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 Topic: My unorthodox Build
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BarkyJ
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 Posted: 07 June 2018 at 7:57am | IP Logged Copy link to clipboard
Well, the equation at the moment doesn't use TSR directly as such, but I guess the whole equation is based around the configuration of the table which used the TSR to figure out RPM's and such, to then determine the loading applied.

So yes the blade TSR is important to get within the ballpark of the intended TSR, else the table either needs to be rehashed to allow for more RPM or more torque etc, and potentially a different point to handle moving it out of the wind when the weather gets wild.

When I get these blades cut, hopefully in the next week, and then the rest of the windmill built, and get a more robust controller assembled, then I guess bolt it all together and do some tests and figure out where we are at, and then adjust the controller to match the blades.

Lots of trial and error to come.
and work is incredibly busy at the moment... sigh.

Warpspeed
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 Posted: 07 June 2018 at 9:08am | IP Logged Copy link to clipboard
It may be a case of approaching the whole problem from the opposite direction.

Get it up and running and take some power measurements at various fairly constant wind speeds, tweaking the loading for optimum power at each wind speed increment.

From that, you can build up a rough curve, then from the curve derive a suitable polynomial to fit.

Once you have the polynomial expression, just apply that to the wind speed to get a continuous smooth adjustment of loading over the whole entire range.

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DaveP68
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 Posted: 07 June 2018 at 10:32am | IP Logged Copy link to clipboard
The TSR isn't the main issue here as such, it's what the Dual F&P stators operating characteristics are that's important when in high wind conditions come along.

The one thing we need to be mindful of is the stators have an up torque limit once reached plateau's out and if saturation is approached drops off again.

My concern is if the blades perform well and end up with a TSR of say 5-6 with say just above 30 % efficiency then there will be too much torque on the shaft for the Dual stators to take. Blade runaway will occur...

The solution then will not try to get as much peak power and angle the blades out of the wind when a certain pre-calculated torque level is reached.

The higher TSR of around 7.5 that I recommend is just a nice to have rather than an end goal.

Also there are other solutions to managing higher torque levels that are easy to implement with a simple modification to the F&P stators.

BarkyJ
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 Posted: 07 June 2018 at 11:45am | IP Logged Copy link to clipboard
Warpspeed wrote:
 It may be a case of approaching the whole problem from the opposite direction.Get it up and running and take some power measurements at various fairly constant wind speeds, tweaking the loading for optimum power at each wind speed increment.From that, you can build up a rough curve, then from the curve derive a suitable polynomial to fit.Once you have the polynomial expression, just apply that to the wind speed to get a continuous smooth adjustment of loading over the whole entire range.

Yep thats right.

BarkyJ
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 Posted: 11 June 2018 at 7:08am | IP Logged Copy link to clipboard
I have been playing around with blades and finally took the plunge to get some cut out roughly.

So the dimensions I went for to start with are 140 at the tip and 200 at the root, then tapering back to the mounting point with a bit of a lug sticking out. So larger than I was originally going to do, but it gives me material to remove, rather than wishing I had more material there later on.

Here are 2 of them roughly dressed

And a big pile of shavings.

Used a jig saw with a suitable PVC blade, and it cut really nicely. Then used an electric power plane to smooth the edges and then start the contouring. Cuts really well with that too.

The part I am stuck on is the 'correct' way to mount these, in terms of trailing edge or leading edge, and how that is actually achieved. All PVC pipe blades I have seen to date, have been mounted on their trailing edge, with the leading edge curling forwards from the mounting plate. This is backwards compared to how 'normal' blades are mounted it seems, they all tend to get mounted with the leading edge and the trailing edge is going behind, but I just don't know if this is achievable with PVC pipe blades.

More thinking and trial and error to come.

Warpspeed
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 Posted: 11 June 2018 at 8:58am | IP Logged Copy link to clipboard

The way the blades have been cut they are essentially straight along their length, without having any twist between the root and the tip. The pitch of the blade remains the same over the whole length.

Now what we want is laminar airflow over both sides of the blade with a fairly small angle of attack, which gives us the optimum lift drag ratio right along the whole width of the blade. Drag is important, because it acts as a brake on our rotor slowing it down and consuming power we would otherwise be able to use as useful torque.

So down near the hub, the blade is hardly moving at all, and the air is rushing past parallel with the drive shaft. The leading edge should be facing almost straight ahead into the oncoming air, and the blade curl away from that slightly to give us our differential front to back pressure to create thrust without hopefully having any flow separation on either side of the blade. From that, we could probably bolt the trailing edge onto the face of a flat hub if it curled around far enough right at the hub.

At some distance along the blade, the blade velocity will be the same as the air velocity, and for the blade to still have optimum lift drag ratio the angle of attack should still be quite small, and the leading edge should be at very close to 45 degrees so that the leading edge again points directly into the effective relative airflow direction over the blade.

Right at the tip, the blade velocity is very high, perhaps ten times the wind velocity. So the relative airflow hitting the leading edge will be almost at right angles to the drive shaft, and the leading edge will appear almost at that same angle.

When we cut our blades out of a round pipe we can curl the blade around the pipe along the length so that the angle of attack varies almost over ninety degrees from tip to root.
That change in pitch is an entirely different thing to the slight curvature across the blade at any point between leading and trailing edge.

That is as I remember my lessons at flying school almost fifty years ago.

Edited by Warpspeed on 11 June 2018 at 9:13am

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 Posted: 11 June 2018 at 9:15am | IP Logged Copy link to clipboard
By making the blade wider at the root and keeping the leading edge in line with the centre of the original pipe it is creating a twist. Having said that though the people using extruded aluminium blades have the angle along the entire blade.

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BarkyJ
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 Posted: 11 June 2018 at 9:16am | IP Logged Copy link to clipboard
Hi Tony

Yeah by placing the template on the pipe on an angle, rather than straight up and down the length, you do get this twist. I had not seen this documented as such in other builds, so I was very conservative with the blades I have cut. Mine are about 3 degrees from straight, which is hardly anything.

I could do it more aggressively, but I just wasn't sure if it was the right thing to do or not.

What is strange though is when you look at the GOE222 blades, they are relatively flat with their mounting, and they are a constant profile. So how do they work so well?

Unsure if you have spotted the new thread I posted the other day
https://www.thebackshed.com/forum/forum_posts.asp?TID=10566&PN=1&TPN=1

I guess all I can do is try and see what happens really. All calculators I have found suggest different to what I have seen people doing in practice, and the shape of the pipe blades these calculators output, involves a massive bulb at the root. Then when you look at diagrams such as this, which is more like what you just posted, it's different again to what people seem to do with pipe blades:

Confusing :)

BarkyJ
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 Posted: 11 June 2018 at 9:21am | IP Logged Copy link to clipboard
 By making the blade wider at the root and keeping the leading edge in line with the centre of the original pipe it is creating a twist. Having said that though the people using extruded aluminium blades have the angle along the entire blade.

Yeah that is essentially what I ended up doing too. The leading edge is in line with the axis of the pipe, the trailing edge is at a slight angle, it worked out to be something like 3-4 degrees only. So when you mount it on the trailing edge, you get this effect of a twist, without having so much curl that the blade isn't ploughing the air.

With the aluminium extruded blades, yeah, quite different concept again, constant profile over the whole length.

In my pics above, you might see one of the blades with the pipe printing on it, PCV blah blah. That in theory is down the length of the blade. You might be able to make out the slight angle between that line under the words, and the leading edge.

Warpspeed
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 Posted: 11 June 2018 at 9:30am | IP Logged Copy link to clipboard
There is probably some happy medium between what is theoretically ideal, and what is practical and easy to fabricate.
There might also be some mechanical tradeoffs such as weight, strength, and stiffness which may vary with the material used.

If you were manufacturing blades professionally, aesthetics and cost are other factors. Noise could come into all this as well, but a well designed blade should not have any flow separation or generate any tip vortices, and should run pretty silent.

I live in the suburbs surrounded by two story houses and large trees, and a not very understanding local council. My interest in all of this is only mainly from curiosity and perhaps learning something new and interesting.

If I was doing it myself, I would probably build and test some scale models, particularly easy to do with PVC pipe of widely varying sizes.

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 Posted: 11 June 2018 at 9:39am | IP Logged Copy link to clipboard
If you search you will find blade calculators that give the cross-section of the blade at intervals. I went through all this a few years ago and made a fibreglass mould off of a wooden template. Never got around to finishing it though, maybe one day I will get back to it. It was a huge amount of work though, what you are doing would take a fraction of the time.

Edited by Madness on 11 June 2018 at 9:40am

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Warpspeed
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 Posted: 11 June 2018 at 9:46am | IP Logged Copy link to clipboard
It might also be possible to make a wooden master, and form some PVC (pipe) blades using heat to conform to the wooden master. If done slowly in a large oven, it may make some pretty radical shapes possible and repeatable.

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