Dang Wind.

The wind made mincemeat out of my hoop house again.  OK, that’s an exaggeration, but we had gusts of 30-40 mph and they were enough to de-skin the hoop house for the second time.   I’m beginning to understand that there is a basic problem with the design, or at least my execution of it.

My version is based on one by Tod Hanley, but I used 20 foot PVC hoops and he used 24 foot square steel tubes.  He needed a tubing bender to build his.  I didn’t.   The hoops are anchored by being placed over rebar lengths that are driven into the ground.  The plastic is anchored at both ends and held down by ropes parallel to the hoops on top of the plastic.  This way the plastic is free to be lifted up for ventilation and is held in place by the rope tension.  You can see how it works in this photo (from http://www.kerrcenter.com).

Sides up for ventilation

The flaw seems to be that the wind can raise the sides up too, and once it gets inside, there can be enough force to turn the thing into a kite.  In my case this past weekend, all the rebars on the east side of the house were torn out of the ground and flung as far west as the ropes would allow.  It must have been quite a sight.

I’m not sure what to do about it.  I could anchor some skirt boards along the ground and fasten the plastic to them, but that would put the kibosh on raising the sides for ventilation.  I may just build some 3 foot sidewalls and fasten the PVC hoops to their tops.  Ventilation could be built into the sidewalls.    Hmmm, not a bad idea.  I’ll have to check out the $$.

Comments on: "Dang Wind." (3)

  1. John Cox said:

    Your shape is a nice airfoil, and holding down the ends isn’t going to do much for it. Most of the lift comes from the low pressure over the top of the plastic. Stronger plastic will make it easier to hold down. A flattish roof with sides might work better, too. Alternately, not tying it down at all would minimize the damage to your poles because the plastic would blow away!

  2. Hi John. Good point about the airfoil. Leave it to an aero engineer to think of that. A video of this hoop house would actually make a good teaching tool, because you can see the lift in the billowing of the plastic as the wind goes over the top. Do you think the wind raising one side and getting inside the house wasn’t much of a factor?

    Standard practice with most of these hoop houses seems to be to run the plastic over curved poles just like I did, but most designs do fasten the plastic down on all four sides and don’t use any ropes.

    I didn’t get a picture of it, but this time the poles stayed put, other than their east side ends popping out. I had run a rope down the center of the house, under the plastic looping it around each pole, and this seemed to keep the lateral movement of the poles to a minimum. And then, yes, the plastic blew away.

  3. John Cox said:

    Us aeros always think of the airfoil: If the front of the airfoil flaps up, the angle of attack is increased and there’s more lift. Overall, I think you’ll find that most of the lift comes from the upper surface rather than the lower. It’s a “half rho-v-squared” thing (Bernoulli’s Law).

    I suppose that points out that tying down the front of the tarp is probably as good as you can do without it getting really expensive. I was also thinking about things that block the air, but that gets to be a lot of work.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: