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Winds aloft/Jetstream?

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Winds aloft/Jetstream?

Postby redneck » Mon Feb 16, 2009 6:46 pm

Just a question, I'm not sure if this is already a part of FlightGear or not (I don't really think it is), but does the real weather fetch simulate the effect of the jetstream? I've flown from KSFO to KJFK before in a B-2 and noticed that my airspeed indicator sometimes differs from my true airspeed, but only up to about 10 kts. From what I know about weather, I should have encountered areas at 35000ft in which I would have had a tail-wind of about 70 kts. I'm guessing FlightGear doesn't have that data, and that it probably comes from a source other than METAR. I'm all for the realism just like everyone else and would like to see this become a part of FG. A west-bound transcontinental flight shouldn't take as long as an east-bound one.
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Re: Winds aloft/Jetstream?

Postby jentron » Wed Feb 18, 2009 5:57 am

No, Jetstream is not modeled, even if it were, it would not affect your true airspeed. It would affect ground speed, but not airspeed. The difference in airspeed you are seeing is because of pressure differences at altitude.

http://en.wikipedia.org/wiki/Indicated_Airspeed

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Re: Winds aloft/Jetstream?

Postby redneck » Fri Feb 20, 2009 3:05 am

So, basically, I would not even save more fuel, if I was flying with the jetstream, or anything like that? I always thought eastbound flights, or westbound near the equator, were supposed to be faster at high altitudes than flights in the opposite direction. I would think that the flight plans were made based on the jetstream patterns for some reason.
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Re: Winds aloft/Jetstream?

Postby Jester » Fri Feb 20, 2009 3:16 am

You are right (EDIT: about shorter travel time and fuel saving). As Ron said, ground speed is affected, and that's what counts when you calculate travel time. However, the instruments usually show airspeed, because that is relevant to aerodynamic behaviour. For example GPS or INS can be used to determine ground speed.

FYI, Torsten Dreyer just made a wiki page about adding support for winds aloft.
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Re: Winds aloft/Jetstream?

Postby redneck » Fri Feb 20, 2009 6:32 am

Awesome, thanks. I think I understand the wiki, but I'll have to look at it more in detail tomorrow or some other day. Right now, It's 1:30 in the morning and I got classes tomorrow at 8:30. I don't have the cvs version, so I probably won't be able to get it to work anyway :( This would be great for those trans-continental flights, and if I ever get around to it, any trans-Atlantic flights. I can't imagine doing any flights around the world, yet, but if I ever have that kind of time and uneeded graphics power, I'll be sure to record that. :mrgreen:
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Re: Winds aloft/Jetstream?

Postby Hooray » Wed Feb 25, 2009 8:16 pm

There were previously discussions about some sort of dedicated "weather" subsystem, which would more completely model the major characteristics of real weather, preferably in a fashion that is integrated and modular enough to allow related characteristics to automatically occur, simply because most of the current weather code is really just about individual features in particular, rather than their inner workings/effect altogether - in other words, thermals are modeled separately from clouds, clouds separately from winds etc - so, for a valid weather scenario, each component has to be set up/configured separately.
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Re: Winds aloft/Jetstream?

Postby MAKG » Fri Mar 06, 2009 12:45 am

FWIW, I doubt folks care much, but global 4D models tend to be a bit coarse and inaccurate, due to the sheer data size.

The one I use (NCEP GFS) lives on a 1 deg x 1 deg grid. There are 0.5 deg grids available (e.g., NOGAPS, which has some obvious upper air "features", and ECMWF which gets bloody expensive and seems to overpredict water vapor), but the data size gets problematic quickly. One needs many pressure levels if one is going to model climbs and descents to low altitude (upper air is a bit easier, as long as one is only interested in altitudes below FL450).

The jet stream is VERY unsteady -- it's not just a constant wind, but it moves around daily. The good folks at KZOA plot transpacific routes based on it, daily. But you can't advect a simulation -- it took a supercomputer to make it, and it will take a supercomputer to evolve it correctly. I run into the same issue in my (very special) flight planner; I cheat (just like the poor Navy OPARS saps do), interpolating timepoints. This smears out the interesting features, like the (very) occasional 200 knot cells. It won't "feel" realistic.

FYI, 70 knots seems a bit on the slow side. It's not at all unusual to see 100+ knots somewhere on any given day, and 200 knots shows up every once in a great while.

The solution for FlightGear will be an analytic/probabilistic model. Not something I'm sure exists, and a whole lot of pain to create. Or maybe folks don't care too much and just want "something" there. That would be easier. I don't know what Jeppesen uses, but I'm skeptical that it can be global in scope and accurate at the same time. The conventional way to cheat is to restrict the domain substantially, such as for CONUS or EPAC.

And while the winds are the biggest factor, there are also issues of temperature. Sound speed is proportional to sqrt(T) where T is absolute temperature. Flying mach numbers (as is commonplace) in the stratosphere will depend upon this in the same proportion. And contrary to common assumptions, the stratosphere isn't -70 deg F everywhere, any more than the ground temperature at sea level is +59 deg F everywhere (ISA is great for generic planning, but sucks for any specific date or location).

That's probably more than you care about; the gist is that it's a difficult problem. I'd like to know how to solve it.
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Re: Winds aloft/Jetstream?

Postby redneck » Fri Mar 06, 2009 2:05 am

MAKG wrote:FYI, 70 knots seems a bit on the slow side.


Yeah, I only realized that after I ahd already posted and the main part of the question was answered. I may have been looking at the wrong weather charts when I posted that, probably 500 millibar instead of 300 millibar, or maybe I was looking at an FL150 chart.

Right now, I could care less about perfectly real weather, b/c that would take some monstrous tech, money, and time to implement something like that into FG, but I was tired of everything above 9000ft ASL being completely ignored by the current weather model. I have a flight planner too, and like to be able to utilize it more, rather than just going, for the most part, from point A to point B. I don't know what your "very special" flight planner is, but I just have the Kelpie flight planner for FG. I can't afford anything expensive that could actually be used inreal life. My goal was basically to be able to look at some weather charts and create a flight plan that utilizes the jetstream to decrease flight time. I usually fly eastbound north of the ITCZ at high altitudes, so I wouldn't have to worry about avoiding it very often.
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Re: Winds aloft/Jetstream?

Postby MAKG » Fri Mar 06, 2009 4:08 pm

The "special" flight planner wouldn't be useful to you.

It has to be special because it's planning segments based on the azimuth of identified stars -- in the absence of wind, they are cycloids on the sphere, quite far from great circles or rhumb lines, and the wind triangle is NOT solved as it is for any other aircraft. It's for an airborne telescope with extremely limited range in azimuth. And the FDM is strictly for a 747SP flying at mach 0.84, which isn't supported by FlightGear (though I'd suppose a 747-100 ought to be "close," as long as the 2.5m x 4m hole in the side is closed....).

It's a bit too limited for general purpose. But I do have to consume weather data, including the jet stream and other upper air winds, to make a sensible plan without pissing off the folks at the Nevada Test Site (R-4806) or getting up close and personal with missile tests in W-291. Or causing an incident with the Mexican government (unplanned foreign airspace incursions are a Bad Thing).
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Re: Winds aloft/Jetstream?

Postby redneck » Fri Mar 06, 2009 5:29 pm

:shock: Sometimes I forget that I'm not just talking to casual simmers and aviation enthusiasts. Your job description is a bit of a mouthful.
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Re: Winds aloft/Jetstream?

Postby MAKG » Fri Mar 06, 2009 6:35 pm

Sometimes I forget the opposite -- though I've been a casual user as well. I like to use the 787 (particularly its autopilot) to figure out what I'm asking our pilots to do. It's been very helpful; it's clear we're not going to get better than 1 deg heading control even in "heading mode," and that's the broad side of a barn for pointing a telescope.
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Re: Winds aloft/Jetstream?

Postby redneck » Fri Mar 06, 2009 7:58 pm

I think in the planes that can be flown by the ap route manager you can get the heading to be as precise as a thousandth of a degree using the AP dialog that won't work in the 787. The only issue is that the autopilot for any of the other airliners is a bit glitchy, and the a/c with really goop ap (I have so far to be the B-2 and the dhc2) don't even come close to the fdm you need to work with. The 737-300 has a much better ap than the airbus family and the 747.
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Re: Winds aloft/Jetstream?

Postby MAKG » Fri Mar 06, 2009 8:20 pm

To be honest, the GUI dialog is irrelevant for this. It's all physical interface.

There is no way to specify a heading more precise than 1 deg.

It might be nice if a real 747SP with vacuum controlled avionics (really! This thing is that old) had a GUI you could type into, but it doesn't.

FG isn't the authority here, it's just a driver for questions. And it's a whole lot more convenient than a 6 hour drive (each way) down to KPMD. Yeah, it's a bit irritating that I work at one airport (KNUQ) and visit another (KPMD, sometimes KEDW and very occasionally KCNW), and have to drive....

And there is the distinction between precision and accuracy. Even if I could specify heading down to the .0003 degrees I need to point a telescope (same deal with pitch and roll angles!), the autopilot wouldn't be able to actually do it.
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Re: Winds aloft/Jetstream?

Postby wookierabbit » Wed Apr 21, 2010 4:21 am

I am dead tired, and I have just a simple question. Are metar winds aloft available on both 1.9.1b and 2.0.0? I know I am being a brat for not reading the forum, but I am seriously dead tired and will need to know hopefully by tomorrow morning. Great, now I am hungry. Thanks.
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