LTA Windpower

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Technology - The Generator Nacelles

In the PowerShip, the generator nacelles may be nearly identical to those efficient, reliable, and cost-effective units used in conventional wind turbines mounted on towers. There is in fact no need to re-engineer these important subsystems from scratch, although the bearings need to absorb their loads as in nacelles for downwind rather than upwind propeller systems. The LTA Windpower design places the propellers downwind from the nacelle for at least three reasons despite the fact that a slight loss of efficiency ensues: (1) This arrangement allows the tether to be attached to the leading edge of the wing instead of to the nose of the blimp, liberating the fragile envelope from carrying propeller drag, while keeping the tether safely and simply clear of the propellers at all times. (2) This enhances the overall stability of the airborne system, since the centre-of-drag is placed behind the centre-of-mass and behind the tether point. (3) When going through the process of tilting the nacelles upward, propellers on the leading edge of the wing would create a downward force (anti-lift) which could precipitate a crash; meanwhile trailing propellers create lift during rotation, which may cause the machine to rise temporarily.

On existing large tower-mounted nacelles, the propeller blades generally have their own pitch controls, which adjust how much air the propellers "bite". These are useful on large PowerShips as well. If the wind becomes too strong, the rotation rate of the propellers can be reduced by using blade pitch adjustments, which also serves to reduce the total drag on the tether.

Incidently, tower-mounted propellers experience large bending stresses during wind transients, but the PowerShip is gentler and smoother to its propellers in that a transient gust is initially absorbed by straightening of the tether and the shifting downwind of the whole airship.