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Lowering terminal velocity by increasing air density

Lowering terminal velocity by increasing air density

(OP)
Hello experts!

This is my first post and it's an 'ask someone with a lot more aerodynamics knowledge than me' kind of post, so forgive me if it needs to be posted elsewhere.

This has been bugging me for some time, explained as follows. I will use point form so that my assumptions / knowledge can be easily snared and corrected:

A question in two parts:

  • Terminal velocity
  • Terminal velocity depends, partially, on air density, i.e. if the atmosphere is denser on planet X, for example, then the same object falling on planet X would have a lower terminal velocity than on Earth
  • Airfoils
  • A basic airfoil is designed to create a pressure differential
  • On one edge of the airfoil there will be a greater air pressure as it is accelerating
  • Visualise, then, an airfoil that was shaped into a 'ring' instead of a flat 'wing' (see attached files)
  • If this 'ring' was falling vertically, and designed with the cross section of such an airfoil, could a greater air pressure be induced inside this ring?
  • If so, would there be a way that this increased air pressure could be harnessed such that the terminal velocity of the falling body could be significantly reduced?
Things I have considered but haven't been able to verify:
  • The effectiveness of such a device would be lower at lower accelerations / velocities. If our falling 'ring' reached terminal velocity - i.e. when acceleration stops, would there be no more pressure differential?
  • Would the overall effectiveness of the 'ring' be less at thinner atmospheres?
  • Could the (higher pressure) air be funneled into a parachute / drag inducing mechanism in order to lower the terminal velocity?
  • Is this re-inventing the wheel i.e. does something like this already exist? I am curious.
Image 1
Image 2

The main reason I ask is that I am wondering whether it is possible for some sort of 'structural parachute' that would work at any altitude and significantly reduce a falling person's terminal velocity. I am predicting that it is possible but it will fall into the 'asking too much from physics' basket - which would explain why I haven't read about it elsewhere bigsmile

Thanks for your time and your thoughts!

RE: Lowering terminal velocity by increasing air density

Watch this video: https://www.youtube.com/watch?v=CAwET3Q9Og4

Notice the function of the ring and how it functions in the lines of the parachute. When the aircraft is at high velocity, the ring closes the parachute to prevent structural (fabric) failure. As the aircraft slows down the ring slips down the lines and that allows the parachute to open fully.

Bronc

RE: Lowering terminal velocity by increasing air density

(OP)
Awesome! Thanks for that link... so the actual ring is designed to channel higher air pressure, or does it use a different technology?

RE: Lowering terminal velocity by increasing air density

No, I don't think so. It's responding to the tension in the lines. What happens when the velocity (relative in this case) of a fluid declines?? It's a VERY elegant design isn't it?

Bronc

RE: Lowering terminal velocity by increasing air density

Or maybe it does, it's a nozzle isn't it? A well defined airflow goes through it directly into the cute. So, so beautiful--simple, yet wonderfully intricate and complex at the same time.

Bronc

RE: Lowering terminal velocity by increasing air density

(OP)
Yeah, for sure... just wondering whether there are applications for the 'ring' itself, that don't involve 'fabric' per se, which presents a tear risk, as the video outlines.

RE: Lowering terminal velocity by increasing air density

this ring reminds me of those Dyson fans ...

another day in paradise, or is paradise one day closer ?

RE: Lowering terminal velocity by increasing air density

"I am predicting that it is possible but it will fall into the 'asking too much from physics' basket - which would explain why I haven't read about it elsewhere"

If so, then you are smarter than the knuckleheads at NASA, who resorted to huge inflatable air bag to land the Mars rovers safely.

TTFN
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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com: http://www.engineering.com/AskForum/aff/32.aspx

RE: Lowering terminal velocity by increasing air density

i doubt the idea has that much merit. what you're looking for is a high drag profile, not so much the pressure differential due to lift. The hardest thing to achieve in the real world is a stable wake, particularly at terminal velocity.

a random thought d'jour ... a rotating streamer would increase the work done by the wake, ie increase the drag, and might be more stable ...

another day in paradise, or is paradise one day closer ?

RE: Lowering terminal velocity by increasing air density

(OP)
If so, then you are smarter than the knuckleheads at NASA, who resorted to huge inflatable air bag to land the Mars rovers safely.

Sorry. I didn't mean any disrespect. I just figured it might be one of those space elevator kind of problems e.g. "you could do X but you'd need a tensile strength of Y which means you'd need a material a thousand times stronger than Z".

i doubt the idea has that much merit. what you're looking for is a high drag profile

Everything I've read about it so far makes me agree with you 100%. If indeed the higher air pressure could be induced, however, surely there could be some way of making it 'work' to create additional drag?

The concept of some kind of tilt-rotor (?) or some mechanism inside the ring occurred to me that it could possibly get the air to do that 'work', but as I understand it, tilt-rotors generate down-force from the above air which wouldn't apply in this scenario.

Again, thanks for the feedback!

RE: Lowering terminal velocity by increasing air density

i think the difference is tilt-rotors are trying to generate lift, but you want to generate drag (much easier !)

another day in paradise, or is paradise one day closer ?

RE: Lowering terminal velocity by increasing air density

It is interesting how this came about. Back in the 1980s I was talking with Boris of BRS about adding a ballistic chute that they were using on hang gliders to a sailplane. The question of parameters came up. I pointed out that an out of control sailplane could accelerate from stall speed to VNE in about 6 seconds, and that the out of control aircraft could easily get up to 200 mph before the pilot could deploy the chute. The conversation ended with "I will have to think about that and get back to you."
The result is what you see today on the Cirrus.
B.E.

You are judged not by what you know, but by what you can do.

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