Why 3 blades? More blades Higher Torque, Thin blades Higher Speed

[Matrix1000]

I've been doing some reading and I noticed a few things.

1. It seems that most axial designs/plans/kits use 3 blades.
   
2. The most 'torque' is generated from many blades such as wind flowers/old water pumpers etc that catch more wind.
   
3. Large wind farm mills use giant blades, and the blade root needs to be large to support the weight.
   
4. The lower 1/3 rd of the blade is the least efficient part of the blade.
   
5. Most people complain about having low wind speeds in their area.
   
   

More blades means more torque, more area to catch the wind. Smaller blades means faster speed. Blade design for small mills don't have the structural requirements that large industrial mills have, so it seems inefficient to copy their blade design.

It seems that 3 blades would be for higher average wind speed areas. Adding just 2 extra turns to a coil requires more torque, and I'm assuming that a coil with 2-4 extra turns will fit in the same area and is so close to the same width as one with 2-4 less it wouldn't decrease the effencicy of the alternator.

Designing a blade set for a 500 watt alternator like for instance Ed's kit, What are your thoughts about using 6 blades, 1/2 to 3/4 the width of the blades on a traditional 3 blade system.

1. This would provide more torque at lower wind speed because there is more surface area to catch wind.
   
2. The thinner smaller blades would require less mass at the blade root because of the strength to weight ratio; eliminating the lower 1/3 of the most inefficient blade area.
   
2. Thinner blades would make less wash area than a thicker blade so by the time the second blade passes the wash area from the first blade, the wash is behind it. Unlike a big fat blade and the density of air and the wind speed.
   
   

I'm trying to design a blade that would be efficient for a 500 watt mill at 5 to 15 mph average wind speed; instead of using blade designs designed specifically for larger mills that have their own structural and aerodynamic needs.

Let me know your thoughts.

[windstuffnow]

   I don't think there would be any real advantage to using 6 blades.  The same power and speed needed to drive the alternator can be achieved with only 1 blade.  The main reason most use 3 blades is primarily a vibration during yaw issue when only using 1 or 2 blades.  The more blades you add the more difficult it gets to maintain a certain speed without running into aerodynamic issues.

   Best advice... Keep it simple.  The windflower types and water pumpers like the old aermotor mills produce a very large amount of torque.  The 8ft aermotor mill toped out at 105 rpm in a 35 mph wind where it then furled.  

   52 ft lbs at 100 rpm = 1 hp and as well 17.5 ft lbs at 300 rpm = 1 hp.  There are many ways to achieve the same output... why not the simplest?

Have Fun

Windstuff Ed

[Matrix1000]

What about harvest efficiency though i.e.

If I had 1 blast of air. How much horse power will the 3 blades catch and pull out of it VS what 8 blades would catch and pull out of it.

In my area wind's aren't constant, which is what I'm hearing from a lot of people and average wind speed is slow.

So...

If wind speed were constant, once a 3 bladed system got up to speed it would of course produce as much as an 8 bladed mill (considering all the high speed aerodynamic inefficiencies of the 8 bladed design at high speed).

The problem is that in many areas winds come in gusts and 8 blades catches more wind than 3. It seems to me that 8 blades can catch and therefore harvest more energy from the gust than 3 blades.

Carving and balancing 5 or 8 thin long blades may be easier but more time consuming than 3 big blades though.

[Matrix1000]

The only reason I say 5-8 is because it seems to me like it would take more structural support, hub bearings, huge root sizes to support 3 x-tra large blades with the same surface area as 5-8 smaller blades.

[ghurd]

This is not from a blade guy, but...

Jacobs, of windmill fame, grandson (?) was interviewed years ago.

He was at the N or S pole, and one of the first mills was STILL spinning!

No maintainace or anything.  Wow.

Anyway, he said it was decided way back that an odd number of blades was always best.  I think he said why, but I do not recall.

This is what I have been doing.  Beats working!

My high torque 5-blade gets noisy and stalls at low RPMs with no load.

Low RPM still being pretty fast.

A 20" box fan blade.

My 3-blade 27" ZubWoofer takes a little more to get it started but not much.

It has gotten loud, but not stalled.

No steady wind after it was finished.

It is 12" active blade, 5" hub, 25' root, 8' tip.

My 4-blade, same as above, starts just a tad easier, but takes a noticably longer gust to get to speed.

Seems to top out a little slower than the 3 blade.

Again, no steady wind after it was finished.

All that is no load.

The 3 and 4 blades go a LOT faster than the 5 blade, but they are not the same either.

The testing was in the same place in a few minutes, one at a time, in low but gusty winds.

All the same height, but the PVCs were hand held, the 20" was on a short 'tower' with a tail.

Tails work much better than a cigarrette and me guessing.

Much better! Learned that with the fan blade.

I'm leaning to the 4-blade for now.

Does OK and I have low average wind.

My cutin is kind of low,

and the alternator tops out fast, too.

A little more torque, less speed. (I can't use too much speed anyway)

Maybe I'll get a bit more amp hours?

Short gusts don't get anything but the 5-blade up to cutin anyway.

Even a strong short gust will not get the 3 and 4 blades up to cutin.

I think the 5 blade has an advantage because it is turning a bit sooner, and has overcome the KE when the main part of the gust arrives (my best guess).

But the fan blade is better balanced than the 3 and 4-blade.

Very basic I know.

I hope something helped.

G-

[Matrix1000]

Thanks for the info...very interesting!

[ghurd]

I should have included...

The gusts are 'maybe' 15MPH,

for less than 5 seconds.

In between there is less than 6MPH, often '0'.

Often the gusts come when the blades are not moving.

This was ALL in a couple hours.

And there was some really weird winds.

The 20" on the tower twisted the clips off of the battery right below.

Maybe 10 or so turns before that can happen.

Strange day.

G-

[Flux]

If your blades are spinning above stall speed then I don't think you will see much difference, there is little energy in gusts, unless there is a reasonable average speed between gusts you won't get much anyway.

If there is iron in the alternator and hence iron loss at start up speeds then a lower tip speed blade will do better but you can easily go down to tsr 5 with 3 blades. There may be a slight constructional advantage by using 5 blades for tsr 5 or lower.

With air gap alternators the starting is so good that this would only be an issue with rotors of 3ft diameter or less, with significant bearing drag.

If you go to the windflower type design the rotor efficiency is high but you will need a very oversize and expensive alternator or gearing with losses that will offset your gain.

Water pumpers have high torque but very poor rotor efficiency even before you add gearing.

It's not a case of copying the big machines, they weren't around when Jacobs found that 3 blades were the best for general use.

If you are basing the argument on the AIR machines, then that is a case of too high a tsr for small blades and an iron cored alternator, cheap design.

Flux

[finnsawyer]

While a thinner blade should have less wash or wake, keep in mind that with six blades a blade passes a given spot twice as often as for a three blade design.  So, I don't think there will be much difference for the same blade profile.  The six blade design would weigh less, though.

As far as the loss of efficiency near the root, I think that comes about because folks prefer to make their blades from 2x6's rather than 6x6's.  That is, they sacrifice getting the right twist or pitch in order to save money and effort.  The blades usually get wider near the root, which tends to increase the drag, as well as the lift, but the effective wind speed decreases which tends to decrease the drag and the lift.  To get the best performance you need the right amount of twist.  How did Jacobs do it?  His mills seem to have been the best for their day.

[Ungrounded Lightning Rod]

What about harvest efficiency though i.e.  If I had 1 blast of air. How much horse power will the 3 blades catch and pull out of it VS what 8 blades would catch and pull out of it.

A blade pulls power out of the air ahead of and behind it.  This changes the speed and direction of the air motion.  If things are designed right, the next blade comes by no later than the moment when the last of the air whose motion was distorted by the previous blade's interaction has moved just beyond the first of the air affected by the new blade.  If it comes by sooner it gets less power because some of the air it's trying to pull power out of has already had the power pulled out of it.  (That's OK, and lets you be less critical with your blade design.)

More blades will pull less power each.  The fraction of the wind power through a given swept area that can be collected by a mill is limited by the Betz limit.  It's pretty easy to design a three-blader to get within a couple percent of that, so there's no need to make more than three blades.  (Unless you really want high torque rather than high speed, like for a direct pumping application.  But for a genny you want high speed - faster means smaller and cheaper genny.)  Two blades have a vibration problem with yaw.  A single, counterweighted blade has additional (major) vibration problems with wind loading.  So you want at least three.  Thus wind genny designs tend to gravitate toward three blades.

For very small mills four may have strength advantages if each opposing pair is made from a single piece of material, taking some of the stress off the blade-hub connection.  (But you have to cut away a bunch of material at the crossover, right where you need the strength the most, to get the blades co-planar - which you want so their wakes interact equally.  So it's not a particularly useful technique for large mills.)

[rotornuts]

Another issue with more than three blades is solidity. As has been stated three blades has been the design of choice because it is the least number of blades you can use without incuring dynamic vibration issues.I've been in the same place as you are now an if you continue your research you'll find that efficiency reportedly starts to decline at above 10% solidity, that's not to say that you should't exceed 10% form what I understand the additional power exceeds the loss in efficiency up to the 20% range. Too many blades and all the air will go around the rotor once the area of the rotor becomes backed up with turbulent air caused by blades and wake interference thus limiting the speed and output of rotors with a high blade count. Nice in theory but impractical in real life.

I'm still working on a single blade design that I believe will function quite well across a broad range of wind speeds and best suited to low winds but as underground points out there are significant dynamic balance issues to work out that some(most actually) believe are insurmountable. I do have to admit that the offset loading issue may break the project.

I think you're better off to look at changing blade diameters and cord width ratios(solidity) rather than add additional blades.

[jamesjones]

"A blade pulls power out of the air ahead of and behind it.  This changes the speed and direction of the air motion.  If things are designed right, the next blade comes by no later than the moment when the last of the air whose motion was distorted by the previous blade's interaction has moved just beyond the first of the air affected by the new blade."

This applies less and less, the bigger the turbine gets. With a commercial megawatt turbine, the rpm can be around 30 (correct me if I'm wrong), there is therefore both a huge space in between the three blades, plus longer and longer in between them passing the point where the previous blade was. Hence huge amounts of wind are simply passing clean through the huge gaps, and the bigger you make the turbine (presuming it's a sail based HAWT) the more energy is lost. This might be one place where it would be better to have more and more blades, to prevent loss of energy.