First, how much torque would be required to yaw a turbine this size?
I don't know that yet, Jeff. I have to talk to Dave Moller first and find out how many rpm his blades run (safely) at the top end, and how much they weigh. Once I know that I can figure the amount of force generate by precession of the blade circle to figure the turning torque. This turbine's thrust will be inline with the yaw shaft, i.e. it will not use an offset mainshaft. As is usual in most designs, I'll add 50% to the calculated minimum torque value and design my yaw drive to have at least that amount of torque.
so what kind of yaw rate would be required?
That I do have figured out, using the motor and pinion/ring gear I plan to use. It will yaw the turbine at 710°/minute. So it will take it 7.6 seconds to yaw it 90°
In high winds do you plan to yaw first and brake only if required, or just brake and yaw together and shut it down
For simplicity (at least at first until I get a chance to do some testing) I'm going to have the furling switch bypass the tail switch to turn it 90° and simultaneously activate the brake motor to apply the brake and stop it.
My concern is a failed actuator or broken wire could disable both your yaw and brake devices.
That could always happen. It's unlikely that the yaw motor and brake motor would simultaneously fail. So I figure that the brake will be powerful enough to stop it even faced into the wind if the yaw motor failed. If the brake motor failed but it still turned 90° it will stall it anyway without the brake with the stator shorted. If the furling switch fails I'll have provisions to control it from the ground in my control box that bypasses both the steer and furling switches on the turbine head.
If the control wires get tore off somehow, the stator is gone so that can't be used to brake it, and nothing works, retreat to a safe distance and just hope the blades and magnet rotors hold. At that point you're at the same place where you'd have a conventional furling tail that failed to work, you got no brake, the stator dropped a coil and the wind is still howling at 50+. All you can do is retreat and watch from a safe distance, and if you see it starting to come apart, take cover.
When I sat down this morning and started drawing out an electric yaw drive on paper, it's not really all that complicated. It's just a matter of building it. Nor am I going to waste time building a brake - I found an old disc brake from a transmission from an International 1640 combine laying in my shed. The nice thing about the brake is that it uses linear actuation - just pull a 1/2" rod that's sticking out of the brake housing and it applies it. Push the rod in and it turns 100% free with no drag. That brake will stop both drive tires on a 45,000 lb combine going 12 mph and make them skid on blacktop. It'll easily stop a 21' turbine. It weighs about 35 lbs though.
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Chris