Power Kite Forum - Wind Meters: what speed do they read?

Wind Meters: what speed do they read?

nate76 - 7-5-2016 at 06:08 AM

This might be more for the pilots out there, but I'd love to hear from anyone.

So a comment came up in the light wind kite forum that got me thinking. Someone mentioned that kite power decreases with altitude, which is true. The lift of a kite is affected by air density, and air density is affected by altitude, temperature and humidity to a small degree (humid air is actually less dense than dry air).

Anyone interested in playing with numbers can use a Standard Atmosphere calculator like this one to see how these factors affect air density:
http://www.digitaldutch.com/atmoscalc/

But that's not really my question. My question stems from a response I gave saying that wind meters typically measure the wind based on what the kite feels. So if you're at a high altitude and your wind meter reads 15mph, that is what the kite is feeling in terms of lift. In aviation this sort of speed measurement is typically called Indicated Airspeed, which is different from True Airspeed or ground speed. True Airspeed takes into account air density and is a more accurate measure of an objects actual speed over the ground.

But I find myself only half believing that answer. Anyone who's kited at a high altitude vs sea level knows it definitely feels different. So I guess my question is: do wind meters measure wind in terms of indicated or true airspeed?

Anyhow, I was surprised to find when I started looking, there are few related threads on this, and people don't necessarily agree. For one, most wind meter/ anemometers don't tell you what they are measuring their speed in (Indicated or True). For two, in other forums, it isn't entirely agreed upon and obvious that wind meters like the ones we use actually measure indicated airspeed, because they are using a different technique to determine speed than an aircraft does (an impellor vs a pitot-static system).

Just as fodder for discussion, I'll include a link to another forum discussing wind speed measurements:
http://www.pilotsofamerica.com/community/threads/wind-speed....

I'll give my initial thoughts, and then I'll shut up. I think our wind meters do measure speed in units approximating Indicated airspeed. However many of the things we do are related to ground speed, so that is why we sense the difference. For example, when we are moving across the water, that is a ground speed. And for two identical ground speeds at different altitudes/temperatures, you will have different amounts of lift and different amounts of power. When we dive the kite for a power stroke, our eyes are accustomed to seeing the actual speed of the kite moving across the sky - so that is essentially a ground speed measurement as well. And since we become accustomed to the kite producing a certain amount of power for a given speed of power stroke based on local conditions, we notice the change in power when we do the same maneuver at a different altitude/temperature.

Anyhoo, not sure if that makes a lick of sense, but would be interested in hearing others' insights. You're never too old to learn something new!

Nate

Windstruck - 7-5-2016 at 06:36 AM

Great discussion Nate! I'm ashamed to say I don't have something to add that helps your specific question. However, I do have an additional personal perspective that may strengthen your point (or not). Like you, I live at higher altitude and do my buggying around home at roughly 7,000 feet above sea level, give or take. I use BigFoot tires and ride on grass fields.

When I go down to Ivanpah with the same buggy and kites (Ivanpah being around 5,000 feet lower, again give or take) I have to go way, way down in kite size for similar feel and speeds. Yes, I know all about the rolling resistance issue and don't contest it, but I swear that doesn't explain away all of the difference. I'd give the hard surface one stage down in kite size, but not the two sizes I seem to need to go down, so I'm going to credit the second kite size shrinkage to the increased density of the air at this relatively lower altitude.

Interesting note that humid air is LESS dense than dry air; asked to guess before reading that and I would have said MORE.

Nice thread. :thumbup:

ssayre - 7-5-2016 at 07:05 AM

If you guys want a free wind meter to play with, I have one that was given to me from weldngod. Only rule is it has to stay in the pkf family when you get rid of it.

let me know. Shipping is included.

bigkid - 7-5-2016 at 07:05 AM

I got into a discussion with an old timer a few years back about windmeters. WOW, did I get schooled. " you damn kids think you know everything, well let me tell you". Never thought that color red was possible in that old of an individual.
Doesn't matter one way or the other, it isn't real air speed anyway. Which meter is best? Is 1mph really 1mph? What about 65mph? Well I stuck the thing out the car window and it was the same as the speedometer.....
I have found they are not equal in quality, cost, or performance. I use my wind meter as a guide for my own use. It is nothing more than a tool I use to gauge the wind at that "moment" so I can pick a size of kite that should work for that area, that surface, and that vehicle.

I still think back to the day when Dave told me where to put my wind meter, sure do miss him.:D

WELDNGOD - 7-5-2016 at 07:56 AM

Despite density, wind speed is still wind speed. Doesn't matter if it is a brick or a feather ,if it travels "x" distance in "y" time , it's going "z" mph. 10 mph at sea level is still 10 mph in Denver But with a less dense medium. Which means it has less ability to hold up the same weight as @ sea level.

IFlyKites - 7-5-2016 at 08:43 AM

Good topic! :thumbup:

Here is my take on it.

I would say wind meters measure indicated air speed. It wouldn't really make sense for an anemometer to measure TAS as they do not have anything built into them to detect density, pressure etc.. and combine that to give us a true air speed.
For TAS to be calculated, (gotten from IVAO):

1) For low speeds, the data required are static air temperature, pressure altitude and IAS

2) Above approximately 100 knots, the compressibility error rises significantly and TAS must be
calculated by the Mach speed.

*Indicated air speed is always lower than True Air speed from what I understand.

nate76 - 7-5-2016 at 03:04 PM

Quote:

Interesting note that humid air is LESS dense than dry air; asked to guess before reading that and I would have said MORE.

Ya, it seems counter-intuitive, but it's because about 80% of the atmosphere is nitrogen, whereas 2/3 of a water molecule is made up of hydrogen, which is really light. At the end of the day though, humidity has a pretty minor effect.

Quote:

Despite density, wind speed is still wind speed. Doesn't matter if it is a brick or a feather ,if it travels "x" distance in "y" time , it's going "z" mph. !0 mph at sea level is still 10 mph in Denver. But with a less dense medium. Which means it has less ability to hold up the same weight as @ sea level.

Believe it or not, that is only half true. The traditional way an airplane measures speed (and to a degree most wind meters), is by measuring differences in pressure. For things that fly, this is what is really important, because it is a direct measure of what the wing (or kite foil) is feeling in terms of being close to stall, etc. But the actual air particle velocity is much faster at higher density altitudes.

That may be hard to believe at first, but I'll give you an example. I'm sure our airline buddies in this forum could tell you that when they are at their cruising altitude, their ground speed (so the speed they are physically traveling across the earth) is something like 450 knots. But the airspeed indicator on the instrument panel will only be registering something like 250 kts.

Another way of putting it is if you had your wind meter out at the ocean beach, and it was reading 20mph, and a dandelion seed flew by suspended in the air, it would also be moving at 20mph. Now lets say you are on top of a nice 12,000ft Colorado plateau and took a wind meter reading of 20mph. That same dandelion seed that just flew in from the coast would be traveling almost 30mph! To your point though, it doesn't really feel like a 30mph wind - at least when the kite is stationary above you - because even though the actual air particles are moving faster, there are much fewer of them.

Now, as soon as you start moving yourself or the kite, all your speed references are based off of ground speed (TAS), and you will start to discern differences in the air density. As an example, if you took your buggy up to this 12,000ft plateau, and your GPS said you were going 30mph, it would feel like you had about 30% less pull than if you were going the same speed at your local beach at sea level.

At the end of the day, none of us really sit around wondering, "Hey, I wonder what the air particle velocity is today?". There are actually really expensive wind meters that use lasers to do that - to track individual air particles - but that's not really what we're interested in. All we want to know is, "What is the general wind speed?" as a point of reference, and then we've become accustomed to semi-consciously making decisions on how much actual power is in the wind that day based where we are at (what altitude we are at and how hot it is).

nate76 - 7-5-2016 at 03:15 PM

Quote:

When I go down to Ivanpah with the same buggy and kites (Ivanpah being around 5,000 feet lower, again give or take) I have to go way, way down in kite size for similar feel and speeds.

@Windstruck - Ivanpah sounds like an awesome place. One of these years it would be fun to meet up with you out there at IBX!

Quote:

2) Above approximately 100 knots, the compressibility error rises significantly and TAS must be calculated by the Mach speed.

@IFlyKites, good point, though I personally am not quite to the level where I need to measure my buggy speed in terms of Mach number. BeamerBob and WELDNGOD on the other hand - well that's a different story!

Methinks a Mach gauge would look pretty cool strapped on a buggy.

Feyd - 7-5-2016 at 06:27 PM

Big Kid nails it. I did the same thing driving around comparing to the speedo.

IMO the bottom line is as far as the specifics of what a given meter reads and shows, I don't know that it really matters much. As long as it measures consistently, the accuracy in terms of true wind speed is irrelevant. After a while most of us stop using meters as we become more familiar with how the wind feels and sounds and how a given kite responds in a set of conditions. We teach our clients to see the tell tails around us and to use a meter. But personally, my ears are my #1 meter.

Even if you have the most accurate data in terms of wind speed, how does relate to the posted wind ranges of kites? Though a good baseline, they are arbitrary at best given the number of variables that come into play when they have to pull something/someone. Line length alone complete changes the wind ranges and the power produced for a given size in a given wind at a given altitude in a given air temp on a given surface. (phew!)

I don't know. It's amazing it works at all and you could go nuts trying to think too much about it. Kiting is a technical activity but also an open skills, visceral and intuitive thing. I "feel" it more than "think" it.

Windstruck - 7-5-2016 at 07:18 PM

Quote: Originally posted by nate76

Quote:

@Windstruck - Ivanpah sounds like an awesome place. One of these years it would be fun to meet up with you out there at IBX!

That sounds like a great plan Nate! I'm sure you know this, but there is a whole contingency of great folks that go to IBX from Colorado.

AudereEng - 7-5-2016 at 09:08 PM

I think I have a different cut on this than most of you.
I have a standard hobby wind meter but almost never carry it out on the sand.

My issue is the local permanent wind sensors are all very different from each other in terms of their physics.
South Beach park has props which are > 20x bigger than the ones on the bridge 2000' away.
These sensors speed up and rotate in direction with very different time constants etc.
So the gusts and wind directional shift data is often worthless because of the physics of the sensors and low speed update rate...
And if you are measuring the speed of rotation of a prop then this is only close to real wind speed.

I want a ultrasonic 3D wind meter.
This system uses 3 to 6 sound wave paths to measure the propagation delay in all 3 axis.
They are mainly used on high value structures like train bridges in windy places in the EU etc.

I considered making a commercial unit for the sail boat market with some upgraded features at a lower price point.
One of the systems features would have been a wireless phone interface to a server with data storage/processing and a html interface so you could get the most info possible before any extended drives to your boat or target sailing area (aka like DC to Florida)
The system was designed to be fairly advanced compared to what exists today which were designed many years ago (maybe 70s to 80s) - it would measure the paths in real time as opposed to multiplexed so you would never loose any gust data which is important for "insurance" related issues.
It was designed to have a reasonable amount of local processing in the unit (aka includes a DSP processor...)
I purchased and tested ultrasonic transmitters/receivers to verify proof of concept which all looked workable but
In the end - the markets I understand look too small at this point in time to be commercially viable.
The insurance angle might make it very workable (wind damage claims are huge) but I have little insight into what the deal would be with this business angle.

Erg - I really want about 20 of these along the coast around me so I can see what is really going down.
LOL - it is not the 10 to 30 minute drive to the sand it is the taking my focus away from what I am working on which makes me want the system so bad.
Sometimes a shift of 5 degrees in wind direction is critical to me as to work-ability of the wind due to the bluffs around here.
It is frustrating when I have a reading off the sensors of 30 gusting 35 but the real wind I get to work with is not that usable due to a slight wind angle issue...

Bladerunner - 7-5-2016 at 10:01 PM

Everything I need to know to chose my kite can be figured out by looking at a spinner wind sock! Best tool by far for judging wind IMHO :yes:

bigkid - 7-5-2016 at 10:56 PM

Quote: Originally posted by Bladerunner

Everything I need to know to chose my kite can be figured out by looking at a spinner wind sock! Best tool by far for judging wind IMHO :yes:

for me it's whether I can scud 10-15 feet at launch. If not, it's time to get out a bigger kite. :cool:

WELDNGOD - 8-5-2016 at 05:48 AM

Quote: Originally posted by bigkid

for me it's whether I can scud 10-15 feet at launch. If not, it's time to get out a bigger kite. :cool:

That's about right!

bigkid - 8-5-2016 at 06:40 AM

Quote: Originally posted by WELDNGOD

Quote: Originally posted by bigkid

for me it's whether I can scud 10-15 feet at launch. If not, it's time to get out a bigger kite. :cool:

That's about right!

This is based on the fact that I buggy with an extra amount of weight. My buggy is over 100lbs so I need to have a kite big enough to move me and the buggy, a total weight package of 340lbs. A smaller person and a lighter buggy might only need to move an amount of weight of 175lbs.
If all things are equal, (wind, area, surface, place) two people being as I said above, the heavy package would need a 12m kite while the lighter package would only need a 6m kite.
The lighter person with a PL bug might not need to scud at all and still buggy without a problem while a heavy person with a 120lb custom race bug would need to scud up to 15-25 feet to buggy with the lighter person.
As with everything else, with all the variables involved this is only a basic guide.
So, your light wind kite is not even close to what would work for me. Get what I am trying to say? Wonder when they will make a 20m fb kite for us big guys?:evil: :lol:

So back to the wind meter, I don't use it anymore. For sale 2 wind meters at a good price. :lol:

Bladerunner - 8-5-2016 at 10:13 AM

I like the wind sock because it not only gives me a good indication of what kite I should chose but once in motion I need only look over at it to see if wind direction or speed has changed.

I can get moving in my little buggy without dragging near as much load as some others. I am certain that my buggy and I come in under 200lbs. Still for low winds and a big beast like my 19m Speed I need room to build on apparent winds.

Chook - 9-5-2016 at 01:38 AM

I was really confused at first as I found that I needed to go up 2 kite sizes in warm humid air on the coast of Queensland compared to kiting at home on the south coast of Western Australia.
My visual indicator was the white caps just starting to form on the ocean at 10 knots here at home. With these same wave indications up above the "27th Parallel" in warm humid Queensland I need at least 1.5m2 more area to do the same job.
I double checked this as a light aircraft pilot, with both the local airfield windsocks as when they have a slight crease across the middle (and 5 degrees below horizontal) this indicates 10 knots as well. The warm humid air has less power for sure.

I'm really enjoying this thread. Cheers guys.

RedSky - 9-5-2016 at 07:01 AM

Quote: Originally posted by Chook

I was really confused at first as I found that I needed to go up 2 kite sizes in warm humid air on the coast of Queensland compared to kiting at home on the south coast of Western Australia.
My visual indicator was the white caps just starting to form on the ocean at 10 knots here at home. With these same wave indications up above the "27th Parallel" in warm humid Queensland I need at least 1.5m2 more area to do the same job.
I double checked this as a light aircraft pilot, with both the local airfield windsocks as when they have a slight crease across the middle (and 5 degrees below horizontal) this indicates 10 knots as well. The warm humid air has less power for sure.

I'm really enjoying this thread. Cheers guys.

Don't forget that altitude also plays a part in air density. Look at any map of Australia and you'll see that the south coast is much lower down than Queensland so you'll need to factor in the differences of altitude between those two beaches.

Windstruck - 9-5-2016 at 07:24 AM

Quote: Originally posted by RedSky

Quote: Originally posted by Chook

:puzzled: Assuming here that you are joking around.... :smug:

RedSky - 9-5-2016 at 10:30 AM

Quote: Originally posted by Windstruck

Quote: Originally posted by RedSky

Quote: Originally posted by Chook

:puzzled: Assuming here that you are joking around.... :smug:

I demand that you take me seriously. :alien:

Cerebite - 9-5-2016 at 10:35 AM

agree Nate, great topic thanks for running with it after I brought it up.

So what we are agreeing is that the air particles are moving at the same speed across the ground in South Padre and in Denver [excluding humidity] but because there are fewer of them up here [density] there is less power applied to the kite [or pressure difference between the "front" and "back" of the kite/ wing].

If density is having as much effect on the power applied by the wind as the speed [relatively speaking] then do we need to start carrying barometers to our kite flies or learn our "home density" for reference when we travel down to Ivanpah or up to Dillon?

abkayak - 9-5-2016 at 11:32 AM

come on...if you need to show up w/ barometers and wind meters your doing it wrong
if you have that many choices in the quiver you got to know what to throw in the air...
of course i like to see a flag or 2 but kite sizing isnt a critical decision unless the soft sand is coming off the ground
i get to hit the NDBC to see what it exactly was

RedSky - 9-5-2016 at 12:03 PM

Quote: Originally posted by abkayak

come on...if you need to show up w/ barometers and wind meters your doing it wrong
if you have that many choices in the quiver you got to know what to throw in the air...
of course i like to see a flag or 2 but kite sizing isnt a critical decision unless the soft sand is coming off the ground
i get to hit the NDBC to see what it exactly was

If I remember correctly, soft sand starts skipping across the beach at 30mph (rough est)

BeamerBob - 9-5-2016 at 12:51 PM

I'll point out that the low rolling resistance at Ivanpah has WAY more to do with kite size choices than temperature, elevation, and/or humidity. Ride at any beach and get over 30-35 mph and you feel like it was a fast session. 30-35 would be a common speed reached by most anyone during their first few days at Ivanpah, depending on skill level. And they will almost assuredly do so with a fb kite < 4m or equivalent. Only the very best pilots can make use of 5m+ kites there except in the lightest of winds.

nate76 - 9-5-2016 at 02:32 PM

Quote:

So what we are agreeing is that the air particles are moving at the same speed across the ground in South Padre and in Denver [excluding humidity] but because there are fewer of them up here [density] there is less power applied to the kite.

What you're saying is correct: for two air masses moving the same speed, the one at SPI will have more power because the air there is more dense.

There was a larger argument that I was trying to get at and don't think I articulated well. It relates best to low winds and critical winds speed required for a kite to fly, and I think it does have some marginal real-world application:

Lets say I have a particularly horrible kite. It launches and flies great at 8mph, but at 7.9mph it won't even come off the ground. And lets assume I live on the beach at sea level. One might assume based on our discussions that if I took this miserable kite to WindstrucS-P-A-M-L-I-N-K-s house in Utah at 7,000ft, and the wind was blowing 8mph - that my kite wound not get off the ground. My argument is that it WOULD take off and fly just fine - albeit with less power. The reason this is true is because most wind meters measure pressure differentials - not differences in speed. The wind speed ends up being more of a back-calculation. If it was measuring just purely differences in the speed of the air mass, I would expect that this kite would not be able to get off the ground since the air is less dense at the higher altitude.

It isn't a particularly bad thing for a wind meter to calculate speed this way; in fact it is kind of nice. It makes kite launching estimations pretty easy: if my crappy kite launches in an 8mph wind at the beach, its going to launch in 8mph of wind in Utah or Colorado or Mt Everest or Mars as well. No mental gymnastics required. What does change though - and what we've all felt when we go somewhere different - is that the amount of power the kite produces as we move along or sine the kite does feel different.

So this is the point at which I had an honest question. Is the scenario I laid out actually true? My gut tells me from my own experience that kites seem to launch in about the same minimum measured wind speed regardless of altitude or temperature. This in my mind make sense because it relates back to my aviation background where an airplane will stall at the same Indicated airspeed regardless of whether its at sea level or 30,000ft. With kites its harder to tell because the speeds we are dealing with are so small to begin with. I'm willing to admit I could be all wrong though and was wondering about other people's experiences and observations.

Like many have mentioned - I rarely use a wind meter any more. Generally the only time I use one is if I'm doing a review on a kite and want to try to be accurate with my wind ranges, or if its gusting really bad and I want to know what the gust differential is. Otherwise my face works just about as well as any wind meter for figuring out what size kite to throw up.

And Bob's right, things like rolling resistance are much more important for actually figuring what size kite to fly. This is just kind of gee-whiz stuff.

Bladerunner - 9-5-2016 at 03:06 PM

Quote: Originally posted by Windstruck

Quote: Originally posted by RedSky

Quote: Originally posted by Chook

:puzzled: Assuming here that you are joking around.... :smug:

Using this logic Sea level must be a whole lot higher way up here ?

Great topic but I think the general consensus is that things like feeling the wind on your face and windicators like flags and socks are the things that really count.

I can find the same wind feels different from day to day locally with the dramatic swings we have in humidity. Or at least that is how I explained it to myself.

AudereEng - 9-5-2016 at 03:22 PM

What is the relationship for a cup or vane anemometer's rotation speed to pressure differentials as opposed to molecular velocity?

Pressure is a very easy measurement to make so why bother with the rotating parts.

abkayak - 9-5-2016 at 05:07 PM

So...wouldn't humid 20 mph wind pack more punch than dry air of the same speed? does it have more grunt/torque behind it?

Wind_dog - 9-5-2016 at 09:03 PM

@Auderenge - Pressure measurement is the basis for
pitot tube wind speed instruments.
https://en.wikipedia.org/wiki/Pitot_tube
The tube still needs to point into the wind (rotate).
I have some very low range differential pressure sensors
if you want to experiment. Send me a U2U and
we can meet later in May at Ona Beach.

As you previously mentioned, ultrasonics are non-moving sensors
as are hot wire anemometers.

AudereEng - 9-5-2016 at 11:29 PM

Thanks Wind_dog - I also have multiple pressure measurement sensors and I am aware of how pilot tubes work in theory.

I really want a 3D system with no moving parts.

It seems like a hot wire anemometer system could be built with multiple wires at different angles to make a 3D sensor.
I am guessing the cooling rate probably is some cosine function of the angle of the wire relative to the wind direction.
But when it blows hard we get flying sea water for hundreds of feet away from the rocks.
This would seem to make this technique interesting...

I liked the ultrasonic's speed (less than 1/100 of a second in the real world) and robustness.
Rain correction was not too hard.
The sound of the waves breaking and hitting the rocks was workable but there was significant energy in the sensor's frequency range.
I was starting to think about the Sea Gull poop issue - it changed the resonance of the transmitters/receivers and takes a long time to wash away and is very corrosive. The joy of real world product design...

U2U me when you are in the area.

Feyd - 10-5-2016 at 03:21 AM

Quote: Originally posted by abkayak

So...wouldn't humid 20 mph wind pack more punch than dry air of the same speed? does it have more grunt/torque behind it?[/rquote

Nope. Humid air is less dense than dry air. :D

abkayak - 10-5-2016 at 06:07 AM

well i guess this is why i didnt become a meteorologist...just thougt it was
and what about density of the air w/ high or low pressure?
im gonna have to google this stuff but im thinking out loud

UnknownAX - 10-5-2016 at 11:51 AM

High pressure, low temperature, dry air is the most dense.

Edit. And obviously air with liquid or solid water (rain,snow,hail) will have even more "grunt'. Also worth noting is that rainy air is more "dense" than haily(?) air, ie. more grunt in your kite. :D

yeti - 10-5-2016 at 04:26 PM

To answer the original question - As I understand it, the average anemometer is efficient enough that air density doesn't affect the reading much. You might want to calibrate your professional grade anemometer for different altitudes and weather you need it to be within less than 5% error.

I've been out kiting with people who have those handheld wind meters and you will feel the difference in power when humidity/temperature are different from one day to the next even though the speed reading is the same for both days. You do have to get a sense of the conditions locally to even have a sense of what a wind speed means power-wise. The wind meters do seem to be reporting pretty close to the actual wind speed, as strange as it may seem with air density as a factor.

Can't seem to find any good information on the web to back this up, but if anyone happens to come across something, I am interested in knowing the truth!