is pushing against it. When you stick your head out to spot, your smiling mug is an airspeed indicator. The amount of wind you "feel" is what determines your indicated airspeed. If you held a child's windmill out the door and counted how fast it was spinning, you would have made a primitive airspeed indicator. As each molecule of air moved past the windmill, it would push it some.

True airspeed is the rate that individual molecules of air move past a specific point. This is a little harder to feel on your face, since you are not good at detecting changes at the molecular level. So TAS winds up getting calculated based on IAS and the density of the air, which is a function of altitude and temperature. TAS is the speed you are actually moving through the air - or the speed at which individual molecules of air are moving past a specific point.

Why not just talk about TAS? Because IAS ("felt" speed) is what generates lift.

Think about the windmill out the door in a plane that will fly a constant true airspeed. As the plane goes higher, the less dense air pushes less and less on the windmill as the air moves past the blades, because there are fewer molecules to actually do the pushing. So the windmill turns more slowly, even though the speed of the plane through the air (its TAS) remains the same. In order to figure out how fast the individual molecules of air are moving through the windmill, we have to take into account the fact that there are fewer molecules pushing on the windmill blades.

Now take the plane up higher still. Eventually, you will notice that the thinner air is pushing less and less on the windmill, until it is hardly turning at all. At this point, you will hear a funny buzzing noise and a bunch of profanity from the cockpit. The buzzing was the stall warning. The profanity was the pilot. The pilot will do some cool pilot stuff, and pretty soon, your windmill will be spinning along quickly again.

For purposes of being a "cool" skydiver, it is important to recognize the distinction between IAS and TAS, but to act as if it does not exist when talking to pilots. This is because they understand this on an instinctive, gut level, and they do not even have to think about it. Arguing with them about it will only make you look silly. And they are used to dealing with people who do not understand it, so they won't really judge you for appearing not to have a clue. It is only when you try to appear to have complete command of the concept but do not that you look like an azz.


Brent


ZoneRat  (D 26968)

Feb 20, 2004, 7:37 AM
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In reply to:
Quote:
The Happy Cone probably doesn't care what's beneath it for the most part.

Quite right, but your happy cone is rather big. An aircraft flies in straight lines (or maybe there is a hook turn in there). This means that in reality the jump run cannot use all of that cone. If the pilot knows there are swamps and crocs to the left of the cone he can fly the aircraft more to the right of the cone.
Quote:
A good spot's a good spot. Right?
A good spot can turn into a bad one if the jumper has a mal.Unsure

Both good points. I would prefer a spotter, Pilot or otherwise, who hedged their bets.


bmcd308  (D 27472)

Feb 20, 2004, 7:37 AM
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Neither


pccoder  (A 43773)

Feb 20, 2004, 7:44 AM
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doesn't it know which direction it is blowing in the air?

and my question had nothing to do with the speed of air on the ground.


(This post was edited by pccoder on Feb 20, 2004, 7:47 AM)


velo90

Feb 20, 2004, 7:48 AM
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Quote:
doesn't it know which direction it is blowing in the air?

Scroll up a few post's and you will see that Kallend has already answered that question.


ZoneRat  (D 26968)

Feb 20, 2004, 8:13 AM
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Brent, I want to thank you for your TAS/ IAS post. Very nicely written... Your examples were excellent. I can see where IAS/ TAS would really effect spotting on a high alt jump. I can also see where your explaination of those two concepts would be a great primer when helping someone understand why a canopy will fly differently at a Colorado DZ vs a sea level DZ.

And, I think it gives me the piece of the puzzle that allows to complete my "Why? Why you do dat?" post earlier up.

There are several concepts that have been brought up so far in my part of this thread. I'm gonna think on them a bit and post a set of tentative conclusions.

That should be good.Wink


sducoach  (D License)

Feb 20, 2004, 8:58 AM
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Re: [diverdriver] Jump Run Direction [In reply to] Can't Post

Chris is the best at explaining the aircraft spotting and Quade gave a great example of the "cone".

Think of spotting in the context of ground speed and time. Upperwinds effect the ground speed of the aircraft which effects the time you have available in the "cone" to exit the aircraft.

More detail if requested

Blues,

J.E.


Premier billvon  (D 16479)
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Feb 20, 2004, 9:18 AM
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>If the planes heading is North and wind is from the North, does the
> plane have to use more fuel to maintain the same altitude as if the
> wind is from the South?

No. This misconception comes about because planes need to use more fuel to maintain the same GROUNDSPEED if they have a headwind. The plane takes the same amount of power (and hence fuel) to maintain a given airspeed, but if the air's moving, airspeed is not the same as groundspeed. That's why you will sometimes hear airline pilots mention that they can compensate for a headwind by burning more fuel. But if you just want to maintain altitude, there's no difference in heading into the wind or away from the wind.


velo90

Feb 20, 2004, 9:24 AM
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Quote:
That's why you will sometimes hear airline pilots mention that they can compensate for a headwind by burning more fuel.

In my experience you just end up late at the airport and miss your connecting flight Unimpressed


pilotdave  (D License)

Feb 20, 2004, 9:32 AM
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Picture yourself on a moving sidewalk or giant treadmill. You can walk normally in any direction you want. Your actual MOVEMENT, relative to the fixed earth, will not be the same as your movement relative to the sidewalk.

Wind is just like the moving sidewalk. Does it take any more energy to walk AGAINST a moving sidewalk as it does WITH the sidewalk? Well, if you walk at a fixed rate (ie the same airspeed regardless of the wind), then no.

I think you're confusing the fact that if you want the plane to fly at the same speed relative to the ground while flying into the wind, then yes, more power will be required. But otherwise, the plane is just an ant on a treadmill. It doesn't care which way the wind is blowing.

Dave


kallend  (D 23151)

Feb 20, 2004, 9:33 AM
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In reply to:
doesn't it know which direction it is blowing in the air?

and my question had nothing to do with the speed of air on the ground.

Imagine you are in a hot air balloon floating above clouds so you can't see the ground. Would you be able to tell the wind speed and direction by, say, smoking a cigarette and watching which way the smoke blows? Or by watching the clouds? Remember, you will be blowing along with the wind, just like the clouds.


pccoder  (A 43773)

Feb 20, 2004, 9:56 AM
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Re: [kallend] Jump Run Direction [In reply to] Can't Post

OK, I guess most of your last few explanations have made more sense to me.

Thanks for the clarification.


hoym  (D 12622)

Feb 20, 2004, 3:19 PM
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Okay, in the past I've scanned the discussions about exit order and spacing between loads. I decided to try to play with some numbers just for fun.
Someone one reading this can tell me if my math is correct or if this has already been considered.

BIG DISCLAIMER!
THESE IDEAS ARE FOR DISCUSSION AND THEIR ACCURACY HAS NOT BEEN VERIFIED!!!

I think the most important thing for skydivers to understand is the concept of ground speed (GS) and also the variables that affect GS. (IAS, TAS, wind direction, direction of flight).

Most pilots measure airspeed in knots (nautical miles per hour). Most skydivers understand statute (normal drive your car type of) miles so my calculations take this into consideration.

One nautical mile is equal to 1.15 statute miles (the unit of measure most of us state side folks use).

What we need to know in advance.
Ground Speed in Knots (GSK). This should be supplied by the pilot.
Distance desired between groups of skydivers. This should be dictated by the DZ, DZO or S&TA.
What we want to calculate.
TIME in seconds. The amount of delay needed between each group of skydivers to give the desired distance between groups.

With the GSK and desired distance information, we should be able to calculate the amount of delay needed between each group of skydivers on a given load.

A knot is a measure of nautical miles per hour. Because we want to calculate time of delay between groups in a measure of seconds, the first thing to do is convert the GSK (ground speed in knots) to a measure of distance of statute miles per second.

So Ö.
If the GSK is 80 then
80 * 1.15 = 92 miles per hour
92 / 60 = 1.5333 miles per minute
1.5333 /60 = 0.02555 miles per second

OrÖ
80 * 1.15 / 60 /60 = 0.02555 miles traveled per second.

Now for discussion purposes, letís say that you want .5 (one half) mile(s) between each group of skydivers. And here is THE BIG QUESTION (for all of us Jaywalk Allstars)Ö How many seconds do we wait to get a half mile of separation between each group of skydivers?

Take the distance desired and divide distance traveled per second and it will give you the amount of time the next group should wait before exiting the airplane.

.5 / .02555 = 19.569 (or about 20 seconds)

Letís look at two more examples with a hypothetical headwind and tailwind component put in to change the groundspeed. To try to keep it brief, Iím not going to change the desired amount of separation distance. You can decide to use whatever you want there.

So for this example, consider a 30 knot headwind so that now our airplane has a 50 knot ground speed.
50 * 1.15 / 60 / 60 = 0.015972
And (for half mile separation distance)Ö
.5 / 0.015972 = 31.3 seconds of delay between groups.

And now turn the airplane around so that thereís a 30 knot tail wind (110 knot ground speed).
110 * 1.15 / 60 / 60 = 0.035138
And for half mile separation distanceÖ
.5 / 0.035138 = 14.2 seconds of delay.


Now, letís go back to our 80 knot GS, half mile separation example where we need 20 seconds between each group. Letís say that there are 10 two-ways on this twin otter. If every group takes exactly 20 seconds between each exit, that should be 180 seconds of jump run from the first group exit to the last group exit.

If the plane with an 80 knot ground speed takes 180 seconds and it travels 0.02555 miles per second, then the jumpers will be spread out over 4.6 miles.

Just some ideas to consider.
If anyone would like to verify the math, that would be great.

BIG DISCLAIMER AGAIN!
THESE IDEAS ARE FOR DISCUSSION AND THEIR ACCURACY HAS NOT BEEN VERIFIED!!!

(Edited to make the incorrect 10 into a 20.)


(This post was edited by hoym on Feb 20, 2004, 7:29 PM)


Premier billvon  (D 16479)
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Feb 20, 2004, 3:27 PM
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Re: [hoym] Jump Run Direction [In reply to] Can't Post

A reasonable approximation. As long as the winds are from the same direction at opening altitude, that formula will give you safe results. The only time it will fail is if winds are from the opposite direction at opening altitude, which is (fortunately) rare.


prost  (D 24959)

Feb 20, 2004, 4:45 PM
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Ok lost in the aerodynamics lesson is the original point.


There are times that jump run is not best into the wind. We have an 6500 foot long runway and the airport goes much farther in each direction. The airport is not nearly as wide. There are a lot of swamps around us. We almost always run jump run north of south (direction of the runway) and off set it in the direction the wind is coming out of. This way worst case scenario, everyone can at least land of the airport.


kallend  (D 23151)

Feb 20, 2004, 6:27 PM
Post #47 of 57 (736 views)
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Re: [hoym] Jump Run Direction [In reply to] Can't Post

Quote:
Now, letís go back to our 80 knot GS, half mile separation example where we need 20 seconds between each group. Letís say that there are 10 two-ways on this twin otter. If every group takes exactly 10 seconds between each exit, that should be 180 seconds of jump run from the first group exit to the last group exit.

I must be missing something here. If they need 20 seconds between each group, why do you then say they take 10 seconds between each exit? If it were me and I needed 20 seconds for separation, I'd wait 20 seconds.

I also think 1/2 mile separation between 2-ways is overkill.


hoym  (D 12622)

Feb 20, 2004, 6:49 PM
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Quote:
If they need 20 seconds between each group, why do you then say they take 10 seconds between each exit? If it were me and I needed 20 seconds for separation, I'd wait 20 seconds.

I also think 1/2 mile separation between 2-ways is overkill.

Oops, the 10 second note was a typo. The math was right though, 9 segments of 20 seconds each equals 180 seconds. (I've fixed that in the other post.)

As for the 1/2 mile for a two way, you may be right. Here is what I said...
Quote:
Letís look at two more examples with a hypothetical headwind and tailwind component put in to change the groundspeed. To try to keep it brief, Iím not going to change the desired amount of separation distance. You can decide to use whatever you want there.

My post was long enough the way it was. I just used a half mile example throughout for consistency sake.

There are a lot of variables as mentioned, and as you suggest, that should be added into the equation to determine the appropriate distance of separation between groups. Size of the formations, type of formations (big way rw, small way rw, freefly, tandem, aff, crw, wing suit, etc.).

That would be quite an administrative task to try to get each type of skydiver to know the subtleties of exit delay calculations to the extent of knowing... IF you are a two way and following another 4 way or smaller (which is NOT a skysurfer or wing suit flyer) THEN take a 10 second delay BUT if you are following an rw group of 5 to 12 then take a 15 second delay or 12 or larger then take a 20 second delay, blah, blah, blah, , blah, blah. (Again, don't use these number, its for discussion. Make your own decision.)

It seems that it might be more simple for the operator to post a note someplace like the loading tent or near the door of the aircraft that for today the delay is X.

Remember one of the things I said was that you need to determine what distance you want and then the math will help with determining the amount of delay. If you don't know what distance you want for separation and you don't know the ground speed, you will never be able to know how many seconds to delay between groups.

Who knows how would be the best way to implement a safe delay between groups. Continue researching and teaching I guess.

(Thanks for finding the typo/mistake.)
-mh.


(This post was edited by hoym on Feb 20, 2004, 8:14 PM)


diverdriver  (D 19012)

Feb 21, 2004, 7:18 AM
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Ok, without looking at any other responses I will try to answer.

Indicated airspeed is just that. What we see on the guage. It DOES NOT mean what we are actually flying through the air. When I'm in the Otter I fly initial jumprun at about 85 knots KIAS (knots indicated airspeed). But I'm ACTUALLY traveling through the air at about 100-105 knots TRUE AIRSPEED. KTAS is corrected for non-standard pressure (at altitude it is less than on the ground) and temperature (combined you get air DENSITY).

So, on a no wind day I will see a groundspeed of about 100-105 knots. This helps me judge my climbout spot, potential "throw", and exit seperation estimation.

Yes, swamp, river, highway, farmer McNasty all plays into selection of jumprun direction / offset. At SDC we have a river flowing north to south along the east side of the airport. Most times when I am flying I run a parallel JR to the river and offset for drift. When someone cuts away it is likely their gear will land somewhere on the airport or at least on the west side of the river. This is important because if you always run into the upper winds you will have people balking at climbing out of the plane on the east side of the river. You have then waisted LOTS of valuable exiting space. People shouldn't have to fear losing their gear so if I can take away one mental factor from the equation I will do it.

Now, if the winds are honking (technical midwestern term for wind speed. We have Honking, Cranking, and Screaming) I will run directly into the upper winds as my "cone" is no longer a cone usually. It has turned more into a narrow "channel". I know it's hard to describe without diagrams really but that's the best wording I can come up with right now.

Help any?


kallend  (D 23151)

Feb 21, 2004, 7:28 AM
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In reply to:

Who knows how would be the best way to implement a safe delay between groups.


-mh.

Me.

http://www.iit.edu/~kallend/skydive/ and click on "Resources"


diverdriver  (D 19012)

Feb 21, 2004, 7:31 AM
Post #51 of 57 (410 views)
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Quote:
Who knows how would be the best way to implement a safe delay between groups. Continue researching and teaching I guess.

Research has already been done! Just gotta read it now. Go to John Kallend's site.


edit: ooops, beat me to it.


(This post was edited by diverdriver on Feb 21, 2004, 7:32 AM)


vonSanta

Feb 21, 2004, 2:12 PM
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In reply to:
In reply to:

Who knows how would be the best way to implement a safe delay between groups.


-mh.

Me.

http://www.iit.edu/~kallend/skydive/ and click on "Resources"

What I love about Kallend is his modesty.

Feel free to add that to your sig Professor Tongue

Seriously though, Kallends resources made me see the light on various things. Excellent stuff.


(This post was edited by vonSanta on Feb 21, 2004, 2:12 PM)


GroundZero  (A 9044)

Feb 22, 2004, 12:20 AM
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Quote:
Bernoulli effect, right?

Bernoulli's is a school teachers easy way to teach what he/she does not understand. Not generally accepted by all. But you learned it above any other aerodynamic theory... and it proves itself well.

Good questions, keep asking...

(Personally I don't buy into Bernoulli, but that's a whole different thread!)


Chris


GroundZero  (A 9044)

Feb 22, 2004, 12:28 AM
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Quote:
Imagine you are in a hot air balloon floating above clouds so you can't see the ground. Would you be able to tell the wind speed and direction by, say, smoking a cigarette

You can't smoke in a balloon! Lots of flammable gas there...

unless you fly with me... shit that big flame overhead is more likely to cause damage than my lil smoke!

Chris


Premier quade  (D 22635)
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Feb 22, 2004, 12:39 AM
Post #55 of 57 (376 views)
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Re: [GroundZero] Jump Run Direction [In reply to] Can't Post

Quote:
(Personally I don't buy into Bernoulli, but that's a whole different thread!)

Like the Knights said when they rode up to Camelot, "It's only a model".

If you consider the fact that a LOT of people fly their canopies at only the most rudimentary and vague understanding of why they fly, then clearly it's not the be all and end all of flight. In fact, I don't think too many birds understand Bernoulli's Principle, but they fly just fine. Further, Bernoulli will -probably- get you through just about all of your civilian flying knowledge requirements other than CFI and ATP.

It's only a model, but it works well enough for most folk's understanding and application purposes.


(This post was edited by quade on Feb 22, 2004, 12:40 AM)


GroundZero  (A 9044)

Feb 22, 2004, 1:01 AM
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Re: [quade] Jump Run Direction [In reply to] Can't Post

Exactly...

Well said Paul.



Chris


kallend  (D 23151)

Feb 22, 2004, 4:29 PM
Post #57 of 57 (349 views)
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Re: [GroundZero] Jump Run Direction [In reply to] Can't Post

In reply to:
Quote:
Bernoulli effect, right?

Bernoulli's is a school teachers easy way to teach what he/she does not understand. Not generally accepted by all. But you learned it above any other aerodynamic theory... and it proves itself well.

Good questions, keep asking...

(Personally I don't buy into Bernoulli, but that's a whole different thread!)


Chris

Lift is produced when downward momentum is imparted to air passing over a canopy or wing: Newton's 2nd law, force = rate of change of momentum, and 3rd Law, action and reaction etc. the air exerts an equal force on the canopy.

Lift is also produced because the dynamic pressure on the bottom of the canopy exceeds that on the top surface, as explained by Bernoulli's principle. It is very easy to verify these pressure differences in a wind tunnel experiment. Of course, these pressure differences also impart downward momentum to the air!

They both predict exactly the same thing. Arguing that one is more correct than the other only shows incomplete understanding.

The derivation of Bernoulli's equation starts with Newton's laws. There's no way one is more correct than the other.


(This post was edited by kallend on Feb 22, 2004, 4:35 PM)



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