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There has to be a better way
By thefinis, Section Reviews Posted on Sat Aug 26, 2006 at 12:21:31 AM MST
Fussing and fessing up to math mistake

Why can't the formulas all be in the same units dang it. I knew better but had not used the tsr formula I got from Ed's site for some time due to having taken the test turbine down. I put up a new version and went to trying to find out what it was doing in the light winds we have been having lately. Well you guessed it when I went and pulled up the formula it was plain as day what to plug into the formula
TSR = rpm * Pi * D / 60 / V so I entered in my wind speed in mph and Diameter in ft and man was the 3 wing doing good. Now I go to another computer days later and have to go to the website and as I am scrolling down I notice wind speed is in mps and diameter is in meters. Duhh I remember now having to enter it in that way when figuring the last turbine. My tsr went from around 3 down to around 2 and I haven't redone but a few of the readings.
What a bunch of bull to have to check and see what each formula is set for. I have lived with miles feet yards gallons etc all my life but it really is about time to change and get the whole world on one set of figures. This isn't the first or the last time I'll do this. I did it before with an online calculator for volume and thought I had gallons but must have had liters. Just wish I could get this math right the first time cause even when I do the figures right I get the units wrong.
Finis

There has to be a better way | 23 comments (23 topical)

Re: There has to be a better way (3.00 / 0) (#1)
by Titantornado on Fri Aug 25, 2006 at 06:45:03 PM MST

Waaaahhhhhhh! If everything in life were easy, no one would appreciate their accomplishments. ;o)

Rod

Re: There has to be a better way (3.00 / 0) (#2)
by windstuffnow on Fri Aug 25, 2006 at 08:26:12 PM MST

Here is one that works in "normal" numbers...

Windspeed x 88 / ( Diameter x Pi ) x TSR = rpm

or in your case...

Rpm x ( Diameter x Pi ) / Windspeed / 88 = TSR

Diameter in feet
Windspeed in mph

The "88" simply converts mph to feet per min since were using revolutions per minute as a standard.

I have a problem converting to the metric system myself, when someone says an inch I can see it but when someone says 25 mm I have to go look it up and convert it to something I understand then say " Oh, about an inch ". I'm slowly getting a handle on it but it still confuses me without my converter handy.

.
Have Fun! Windstuff Ed

Re: There has to be a better way (3.00 / 0) (#3)
by willib on Fri Aug 25, 2006 at 11:25:49 PM MST

http://www.digitaldutch.com/unitconverter/

Carpe Ventum (seize the wind)

Re: There has to be a better way (3.00 / 0) (#4)
by mikeyduk on Sat Aug 26, 2006 at 12:03:12 AM MST

Go to this link http://www.csgnetwork.com/convfactorstable.html
To convert just about anything.
Scroll to the bottom of the page for more.

Mike

Re: There has to be a better way (3.00 / 0) (#5)
by Flux on Sat Aug 26, 2006 at 12:39:51 AM MST

Instead or relying on formulae for everything it is far easier to go back to basics.

Once you understand what tip speed ratio is, it is obvious that tip speed and wind speed have to be in the same units.

Some of the metric units are awkward and you have to watch the difference between Newton and kg but for most windmill calculations metric is far simpler Watt is the unit of power direct without having to go via hp.

I would absolutely refuse to use a formula unless it specified all variables and their units.
Flux

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#7)
by dinges on Sat Aug 26, 2006 at 04:41:57 AM MST

"Some of the metric units are awkward "

Being educated in metric, I tend to disagree; I find the imperial units to be awkward :)

Not only will you have to watch the difference between Newton (force) and kg (mass), you'll have to watch the difference between meter, second, etc... Newton and kg are completely different things, not just different units scaled by 10 (9.81).

"I would absolutely refuse to use a formula unless it specified all variables and their units."

I go further than this: I refuse to use equations/formulas that I don't know the theory that lies behind it, of. Often the theory makes lots of assumptions about the real world, which seep into the equation. And from a simple, mathematical-looking equation one can't determine what were the assumptions that lie beneath it. And whether the assumptions are valid for your specific problem.

As for as those silly formulas with constants in them to make up for the use of funny units (feets, inches, pounds, square yard per foot-poundal, etc.), they're errors waiting to happen.

And as far as horsepower goes: which of the three (3!) definitions and values do you refer to...

Keep it simple, keep it metric & SI.

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#8)
by dinges on Sat Aug 26, 2006 at 04:48:04 AM MST

Oh dear... I knew of 3 definitions and values for HP. Seems I'm not infallible. Look what wikipedia coughed up:

http://en.wikipedia.org/wiki/Horsepower#Measurement

"There have been many definitions for the term over the years since James Watt first coined the term in 1782. The following metrics have been widely used:

Mechanical horsepower -- 0.74569987158227022 kW (33,000 ft·lbf per minute)
Metric horsepower -- 0.73549875 kW
Electrical horsepower -- 0.746 kW
Boiler horsepower -- 9.8095 kW "

This is just a few of possible values, btw.

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#6)
by Darren73 on Sat Aug 26, 2006 at 04:05:58 AM MST

You think that's bad, you want to try being a controls engineer for a while, there typically a dozen or so different units for the same variable to deal with, eg even when someone quotes gallons on here are they talking in us gal or imp gal, yes they are different, the us one is short

1 imp gal = 1.2025 us gal.

regards
Darren
(Spell checked using UK English)

Re: There has to be a better way (3.00 / 0) (#9)
by Flux on Sat Aug 26, 2006 at 05:10:49 AM MST

Yes Darren, that gallon issue crops up regularly with hydro questions and it makes a lot of difference if it is imp or us gallon.

North Americans seem to like big numbers and seem to have smaller units to make it possible.

In general I go along with Peter about the SI metric system, but things like the Pascal and the Weber are really awkward things.

It's a good idea to separate the Newton from the kg but for those brought up on the pound force it takes some thought to get the g in the right place.

Most of the electrical industry grew up on crazy units with kilolines per sq inch and worse. I like the idea of all using the same, so why in this country do we teach the cm in school and use mm in industry, we never learn.
Flux

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#10)
by vawtman on Sat Aug 26, 2006 at 06:30:27 AM MST

Flux you made your first mistake
North Americans have bigger units LOL

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#11)
by jmk on Sat Aug 26, 2006 at 07:08:23 AM MST

And it doesn't make a diferance where you live . All the girls like to work with bigger units. There should be some formula to discribe woman. Something to do with mass velocity and pi.
jmk
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#12)
by elvin1949 on Sat Aug 26, 2006 at 10:30:04 AM MST

Nope
bigger number's smaller units.
55 gal drum will not hold 55 imperial gallon's.
later
elvin

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#13)
by thefinis on Sat Aug 26, 2006 at 12:04:46 PM MST

Thanks guys I will add some of these to my page of formulas and to my bookmarks but mostly I have gone back and made sure I added a list to each formula of what units were used instead of relying on my old brain to remember. Something I should have done the first time. It just seems to me that as the world becomes more of a global market and community that it really is time to start pushing again for a common set of measuring units.

The best/worst part is that the 3 blade tsr is down but it is following the math better and doing about what the 2 blade did but should have more torque even if it is running at near the same tsr. I know that tsr isn't really a good measure of how a turbine is doing but it is better than comparing rpms on varying size turbines.

If it will just rain so I don't start a fire and burn the whole place down I can cut a hydraulic system out of an old cotton stripper and use it to measure power being made. Of course if it does rain I will have a half a year's worth of work to do plowing out failed summer crops and planting winter grain crops.

Finis
Texas born and bred

Re: There has to be a better way (3.00 / 0) (#14)
by vawtman on Sat Aug 26, 2006 at 01:20:27 PM MST

Hello Finis
I think we should just throw the math out the window with these and design a gadget to measure whats happening but isnt there a formula for ft/lbs.A 200lb turbine spinning at 60rpms in a 10 mph wind at 8ft diameter.How many ft/lbs of torque is there?The extra blade you added is creating more lift thus increasing torque i think.Maybe thats what we need to base these on instead of tsr values.

An 8ft hawt in a 20mph wind can be slowed alot easier than a 8ft lift based heavy machine in the same wind.I know a hawt would spin faster and is a good match for direct drive but you could gear a vawt to that speed easily.

Get to work on that stripper and if the place burns down youll have nice winds.
I meant the cotton one:+)
v.....

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#17)
by thefinis on Sat Aug 26, 2006 at 05:33:14 PM MST

There are some formulas for finding amount of kinetic energy in a moving mass and some for torque that might apply but until I put a load on it and get some measurements it is all just guessing. The wings will not fly the same loaded as unloaded and rpms will drop which might make them work better just have to wait and see.

Hawts have many advantages but vawts have a place too. It is an argument I will try to stay out of till I have some working turbines of both.

<grin> If I work on the wrong kind of stripper my wife will burn the place down with me locked in it. She takes that till death do we part line real serious.

Finis
Texas born and bred
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#22)
by vawtman on Sun Aug 27, 2006 at 03:09:17 PM MST

Finis back to the stripper again,You said you could cut her apart and measure power being made.How?convert psi to ft/lbs another darn equation.LOL Hmmm

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#23)
by thefinis on Mon Aug 28, 2006 at 07:03:34 PM MST

Hey I have started to answer several times but kept going and searching for stuff to use for this. The formula for fluid power as horse power is
hp = gpm(flow) times psi(pressure) divided by 1714
The best way is to use a flow meter and pressure gage with an adjustable flow valve(for loading the turbine). Your pressure gage has to be on the high pressure side of the restriction(valve) the flow meter can be in either the supply line(high psi) or the return line(low psi). You want to close the valve(raising psi) till you just see a reduction in flow take a reading then increase psi by 5%(hmmm I am guessing here) take another reading and keep going till the equations show a drop in hp.

The formula here is flow which is based on rpms and displacement of the pump and psi which relates to torque the dividing number just factors it to horse power. If you want to change the hp to watts 1 hp = 746 watts. The smaller the displacement of the pump the less flow you will make but it will be able to give a higher psi number from the same turbine.

Another way to determine flow if you know the rpms (with a meter not guessing) then you can multiply the displacement per rev of the pump times the rpms and get your flow rate but it will be without the efficiency losses(internal leakage) of the pump. These losses can be rather high at low rpms with some types of pumps. Check and see if pump specs have them graphed

There are more ways to come up with a flow measurement too but this is a start.

Finis

Texas born and bred
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#18)
by windstuffnow on Sat Aug 26, 2006 at 06:02:01 PM MST

There is a formula for Horse power based on torque and rpm. RPM x torque / 5252 = HP. ( torque in ft/lbs ). A very simple piece of plywood with a hole bored in it that matches your output shaft and a spring scale. Slot the wood in the center of the hole and use a clamp to squeeze the shaft slightly. Attatch the spring scale at 1 ft from the center of the shaft and the reading will be in ft lbs. If you know the rpm and ft/lbs you can find out how much power the turbine is making.

Another simple way to measure torque is by using an alternator connected to the shaft and a variable resistor ( pot ). Clamp an arm to the alternator and allow the alternator to move freely, hold it in place with the arm and a spring scale. Slowly adjust the resistance and read the spring scale.

Weight of the machine doesn't matter in relation to the output only how the machine will respond to wind changes, its the power it will produce continuously in a given wind without stalling or slowing down. There is no advantage to the extra weight, extra weigh will simply hold the machine back from comming up to speed quickly... based on the weight.

Math is your friend, it's the only way you'll know if the changes you made or make are doing better or worse otherwise its simply a guess... guesses don't count.

Have Fun! Windstuff Ed
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#19)
by vawtman on Sun Aug 27, 2006 at 08:47:59 AM MST

Hello Ed
Any chance for a quick drawing of the spring scale method and is this scale like one you use to weigh fish?Thanks

[ Parent ]

Re: There has to be a better way (3.00 / 0) (#20)
by windstuffnow on Sun Aug 27, 2006 at 10:18:03 AM MST

Below is a quickie graphic of a prony brake. I've used a fish scale for some of the early tests... don't know how accurate they really are but it would get you a ball park figure. As long as you use the same scale for testing you'll at least have an idea if any changes you make have improved or lowered the output.

Being a large diameter turbine you may want to use a longer arm unless you have a spring scale that would go up to 400 lbs or so. A longer arm would allow you to use a smaller spring scale then just multiply that by the arm length to get ft lbs.

Using a prony brake gives you direct shaft output which is nice in finding actual blade efficiency. The trick is to get enough force on the arm by adjusting the drag and maintaining rpm at a given wind speed without slowing or stopping the machine. It also helps to find the speed it will produce the best power in a given wind.

.
Have Fun! Windstuff Ed
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#15)
by ghurd on Sat Aug 26, 2006 at 01:39:23 PM MST

I only use "Venusian" units ever since `the UFO incident' that I never speak of.
He told me "If you don't like The Venusian units, look up Uranus!"

Ghurd.info

Re: There has to be a better way (3.00 / 0) (#16)
by thefinis on Sat Aug 26, 2006 at 03:43:23 PM MST

I heard the Plutoians refused to use any thing but their own units based on the number of 3 legged steps it took to circle their world so they kicked them out of the planet system. Darn those Venusian slugs.

Finis
Texas born and bred
[ Parent ]

Re: There has to be a better way (3.00 / 0) (#21)
by zapmk on Sun Aug 27, 2006 at 02:02:54 PM MST

Using the prony brake is a good way to find the torque.

But its a lot harded to use it on a vertical shafts.

One of the tricks that I have been using is to use a thrust bearing and a couple of harden washers and a locking collar and put it under the prony brake to keep it from moving down the shaft.

How if I just had a wind tunnel, it sure would make it a lot easyer. :)

-Zapmk

There has to be a better way | 23 comments (23 topical)

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