LEFT: Gas Turbine Lab manager Jim Letendre (left) and AeroAstro technical instructor Dave Robertson prepare to lift the blades from the tunnel top. (William Litant /MIT photograph) RIGHT: The fan blade pitch gearbox in place before removal. The three planet gears in the center (and three more behind them) turn the surrounding sun gear, which then turns the six outer gears. The outer gears turn shafts that twist the fan blades, changing the pitch. Lateral shifting of the planet gear shafts has worn the shaft and nut in the center of each down to the cotter pins that secure them. (David Robertson/MIT photograph)
Working in the depths of the tunnel during the height of summer was “like working in a pizza oven wearing a toaster-oven hat.”
WBWT wind speeds are controlled in two ways: The 1 megawatt DC electric motor that spins the tunnel fan has three fixed rotational speeds and the pitch of the six (length) wooden blades may be varied. Something was preventing the blades from changing pitch. “We ruled out electrical problems,” Robertson said. “You could hear the pitch motor laboring.”
The blade pitch is controlled by a small electric motor that actually spins with the fan. The motor operates a series of six planet gears that turn inside a larger sun gear. The sun gear, in turn, rotates six more gears that, through a worm-and-gear system, simultaneously twists each blade. Robertson believes it had been at least 30 years since the pitch control was last serviced.
68
AEROASTRO 2015-2016