# Helicopter - Variable Air Density

Gold Member
This is not homework.

If you descend toward the ground in a helicopter, the density of the air is going to increase. I'm curious: Is the amount of energy required to turn the blades directly proportional to the density of the air? For instance: If the air density doubles, does the energy to turn the helicopter blades double (with a constant rate of blade rotation through both scenarios)?

Thanks,

Jake

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russ_watters
Mentor
Not exactly. Because the chopper needs the same amount of lift at all times accept when accelerating up or down, as it decends, it needs to move less volume of air. Drag will probably increase a little, but not in direct proportion to the air density.

AlephZero
Homework Helper
It's possible the amount of energy would decreases as you approach the ground, rather than increases. The reason is that the lift force is related to the change or momentum of the air, not the change of energy, and in general it is a more efficient use of energy to create thrust by moving a large mass or air slowly, rather than a small mass of air fast.

But I don't claim to be an expert of helicopter rotors, so I'm only saying that is "possible", not that it's a "fact".

First of all, your question is obviously a very abstract model. If any half-way realistic approximation tried to maintain the same rotation speed in denser air, it would simply shoot away with increased lifting power.

Couple that with the fact that the airflow around the blades isn't laminar in nature, it is or can be very much turbulent, thank you. This means that if you find an answer to your question which can fit in this space, please inform Sykorsky and you are set for life :-)

With those caveats, the answer to you question has to be yes - simply because the blades need to displace a larger mass of air which will take away kinetic energy directly proprtional to the mass of the displaced air. However, this is so far removed from reality, I hesitate to even call it an answer.

Danger
Gold Member
I'm not a round-wing weirdie, but I would love to get my mitts on one sometime before I croak (preferably not immediately before I croak...) While I'm familiar with autorotation and other principles, there is one thing that I've never seen addressed. Does "ground effect" influence whirlybirds in anything similar to the way it does fixed-wing critters?

Does "ground effect" influence whirlybirds in anything similar to the way it does fixed-wing critters?
Absolutely. As I remember from the history of the issue, one of the major challenges for tne early attempts was to demostrate that what they got is a true chopper as opposed to hovercraft which depends on the condensed/compressed air cushion to stay afloat.

Danger
Gold Member
Cool. Thanks, Grizz.