tail pivot angle

[imsmooth]

At this link http://www.otherpower.com/metalwork2.shtml there is a handwritten drawing showing the tail pivot making a 45 deg angle with the assembly.  The other angle is the tilt which allows the tail to pivot upwards with high winds.  I am referring to the 45 degree angle.  Why is this there instead of letting the pivot come straight off the back?  Is it to give more space?  Does it have something to do with giving room for that 10deg offset?

[imsmooth]

I'm sorry to post again, but I don't see how to edit.  I can see now why the 45 angle is there: it seems to allow room for the stop bumber to hit the yaw bearing.  If the pivoit came straight out there wouldn't be enough room to furl completely.

The moderator can edit these two posts as needed.  Sorry.

[TomW]

I'm no builder, but that geometry of the offset and tilt back is what drives the furling. I will let the others explain the details. It has to do with gravity acting against thrust on blades and tail.

It is fairly well documented over the years, is durable,reliable and pretty well refined.

Thats all I can tell you with certainty.

Tom

[imsmooth]

Thanks Tom, but I wasn't talking about the "tilt" angle which concerns the gravity.  I was referring to the other angle: the 45 deg angle that offsets the entire tail pivot from the assembly.  This angle sweeps parallel with the horizon.

[electronbaby]

If you look at the axial flux design from an overhead view, you can see that the 45 degree tail pivot is placed almost 180 degrees from the alternator offset in front of the tower. The tail pivot offset angle helps to balance some of the tower top weight and keep it centered over the yaw bushing under normal operating conditions. This allows minimal friction and wear from yawing over time. It also allows the tail pivot to be moved further forward of the yaw bushing tube when furled, and as you stated, it allows the tail bump stop to reach the yaw tube and prevent the tail from swinging too far closed and possibly hitting the prop. When in the furled position, this offset also allows for less force required to keep the tail in the upward, furled position. This has to do with the fact that by bringing the alternator and prop around the tower, you are bringing this tail hinge almost forward of the yaw bushing. In essence, you are decreasing the moment of the tail boom with regard to the yaw bushing and allowing it to stay furled more reliably (if its done right I suppose). The reason I think this needs to be accomplished is the fact that the prop (in its furled position) has much less cross sectional area into the wind and therefore is capable of much less thrust.  There is always a balance in every design, but with so many variables, sometimes it can be hard to replicate exactly. Good thing, many of us like a good challenge.

[Flux]

If you don't have the 45 deg( ish) angle then the tail will fall to its lowest point when directly down wind with the blades at right angles to the wind. It will have no restoring force and will just flop around. There is no force on any stop which has to be overcome before the boom parts company from the stop. The maximum restoring force would be when furled to 45 deg. At full furl the tail would be at highest point again with no restoring force, again useless.

By shifting the whole thing round 45 deg you get a decent compromise with reasonable restoring force against the stop in the unfurled position and you still haven't hit the dead centre when fully furled.The actual angle can be changed to alter furling characteristics, I am not sure 45 is the best compromise.

There is no need to physically put the pivot at 45 deg, you can put it directly at the back if you want but instead of a single 20 deg or so tilt from vertical the hinge pin needs to be at a compound angle that includes the 45 deg displacement of the scheme as normally done.

In view of the near impossibility of getting anyone to understand the thing in its simplest form I think Hugh Piggott was very wise to separate the two angles for easy construction. It would be incredibly difficult to explain the compound angle. If you want examples of this look at Bergey and an even better example is the Sencenbaugh machines.

Flux

[pepa]

 hi imsmooth, as everyone said it not simple but if you understand that the wind pressure on the angled tail in the unfurled possition keeps the offset of the blades in check and keeps the tower head in line with the wind without a lot of movement as the wind speed changes. no matter what, it works if done properly, pepa

[imsmooth]

FLux, I'm missing something...

If you have a 45 deg angle offset, won't the blade be 45 deg away from the wind, and decrease power?  When furling begins, 45 degrees has to be overcome before the blades are square with the wind.

If there is no offset why do you need a restoring force?  If there is a stop it will stop the boom when it is directly behind the blades.  Isn't this what we want for low wind speeds in order to capture the most energy?

[imsmooth]

Oh.  thanks pepa.  Flux, are you refering to the restoring force from the tail against the rotor?

[electronbaby]

The restoring force Flux talks about (and Im sure he will answer this question himself) is Gravity. Gravity is the force that mainly is responsible for unfurling the machine into its normal operating position when the thrust on the front of the prop is minimal (less force than required to furl).

There are two "stops" we must understand. Stop1 would be the obvious stub welded on the side of the tail boom that contacts the yaw tube under "fully furled" conditions. This stop will prevent the tail from hitting the blades when the machine is fully furled.

Stop2 is the stop that is built into the tail hinge. It is not very obvious, and its features dont stand out, unless you have built a machine. This "Stop2" is determined by how much you notch out of the tail hinge. When it is normally built, this hinge has a certain degree of travel. This total degree of travel is responsible for allowing the tail to fold closed and up (fully furled) till it hits the yaw tube ("stop1"), and in the other direction, to UNFURL only to the point of contacting the other end of the stop. This "notch" you remove is normally 60 degrees, and forms what we call a "pre-loaded" hinge. In the unfurled position, the hinge rests on one end of the "notch" you removed from the outer sleeve of the hinge, and in the other direction it does not necessarily contact the notch, but the tail is allowed to hit the bump stop preventing blade damage. Normally there is slightly more material removed from this "notch" as to allow the tail boom as a whole, to rest at 100 degrees with respect to the prop (not 90 like normally pictured). This allows the machine to run more true into the wind and keeps the tail out of the turbulence caused by the drag of the hub and alternator.

Another way to picture this is if you think in terms of drag. A spinning prop regardless of how efficient it is, will have drag. If you mount the spinning prop and heavy alternator offset from the point at which it pivots, the wind will want to always push the turbine around the tower. NOW, picture how we will prevent this from happening under normal circumstances: We will mount our tail and hinge 180 degrees out from this force (to help counteract it) and we will give out tail boom a loaded hinge. Gravity will keep the hinge open all the way (determined by the notch removed from the outer sleeve) until the oncoming wind exerts so much force that it wants to turn the prop and alternator out of the wind. At this point, out tail lifts up and still follows the wind. If the machine is fully furled, the tail boom bump stop will hit the yaw tube preventing tail damage, until the wind dies down. At this point, the force is diminished on the front of the prop and gravity pulls the tail back down which allows the turbine to return to its unfurled position facing the wind again.

There is a very good image on The Back Shed explaining this. http://www.thebackshed.com/windmill/Docs/Furling.asp I also have an animated drawing...Ill try to dig it up and post it.

[electronbaby]

Correction, "notch" should be 160 degrees. Quite often I make them slightly less and have no problems.

[imsmooth]

Roy, I appreciate your answer, which is very good, but I do understand these points.  The 20 to 30 degree tilt back allows gravity to restore the tail to the rest position.  I am still not sure why this 20 degree tilted pivot needs to be rotated 45 degrees counterclockwise.  In the rest position, the tail will be at its lowest position due to gravity, and 45 degrees counterclockwise from being perpendicular to the blades.  I don't see this purpose other than giving the rotor more time in the wind before being moved out by the thrust.

[electronbaby]

The reasons for the 45 degree offset are threefold:

1. -- it helps to balance the turbine on the yaw pivot to decrease wear over time. This was stated earlier.
   
2. -- it helps to locate the tail (in its unfurled position) in less disturbed wind cause by turbulence around the alternator (which is probably not that big of a deal).
   
3. -- it helps to give the turbine a wider range of furling movement (and yes increase distance so you are right about the "time" issue. I feel it is more "responsive".
   
   

My thoughts are: there probably is no reason why you couldn't mount the hinge directly behind the yaw tube. I have never done it this way, but Hugh has. Take a look at his 12 foot plans. This might be what you are talking about.

[Flux]

Let me try this another way. I seem to have a file uploaded here that must have been used to explain this before.

Imagine the thing with no stops and the pivot at the back with just the vertical inclination. The tail will drop under gravity to its lowest point and would come more or less to the position labelled normal tail position. As it is at its lowest point it hangs loose and would just flop about.

Now move the hinge round 45 deg and its lowest point then would be the one labelled tail with no stop. Now if you impose a stop to hold it in the first position it will try to drop to the unstopped position and gravity will force it positively against the stop.

The first example would not steer the machine but in the second case the tail boom forced against the stop will let the boom and stop act as a rigid structure and the tail will steer the machine. Regard the tail as anchored down wind and the prop thrust acting against the alternator offset is trying to rotate the tail. As it is anchored down wind nothing happens until the turning moment is greater than the gravity force holding the boom against the stop. Beyond this thrust the tail still stays anchored down wind but the rest of the machine rotates about the yaw axis with the moment of prop thrust x alternator offset being balanced by the tail weight acting on the hinge inclination which is trying to get the tail back against the stop. This is the basis but as Roy discussed other factors about the stop I will complete the discussion about why we don't stop the tail in line with alternator axis.

In simple theory we regard the tail as anchored down wind but a real tail has no force on it when exactly down wind. It will only develop a force when the alternator offset tries to rotate the yaw head so without an infinitely large tail. The whole thing balances when the force on the tail vane x boom length balances the other moment of prop thrust x alternator offset. The whole thing runs crabwise with the prop at an angle to the wind. We don't set the back stop with tail in line with generator axis but let it move away from the furled direction as shown so that the vane force is great enough to keep the blades square to the wind . We try but in reality this needs to be a far greater offset than generally imagined and most machines run a an angle to the wind below furling. If you make two machines with alternator offsets on either sides and see them running together you will be surprised how bad this issue is but it has negligible effect on power capture so best forget it and carry on regardless.

I hope this last attempt at explaining this 45 deg angle works. I realise that many find it confusing but it is simple enough and it would help if you met the basic concepts that this idea is derived from. Thinking of spring controlled schemes before thinking of the gravity replacement for the spring would probably help. The first scheme I met as a kid was very simple to understand and the gravity derivatives of that seem logical once you have got the idea of the basics with a tail boom forced against a stop acting as a rigid lever until there is enough force to make them part company.

Flux

[imsmooth]

With all the explainations I understand.  thanks everyone.

[ghurd]

"Strong winds, almost fully furled" (furl3.jpg) often confuses people,

because it shows the tail Always straight to the wind.

The tail gets a bit sideways as the blades get a bit sideways.

Sorry if that is obvious.

G-

[electronbaby]

I just thought I should post this to clarify what I said earlier. It is possible to not use the 45 degree offset. I hope Hugh does not mind. This is from one of his books. I gave him credit in the image. This is a drawing of the tail pivot directly behind the yaw tube. I believe in the original build, a chain was used as the stop for the tail hinge. This still allowed the tail to operate normally under furling conditions, and kept the chain taught when the machine was unfurled.

[Ungrounded Lightning Rod]

You'll note that the tail pivot axis between in this design is still nodding off to the side rather than straight back, even though the tail pivot is right behind the yaw axis.

The important thing is to get that lean right.  It has to lean to the side so the tail is already trying to hold itself against the unfurled stop by trying to pivot lower even in no wind.  The tail starts to fold up when the forces it's fighting from the offset prop become larger than the force holding it against the stop (which is derived from its weight and the pivot angle).

When you're fabricating such a tilted-out-and-to-the-side pivot it's easier to build it on a piece of sheet metal for a support and angle the support off to the side in such a way that the pivot ends up just leaning outward on the end of the vertical sheet metal support.  It's a fabrication simplification to get the right lean angle, not a need for the pivot axis to actually be located off to the side.

[Ungrounded Lightning Rod]

The tail's a bit sideways even (especially) if the mill is NOT furled.

The tail always flies CLOSE to straight downwind.  But the offset of the turbine creates a force trying to turn it away from the wind any time the wind is blowing on it (and it's not already turned-away until it's behind the yaw axis).  So when the yaw is in equilibrium (furled or not) the tail is off a bit to the opposite side, picking up a yaw force equal-and-opposite to that picked up by the offset turbine.