This is coolbert: Another drawback to the helo is the noise level. Very loud and can be heard from a long distance. In a counter-insurgency type environment, this can be a big drawback, when stealth may be required for success. During the Nam war, the communists forces would construct a specially shaped structure at ground level just large enough for a sentry to enter and sit in. Looked sort of like a beehive. Had sloping walls of just the right angle so that the structure acted as an amplifier to the sound of the helo blades as they beat the air and give off the wop-wop sound peculiar to the helo. The sentry was able to hear a helo from as far as fifteen (15) miles away, and not only detect the bird, but also tell from what direction the sound was coming from, and also whether or not the bird was approaching or moving away. Some sort of doppler effect at work here. Gave the commies enough advance warning to take protective measures, camo, defensive posture, etc. Some smart commie in Moscow or Peking figured this one out and told the Vietnamese about this, I would bet.

coolbert.