Winds at altitude - RC Groups
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Jun 14, 2014, 04:38 PM
Registered User
RPinCA's Avatar
Discussion

Winds at altitude


The background: Tuesday morning I went to SVSS for a quick flight. Tom was just leaving after Bruce searched for his plane from the Flying Lawn Chair. As I pulled out my Graphite, Bruce advised against flying. He had just been in the air, but landed after only 20 minutes due to extreme winds. They didn't seem that bad on the ground, around 10mph, but Bruce said as soon as he got off the ground he was hit with bad turbulence.

Look what I found while searching for the UCD Wx Station:


Note the entries for wind at .5m and wind at 10m (today’s readings, 14 Jun 2014). At Russell Ranch, winds at standing elevation are 4.66mph, but at 10m (33’) it jumps to 10.46mph – more the double. Gusts: 5.78 go to 13.22, almost exactly the same ratio. Campbell Tract is not quite double, but close enough for horseshoes. This tells me two things.
1. On approach, winds will decrease as the plane nears the ground. This explains some of the overshoots I have not understood. This is classic wind shear. As a GA pilot in my Bonanza, I never really noticed this shear. We are trained to be alert for shear at 200-500’ that has caused crashes, but here I am seeing something new that happens at ground level.

2. When we look at the wind sock or use a hand-held anemometer, we are seeing the slower wind at the surface. It is likely-to-certain that the wind at 10m will be stronger – and if this sample represents the norm, the wind will be twice as strong at 10m. This dovetails directly into Bruce’s report: “as soon as I got off the ground...”
Following Bruce’s advice, I packed up my plane and didn’t fly Tuesday. I would not have known without his report.

I doubt there is any way we can erect an anemometer at 10m on our field. I will continue to look at this information over time, to see if the 2x correlation is consistent. It’s important if it’s true.

There is a nice video by Joe Wurts on YouTube:

Joe Wurts on soaring (1 hr 22 min 27 sec)

At time 1:33 and throughout the video, we see a pair of streamers on a pole. There are two poles, and perhaps more. The streamers look very helpful, especially if you take the time to watch the video and think about having them at the four corners of SVSS. I have noticed that the streamers are fairly high for a flying field, much higher than our wind sock. Might they be at that height to show the “true wind” rather than the wind slowed by weeds and trees?

We could have our own weather station using cellular service to make readings available. This seems like a valuable investment for our club.
Vantage Vue Wx Station........ http://www.vantagevue.com/products/p...asp?pnum=06250
Vantage Pro2 Wx Station...... http://www.davisnet.com/weather/prod...asp?pnum=06152
Wireless Vantage Connect... http://www.davisnet.com/weather/prod...asp?pnum=06620
The total solution is about $1000-$1200 in parts. There are monthly charges for the wireless service, unless we can piggyback onto a local farmer’s network in return for sharing the information with the farmer. Must be within 1000’, though, so this seems unlikely.
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Jun 15, 2014, 01:30 PM
Registered User
Be nice to set up some poles (2 or more-1" sch. 40 20 ft. length and some crate paper ribbons or fabric ribbons) and have some of the LS5 guys lead a work shop.
That is doable for $20 or so.
Jun 15, 2014, 02:36 PM
Registered User
About two years ago the club looked into setting up our very own weather station, which would be very nice if it were practical. Unfortunately lack of internet access and the need to use an expensive alternative such as cell service as well as the need for 24/7 power made for complicated and/or impractical solutions that the club as a whole found not worth the investment.

Fortunately, we have available two good sources of nearby weather that are, in my experience, excellent predictors of wind speeds.
The first it the UC airport (about 6 miles away):

http://www.wrh.noaa.gov/zoa/getobext.php?sid=KEDU
This is called OBS: University Airport
It is reliable and a very good predictor of what our field conditions will be like
as the wind usually comes up at the airport about an hour before is moves over to our site.

The second source is Finch Farms which is about a mile or so to the south east of our field.
http://www.wunderground.com/personal...MFFAC1#history
This one tells us what it is like right now at our field. Unfortunately, it seems to go down occasionally and defaults to another weather station in El Macero which must be located in an enclosed area as wind speed reads much below our field. If you see that the correct web site is listed on the header listing you can rely on the numbers.

I have found these two sources most adequate for knowing our field conditions.

JPH
Last edited by jpherit; Jun 17, 2014 at 10:31 AM.
Jun 16, 2014, 12:44 AM
Team Hello Kitty
SoaringDude's Avatar
A few related links:

SVSS Weather Station Design Website -- This presents the various design choices and exploratory tests that were done re: the SVSS weather station design. A number of Internet connectivity options were explored including satellite service, ham radio 2mtr packet radio, and several others. Since we had a relationship with the Yolo Bowmen archery club 3 years ago they offered to let us set up a satellite dish on their clubhouse roof for free (clubhouse has AC power). By the way, the Bowmen vacated the park a few years ago.

SVSS Forum Weather Station thread -- This thread tracked the discussion of a few weather station topics. While the project was formally approved the funding was withheld because the club found itself in limbo regarding the Yolo County lease agreement annual costs.
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I agree with Jonathan re: the Davis airstrip data. After using it every week for 3 years it's rare when the wind conditions on our own field differ much from the airstrip numbers. The ONLY benefit I would see to our own weather equip is to discover fog conditions at our field. The visibility data from the Davis strip often does not reflect the fog conditions at our field. A fog "measurement" could be handled by a simple webcam which could also double as a security camera looking out over our storage containers (as we discussed 3 years ago). But I don't think the recurring costs would be justifiable unless we start suffering lots of vandalism at the field.

Regarding winds aloft over our field, three years ago I too was curious about our field's wind gradient. Turns out the composition of ground objects has a big influence on the wind gradient ("shear"). So if you implement this wind shear formula and plug in the shear exponent for flat terrain/tall grasses on our field you get a chart (see attached) that can be used to estimate the wind speed at our field at a chosen altitude by first taking a wind reading from a small wind meter at 6 feet off the ground. From the chart the wind speed at launch altitude is probably ~2x the 6' value. After doing lots of winch launches in wind and following thermals via circling at launch altitude this seems like a decent first order estimate.

The wind shear data from the Davis airstrip instruments may be significantly different from our field's given the instrument tower's proximity to buildings, trees, etc. Only way to have a better guess is to look at their instrument tower and see what kind of terrain and objects surround it.

Chris B.
Jun 16, 2014, 11:34 AM
Team Hello Kitty
SoaringDude's Avatar

How to measure your own winds aloft speeds


The question comes up occasionally about how windy it is at flying altitudes on our field. Everyone pretty much understands that wind speeds get higher with altitude, but by how much? Turns out this is pretty easy to estimate.

Both our north and south winch lanes are approximately 700' long. Using that as our "yardstick" you can estimate the wind speed of the air mass at your plane's altitude by first launching or flying your plane to the upwind end of the winch lane, put your plane into a reasonably tight circle (which in brisk winds will look quite elongated), then count the seconds that it takes for the center of your plane's "circle" to reach you, the R/C pilot. Then use the chart below to convert your seconds into MPH (where 1 ft/sec = 0.681818 MPH).

Obviously if the wind is not parallel with our winch lanes you have to estimate the equivalent distance but this method should yield a good ballpark estimate.

I'm interested in your results if you have a handheld wind meter to measure the ground wind speed and then use this method at various altitudes, especially at winch launch height (~500 ft).

Chris B.
Jun 17, 2014, 11:39 AM
Good for what ALES you
awilmunder's Avatar
I have found this NOAA site very good at predicting wind speeds at the SVSS field. It also charts projected temperatures, sky cover, and gusts over 20mph.

http://forecast.weather.gov/MapClick...Type=graphical
Jun 17, 2014, 12:20 PM
Registered User
RPinCA's Avatar
Hi Aric,

I like that site - I think it was you who introduced it to me. I like it so much I wrote an app that lets you choose any site super-easily, posted here on the SVSS rcGroup forum. It uses the Google map engine to find Lat/Lon from a name or address. At Sheldon's suggestion, it is able to be started by a shortcut with a parameter such as "SVSS", "Visalia", "Montague" or almost anything. For example, you could make a shortcut such as:
LocalWx "1600 Pennsylvania Avenue, Washington, DC"
...and it will bring up the safe data for the White House.

I have a desktop icon for "SVSS Weather" that brings up the page in the link, as well as an "EDH Weather" for a site that's often more calm when it is windy in the valley.

I noticed the NOAA page says "Surface Winds". It's possible that the winds at 10m are twice what's shown on the page.
Jul 28, 2014, 11:41 PM
E sailplane thermal hack
And,,,,, occasionally ground wind speeds DON't relate to higher wind speeds aloft!!
Just this last weekend ( July 28) me and my soaring protege Noah experienced the EXACT opposite. At ground level wind and turbulence very noticeable ,,, but once above approx 300ft the air was noticble smoother and Lower wind speeds


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