Wind load when Furled

[(unknown)]

After playing with a few calculators online http://www.alton-moore.net/wind_calculations_english.php it gives a thrust figure for the blades. Once the blades furl what does it do to the thrust? Trying to figure out the windloading for the tower I have. It is very similar to the MA40 listed here http://www.texastowers.com/maseries.htm. I am going to add some guys to it and I have the benefit of being ably to retract it to 22 feet in really high winds. But I need to figure out the loading I am going to see from 6'8" blade diameter.

[Flux]

The thrust is related to the power extracted from the wind ( mechanical power from the blades).

Up to furling it is typically about 1/2 of the thrust on a solid disc.

Once you start to furl the power extracted stays fairly constant and so does the thrust loading on the blades. The wind loading on the rest of the structure ( other than blades) still rises and depends on the projected area.

For safety I would take the full disc loading at furling speed.

Flux  

[Mary B]

The problem is this doesn't fit the thrust calculated with Alton's online calculator. The rest of the windload is easy to figure as a matter of square feet. If you take the blades as a solid disc you would have 36 square feet in my case and that is a huge windload. The tower itself already has the windload calcs done on it so all I would have to add in would be whatever small static load the frame and tail will represent. The dynamic windload from the blades has me stumped. Acording to the calculator a 6.85 foot diameter blade set is going to produce 68 pounds of thrust at 26 mph which is going to be close to where I will want to furl. If this stays constant my tower will handle the wind loading without guys. It previously had 2 five element 6 meter amateur antennas on it. Bottom one at the tower top, second one 15 feet above the bottom one on 2 inch schedule 80. Both antennas were 5 square feet of windload and that top one had a huge lever arm from the 15 feet of mast.

[Flux]

Is that calculator right? are you using it right. I don't understand the things anyway.

Using your figures and approximating a bit, diameter 2m area 3.12m^2. wind speed 11.8m/s.

Thrust on a solid disc is 1/2 x rho x A x V^2.

Using 1.2 kg/m^3 for rho

thrust is .6 x 3.12 x 11.8 x 11.8   = 260N or about 57lb.

For a prop you can expect half that or even less. It is unlikely that you will be within rating of your alternator at 26 mph so you will likely furl earlier.

For safety think about 50 lb but expect less. If the tower can't stand this then you have a problem.

Flux

[Mary B]

50 pounds is very little wind pressure compared to an antenna. I might be able to use the tower unguyed since it was designed to be freestanding.

[Flux]

You know your wind area, 26 mph is very low. What happens when the wind reaches 90 mph.

I don't know how you work out the effective area of an antenna array, but I would have thought that the wind generator was a far bigger load.

 I don't know anything about your tower, if it was designed to be free standing then it may be ok, but in high winds I think you need to watch it.

Flux

[Mary B]

It has survived several 90+mph storms :-) And once furled does the thrust produced on the blades stay the same or does it increase by some amount? If the blades are turned out of the wind they are presenting a much smaller area to the direct wind. And I am going with a tilt back furling system that is hydraulically dampened so wind gusts from other directions will have less impact on things.

[wdyasq]

90. ^3 = 729,000
    
26. ^3= 17576
    
729000. /17576 = 41.477
        
        

There will be ~41.5 times the load at 90mph as 26mph, regardless of 'furl/no furl'.

Ron

[Flux]

Thrust from the blades will not increase much, it may even fall when furled hard.

All the area dependent thrusts will increase with wind speed cubed.

In very high winds if you can stop the blades ( depends on your alternator), they will only present their frontal area.

Flux

[Mary B]

yes but that equals 2,822 pounds which is well within the design limits of the tower. I realize load goes up as a cube of the wind. As long as the intial furled thrust stays the same then 41.5 times that is what I need to worry about.

[Mary B]

most antennas treat the windload as a flat plate calulation of all the parts. So a 10 foot long 3 inch diameter boom would be 2.5 square feet. Add the rest of the lements etc to come up with the total windload. My 3 blades plus the area of the hub and generator will be 5.5 square feet give or take 1 square foot.

[southpaw]

If you have a blade diameter of 80" your swept area is pi.r^2 or 3.1416 *40"*40" =5026sq." or 34.9 sq. ft. this is the area you need to use for wind loading. If your furling system is working properly the load would be fairly constant from the time it starts to furl untill it is completely furled because if the load starts to drop it starts coming out of furl. The furling system relies on the pressure against your blades to work.

southpaw

[Mary B]

This made more sense, if I have a thrust of 68 pounds at 28mph and it is fully furled the force against the tower stays at 68 pounds. This is well within the ratings of the tower I have which will support 100 pounds of thrust at 50mph.