Stealth helicopters are helicopters that incorporate stealth technology to avoid detection. In recent years, designs for blades have emerged that can significantly reduce noise, which is a major issue for clandestine use of helicopters. A raid on the compound of Osama bin Laden in May 2011 utilized what appeared to be two Sikorsky UH-60 Black Hawks, heavily modified for quieter operations and employing stealth technology to be less visible to radar.