The Huey's twin-blade rotors with their distinctive whop, whop, whop provided the soundtrack to the Vietnam War. But in January 1961, Bell engineer Bart Kelley was considering a 3-bladed version, as he wrote in his diary: "Thinking about 3-blade lack of collective response. Could it be that each blade hits downwash of its predecessor? Lower rpm might be better?" By March, he wrote: "We have been studying 3-bladed versions of the standard semi-rigid rotor system.... We have not yet found enough advantages in the three-bladed system to compensate for the somewhat greater weight and complexity, but are continuing to push the program as an insurance policy for the future."