The Air Force is asking Congress to restrict further construction of the towering wind turbines that have edged closer to its nuclear missile sites in Montana, Wyoming, North Dakota, Nebraska and Colorado. The underground silos share space on vast private farmlands with the turbines, which have grown in size and number as the country's energy needs have increased. But the turbines make it dangerous for military helicopter crews, which need to fly in low and fast when a site alarm triggers. The Air Force wants Congress to pass legislation to create a buffer zone of 2 nautical miles around each site. The legislation has the support of wind energy advocates. Existing towers would be unaffected.