Sq ft for tail

[philk]

I am building a 10' machine, how many sq feet should the tail be?

I found the info on the tail boom but could not find how many sq feet there is in the tail.

Thanks.

[spankytree]

Good question, I just finished a 20' and the plans called for roughly 20 sq ft. It LOOKS small but seems to work.

Spanky

[Sly]

Hi,

I used 2'wide x 3'high made out of 3/8" thick plywood and rounded the corners. This is based on the information in Hugh's manual. Maybe you should consider going with 1/4" thick as recommended by experienced builders on this site to furl earlier.

sly

[Flux]

I hadn't realised that this was not covered and probably a lot of the furling issues stem from this. The area is not very important but the weight certainly is.

You need a reasonable size to get it to steer reasonably against the alternator offset thrust, but even with a monster vane it will still run at an angle.

The 3 x 2 ft seems a sensible sort of size  and I doubt that you could go much bigger and still keep the weight to a sensible value.

If you want to get it to face somewhere into the wind you will need to offset the tail away from the furling direction and even then it will be much of a compromise between keeping it roughly into the wind and looking sensible and not messing up the gravity restoring force too much. Offsetting it more than 20 deg looks a bit silly and may adversely affect the range of restoring force from the inclined hinge. If it runs facing the wind within 30 deg you will not see any difference but if you compare two machines on the same site with furling offsets on either sides you will realise that it doesn't face square on to the wind.

Flux

[imsmooth]

I thought 3% of the rotor swept area is about right.  You want enought area to balance an area of the blades so the vane does not get pushed out of the wind before it furls.  I look at it this way:

You have three blades.  Two blades surface area cancel out leaving a third that can act as a lever arm when the wind blows.  For a 10' rotor the blade probably has a surface area close to 2 sq feet.  A 10' rotor has a surface area close to 75 sq feet.  3% of 75 is close to 2.2.  So, as long as you are greater than this the tail will always stay in the wind and the only way the assembly will yaw is if the tail furls up.

[Flux]

That seems very low. It will no doubt steer a non furling machine quite easily such as a pitch controlled one but it would let a furling machine run at a fair angle to the wind.

The thrust loading is typically about half that of a disc of the prop swept area.

A 10 ft rotor has a swept area of about 78 sq ft and the suggested tail area of 6sq ft is not wide of the mark, that gives about 7.7 %. If someone said 20 sq ft for the 20ft machine then that is about 6.4 %. On that basis 7% would deem a reasonable starting point.

The vane has no restoring force until it is at an angle to the wind so the smaller the vane the larger angle it must run to the wind to balance the alternator offset thrust. with only 3 % I think it would be up towards 45 deg to maintain balance.  In theory such things can be calculated but the seeking force of the prop is not known and it is a bit uncertain what sort of wind the tail sees in the wake of the prop  so it seems a bit of worthless calculation that will most likely add more confusion.

Flux

[mgtd]

Vane size was a sticky point during my build also--no mention of size is in the instructions.  Finally in the "Operation" section for the 10 foot mill I found a photo that I interpolated dimensions from, knowing the dimensions of the metal work specified in the build instructions.  I came up with approximately 18" by 38" which comes to about 4.75 Sq feet of area. Since I was using 1/4" sheet PVC (for durability and lack of maintenance), which is somewhat heavier than the recommended plywood, I scaled back to a vane that was 15" by 35",or about 3.65 sq. feet.  I believe this smaller vane was still too heavy and contributed to my recent post "10' Smoked in High Wind", Oct-28-2008. I'm going to go to 18" by 40" when I put my mill back up.  

On another note--but related to overheating, I have made some modifications to my rotors, hub and a few other parts to increase airflow across the stator and rotors.  I will be posting pictures and details soon.  MGTD  

[imsmooth]

I'm not sure if 3% or 6% is the absolute minimum, but went higher with mine.  For the 10' I'm working on I have 8 sq feet (2 x 4) for my size. .

[scottsAI]

Posted "Floppy tail" couple years ago with tail design requirements:

http://www.fieldlines.com/story/2006/10/7/6296/37753

Researched many designs into general equations.

Have fun,

Scott.

[Ungrounded Lightning Rod]

You have three blades.  Two blades surface area cancel out leaving a third that can act as a lever arm when the wind blows.

Bad way to look at it.

 - The blades, when "flying", decelerate all the air in the area they sweep.  They don't quite stop it.  But they have a wind resistance much greater than a flat board of their area.  It's almost as if they almost filled the entire swept space.

 - But they're balanced.  So you can treat it as a slightly leaky disk with the resulting wind resistance concentrated at the center of the axle, which is slightly offset from the center of the yaw pivot.

 - AND there's the "wind seeking force" - an additional force on the blades that tries to pull them back to face the wind when they're spinning and turned a bit away from it.

So figure it's a slightly porous disk a little offcenter and make a tail big enough to compensate for the crescent of swept area that appears on one side and not the other, plus some extra to overcome the wind-seeking force.