How Much Air Mass for a Hovering Helicopter?

In summary, the conversation discusses the mass of air that must pass through the blades of a hovering helicopter every second in order to produce enough thrust. The calculation involves considering the upward force of the air on the helicopter, which is equal to the downward force of the helicopter's weight. The air is accelerated by the blades from a state of rest to a speed of 70m/s.
  • #1
henry3369
194
0

Homework Statement


During a rescue operation, a 5200-kg helicopter hovers above a fixed point. The helicopter blades send air downward with a speed of 70m/s .

What mass of air must pass through the blades every second to produce enough thrust for the helicopter to hover?

Homework Equations


ΣF = 0

The Attempt at a Solution


Since the helicopter is hovering, the magnitude of the downward forces have to be the same as the upward forces. The only downward force on the helicopter is the weight = mg. So this must equal the upward forces. I can't figure out what the upward force is. I tried using impulse and momentum to find force, but the change in momentum is zero, so impulse is zero so that doesn't help me find the force of the air on the helicopter. I know that the force the blades exert on the air, the air exerts on the helicopter, and all I have is the velocity of the air from the blades.
 
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  • #2
Looks like the air is changing momentum: vi=0, vf=70 m/s.
 
  • #3
lewando said:
Looks like the air is changing momentum: vi=0, vf=70 m/s.
Makes sense. I was thinking about the system as a whole. Thank you!
 
  • #4
lewando said:
Looks like the air is changing momentum: vi=0, vf=70 m/s.
Hmm. How did you conclude that the air was changing speed if it says that the blade sends air down at a speed of 70m/s.
 
  • #5
A blade, not unlike a fan, accelerates air from a state of not moving to moving.
 
  • #6
You can assume there is no wind down- or upwards, so initially the air is at rest.
 
  • #7
Alright, in a wind tunnel, vin = vout, but in free air this is much less so. Even though there would seem to be some continuity of air flow near the blade, at some point far enough away, the air is not moving.
 

1. How much air mass is required for a helicopter to hover?

The amount of air mass required for a helicopter to hover depends on several factors, including the weight of the helicopter, the size and shape of the rotors, and the atmospheric conditions. However, on average, a helicopter needs to displace at least its own weight in air mass to maintain a stable hover.

2. Can a helicopter hover in high altitude or thin air?

Yes, a helicopter can hover in high altitude or thin air, but it may require more power and thrust to maintain a stable hover. This is because the air is thinner at higher altitudes, which means there is less air mass for the rotors to push against. Helicopters designed for high altitude operations may have larger or more powerful rotors to compensate for this.

3. How does air density affect a helicopter's ability to hover?

Air density plays a crucial role in a helicopter's ability to hover. The denser the air, the more air mass there is for the rotors to push against, making it easier for the helicopter to maintain a stable hover. In thinner air, the helicopter may need to fly at a higher speed or tilt the rotors at a different angle to generate enough lift to hover.

4. What happens if a helicopter runs out of air mass while hovering?

If a helicopter runs out of air mass while hovering, it will start to descend unless the pilot can quickly increase the power and thrust to compensate for the loss of air mass. This can happen in situations where the helicopter is hovering near the ground or in confined spaces, where the air is churning and turbulent.

5. Can a helicopter hover in zero gravity?

No, a helicopter cannot hover in zero gravity. Helicopters rely on the downward force of gravity to provide stability and control. In a zero-gravity environment, there is no gravity to provide this force, so the helicopter's rotors would have nothing to push against, and it would simply spin out of control.

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