Windmill Design

Off Topic
61

[quote]Vegita wrote:
Bill Roberts wrote:
That is not a different mechanism.

In either case it is pressure differential.

In either case pressure is below ambient above, and above ambient below.

As opposed to that being true for one but not the other.

In either case the force results from, as Vegita put it, “deflecting” (or accelerating) air away from the wing.

As opposed to being true for one but not the other.

With regard to angle of attack: This does not indicate different mechanism. It is also a product of definition. If for example you define alpha relative to the trailing tangent of the upper surface, rather than leading edge to tip – and it’s arbitrary how one defines it – then the curved surface also provides no positive lift at zero alpha.

But in any case that has nothing to do with differing mechanism, which in either case is generating force by accelerating air away from the wing, opposite to the direction of lift, concurrent with generating a pressure differential between upper and lower surfaces.

I’m not sure why you are still arguing this, It’s semantics. One works better at higher wind speeds, the other works better at lower wind speeds. That was the main point I tried to make like 30 posts ago.

If you feel like aerodynamic lift is responsible for both blades moving your entitled to. But to say difference in pressure is causing the movement, well shit, difference in pressure causes all movement. When I jump, I exert pressure on the ground, when the pressure is sufficient to overcome the pull of gravity I leave the ground, Yet I am still not an airfoil.

An internal cumbustion engine fires gas and oxygen in a cylender and creates pressure from expanding gas that drives a piston, which drives a crankshaft, which eventually drives wheels. This is not aerodynamic lift, but pressure differences are still responsible for the motion.

V [/quote]

You had said that one worked by deflecting air and the other not, that one worked by pressure differential and the other not.

Now that we agree that there is no difference in these, that both do these things, if we do agree on that, then we are agreed overall. Except that I know of no evidence that a flat board is the optimum shape for any condition. (EDIT with regard to DoubleDuce’s below post: Except where cost and manufacturing ease are accounted for.

In the previous sentence I was referring purely to performance. DD is absolutely correct that convenience may far outweigh a performance difference in various situations.)

DoubleDuce: No, the parachute is drag, not lift.

With regard to the remainder: see my previous post.

62

[quote]Bill Roberts wrote:
You had said that one worked by deflecting air and the other not, that one worked by pressure differential and the other not.

Now that we agree that there is no difference in these, that both do these things, if we do agree on that, then we are agreed overall.[/quote]

Yes. I just mostly disagree with that one is optimized or better than the other. Any time you have to deal with manufacturing capabilities you quickcly learn the best idea isn’t always the best idea.

63

[quote]Bill Roberts wrote:

DoubleDuce: No, the parachute is drag, not lift.

[/quote]

LOL. Exactly what I’ve been trying to get at. A flat wing I see more as a drag force than true lift. Step angle of attack, turbulent low pressure zone on top. Just a matter of perspective i guess.

64

[quote]Bill Roberts wrote:
Vegita wrote:
Bill Roberts wrote:
That is not a different mechanism.

In either case it is pressure differential.

In either case pressure is below ambient above, and above ambient below.

As opposed to that being true for one but not the other.

In either case the force results from, as Vegita put it, “deflecting” (or accelerating) air away from the wing.

As opposed to being true for one but not the other.

With regard to angle of attack: This does not indicate different mechanism. It is also a product of definition. If for example you define alpha relative to the trailing tangent of the upper surface, rather than leading edge to tip – and it’s arbitrary how one defines it – then the curved surface also provides no positive lift at zero alpha.

But in any case that has nothing to do with differing mechanism, which in either case is generating force by accelerating air away from the wing, opposite to the direction of lift, concurrent with generating a pressure differential between upper and lower surfaces.

I’m not sure why you are still arguing this, It’s semantics. One works better at higher wind speeds, the other works better at lower wind speeds. That was the main point I tried to make like 30 posts ago.

If you feel like aerodynamic lift is responsible for both blades moving your entitled to. But to say difference in pressure is causing the movement, well shit, difference in pressure causes all movement.

When I jump, I exert pressure on the ground, when the pressure is sufficient to overcome the pull of gravity I leave the ground, Yet I am still not an airfoil. An internal cumbustion engine fires gas and oxygen in a cylender and creates pressure from expanding gas that drives a piston, which drives a crankshaft, which eventually drives wheels.

This is not aerodynamic lift, but pressure differences are still responsible for the motion.

V

You had said that one worked by deflecting air and the other not, that one worked by pressure differential and the other not.

Now that we agree that there is no difference in these, that both do these things, if we do agree on that, then we are agreed overall.

Except that I know of no evidence that a flat board is the optimum shape for any condition. (Edit with regard to DoubleDuce’s below post: Except where cost and manufacturing ease are accounted for.

In the previous sentence I was referring purely to performance. DD is absolutely correct that convenience may far outweigh a performance difference in various situations.)

DoubleDuce: No, the parachute is drag, not lift.

With regard to the remainder: see my previous post.

Finis.[/quote]

I don’t view an airfoil as “deflecting” air. I view it as re-routing air on one side to make it travel farther/faster. This is not deflection, the air moves along the contour of the air foil and continues to travel at the same direction it origionally was, just at a different speed, which is why there is turbulance when the two sides meet back up.

Just like a sail on a ship catches airflow and transfers the energy to the ship, a flat blade does the same thing, it is just transferring the captured energy into a rotar instead of a shipmast.

V

65

Sails on a ship are an aerofoil. These days they only operate purely as a drag device when running directly downwind, and most of the time it is faster to gybe and use them as foils (sailing the apparent wind).

As I recall a flat blade at an angle of attack to the wind is not operating solely as a drag device, it is an inefficient foil with high drag. Despite the lack of ‘shape’, air still travels roughly the same way, it just stalls much earlier.

Making a foil out of wood is not difficult, it doesn’t need to be perfect to be many times more efficient than a flat blade.

A better idea - google “Savonius Rotor”. They are surprisingly efficient in the sort of turbulent wind you have described in your area, because they do not need to face into the wind. Very compact, very low maintenance and very cheap to build (a couple of cans or drums cut up do the job)

66

Sails on a ship are an aerofoil. These days they only operate purely as a drag device when running directly downwind, and most of the time it is faster to gybe and use them as foils (sailing the apparent wind).

As I recall a flat blade at an angle of attack to the wind is not operating solely as a drag device, it is an inefficient foil with high drag. Despite the lack of ‘shape’, air still travels roughly the same way, it just stalls much earlier.

Making a foil out of wood is not difficult, it doesn’t need to be perfect to be many times more efficient than a flat blade.

A better idea - google “Savonius Rotor”. They are surprisingly efficient in the sort of turbulent wind you have described in your area, because they do not need to face into the wind. Very compact, very low maintenance and very cheap to build (a couple of cans or drums cut up do the job)

See:
http://www.angelfire.com/ak5/energy21/microsavonius.htm

67

[quote]DragnCarry wrote:
Sails on a ship are an aerofoil. These days they only operate purely as a drag device when running directly downwind, and most of the time it is faster to gybe and use them as foils (sailing the apparent wind).

As I recall a flat blade at an angle of attack to the wind is not operating solely as a drag device, it is an inefficient foil with high drag. Despite the lack of ‘shape’, air still travels roughly the same way, it just stalls much earlier.

Making a foil out of wood is not difficult, it doesn’t need to be perfect to be many times more efficient than a flat blade.

A better idea - google “Savonius Rotor”. They are surprisingly efficient in the sort of turbulent wind you have described in your area, because they do not need to face into the wind. Very compact, very low maintenance and very cheap to build (a couple of cans or drums cut up do the job)[/quote]

That’s how all the pond aerators worked where I grew up in the Ozarks. If hillfolk can do it, I mean…

68

[quote]DragnCarry wrote:
Sails on a ship are an aerofoil. These days they only operate purely as a drag device when running directly downwind, and most of the time it is faster to gybe and use them as foils (sailing the apparent wind).

As I recall a flat blade at an angle of attack to the wind is not operating solely as a drag device, it is an inefficient foil with high drag. Despite the lack of ‘shape’, air still travels roughly the same way, it just stalls much earlier.

Making a foil out of wood is not difficult, it doesn’t need to be perfect to be many times more efficient than a flat blade.

A better idea - google “Savonius Rotor”. They are surprisingly efficient in the sort of turbulent wind you have described in your area, because they do not need to face into the wind. Very compact, very low maintenance and very cheap to build (a couple of cans or drums cut up do the job)

See:
http://www.angelfire.com/ak5/energy21/microsavonius.htm[/quote]

No, a flat blade with an angle of attack has airflow detachment on the leading edge of the wing. It is essentially a constantly stalled air foil. That in my book makes it different.

The biggest advantage of a modern foil over a flat wing is drag because it maintains laminar flow (mostly) around the wing. However. in a fixed machine like a turbine drag doesn’t really matter, other than it may generate a little more force on a thrust bearing.

In addition the smaller the scale, the less it matters. I think you can look at how they do the large turbines, but take it with a grain or 2 of salt. The difference in scale is several orders of magnitude. You cannot just assume that the optimal design for a windmill the size of a commercial air liner is going to be best for what you make in the backyard.

Further if you can afford to buy 15 flat blades vs. 2 or 3 bought/made airfoil blades, you can generate more power with less blade efficiency at the same price. Or maybe you can make a larger flat blade version for the same price.

There are 100s of additional factors than which blade design is more efficient that should be considered.

69

[quote]Vegita wrote:

I really wished I had a stream or small river running through my property, I think a water turbine would be much more stable and provide more energy than a wind turbine. But you have to work with what ya got.

V[/quote]

I have the perfect position for a water turbine. About 300 metres away from my house there’s an old mill that hasen’t been used for decades. Great flow of water and about 3m drop. Do you know any simple methods I could use to harness some of that power? I have no idea just how much water flows through there, but I’d do a set of 20 rep squats if it isn’t enough to power my house, even with a more primitive system. I can get some pictures tomorrow if you’d like to see it.

70

[quote]shoo wrote:
Vegita wrote:

I really wished I had a stream or small river running through my property, I think a water turbine would be much more stable and provide more energy than a wind turbine. But you have to work with what ya got.

V

I have the perfect position for a water turbine. About 300 metres away from my house there’s an old mill that hasen’t been used for decades. Great flow of water and about 3m drop. Do you know any simple methods I could use to harness some of that power? I have no idea just how much water flows through there, but I’d do a set of 20 rep squats if it isn’t enough to power my house, even with a more primitive system. I can get some pictures tomorrow if you’d like to see it. [/quote]

Most likley a simple paddlewheel is the easiest to manifacture yourself. I would pur a decent concrete footer close to the streambed, then build somthing the size of a small shed. Have an axle go through the sides of the shed and out over the water. Then you have your wheel mounted on the axle and have it so the blades a submerged a bit when the water is at it’s lowest. Most streams don’t rise and fall too much especially if you don’t have a spring snow melt. If you do, you may want to have some way of raising and lowering the wheel.

V

71

[quote]DoubleDuce wrote:
No, a flat blade with an angle of attack has airflow detachment on the leading edge of the wing. It is essentially a constantly stalled air foil. [/quote]

A flat blade stalls much earlier than a foil (and produces no lift at zero degrees, but that is contentious because you can get zero lift from a foil if you tilt it down, too), but it is certainly not “constantly stalled”. The separation bubble behind the leading edge actually helps flow over the top surface of the wing, and when that breaks down it stalls.

The Bernoulli effect is not a big contributor to the sum forces on a wind turbine (from memory, don’t quote me on that!)

[quote]
In addition the smaller the scale, the less it matters. [/quote]

Agreed, especially at the tip which is moving relatively slowly compared to a commercial turbine.

[quote]
Further if you can afford to buy 15 flat blades vs. 2 or 3 bought/made airfoil blades, you can generate more power with less blade efficiency at the same price. Or maybe you can make a larger flat blade version for the same price.[/quote]

Sure, no argument here. Having some experience shaping foils in the past I’m just letting Vegita know that it’s really not difficult to do (belt sander, plywood, epoxy). Then again, you will probably have to twist them to produce an efficient blade, and that is a whole new kettle of fish.

Flat blades will have their own confounding factors, especially if you find yourself adding blades. Certainly the best for pumping water, not sure about the more demanding design parameters of electricity generation?

I still think the Savonius is the best design for this sort of application, particular in a turbulent valley, close to the ground.