Flat Plate Axial Mill Furling Results

[brianc4]

Dan B had asked for info on how many pounds of force on the spindle were required to start furling considering the 11 ga. tail.

I got the scale today and got the following results. I hooked the scale to one of the stainless steel all threads & had Dad hold the tail. It only takes 15 Lbs. to start furling the tail & achieves fully furled at 40 Lbs.

Any input on wheather this is too heavy or too light would be greatly appreciated.

Brian Clark

[Ungrounded Lightning Rod]

I trust by "hold the tail" you meant "put sideforce only on the tail" - like by standing in front of it or holding up a cloth-covered forearm vertically for the center of the tail vane to bump into and slide up on.

The downward force from the weight of the tail is a critical parameter of the furling.  If the tail is "held" in a way that produces any additional vertical force, it will change the amount of force on the spindle that is necessary to furl the mill.

Similarly, once the assembly is furling the force applied to the spindle should be directed in the direction the tail is pointing and applied to the spindle at about the location that will be in the center of the blade assembly (to approximate the air-resistance center-of-effort).  Pushing it in another direction or applying the force at a different point on the spindle will apply extraneous torques or change the lever arm of the force, again fouling the measurement.

[Flux]

Seems a large difference between start and final figure. I know with this scheme the force does vary with it being a maximum with the tail somewhere beyond 45deg, but if there is a reasonable preload against the stop I would have thought the break away force would be much nearer the final value.

The first 30 deg of tail movement doesn't contribute much to a reduction in power anyway so it wouldn't hurt if the force was much more nearly constant.

As to the actual values I will leave that to Dan, it all depends on the prop seeking force and he is the one most likely to have experience with this prop and alternator set up.

Flux

[SparWeb]

I think the test method we're more familiar with is to push on the spindle, not the tail.  The tail can be rested against a vertical post, and the push force on the spindle is measured with, say, a bathroom scale.  Or you could use a spring-scale and pull on the back of the spindle.  Once the spindle starts to fold up, it moves back and the tail will need to slide upward on the vertical post, so you don't want much friction there.

Since you were pulling 15Lb on the tail, not the spindle, I fear it's too much.

Your windmill is in the size category of Dan's, not mine, so I won't venture a target number.

[brianc4]

I didn't do a very good job of explaining the details of the test!

When I said I hooked the pull scale to the stainless steel all thread I was refering to one of the all threads on the spindle so in fact I was pulling on the spindle not the tail. And when I said I had dad hold the tail he just put his arm in front of it to stop it not hold it down.

I double checked the results by letting the tail hit against the mast of the forklift and got the same results.

Brian Clark

[scorman]

Brian,

seems like you are using the pull scale in the correct manner, as long as the bolt you are attaching to, isn't sticking out to far from the rotor plane, since that would add a small lever arm ( but probably insignificant). The lever arm you are supposed to be using is the offset of the rotor axis to the yaw pivot x force exerted by the wind on the rotor. So you should be pulling in the direction of the axle axis.

I contributed to an older thread on the question of how much thrust is exerted by the wind on a rotor that is not furled. There were several equations which contradicted each other, including one that was higher than that of the wind itself.

If I can summarize the various comments, the efficiency of the turbine at any particular WS, relates to what % of the total avail power in that WS for a specific diameter rotor is then exerted along the axis towards the tower.

As an example, for a 16 foot mill like DaveB's, in a 25mph WS, the total power in the wind is 326 # force, but if running at Cp=0.35, then about 100# is pushing backwards

Total force = 0.0026 X (Area) X (wind velocity ^2)

If the axis is offset, then a yaw force pushes against the tail, wherein the mounting angle and weight of that tail gives the reaction force you are measuring statically in your test setup.

Changing subjects slightly:

There was also an earlier thread by you late 2007 asking about blade designs and sheet metal construction ..what was your final design as far as material, diameter, pitch, profile, taper, twist??

Stew Corman from sunny Endicott

[brianc4]

Stew,

Thanks for the input & formulas on the furling.

The first PMG mill I built we used PVC blades cut out of a 12" dia pipe. They worked really well & had excellent low wind start up & achieved good speed without too much drag.

Due to time constraints I have picked up a set of 10' 6" fiberglass blades to to try on the axial mill. The startup speed & overall performance may not be as good as a custom carved wood blade but I am going to give them a try.

As for metal blades I am still planning at some time to give them a try. The design will be about like the profile we used on the PVC blades but with thinner tips to get the tip speed up. I plan on using 4130 annealled chrome moly for the blades when I get a chance to fool with them.

Brian Clark