Describe in terms of the conservation of momentum how a helicopter hovers

Which of the following ziegler nichols tuning methods the response of the controller to a step input should exhibit an s-shaped curve? a)-open loop mode b)-closed loop mode c)-both modes (open & closed) d)-none of the modes (open & closed)

Vibration monitoring this technique uses the noise or vibration created by mechanical equipment and in seme cases by plant systems to detemine their actual condtion. a)- true b)- false

What is the heat treatment of metals? what is the benefit of it? why and how it's useful? answer in details, do not write by hand.