Feb 19, 2004, 12:42 PM
Post #1 of 57
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Jump Run Direction
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This is something I probably should already know... but don't.
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...
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.
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. ???
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. ???
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.
This is something I probably should already know... but don't.
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.
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...
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)
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.
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...
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.
quade (D 22635)
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Feb 19, 2004, 2:16 PM
Post #17 of 57
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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.
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...
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.
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.
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.
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