# 11 Questions on Airfoils (take your best shot!)



## AgrarianDr (Mar 25, 2011)

There are some things, like Animal husbandry or microbes in the soil, where I consider myself a relative expert - 
but when it comes to this? Dumb as a box of rocks!

So here, in no particular order, are my Newbie questions about airfoils. 
And don't worry about offending me - I'm way beyond that.

1.	Why is speed important in wind energy but with water torque seems to be the goal?

2.	Why do water turbines and wind turbines have such different blade designs?

3.	If faster air is created just after the camber in an airfoil why aren&#8217;t there more designs with a leading ball, that would essentially create and airfoil prior to the blades and dramatically improve performance for such little effort?

4.	If the primary thrust of turbine blades are at the tip and at least a fair part of the remaining blade is simply friction producing, then why aren&#8217;t all turbine blade designs bladeless in the center?

5.	If bigger, fatter blades create more torque, doesn&#8217;t that have just as much potential as speed? Or is that a judgment call on the designer, or is each blade designed for specific use (low cut in vs. high speed)?

6.	I think Ram towers are a stroke of brilliance - but apparently they are not that common, and considering they seem to take the best of both HAWT and VAWT designs I really have no idea why - And why wouldn&#8217;t you want the exhaust out the top vs. the bottom?

7.	Why wouldn&#8217;t a Pelton wheel design (low cut in speed) work well for wind power?

8.	If a squirrel cage is so "in"-efficient, then why in the world does virtually every-single HVAC unit on the planet use them?

9.	Why wouldn&#8217;t an obstructive tower be more beneficial than an open blade (where the tower itself acts as a vertical air foil?

10.	Do the same laminar vs. boundary airfoil principles apply in water as they do in air?

11.	If a higher alternator speed produces more power and is therefore the goal, and if fewer blades = less resistance = higher speed (like a Cessna airplane prop) , then why do all turbines have so many blades? Is it because of the torque needed to start &#8211; and if more blades provide more torque, why not 20 blades vs. 120?

Good luck with those! And if I can help you with animal, dirt or fertilizer questions, ask away! 
But THIS stuff? WAAAAY over my pay-grade - at least for now. :grin:


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## Jim-mi (May 15, 2002)

I don't believe that there are any folks on here that are in the air foil designer / engineer pay grade so most all you will get is speculation . . . . . . .

I have been involved with a company that has installed wind farms. 
I have no reason to question the blades----that the engineers developed--- on these 6 (yes thats six) million dollar turbines . . . .The towers are an extra 1.5 million bucks.

At the time these 2.5Mw turbines were the most powerful in north America . . . . . Not sure if larger have been installed since.........
5Mw are available for offshore . . . . . . 

So what I'm trying to say is some MIGHTY BIG bucks have gone in to the research and development of these units and humble ole me don't feel qualified to ask "why".

I do know the three blade has the best over all 'ballance' . . beats two badly.
They even tried a one blade--with a counter weight . . . problems with that.
And of course there has to be a 'compromise' design for the hi and low winds . . .
Yes water pumpers have mucho blades . . they say for torque hot speed..

Some interesting questions and the yap yap could go on for some while about yea and no.
But for now I am very pleased with the performance of my tried and proven three bladed wind turbines . . . . Many folks have gone way before me proving they work.


rant off . . for now.................


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## AgrarianDr (Mar 25, 2011)

Jim-mi said:


> I am very pleased with the performance of my tried and proven three bladed wind turbines . . . . Many folks have gone way before me proving they work.


Yeah, have seen some of those monsters up around Traverse City. Behemoths might be a better word - but my neighbors might have something to say about me sticking one of those in the front yard - "if" I had the money, which I don't. (Actually, as huge as they are the neighbors in the next town might have something to say)

To the point though, yes, far smarter people design these things for a living, but that has never stopped me from asking, and listening, even to "speculation" as you say. For me, (as is true for most of my decisions) you don't know what you want - until you know what you DON'T want, and I haven't gotten there yet


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## pancho (Oct 23, 2006)

Can't answer much except give a little info about water turbine bladed.
Water turbines have wicket gated to control the amount of water flow. The turbine blades pivot to provide the torque needed. Blade size will depend on the head, length of penstock, head and volume of water.
Most water is very corrosive and there is cavitation to contend with.
A wind turbine blade would not catch enough water to create the needed torque.


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## Ky-Jeeper (Sep 5, 2010)

From what I read from your post, you won't have very much trouble understanding the efficient use of hydraulics and aerodynamics. 


I do have a question for you soon on preheating the root zone in a raised bed.


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## AgrarianDr (Mar 25, 2011)

Ky-Jeeper said:


> I do have a question for you soon on preheating the root zone in a raised bed.


You should probably shoot me a PM, 'cause it sounds like there may be more to the question. There are several reasons for raised beds but the primary reason is that being raised will heat the soil far earlier than normal, so you are already ahead of the curve. What am I missing? 
If you plant them too soon you are going to run into other environmental issues - unless you throw a removable hoop over the bed. But there could even be some "timing issues" with that option as well.


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## Ross (May 9, 2002)

I know HVAC units use squirrel cage fans primarily because they are quiet. Circ pumps don't have this problem they just live to break inertia and move the water coming in to just outside the exit shoving what's in the way over. Most of those are vane pumps.


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## farminghandyman (Mar 4, 2005)

I will give a try, no expert. but have been do some reading and have water pumpers and designing a Wind turbine for my own use.


AgrarianDr said:


> There are some things, like Animal husbandry or microbes in the soil, where I consider myself a relative expert -
> but when it comes to this? Dumb as a box of rocks!
> 
> So here, in no particular order, are my Newbie questions about airfoils.
> ...


the generator needs speed or many many poles in the generator to effectively generate electric,

water need to be pumped, and it is usually via reciprocating pump, there is only so fast one can move that sucker rod up and down, the longer the stroke the slower it needs to move, 
the water pumper is like a farm tractor, and the electrical turbine is like a race car, two different need and two different designs,



> 2.	Why do water turbines and wind turbines have such different blade designs?


the water pumper is a drag type blade moves by wind sliding off the blade, and the wind turbines have the lift type blade and it flys into the wind like a wind on a airplane,



> 3.	If faster air is created just after the camber in an airfoil why aren&#8217;t there more designs with a leading ball, that would essentially create and airfoil prior to the blades and dramatically improve performance for such little effort?


 IF I am under standing the question, I think you will see a much more rounded leading edge on the base of the machine where the slower moving blade is.



> 4.	If the primary thrust of turbine blades are at the tip and at least a fair part of the remaining blade is simply friction producing, then why aren&#8217;t all turbine blade designs blade less in the center?


the modern turbine usually has a number of different profiles on the blade so it can take or make the most effective use of the wind at the speed it is designed to function, (the tip of the blade is moving faster so the bulk of the power is built up there),



> 5.	If bigger, fatter blades create more torque, doesn&#8217;t that have just as much potential as speed? Or is that a judgment call on the designer, or is each blade designed for specific use (low cut in vs. high speed)?


tip speed is usually one of the main design permanters, (to minimize erosion on the blade), so the diameter of the blade tip speed limits the design RPM, and the profile is some what by if the type is for a adjustable blade or fixed, what type of over speed protection is designed into the unit, some use a profile to stall out when the speed is over the safety limits of the design of the turbine, 



> 6.	I think Ram towers are a stroke of brilliance - but apparently they are not that common, and considering they seem to take the best of both HAWT and VAWT designs I really have no idea why - And why wouldn&#8217;t you want the exhaust out the top vs. the bottom?


not sure what your referring to, if your talking about a collar to help make a tunnel to increase the volume of wind into the blades (a lot of structure and bracing for minimal improvement, a few more feet of blade is cheaper and easer,




> 7.	Why wouldn&#8217;t a Pelton wheel design (low cut in speed) work well for wind power?


the wind speed is usually improved and turbulence is removed by getting the turbine in the air, most likely the structure or tower needed to implement that type of design would be very expensive,





> 8.	If a squirrel cage is so "in"-efficient, then why in the world does virtually every-single HVAC unit on the planet use them?


there is a difference in moving air and capturing air movement, the squirrel cage fan is normally very quiet and is reasonable efficient in moving air. 



> 9.	Why wouldn&#8217;t an obstructive tower be more beneficial than an open blade (where the tower itself acts as a vertical air foil?


(I may be miss under standing this)
the tower would be best to be invisible and not create any turbulence, 
and the tower is not producing power so besides making it so it is as minimal in creating turbulence why or how could it produce power, 



> 10.	Do the same laminar vs. boundary airfoil principles apply in water as they do in air?


 I have no idea, never studied the fluid dinamics of props or pumps,



> 11.	If a higher alternator speed produces more power and is therefore the goal, and if fewer blades = less resistance = higher speed (like a Cessna airplane prop) , then why do all turbines have so many blades? Is it because of the torque needed to start &#8211; and if more blades provide more torque, why not 20 blades vs. 120?


most wind turbines have 2 or 3 blades, most water pumpers have many blades,
the more blades they create drag and turbulence, the more blades they have they become self limiting as at higher speeds the turblance will start to interfere with the following blade, the same thing happens with wind turbines if many blades are used, Water pumpers are self limting in the speed even if they did not turn out of the wind, 
The water pumper used a drag blade as well and like said design for a tractor type use, and the wind turbine is like the sports car, fast and lean, they may both capture the same horse power, but out put design is different, 

a water pumper (aermotor) is geared down close to 3 to 1 for the most part.
most wind turbines (unless using multi pole generators) gear up the speed many times 10 to 1 or many times more, 

(for example take a cars alternator, your going to need about 2000 rpm to make it work effectively, you have a turbine with 200 rpm main shaft, that is 10 to 1 gear up, 

so to take a drag type blades (water pumper), that is turning under 100 rpm that would take most likely a dual gear up, to get the alternator to make electricity, because your looking at 20 or more to 1 to make it work, 

The larger diameter the blades the slower it will turn (by design),
so to get more power (larger blades) the slower the main shaft, so if using a 4pole generator your going to need about 1800 rpm to make work effecenty if it based on a 60 cycle design, if it is 2 pole your looking at about 3600 rpm, 





> Good luck with those! And if I can help you with animal, dirt or fertilizer questions, ask away!
> But THIS stuff? WAAAAY over my pay-grade - at least for now. :grin:


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## Jim-mi (May 15, 2002)

Yes I was referimg to the turbines at the "Stoney Corners" wind farm area ---south east of Cadillac Mich.

Those blades are 140 feet long ...each
A man could walk into the end of the blade. . . . .(before they were mounted of course)
The max 'hub' speed is 14 rpm.............
I'll let you figure out the blade tip speed . . . . . .
The gear box is mighty fancy/expensive.............

The wind "farm" in the upper P (yes I set a tower there) is getting direct drive turbines........


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## Harry Chickpea (Dec 19, 2008)

There are some things, like Animal husbandry or microbes in the soil, where I consider myself a relative expert - 
but when it comes to this? Dumb as a box of rocks!

So here, in no particular order, are my Newbie questions about airfoils. 
And don't worry about offending me - I'm way beyond that.

1.	Why is speed important in wind energy but with water torque seems to be the goal?

Energy increases as a square of the speed. With water, head increases the pressure. Flow past the turbine can be fast (peltons) or slow and with a lot of torque. "Saving" water is important. You can't "save" wind in a reservoir. Designs reflect this.

2.	Why do water turbines and wind turbines have such different blade designs?

Wind has far less mass. Water turbine blades require more strength given the same diameter of turbine. Water turbines are also usually in some sort of raceway or pipe.

3.	If faster air is created just after the camber in an airfoil why arenât there more designs with a leading ball, that would essentially create and airfoil prior to the blades and dramatically improve performance for such little effort?

Collapsing the low pressure zone in an unorganized fashion wastes tremendous amounts of energy.

4.	If the primary thrust of turbine blades are at the tip and at least a fair part of the remaining blade is simply friction producing, then why arenât all turbine blade designs bladeless in the center?

If you have to have a structural support for the tip along the blade, why not make it aerodynamic and try to gather just a little more energy or minimize the friction?

5.	If bigger, fatter blades create more torque, doesnât that have just as much potential as speed? Or is that a judgment call on the designer, or is each blade designed for specific use (low cut in vs. high speed)?

Speed and air density are a couple factors, smoothness of the airflow is another. There are recognized good shapes for various speeds and situations.

6.	I think Ram towers are a stroke of brilliance - but apparently they are not that common, and considering they seem to take the best of both HAWT and VAWT designs I really have no idea why - And why wouldnât you want the exhaust out the top vs. the bottom?

Ram towers, if you examine them more closely, do almost nothing. You can begin to see this if you draw straight lines indicating the wind going into a ram. The direction of force has to change from straight to as much as 90%, which then works in opposition to the wind coming in from the opposite side of the ram. There is more, but it gets more complex. In water, what might appear to be a ram design is more to eliminate issues with friction with the walls of the intake pipe.

7.	Why wouldnât a Pelton wheel design (low cut in speed) work well for wind power?

Peltons work with high mass and high pressure situations. Wind is neither.

8.	If a squirrel cage is so "in"-efficient, then why in the world does virtually every-single HVAC unit on the planet use them?

They are quiet, the air against the vanes is a consistent speed, they work well with standard AC motors, and they build up a nice controllable pressure within the ductwork.

9.	Why wouldnât an obstructive tower be more beneficial than an open blade (where the tower itself acts as a vertical air foil?

Not sure what you mean.

10.	Do the same laminar vs. boundary airfoil principles apply in water as they do in air?

Water is effectively non-compressible. IIRC, friction is greater with water. Other than that, the principles are similar, but not quite the same due to that difference in compressability.

11.	If a higher alternator speed produces more power and is therefore the goal, and if fewer blades = less resistance = higher speed (like a Cessna airplane prop) , then why do all turbines have so many blades? Is it because of the torque needed to start â and if more blades provide more torque, why not 20 blades vs. 120?

Slow speed, lots of blades to intercept the wind flow and generate torque. Think the old water pumping windmills. At high speeds, there is MUCH more power available. It is better to throw away low speed energy and focus on designing for the high speeds where more power can be extracted.


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## farminghandyman (Mar 4, 2005)

sorry I miss understood a few of your questions, I took water turbine as a air powered water pumper, as air foils have little to do with water, so I was thinking AIR, powered machines,


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## AgrarianDr (Mar 25, 2011)

Thank you all, especially Harry Chickpea and farminghandyman.
Still have a lot of questions, but am certainly a far cry from where I was.

For the record, "obstructions", be it a tower or ?, create turbulence and therefore a far stronger airflow. If you look around you can see where wind turbines are being installed on the sides of silos - and not just for convenience. It's kind of like standing just around the corner of a building in a strong wind. The actual wind speed is increased substantially.

As to the Ram Tower, it works a lot like a dam. Imagine a tall, narrow pyramid with louvered walls - louvers collecting and forcing wind internally towards the peak - with a HAWT (typical horizontal axis design) at the top, but face down. Have found very little information on this design, and probably for good reason, but the concept is sure intriguing! I'm guessing the advantages are the forced collection and immediate response to directional winds, but the downside is probably the same as most VAWT designs, that being that only 1/3 of the blade spread (or in this case maybe 1/4) is actually being used at any given time

Fun to think about though!

Thanks again for the responses


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## artificer (Feb 26, 2007)

I have a slightly different take on #1.

A column of water has a certain amount of energy, just as it sits there. You can leave the energy stored in the column, or extract it fast or slow. The speed at which you extract the energy is going to be dependant on you generator. Most water generating systems let the energy be converted from static/still water to the dynamic energy of russing water. The generator extracts as much energy as possible by slowing the water down or being reacted upon by the head pressure of the column of water.

Air, on the other hand, only has useable energy while its moving. The faster it moves, the more energy it takes. If the air is slower than 10mph, there really isn't much energy available to use. Going to just 12mph almost doubles the energy available. So... you want the highest wind speed possible.

Michael


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