txnetcop
March 18th, 2015, 18:01
Controlling the Wind in X-Plane
By Chuck Bodeen
Just suppose you are sitting at the end of runway 09 at KCYS (http://www.cheyenneairport.com/), Cheyenne Wyoming Regional, and you want to simulate a takeoff into head wind.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-18293300-1425663775_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-18293300-1425663775.jpg)
While I want to keep this article on topic, my memories of Cheyenne [my birthplace] have taken over. Before World War II, my father was a Douglas DC-3 (http://en.wikipedia.org/wiki/Douglas_DC-3) mechanical inspector for United Air Lines and worked in a hangar at <A>. We lived in a house at <B> which was (at the beginning of the War) bought by the government, moved into town, and replaced by a couple of large hangars where, "The tail gun position [on B-17s] underwent one major revision, based on a design proposed by workers at the United Air Lines Cheyenne Modification Center — hence, the Cheyenne Turret (http://en.wikipedia.org/wiki/Tail_gunner#mediaviewer/File:B17_tail_turret.jpg). "
In the summer, as a young man, I had a job renting seat cushions at <C> for the annual Frontier Days Rodeo (http://www.cfdrodeo.com/rodeo-main/) . Runway13-31 <D> did not exist and 9700 foot 09-27 was only about half that long…which was plenty for the DC-3s which needed only about 6000 feet <E> for takeoff on a hot day at 6160 feet MSL.
I well remember the day the first DC-4 (http://en.wikipedia.org/wiki/Douglas_DC-4) came in. That eventually put an end to the need for the DC-3 facility at Cheyenne as well as my Dad’s job.
Using X-Plane as it is
The heading of runway 09 is 85 degrees magnetic; 95 degrees true. Let’s say the wind is coming from 90 degrees – with a slight crosswind component.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-58009100-1425663824_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-58009100-1425663824.jpg)
On the X-Plane “environment/weather page you will find the input area on the left. I used this to create the wind speed profile on the right. The solid red lines are interpolations between the input data points. The dotted lines are constant wind speed extensions. The blue represent what the actual profile might have been.
The rules are that the lowest-altitude must be at least 1000 feet. The other two must be separated by at least 1000 feet. This is explained rather briefly here (http://wiki.x-plane.com/Chapter_4:_Flight#Wind_Layers).
I’m going to concentrate this discussion to wind speed because wind shear or “gradient (http://en.wikipedia.org/wiki/Wind_gradient)” is necessary for “Dynamic Soaring (http://en.wikipedia.org/wiki/Dynamic_soaring)”…and that’s what brought me to the current subject. I have kept the direction constant because the majority of technical papers on DS and wind shear do that.
Dynamic Soaring is used by the albatross to fly for as far as 1500 km at a time without flapping a wing and by radio controlled glider pilots to reach speeds over 500 mph. An article on that subject will be coming…eventually.
Besides wind speed, X-Plane can set gust increase, shear direction, and turbulence.
Gusts:
On the X-Plane.org forum, “Bret S” (http://forums.x-plane.org/index.php?showtopic=67872) explains that this feature “adds random wind gusts up to the value you set…and in the direction of the wind that you set.”
“Morten” posted a comment on gusts and shear direction variation:
It starts with this video (http://www.xplanefreeware.net/morten/DIV/wind.mov) of a panel flying through gusty air and says:
“As you can see, XP pretty much does what it should and stays within the peaks.
That’s not the problem. The problem is the RATE/FREQUENCY it does this it!
It can go between peaks multiple times per second! If this was the real world I doubt there would be many sailboats or light aircraft around...”
Shear directionAustin Meyer says this, “ Interpolate between the shear should probably be labeled as “variation”, since I actually change in direction over TIME, NOT SPACE. So you enter the direction and speed variation at each altitude, and x-plane interpolates in between them.The wind changes slowly and smoothly over at least 1,000 feet… far too little shear to use.The shear is the variation in direction… it changes on a frequency of about 1 or 2 seconds or so.. It is a random change, so could not be used to drive an airplane, I think”
Turbulence
Still on the Forums, Andy Goldstein (http://home.earthlink.net/~x-plane/FAQ-Author.html) says “The turbulence calibration has been a sore point for years. Best as I can make out, the turbulence scale goes from 0 to 10, and each unit represents 10 kt variation in wind speed. Also, the turbulence is continuous chop, vs the intermittent bumps you normally get in real life. Put together, a little goes a long way.
And Chris Serio (http://developer.x-plane.com/author/cjserio/) adds: “I received many bug reports about turbulence this week. Austin responded by making some tweaks which will be in the future 10.04Beta2. When it's released it should be a step in the right direction though I haven't had a chance to try it myself.”
For the applications in which I am interested the first thing I had to accomplish was to eliminate the 1000 foot spacing restriction. The datarefs used to set the three altitudes and set direction and speed are writable:
sim/weather/wind_altitude_msl_m[0] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_altitude_msl_m[1] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_altitude_msl_m[2] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_direction_degt[0] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_direction_degt[1] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_direction_degt[2] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_speed_kt[0] float y kts = 0 The wind speed in knots.
sim/weather/wind_speed_kt[1] float y kts = 0 The wind speed in knots.
sim/weather/wind_speed_kt[2] float y kts = 0 The wind speed in knots.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-17627100-1425663944_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-17627100-1425663944.jpg)
With the use of a plugin, the wind profile parameters can be released from the 1,000 foot spacing rule. Using altitudes, direction, and speeds fairly complex wind profiles can be produced. Of course, this means that we’ll need a C++ plugin or a Python script, but once we make that move there is a world of possibilities.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-60014300-1425663952_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-60014300-1425663952.jpg)
This diagram shows what you can do by varying both the speed and direction of the wind. Notice that the 1000 foot rule need not be enforced.
Continued tomorrow...
By Chuck Bodeen
Just suppose you are sitting at the end of runway 09 at KCYS (http://www.cheyenneairport.com/), Cheyenne Wyoming Regional, and you want to simulate a takeoff into head wind.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-18293300-1425663775_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-18293300-1425663775.jpg)
While I want to keep this article on topic, my memories of Cheyenne [my birthplace] have taken over. Before World War II, my father was a Douglas DC-3 (http://en.wikipedia.org/wiki/Douglas_DC-3) mechanical inspector for United Air Lines and worked in a hangar at <A>. We lived in a house at <B> which was (at the beginning of the War) bought by the government, moved into town, and replaced by a couple of large hangars where, "The tail gun position [on B-17s] underwent one major revision, based on a design proposed by workers at the United Air Lines Cheyenne Modification Center — hence, the Cheyenne Turret (http://en.wikipedia.org/wiki/Tail_gunner#mediaviewer/File:B17_tail_turret.jpg). "
In the summer, as a young man, I had a job renting seat cushions at <C> for the annual Frontier Days Rodeo (http://www.cfdrodeo.com/rodeo-main/) . Runway13-31 <D> did not exist and 9700 foot 09-27 was only about half that long…which was plenty for the DC-3s which needed only about 6000 feet <E> for takeoff on a hot day at 6160 feet MSL.
I well remember the day the first DC-4 (http://en.wikipedia.org/wiki/Douglas_DC-4) came in. That eventually put an end to the need for the DC-3 facility at Cheyenne as well as my Dad’s job.
Using X-Plane as it is
The heading of runway 09 is 85 degrees magnetic; 95 degrees true. Let’s say the wind is coming from 90 degrees – with a slight crosswind component.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-58009100-1425663824_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-58009100-1425663824.jpg)
On the X-Plane “environment/weather page you will find the input area on the left. I used this to create the wind speed profile on the right. The solid red lines are interpolations between the input data points. The dotted lines are constant wind speed extensions. The blue represent what the actual profile might have been.
The rules are that the lowest-altitude must be at least 1000 feet. The other two must be separated by at least 1000 feet. This is explained rather briefly here (http://wiki.x-plane.com/Chapter_4:_Flight#Wind_Layers).
I’m going to concentrate this discussion to wind speed because wind shear or “gradient (http://en.wikipedia.org/wiki/Wind_gradient)” is necessary for “Dynamic Soaring (http://en.wikipedia.org/wiki/Dynamic_soaring)”…and that’s what brought me to the current subject. I have kept the direction constant because the majority of technical papers on DS and wind shear do that.
Dynamic Soaring is used by the albatross to fly for as far as 1500 km at a time without flapping a wing and by radio controlled glider pilots to reach speeds over 500 mph. An article on that subject will be coming…eventually.
Besides wind speed, X-Plane can set gust increase, shear direction, and turbulence.
Gusts:
On the X-Plane.org forum, “Bret S” (http://forums.x-plane.org/index.php?showtopic=67872) explains that this feature “adds random wind gusts up to the value you set…and in the direction of the wind that you set.”
“Morten” posted a comment on gusts and shear direction variation:
It starts with this video (http://www.xplanefreeware.net/morten/DIV/wind.mov) of a panel flying through gusty air and says:
“As you can see, XP pretty much does what it should and stays within the peaks.
That’s not the problem. The problem is the RATE/FREQUENCY it does this it!
It can go between peaks multiple times per second! If this was the real world I doubt there would be many sailboats or light aircraft around...”
Shear directionAustin Meyer says this, “ Interpolate between the shear should probably be labeled as “variation”, since I actually change in direction over TIME, NOT SPACE. So you enter the direction and speed variation at each altitude, and x-plane interpolates in between them.The wind changes slowly and smoothly over at least 1,000 feet… far too little shear to use.The shear is the variation in direction… it changes on a frequency of about 1 or 2 seconds or so.. It is a random change, so could not be used to drive an airplane, I think”
Turbulence
Still on the Forums, Andy Goldstein (http://home.earthlink.net/~x-plane/FAQ-Author.html) says “The turbulence calibration has been a sore point for years. Best as I can make out, the turbulence scale goes from 0 to 10, and each unit represents 10 kt variation in wind speed. Also, the turbulence is continuous chop, vs the intermittent bumps you normally get in real life. Put together, a little goes a long way.
And Chris Serio (http://developer.x-plane.com/author/cjserio/) adds: “I received many bug reports about turbulence this week. Austin responded by making some tweaks which will be in the future 10.04Beta2. When it's released it should be a step in the right direction though I haven't had a chance to try it myself.”
For the applications in which I am interested the first thing I had to accomplish was to eliminate the 1000 foot spacing restriction. The datarefs used to set the three altitudes and set direction and speed are writable:
sim/weather/wind_altitude_msl_m[0] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_altitude_msl_m[1] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_altitude_msl_m[2] float y meters = 0 The center altitude of this layer of wind in MSL meters.
sim/weather/wind_direction_degt[0] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_direction_degt[1] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_direction_degt[2] float y [0 - 360) The direction the wind is blowing from in degrees from true north clockwise.
sim/weather/wind_speed_kt[0] float y kts = 0 The wind speed in knots.
sim/weather/wind_speed_kt[1] float y kts = 0 The wind speed in knots.
sim/weather/wind_speed_kt[2] float y kts = 0 The wind speed in knots.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-17627100-1425663944_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-17627100-1425663944.jpg)
With the use of a plugin, the wind profile parameters can be released from the 1,000 foot spacing rule. Using altitudes, direction, and speeds fairly complex wind profiles can be produced. Of course, this means that we’ll need a C++ plugin or a Python script, but once we make that move there is a world of possibilities.
http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-60014300-1425663952_thumb.jpg (http://forums.x-plane.org/uploads/monthly_03_2015/post-1379-0-60014300-1425663952.jpg)
This diagram shows what you can do by varying both the speed and direction of the wind. Notice that the 1000 foot rule need not be enforced.
Continued tomorrow...