Author Topic: 5 blade vs 11 blade design  (Read 11968 times)

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fabieville

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5 blade vs 11 blade design
« on: September 01, 2016, 11:15:10 PM »
would a 5 blade design consisting of (5) 35" Blades giving you a 80" swept area have similar performance with speed and torque like a 11 blade design consisting of (11) 29" blades with a swept area of 59"?

joestue

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Re: 5 blade vs 11 blade design
« Reply #1 on: September 02, 2016, 02:28:52 AM »
more blades means more torque, less efficiency. more torque, less rpm means a less efficient generator.
My wife says I'm not just a different colored rubik's cube, i am a rubik's knot in a cage.

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #2 on: September 02, 2016, 03:17:24 AM »
The effect of the number of blades on the maximum theoretical Cp is explained in chapter 4.3.2 of my free report KD 35 (see www.kdwindturbines.nl). A rotor with 11 blades will have a very little higher theoretical Cp than a rotor with 5 blades if the airfoil used for the blades has the same drag lift ratio (see figure 4.3 in KD 35). However, if both rotors have the same diameter and design tip speed ratio, the rotor with 11 blades needs chords which are less than half the chords of the rotor with 5 blades. Smaller chords result in lower Reynolds numbers and the drag lift ratio increases if the Reynolds number is lower. The higher drag lift ratio finally results in a lower theroretical Cp (see chapter 4.3.3) for the rotor with 11 blades. If the rotor with 11 blades has a lower design tip speed ratio than the rotor with 5 blades, more power will be lost in wake rotation (see figure 4.2 in chapter 4.3.1) and this is a second reason why the theoretical Cp will be lower. If the rotors have different diameters, the smaller diameter will be the biggest reason why the mechanical output of the smallest rotor at a certain wind speed will be much lesser. The electrical output finally depends on the generator efficiency.

The same effect of the negative influence of the smaller Reynolds number is also valid if a rotor with three blades is compared to a rotor with five blades and if both rotors have the same diameter and design tip speed ratio. To my opinoin there is no good reason to chose for five blades if the rotor has a design tip speed ratio larger than about 4. Rotors with many blades are only required if a very high starting torque is needed like this is the case for water pumping with a single acting piston pump.
« Last Edit: September 02, 2016, 03:27:44 AM by Adriaan Kragten »

Ungrounded Lightning Rod

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Re: 5 blade vs 11 blade design
« Reply #3 on: September 02, 2016, 03:51:52 AM »
No.  More blades isn't better, and smaller swept area is worse.

In essence it's the swept area that matters, not the number of blades.  (Edit:  But see Adrian's post above for reasons why more blades can be somewhat worse aerodynamically.)  Fewer blades just means they have to be designed to decelerate a thicker slice of air ahead of and behind them on each pass.  Once all the air between two consecutive blades is decelerated the appropriate amount, you've got what you can get and you're done.  It doesn't matter if the next blade is 45 (8 blades), 90 (four blades), 120 (three blades), 180 (two blades), or 360 (one blade) or whatever degrees around the shaft from its predecessor.  Fewer blades, wider blades (to affect a deeper "cut" of the wind).

One blade is a pain to do right.  Only a handful of people have attempted it.  You end up with a big counterweight, a shape something like a maple seed, an even more extreme version of the two-blade yaw vibration issue, and a few problems of its own.  Plus extreme downwind subsonic vibration.  Keeping unbalanced wind forces from shaking the mill apart at different wind speeds is a pain, too.

Two blades have an issue with vibrating during yaw - turning to track a changed wind direction - while spinning (and they'll always be spinning when they yaw).  They have a lot of inertia against yaw when the blades are horizontal, essentially none when they're vertical, so tracking of wind changes varies as the blades turn, causing the mill to shake.

Three or more blades balance nicely.  There's no substantial advantage to more blades (except for four on small mills, where it may be easier to fabricate two opposing blades out of a single piece of material).  But more blades on the mill means more blades to make.  So horizontal-axis mills usually end up with three blades, or occasionally four on small ones.
« Last Edit: September 02, 2016, 04:07:13 AM by Ungrounded Lightning Rod »

fabieville

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Re: 5 blade vs 11 blade design
« Reply #4 on: September 02, 2016, 07:13:05 AM »
The PMA that I have is a ARI 48v 750watt that I am using to charge a 12v battery bank and in the near future its going to be charging a 24v battery bank instead of the 12v. It reaches cut in charge very quickly and easily now under load.

I have it set up with (3) 35" windgrabber blades from windynation with a total swept area of 80". And it is performing excellent in my low wind area of about 6-12mph. I get on a average of about 1-4amps and periodically 6-8amps. When I get wind gust now and then I get about 9-11amps. The highest I clock was 16amp since I have it set up and my area is not a high wind area. I would say between low and high. I was just wondering if it would be best to upgrade to the 5 blade design using the same 35" windgrabber blades or upgrade to a 11 blade design using the 29" blades or just keep my current configuration that I have? Please give me your thoughts on this regard.

Here are sum pics of my design:

kitestrings

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Re: 5 blade vs 11 blade design
« Reply #5 on: September 02, 2016, 08:30:29 AM »
I'd suggest staying with three blades, unless you're planning to pump water, compress air, etc. for the reasons above.  I don't think an "upgrade" to more blades with a smaller rotor diameter will pan out to be an upgrade.  If you change voltage you can probably optimize the (3) blades for the new parameters (cut-in, rpm range, TSR)

All things being equal, nothing will influence output more than wind swept area.

Nice looking turbine.

~ks

fabieville

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Re: 5 blade vs 11 blade design
« Reply #6 on: September 02, 2016, 09:46:08 AM »
What about if I upgrade to the 5 blade design using the same 35" windgrabber blades and maintaining the same 80" swept area would I get a better performance in regards to staying in the cut in charge rpm a little longer with the added torque, better momentum and lower start up?

So in all a slight increase in wattage production from the 5 blade design??

kitestrings

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Re: 5 blade vs 11 blade design
« Reply #7 on: September 02, 2016, 04:00:31 PM »
Doubtful.

Does it start hard (from a  stop)?  Otherwise, I see no real advantage, and the disadvantage of remaking the hub plate and maintaining more blades.  I don't see a win.

Smithson

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Re: 5 blade vs 11 blade design
« Reply #8 on: September 02, 2016, 09:00:03 PM »
     I don't mean to hi jack this conversation but I was wondering about something.  Please excuse my poor manners.  How do you prevent over speed.on. that machine?  I emailed another distributor of windy nation aluminum blades and they said that at a certain rpm the blades spill the wind. I never heard of such a thing. Does it work? How would you cut the blades to have that effect?
    That is a beautiful alternator, by he way.  ARCH

fabieville

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Re: 5 blade vs 11 blade design
« Reply #9 on: September 03, 2016, 12:21:12 AM »
Doubtful.

Does it start hard (from a  stop)?  Otherwise, I see no real advantage, and the disadvantage of remaking the hub plate and maintaining more blades.  I don't see a win.

No it does not start hard from a stop. I already have a 5 blade hub for using these same size blades.I bought the turbine used on ebay and got 2 hubs from the seller and I bought the blades from windynation. All i can tell is that it spins quite easily with a very low wind start up speed and once it starts to spin it rarely stops even with load on it.

I was just thinking that maybe the 5 blade design would keep it in the charging mode for a longer period. Because now as it reaches cut in speed it stays in it for couple seconds and then it cuts out when the rpm drops due to battery load or perhaps a decrease in the wind speed. So i am guessing that 5 blade design would keep me in the cut in charge rpm longer as it it better to get a small constant charging current than some periodically current that comes on now and then stays in the charging zone less time than the 5 blade design.

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #10 on: September 03, 2016, 02:57:24 AM »
Blades designed for a rotor with three blades can't simply be used for a rotor with five or more blades. If the rotor is designed to the aerodynamic theory and if it has a certain design tip speed ratio, diameter and airfoil, a blade must have a certain chord and a certain blade angle at a certain radius. If you design a rotor with five blades with the same variation of the chords, it must have a lower design tip speed ratio as the solidity is a factor 5/3 higher. This lower design tip speed ratio will require larger blade angles but if you use the original blades you get the same blade angles as for the rotor with 3 blades. If the blade angle is too small, the angle of attack will be too large and this means that the generated lift coefficient will be too large and that the airfoil may even stall. So the maximum Cp of the rotor with 5 blades will be a lot lower.

Report KD 35 gives the aerodynamic theory to design a windmill rotor and the formulas given in chapter 5 give the relation in between the chord, the blade angle, the number of blades, the design tip speed ratio, the diameter and the used airfoil, so with these formulas you also can check if a given rotor is designed properly.

fabieville

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Re: 5 blade vs 11 blade design
« Reply #11 on: September 03, 2016, 09:00:31 AM »
Blades designed for a rotor with three blades can't simply be used for a rotor with five or more blades.

The blades are design to work on a 3 blade hub and also on a 5 blade hub. On windynation website they sell the 2 hubs for these blades that is why I am asking if you would recommend the 5 blade design over the 3 blade design for my low wind area. The 3 blade design is working very good so far.  I am just wondering if the 5 blade would give me better performance?

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #12 on: September 03, 2016, 11:25:27 AM »
If the supplier of the blades says that the blades can be used for a rotor with 3 blades or for a rotor with 5 blades it is a clear indication that at least one of the rotors is not designed according to the aerodynamic theory. The blade angles for a rotor with 5 blades must be much larger than for a rotor with 3 blades. The starting torque coefficient for the rotor with 5 of the given blades will be about a factor 5/3 higher, so the starting behaviour will be better. But a high starting torque is only needed if the generator has a high peak on the sticking torque. If the starting behaviour is good for a rotor with 3 blades there is no reason to take 5 blades. The power output for 5 blades may even be lower than for 3 blades if the blades are designed for a rotor with 3 blades. The rotor with 5 blades may turn too slowly because the wrong blade angles result in too much aerodynamic drag. Another point is that the matching in between rotor and generator can only be good for one rotor.

You should check the rotor geometry to find out for which number of blades the design is optimal. But there is a problem as the airfoil varies. From the photo I see that the chord increases at decreasing radius. It seems that the blade is a part of a cylinder, so the bending radius is constant for the whole blade. This means that the camber is small at the blade tip and large at the blade root. In my free report KD 398 I give the aerodynamic charactristics for the cambered plate airfoil for three different cambers. If the camber differs from the camber for which the characteristics are measured, you have to interpolate.
« Last Edit: September 03, 2016, 11:30:28 AM by Adriaan Kragten »

hiker

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Re: 5 blade vs 11 blade design
« Reply #13 on: September 03, 2016, 12:06:04 PM »
Try carving out some simple wood blades...your blades look similar to Pvc pipe blades...more rpm..with wood blades...
WILD in ALASKA

stofanel

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Re: 5 blade vs 11 blade design
« Reply #14 on: September 03, 2016, 06:53:36 PM »
I concur with hiker.

Your main problem is the non-existent blade airfoil. Should you choose to increase performance via better aerodynamics, you must get better blades. This means turbine blades where the cross section is an actual airfoil instead of a bent piece of metal. And no, adding more blades will not increase the power of the turbine. It will decrease it.
 

MattM

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Re: 5 blade vs 11 blade design
« Reply #15 on: September 04, 2016, 12:39:44 AM »
I've experimented with sheet metal and had no problems with getting consistent weights and shapes between blades.  I just never found steel to last over a year.  I used an ogee shape rather than a scoop as the ogee was more airfoil like.  Absolutely lightning quick responses to wind.  A real blur in light winds.

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #16 on: September 04, 2016, 04:39:47 AM »
A cambered sheet airfoil is not a bad airfoil if the camber is small. For 7.14 % camber, a minimum drag lift ratio of 0.03 can be reaslised (see KD 398 figure 2). For low Reynolds values, a cambered sheet airfoil is even better than a normal Gottingen or NACA airfoil. The sharp nose makes the boundary layer turbulent and this turbulence prevents stalling at low Reynolds values.

Already in 1978, when I was working at the Wind Energy Group of the TU-Eindhoven, I have built a 2-bladed, 1.8 m diameter rotor with blades cut from a cylinder and with an increasing chord and so an increasing blade angle and camber at reducing radius. The rotor was tested in the wind tunnel of TU-Delft and the measurements are given in the TU-Eindhoven report R 398 D (no longer available). We have tested three different materials PVC, aluminium and stainless steel. PVC could not be used for a sheet thickness of 3 mm because of stall flutter already at a very low wind speeds. Aluminium also suffered from flutter, even for a sheet thickness of 3 mm. Stainless steel worked nicely for a sheet thickness of 2 mm. The maximum Cp of the stainless steel rotor was more than 0.4 for a design tip speed ratio of 6.

Making cambered blades with an increasing blade chord is more difficult than making cambered blades with a constant chord and for some of my small VIRYA windmills I therefore use 7.14 % cambered blades with a constant chord. These rotors have a maximum Cp of about 0.38. Because of the low torsion stiffness of cambered sheet blades, the thickness must be rather large to prevent flutter at high wind speeds. The maximum free blade length is 0.8 m for a sheet thickness of 2 mm.

MattM

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Re: 5 blade vs 11 blade design
« Reply #17 on: September 05, 2016, 07:42:10 AM »
2mm = 12 gauge

Not your typical sheet metal thickness.  In contrast I used 16 gauge and that was the maximum I'd use on the machinery

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #18 on: September 06, 2016, 12:12:43 PM »
The sheet thickness is relative. So for 2.5 mm thickness, the free blade length can be 1 m. For 3 mm thickness, the free blade length can be 1.2 m. Flutter happens at a certain tip speed. So for a rotor with a high design tip speed ratio, flutter will occur at a lower wind speed than for a rotor with a low design tip speed ratio. The rule of thumb of the minimum sheet thickness is valid for a design tip speed ratio of about 5. The maximum tip speed has to be limited by a correct safety system.

If the blade is connected to a spoke mounted at the hollow side of the blade, this spoke is stiffening the blade a lot. So the free blade length is the length of the blade outside the spoke.

stofanel

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Re: 5 blade vs 11 blade design
« Reply #19 on: September 06, 2016, 04:38:05 PM »
Here is some actual wind tunnel data for cambered flat plate airfoils:

http://www.personal.psu.edu/lnl/097/belgium.pdf

Looking at pages 14 and 15, and reading the Cl and Cd graphs, one can easily infer L/D ratios of less than 10:1. This means very large drag penalties in the power producing flight envelope. I do not mean to rain on anybody's  parade, but cambered flat plate airfoils are only good for lawn ornaments and perhaps low RPM water pumps.


kitestrings

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Re: 5 blade vs 11 blade design
« Reply #20 on: September 06, 2016, 09:09:19 PM »
fabvl,

The other thing you might consider is getting it up in the air a bit more.  It looks pretty cool, but it is hard to imagine that the wall doesn't have some negative effect on a laminar stream of air getting to the prop.  ~ks

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #21 on: September 07, 2016, 04:56:31 AM »
I have looked at the chapter about cambered plate measurements in the report of Thomas J. Mueller. The first thing I noticed is that one speaks about cambered plates but the camber is not specified. The minimum drag lift ratio depends very much on the camber (the ratio in between the thickness and the chord in %) and probably also somewhat at the ratio in between the sheet thickness and the chord. The characteristics given in my report KD 398 for three different cambers originate from measurement performed by Imperial College and I think that these measurments are very reliable.

I have tested several rotors with cambered plates in the open wind tunnel of TU-Delft and the measured maximum Cp corresponds to what may be expected theoretically based on the measured characteristics of cambered plate airfoils. I have also tested real wind turbines with 7.14 % cambered plates on my testfield in The Netherlands and the measured output corresponds to the estimated Cp-lambda curve and the measured generator characteristics. So a good small wind turbine can be designed with 7.14 % cambered plate airfoil.

stofanel

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Re: 5 blade vs 11 blade design
« Reply #22 on: September 07, 2016, 03:18:34 PM »
I can't comment any further on the reliability of the thin airfoil data. Assuming though that a 7% camber does deliver a L/D of 30:1, it will take a CNC machine to cut a cylindrical shape to those specifications. Furthermore, unless the turbine spins at 300RPM+, the blades will not fly at the Reynolds numbers reported in the wind tunnel tests (I believe the data is valid for Re>150000).

When designing a turbine blade, one must tightly control: c * Cl, (blade chord and local lift coefficient). Cl is strictly a function of Re, twist, and camber. When using a cylindrical surface to cut the blades, one has control over the chord OR camber, but not both. The whole design process is one big compromise. It would be an interesting project to compare flat plate performance vs thick airfoil performance for same diameter blades. And no, wind tunnel tests are invalid. The limited cross sectional areas of wind tunnels skews the axial induction factor of the turbine disk area, making the windmill perform much better than in the real world.

richhagen

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Re: 5 blade vs 11 blade design
« Reply #23 on: September 07, 2016, 08:53:23 PM »
5 or 11 huh?  Sticking to that question and not the relative merits of sheet metal blades, I think that when you look at the formulas for angle and chord for turbines with larger numbers of blades optimized for a given wind speed you find that for each blade you add the depth of the material in the blade in the axis of the approaching wind is less.  That means more flexing or a stronger blade material is needed.  I suspect that this is one of the main drivers behind why almost all modern commercial wind turbines have three blades.  It is the minimal odd number of blades that can be balanced without a counter weight.  Even numbers of blades tend to have vibration issues which is why we don't see many two bladed machines made now days.
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Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #24 on: September 08, 2016, 04:47:24 AM »
It is true that many wind tunnel tests are unrealiable because of wind tunnel blockage if the scale model of the rotor is large with respect to the cross sectional area of the wind tunnel. This tunnel blockage prevents expansion of the wake around the rotor like this happens for real wind. However, the 2.2 m diameter wind tunnel of TU-Delft is an open tunnel which means that the wake can expand around the rotor. The wind is blown away in a free space and is not sucked to the entrance of the tunnel like in most other wind tunnels. Extensive tests with rotors of different diameters have been performed to determine the maximum rotor diameter which can be tested in this wind tunnel. Rotor measurements from this wind tunnel are therefore very reliable.

In my report KD 599 I give an overview of measurements of Savonius rotors which can be found on the Internet. Most measurements are performed in closed wind tunnels and the maximum Cp which is measured is therefore much too high. The few measurements which are performed correctly give a maximum Cp of only about 0.2 at a tip speed ratio of about 1.2.

In all my design reports of rotors I calculate the Reynolds values for different cross sections for a reasonable wind speed of about 5 m/s. I take the measurements with Reynolds value closest to the calculated value. The Reynolds values for constant chord blades decrease about linear with the radius but are much higher than 100000 for the outer halve of the blade if the chord isn't chosen very small. The outer halve of the blade contributes most to the generation of the power.
« Last Edit: September 08, 2016, 05:03:11 AM by Adriaan Kragten »

MattM

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Re: 5 blade vs 11 blade design
« Reply #25 on: September 08, 2016, 07:57:00 AM »
The wind tunnel also basically measures a static blade, not one rotating around a rotor.

While the flat blade looks like a good idea, it really seems to warp as pressure against it increases, robbing it of potential.  To counteract this propensity for blade twist the blade requires tapering.  It also requires a rotor that allows it to attach at a sloped angle.

It worked much better using an ogee shape.  It mounts flat to a rotor.  And blade twist actually works into your favor as it decreases blade pitch at the tip, but less so at the root.  And your blade is stiffer with opposing folds whereas a blade shaped like a swoosh loses strength with each fold.  Plus the twists are at the trailing edge rather than at the leading edge, reducing drag during rotation.  And you don't taper ogee blades.
« Last Edit: September 08, 2016, 08:01:37 AM by MattM »

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #26 on: September 09, 2016, 03:22:15 AM »
The wind tunnel measurements I refer to are done for a complete rotor, not only for a cambered sheet airfoil. It is true that the torsion stiffness is larger if the blade is tapered. For the rotor with tapered blades which was tested in the wind tunnel, the free blade length was 0.9 m for a sheet thickness of 2 mm. For a constant chord blade I found that it should not be longer than 0.8 m. For a tapered blade one will get a higher maximum Cp than for a constant chord blade but manufacture of constant chord blades is much simpler as the blade is cut to a certain length and width and cambered afterwards. The tapered blades were cambered first and then sawn to the correct size.

MattM

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Re: 5 blade vs 11 blade design
« Reply #27 on: September 09, 2016, 07:33:47 PM »
What kind of brake are you using to bend your 2mm steel?

MattM

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Re: 5 blade vs 11 blade design
« Reply #28 on: September 11, 2016, 08:24:04 PM »
I know this is off topic, but I just wanted to demonstrate the ogee blade concept.

By using opposing bends near the mounting point you keep the overall camber to a minimum.
The scoop shapes create a draggy front leading edge in comparison.

Adriaan Kragten

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Re: 5 blade vs 11 blade design
« Reply #29 on: September 12, 2016, 04:41:01 AM »
I have developed an hydraulic blade press which makes use of two 6 ton's car jacks. First you have to find out around which radius the blade has to be folded to end up with the correct radius for 7.14 % camber. This is because the blade bends back in the elastic region. So the radius of the press must be much smaller than the final radius of the blade. You find the correct radius by using a small strip of 2 mm stainless steel and bend it around cylinders of different diameters until you find the cylinder with the correct radius.

The blade press consist of a heavy lower beam with the correct radius at the upper side and which has the length l of the blade. The two car jacks are placed below this beam at 1/4 l of both ends. The car jacks are placed on a heavy bottom strip. Next you need a frame which supports two beams with make contact to both sides of the upper side of the blade. This frame must connect the bottom strip to the two upper beams. These two beams must be beveled with such angle that the blade is touched only at the most outer sides. This is to make sure that the bending radius is made over the whole blade width. The whole construction must be very strong and stiff because cambering of a whole blade in one stroke requires a very large force. Once this hydraulic blade press is availble, cambering of a blade takes only about one minute and can be done by one man. The hydraulic blade press which I have developed is ment for a sheet thickness of 2 mm, a sheet width of 125 mm and a sheet length of 800 mm.

I have also designed a blade press for a 3 mm stainless steel blade with a length of 1.5 m and a sheet width of 200 mm. This press uses four car jacks powered by two men. For this press, the lower beam is an I-profile with many separate ribs bolted on top of it. The upper side of the ribs have the correct bending radius and the distance in between the ribs is that small that no wobble in the blade camber is created. This blade press is made in the Philipines for the VIRYA-4.1 windmill. The drawings of these blade presses belong to licences of the concerning windmills and are not free available.

A simple blade press for the aluminium blades of the VIRYA-1.04 and the VIRYA-1 is given on drawing 1302-01 which is incorporated in the free manual of the VIRYA-1.04. For this blade press, the force is supplied by ten M10 socket head cap screws which must be turned simultaniously. Making of a blade with this blade press takes much more time than with a hydraulic blade press.

MattM

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Re: 5 blade vs 11 blade design
« Reply #30 on: September 12, 2016, 06:50:01 AM »
I imagine that your minimum distance between folds is fairly large unless your anvil you press against has a relatively small gap.

I used to have access to a 16 ton hydraulics brake that could put folds a quarter inch apart.  Ended up trading it out for an Autobrake2000 limited to 12 gauge.  The latter had two leading edges, one 3/8th or 1/2 inch dependent on how it was flipped, the other a 1/4 inch on both sides.  Since most of the material was quite thin, it required a change in the blade gap vertically for thicker metal.  It wasn't a fast change to work with thicker material, so I didn't change it often.  16 gauge was the heaviest I ever used on it, with blade lengths up to ten feet.  It was simpler to use 20 gauge in 30 inch lengths, because it didn't require altering the vertical height of the brake blade. 20 gauge obviously isn't nearly as thick as 16 gauge, but it was easy on the wallet.  Hydraulic presses don't really have that issue.  The biggest issue with hydraulics is consistent bends.  We had to constantly reset gaps and press depths with micrometers every 100 pieces.  I never cared for staring at the tiny numbers

Ungrounded Lightning Rod

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Re: 5 blade vs 11 blade design
« Reply #31 on: September 26, 2016, 06:03:01 PM »
It is the minimal odd number of blades that can be balanced without a counter weight.  Even numbers of blades tend to have vibration issues which is why we don't see many two bladed machines made now days.

I thought it was just two blades (or one blade), not all even numbers of blades, that have vibration issues.  The vibration comes from a sinusoidal variation of the moment of inertia with the angle of the blade as it rotates, causing resistance to yaw to vary cyclically as the blade spins.  For four blades (sin^2 + cos^2 = 1) this should balance out exactly, six blades should be much like three, and so on.

Is there another mechanism I'm not aware of that would create vibration with even blade counts greater than two?

Assuming I'm correct, the typical three blade design is because three is the minimum number of blades greater than two, not because it's the minimum odd number of blades greater than one.

stofanel

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Re: 5 blade vs 11 blade design
« Reply #32 on: September 28, 2016, 03:15:42 PM »
It is the minimal odd number of blades that can be balanced without a counter weight.  Even numbers of blades tend to have vibration issues which is why we don't see many two bladed machines made now days.

I thought it was just two blades (or one blade), not all even numbers of blades, that have vibration issues.  The vibration comes from a sinusoidal variation of the moment of inertia with the angle of the blade as it rotates, causing resistance to yaw to vary cyclically as the blade spins.  For four blades (sin^2 + cos^2 = 1) this should balance out exactly, six blades should be much like three, and so on.

Is there another mechanism I'm not aware of that would create vibration with even blade counts greater than two?

Assuming I'm correct, the typical three blade design is because three is the minimum number of blades greater than two, not because it's the minimum odd number of blades greater than one.

You are basically correct. For the most part, more blades increase the main harmonic frequency, which tends to attenuate resonance. Also, more blades reduces the overall loading of the individual blade, and therefore the amplitude of the vibration.