Puttin the Brakes on !

[Dave B]

We are getting hit with an early Winter storm here in the North East. 30-40 MPH winds most of the day highest gust so far has been 42 MPH. I've been watching closely my 18' and experimenting with the load trying to get an idea of the furling range. It seems set a bit higher than I'd like watching it fully furl and hold only after a constant 25+ Mph. Here it is running over 200 rpm and 100 VAC. 3 phase. Measured output at my hot water heating elements has peaked over 4500 watts. The Gottingen 222 blades I have create a lot of torque seem to want to start and run even with the alternator shorted with 30-40 MPH gusty winds. Not wanting to put that to the complete test for a shut down mode I slowed the machine by shorting it during a lull in the wind and gradually pulled on my parking brake to a stop. Spring loaded tension on the cable is holding the blades stopped, this is a scrub brake arrangement against the rear rotor. The weather is very nasty and getting worse and it is a great feeling being able to lock down the machine, so far so good. If you haven't been through the panic or self destruct mode that the winds will bring (it's just a matter of time) you cannot appreciate how great it feels to shut things down. I'll report back, looks like a couple days of this ahead. Dave B.    

[harrie]

Wow, 200 RPM at 100 volts AC! I dont think my heart would stand watching it spin that fast. How far do you think your blades are bending back at that RPM?? IM glad your brakeing system works for you, and I can beleive you needed to wait for a break in wind volocity to get it slowed down.

[Flux]

Harrie

To you with your stall operated machines you would be frightened by that speed, but to anyone else it is quite modest. Speed doesn't cause the blades to bend back, but operating near the peak of the prop's power curve and extracting more energy does increase thrust and may cause the blades to deflect more than in hard stall.

For you in stall it doesn't matter when you brake it, but when operating in normal mode it makes good sense to wait for a lull even if the thing is actually capable of stopping from full flight.

It's not possible to operate a heating scheme in stall mode and get any reasonable power, just a nice peaceful luxury if you can manage with a modest output for battery charging.

Flux

[DanB]

Yes - as flux says 200 rpm is not that much for a machine that size.  I know my 20' machine exceeds that at times.  I don't have a brake on that one - I rely on shorting it, but it always works out no matter how fast it's running.  Actually - for about the last 6 months I've only been shorting 1 phase to stop it (and it always stops quickly even if its running at high output)...  which is nice because it makes me feel like I have a redundant brake (if one phase fails for any reason I can still stop the machine in high winds).  It sounds a bit silly...  (that I only short one phase)  - the reason is simply that I use an old 'safety switch' that has 2 fuses in it and one fuse is missing.  I've not bothered to replace it or by pass it yet.

Daves machine is 18' diameter.  For referance - I believe that south west windpowers H500 (a 15' machine rated @ 3kW) reaches its rated power at 800 or 900 rpm!!!  (seems a bit nutty to me but that is the case)

[harrie]

Yes, one of my first machines 12 foot dia at 12 volts, did well in excess of 200 RPM, and it did fine. I think the reason I felt the larger dia machines would tend to bend back was due to my 18 foot at one time was in 24 volt, running at about 200 or so RPM.

The tips of the blades were at 12 inches from the tower, and they bent back far enough to strike the tower. After reading your posts, maybe, and that could very well be that the blades were not made with the correct airfoil? could that be the case?

hope we are not stealing daves thread!

[scorman]

Dave,

"It seems set a bit higher than I'd like watching it fully furl and hold only after a constant 25+ Mph. Here it is running over 200 rpm and 100 VAC. 3 phase. Measured output at my hot water heating elements has peaked over 4500 watts"

Have you updated your performance chart?

Your 240v/3600w heating elements have a 16 ohm resistance.

To get 1500watts per leg as in your quote, you'd have to be cranking 155v, and that isn't even on your chart and way higher than 200rpm.

But, your furl is around 25mph ?, so how can you be cranking that high rpm or volts??

Is it still spinning like a top while fully furled and that is why you applied brake?

Keep up the good work ..encouraging to see someone making that much juice and not having it blow apart in strong wind ..overbuilding is the way to go ..nothing marginal would have still been flying!

Stew Corman from sunny Endicott

[Dave B]

Stew,

 I have been working with different heating elements in preperation for designing my controller. My previously posted chart(s) are just a fraction of data I have logged over time with different load / wiring schemes. 200 RPM is not considered fast with respect to other machines as Dan mentions, in run away mode my previous 12' measured over 800 RPM. My whole design from the start was decided on and wired for approx. 100 VAC at 200 RPM and I hit it right on the nose. Back figuring 200 RPM from the earlier shorter Wincharger blades for my 18' put me at a tip speed slightly less than that at which the wincharger's air brake started governing the speed. My plan was to run the profile near the tip speed range at which it was previously designed for but not to exceed it, for 18' that took some figuring. There is meaning behind my madness at times and for my set up heating water it appears I made some very good design choices. The brake is off now and held fast in 40+ MPH last night so I'm happy with that besides. Thanks for the compliments.  Dave B.