Blade design help
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The Blade Designer Program - A Free Basic Help Tutorial in Blade Design

      

 

The inexpensive "blade designer program" is available for purchase from http://www.windstuffnow.com

The Blade Designer program was written by Ed Lenz. 

This basic tutorial is written by Fred Tonch on May 28 2003 from http://www.internetfred.com

Table of contents:

  1. introduction
  2. hints
  3. the program input parameters
  4. adjusting the ratios (added features)
  5. all adjustments and effects list
  6. estimated propeller performance and calculated generator performance
  7. using the tables to produce a propeller (optimizing the design)
  8. blade terminology
  9. building with the design and building the blade
  10. credits

 

Introduction

About a year and a half ago, I began the daunting task of wanting to create a wind turbine for the sole purpose of producing power. I quickly found out this year that a basic understanding of aerodynamics is a requirement if your intention like mine, is to build and design such a device. Without the proper propeller matched to the generator, the power achieved will be substantially lower then with a matched propeller. The generator and propeller go hand in hand.

I am writing this basic help tutorial from the standpoint of a beginner wind turbine builder. I also suggest purchasing "Windpower Workshop" from Hugh Piggott also available from windstuffnow, as it has the basic essentials and guidelines necessary to building a wind turbine. The book has fairly extensive information regarding blade design and parameters required in design of a turbine. Only the basics will be covered in this help document.

The blade designer program has many added features which are not apparent at first glance. Armed with the book and this program, basically any small wind turbine and blade can be designed, built and matched to produce maximum output.

For all types of metric and standard number conversions, use the free program available for download called "win-convert express". This is a simple zip file. Note: if you are using win95/98 then you must download visual basic 6  runtime if you do not have it installed already or the converter will not work. This program is called VBRun60.exe. Do a search on www.google.com to find it.

Hints:

In this help document I use the words "blade" to mean a single blade and the "propeller" a unit composed of individual blades and a hub connected together as a single unit. Making blades into a propeller is a replication process. Many blades can be produced at once with this program and assembled into  a propeller. Also note that some people call the propeller a rotor. Actually the a rotor is a part of the generator, hub housing and spindle assembly, but anything that rotates can be considered a rotor so it's ok to use the term rotor in use of propeller or it's short form "prop".

In the program the highlighted areas marked in a darker gray color are for inputting parameter variables or values used to design a blade. A calculated value located in a table is not changeable except by the adjustment menus. All adjustments and effects where found by adjusting with each input and watching the result or effect. It is highly suggested that you do this also, since it will tell you how the program functions and calculates. 

The web site http://www.windmission.dk/workshop/BasicBladeDesign/bladedesign.html also offers a very good tutorial on blade design and number crunching.

The Program Input Parameters

This is the basic area for inputting parameters to create the blade. This is the first menu that will be discussed into it's simpler parts below. 

bulletRotor diameter in meters - the total diameter of the propeller. Here are some standard numbers. To find rotor size in meters you can convert ft to meters by multiplying ft x .3048
bulletThe Diameter is two times the distance from the center of the hub to the tip of the blade. It also can be looked at as the distance across the circle that the propeller would make when rotating. This is also know as the total swept area.

Standard Meters to Feet

.04 m  = 0.131 ft
.08 m  = 0.262 ft
.5 m  = 1.46 ft
1 m   = 3.28 ft
1.5 m= 4.92 ft
2 m   = 6.56 ft
2.5 m= 8.20 ft
3 m    = 9.843 ft
3.5 m = 11.48 ft
4 m    = 13.12 ft
4.5 m = 14.76 ft
5 m    = 16.40 ft
bulletTip Speed Ratio (tsr)- How many times faster then the wind speed, the blade tip is designed to run. The tip of a blade can travel faster then the wind. The tip speed "ratio" is the optimum between stall and runaway. Typical tip speed ratios are 1 thru 10. Tip Speed Ratio (tsr) = (tip speed of blade) / (wind speed). 
bulletNotes: Rotors are designed to run best at a particular 'tip speed ratio', but in reality they run at a speed which also depends how they are loaded. If the generator draws more power than the rotor has to offer then it slows, and often stalls. 

bulletNumber of  blades- 3 blades is most preferred and most often used. 2 blades  produces noise, rattle, imbalance, harder to start in low winds and other problems (tsr>4). (check out  http://www.windmission.dk for multi bladed designs (tsr<2) at lower wind speeds)
bullet 
bulletAngle of attack.- in degrees. This is the propeller efficiency. 4 degrees is standard. Numbers here can range from 1 deg to 15 deg typical. The angle between the chord line of the airfoil and the flight direction is called the angle of attack. Angle of attack has a large effect on the lift generated by an airfoil. 

bulletLift Coefficient - the lift coefficient is approximately two times pi (3.14159) times the angle of attack expressed in radians. Standard setting  =.8  Typical numbers range from .8 to 1.25
bulletNumber of stations - This is used to create the blade on a piece of wood. The stations are markings spaced at intervals.

Mechanical losses in the propeller, hub and spindle and electrical and magnetic losses such as usage of iron and wire resistance in the generator effect overall efficiency. All of this is calculated and included in the overall efficiency of the generator.

Typically on a propeller type it ranges from 25-35% overall and on Savinous type about 15%. So if you have a prop making say 45% efficient use converting wind into power and an alternator that is 75% efficient then
the over all efficiency would be about 33% ( .45 x .75 = .3375). This doesn't include any transmission line losses though. 

bulletOverall Efficiency - Overall efficiency is basically the power you will get out of the generator. 

Adjusting the ratio's (added features)

Adjusting the above calculated values for generator performance. 

This menu is for adjusting the calculated generator performance values. 

bulletThe Recommended Ratio - This number is a calculated value ratio of all factors of wind speed.
bulletThe User Ratio - This number is an input option, the user ratio is used to adjust the "Watts at recorded  ratio"
bulletThe Open Voltage is the open voltage from either a "to be built" or "already built" generator 
bulletThe Ohms is the total ohms from either a "to be built" or "already built" generator 
bulletThe Regulated Voltage measured in DC is the voltage from either a "to be built" or "already built" generator 

This menu allows you to input the wind velocity and adjusts the following calculations.

Regarding the tail size, the number represented in the program is the minimum size in square ft you should have. It can be any size or shape as long as it has the minimum amount of sq ft area .

bulletRotor thrust in pounds - This is the amount of thrust exerted on the blade during operation calculated by the wind velocity.
bulletRotor offset is for the furling system. This offset is the number of inch's to offset the furling system from the center rotor of the windmill.
bullet Tail Size - When thrust is applied to the rotor this is the tail size required by the turbine to keep it stabilized.

All Adjustments and Effects List

bulletAdjusting the rotor diameter effects all numbers in all the tables calculated.
bulletAdjusting the tip speed ratio effects the stations ratios, estimated propeller performance rpm, calculated performance generator - ratio;  and  calculated performance generator -watts recorded ratio.
bulletAdjusting the number of blades effects the stations ratios -cord, thickness and drop.
bulletAdjusting the angle of attack effects the stations ratios - Bld angle (blade angle)
bulletAdjusting the lift coefficient effects the stations ratios - cord, thickness and drop.
bulletAdjusting the number of stations effects the stations ratios - station number
bulletAdjusting the overall efficiency effects the estimated propeller performance -watts &  rpm, all calculated performance generator variables and the recommended ratio.
bulletAdjusting the user ratio effects the calculated generator performance -watts recorded ratio
bulletAdjusting the open voltage  effects the calculated generator performance - rpm, ratio & watts recorded ratio
bulletAdjusting the measured rpm effects the calculated generator performance - rpm, ratio & watts recorded ratio
bulletAdjusting the measured ohms effects the calculated generator performance - rpm, ratio & watts recorded ratio and the open voltage
bulletAdjusting the regulated voltage effects all calculated generator performance variables
bulletAdjusting the wind velocity effects the rotor thrust in pounds
bulletAdjusting the rotor diameter adjusts the rotor thrust, rotor offset and tail size.

Estimated Propeller Performance and Calculated Generator Performance

The next menu is the calculated "estimated prop performance and the calculated generator performance". These numbers are not input variables, they are calculated from the above input parameters. 

Estimated Propeller performance 

bulletThe estimated propeller performance numbers given in watts and rpm is the amount of power that can be achieved by the propeller doing work.

The Calculated generator performance

bullet Amps - How much amperage can the generator deliver
bulletOpen Voltage - how much voltage can the generator deliver without a load connected
bulletRpm - this is the calculated speed in revolutions per minute of the generator rotor.
bulletRatio - This is a calculated drive ratio. If your going to use a belt or chain drive instead of direct drive. This example picture above shows almost a 3 to 1 drive or basically running the alternator 3x faster than the prop rpm. What it does is figures out the best possible match for each wind speed then averages it out in the bottom as the recommended ratio. Or you can simply click on the user ratio button and put your number in. Direct drive would be 1 or any other ratio and the watts will show what the unit would do with that ratio. 
bulletWatts Recorded Ratio - This is the generator performance ratio for watts. This ratio reflects the alternator speed in relation to the blade speed. In the program when the user ratio is clicked the recommended ratio is no longer of any use. "watts recorded ratio" is actually "recommended" ratio.

Using the tables to produce a blade

(optimizing the design)

I have asked Ed to give a brief note on using the program to create a blade. Ed will now describe the principles behind optimizing the blade design and what to look for.

In the break down of data tables, the perfect blade would reflect the watts from the prop and the watts in the generator to match perfectly. For instance if the blade is making 100 watts and the generator is making 100 watts that's a perfect match. Unfortunately this is very unlikely. We have to deal with a wide range of rpm's and there is always an imbalance somewhere in the range. 

You have to design the blade to work within the average wind speed of your area. So if your area was in the 10-20 mph wind range you would try to match the prop and generator to that wind speed as best as possible. If the prop watts are lower than the generator watts the blade will stall and never reach its designed TSR and ultimately never reach its power at that speed. If the prop watts are higher than the generator then the blade will start to spin faster to meet the generator load. This is the optimum, where the the generator and the prop meet.

There is a point where the prop will "run away" from the generator. This means the load provided by the generator isn't enough to keep the blades at their optimum designed speed. 

This can cause a couple problems... 

  1. The prop is now running beyond the TSR it was designed for and can cause blade erosion, where the leading edge close to the tips and the tips of the blades will start pitting and cracking... especially in rain or adverse weather. 

  2. Since the blade is producing more power than the generator, the generator is trying to produce more than it can... this causes heat which ultimately will burn up the windings and possibly ruin a good set of magnets. 

If we keep them matched as close as possible the load will control the prop speed and all is well. Usually, where the prop starts to run away from the generator is where its best to start furling the blade. The cause of this imbalance is because the power coming through the prop is cubed and  the generator output is linear. 

So its a give and take situation... either give up the low end or give up the high end.

Blade Terminology

Camber is often used but misunderstood. Defined as curvature in the mean thickness line of the blade section.

 

Building the blade

This table is composed of the following:

bulletThe station number - stations are markings spaced at intervals.
bulletThe Radius in inch's - The radius of the blade at each station. Section length is the same as blade width. Each station is expressed as a percent of radius increment ( i.e.: 40 radius is 40% of the blade radius) .
bulletThe Blade Angle in degrees - Basically the angle of that portion of the blade when its carved. This is the angle of the blade based on the "drop" and "chord width"
bulletThe Chord in inch's - This is the chord width
bulletThe Thickness in inch's
bulletThe Drop in inch's

This is the calculated station numbers used to produce the blade as follows

bulletPlease Note: At this point it is highly suggested that you consult the windpower workshop book for greater detail into the building of the blade. This is only a very basic layout. 
 
bulletA blade is thickest at the root for structural integrity
 tip 
  
bulletMark out the stations, draw the lines completely around the material


 

bulletTaper the blade - mark the width out and cut out parts marked 


 

bulletMark the drop of each station

bulletAfter carving it looks like this
bulletNext measure the thickness of each station on both sides and remove the material from the other side

bulletA cross section of the blade

bulletAnother cross section of the blade

 

I wish you good luck on your project and please send me pictures and web links of your latest projects!

www.internetfred.com 

Credits

I would like to thank the following people that contribute their talents, inspiration and knowledge each day!

THANK YOU ALL FOR THE FANTASTIC WORK, WEB PAGES AND IDEAS!

Ed Lenz -www.windstuffnow.com  

Hugh Piggott http://www.scoraigwind.com/ 

Tom S - http://home.cogeco.ca/~tszaran/indexz.html  

Dan B and Dan F - http://www.otherpower.com/  

JK TAS Jerry  http://www.dplusv.com/Photo-03.html