I think he means the pressure against the prop hub that turns it into furling?

Not positive, but I do know DanB provided that information at one time although I cannot say for sure what value he stated.

Terminology can get confusing at times, indeed.

Cheers.

TomW

The Truth is the Truth, even if no one believes it; and a lie is a lie even if everyone believes it

I have now found a figure from Dan suggesting that his 10 ft machine needs a force of about 25lb to furl at 1kW output. This is interesting and seems to indicate that the prop seeking force is much greater than I imagined on Dan's machines.

Firstly I have no idea what wind speed Dan's 10ft machine needs to produce 1kW but it must be up in the 30 mph region.

I have found some figures for my 10 ft machine, which furls about 25mph.

The thrust on a disc is found from 1/2 rho A V^2. The thrust on a rotating prop will be less than this and depends a lot on the power coefficient at that point but I seem to have assumed that .7 x the disc thrust would be near.

At 25mph this gives just under 100lb. In my case the offset is 5" so the torque trying to turn the thing out of the wind is 100 x 5/12 = 41 lb-ft

The moment of the tail against its stop would need to produce this torque to balance it.

I found that the seeking force on my prop produces a torque of about 18 lb-ft  so the tail needs to balance 41-18 or 23 lb-ft.

This torque equates to a thrust of 23 x 12/5 = 55lb of thrust on the prop.

This is much higher than Dan's figure and I suspect Dan's figure is at a higher wind speed, making the difference even greater.

I do know that the seeking torque is very dependent on the prop design and loading conditions and it is all to easy to muck things up and never be able to overcome that seeking force ( it will never furl). It looks as though Dan's seeking torque is rather higher than in my case ( I have been caught and play very safe now).

I have no idea how these figures relate to Hugh's design, and again prop construction and alternator loading would vary from one machine to another, with different line resistance etc.

None of the details of my tail would help, it uses sheet metal rather than plywood and doesn't use an inclined hinge but uses a wire rope round a drum to convert weight to restoring torque. The restoring torque is about 23 lb-ft as I mentioned so you could compare that with a hinged tail quite easily. ( torque needed to move tail from its stop measured horizontally).

I always seem to have to add weight to the tails of my machines to raise the furling speed, which is safe. I still wonder if some of the machines mentioned here really do furl at a safe speed, room for more investigation.