5 ms·

Ram air turbine

37 points by Hooke 11y ago

InclinedPlane 11y ago
Deep ocean sailing yachts used to use these for electricity generation (for powering things like navigation equipment and radios) but now it's more common to use battery backed solar power.
- HerpDerpLerp 11y ago
  I would say that boats have Wind Turbines rather than RATs. https://en.wikipedia.org/wiki/Wind_turbine https://en.wikipedia.org/wiki/Wind_turbine The power comes much more from the wind than the vehicle moving through the air (though I am very aware that the speed of the boat affects the apparent wind the generator will experience)
  - bnegreve 11y ago
    Actually water turbines (aka hydrogenerators) are also an option, but it's less common. http://cdn.streamlike.com/secure/Covers/68/17853.jpg http://cdn.streamlike.com/secure/Covers/68/17853.jpg
dingaling 11y ago
They make an horrific noise when deployed, the blade tips are highly supersonic. I've heard one once as a groud observer and I can only imagine the racket they make in the cabin. For that reason they're not tested in routine service but there are ground-testing rigs that spin them up to a fraction of their maximum rotation speed. The A320 series have RATs but the 737s ( all series ) don't since they remain manually-flyable without electrical power.
unwind 11y ago
I'm confused. It looks like a turbine powered by the movement of ambient air, in this case due to the aircraft's movement through said air. But the article says: The RAT generates power from the airstream by ram pressure due to the speed of the aircraft. How is this dependent on the pressure, rather than just the speed of the air? Would a RAT from an aircraft not work on a stationary object subject to wind? Or is such an object also experiencing ram pressure (just the other way around due to reference frame shifting), i.e. all (wind) turbines work due to ram pressure?
- jonsen 11y ago
  "ram pressure" is a link to its own explaining wiki-page.
  - unwind 11y ago
    Yes of course, I read that and it didn't help. It talks about the drag experienced by an object moving in a fluid. I guess I'm just dense.
    - jonsen 11y ago
      You are right, not really informative. Maybe Ram Air Turbine is a bad name. A turbine driven by the airflow from "ramming" through the air?!
- JorgeGT 11y ago
  Aerospace eng. here, you are right in that is a confusing terminology. Ram pressurein this context is a way to refer to the available kinetic energy due to airflow: 1/2 x density x speed^2 (also called dynamic pressure). A point to stress is that the energy delivered to the plane systems by this auxiliary turbine is thus varying with the square of the speed of the plane. That means that as you slow down for landing, the RAT delivers less and less energy... and the commanding authority of the hydraulic-actuated control surface lowers, making the plane less and less controllable. Not fun!
  - toomuchtodo 11y ago
    You'd think the RAT would charge a battery acting as buffer. But who am I to armchair quarterback an entire aerospace industry.
    - astrodust 11y ago
      Having a sufficiently large battery introduces another point of failure, something undesirable in a situation where you're already in bad shape. You don't deploy the RAT unless shit is going sideways.
PhantomGremlin 11y ago
Be sure to follow the link to Pinnacle Airlines Flight 3701. That's a double Darwin Award, well deserved.
- goldenkey 11y ago
  Wow, that was quite a legendary chain of fuckups. Link for others: https://en.m.wikipedia.org/wiki/Pinnacle_Airlines_Flight_3701 https://en.m.wikipedia.org/wiki/Pinnacle_Airlines_Flight_370...
  - astrodust 11y ago
    It takes a lot of gall to double down on your mistakes in a situation like that. These two should've been stripped of their license had they survived that stunt.
    - goldenkey 11y ago
      It seemed like they were hoping the 2nd engine wasn't totally cashed. So they lied to the ground about having only 1 engine failure. I would imagine being alive is worth embarrassment but given the prestige of a pilot, I really can't say more than, bad choice.
- blackflame7000 11y ago
  The one above it is also pretty dumb.
chinathrow 11y ago
Here's one of those RATs in motion on a 777: https://www.youtube.com/watch?v=FfB2EP71hqY https://www.youtube.com/watch?v=FfB2EP71hqY
boxctim 11y ago
Hooke was this by any chance inspired by a reddit discussion?

37 points by Hooke 11y ago

Deep ocean sailing yachts used to use these for electricity generation (for powering things like navigation equipment and radios) but now it's more common to use battery backed solar power.

HerpDerpLerp 11y ago
I would say that boats have Wind Turbines rather than RATs. https://en.wikipedia.org/wiki/Wind_turbine https://en.wikipedia.org/wiki/Wind_turbine The power comes much more from the wind than the vehicle moving through the air (though I am very aware that the speed of the boat affects the apparent wind the generator will experience)
- bnegreve 11y ago
  Actually water turbines (aka hydrogenerators) are also an option, but it's less common. http://cdn.streamlike.com/secure/Covers/68/17853.jpg http://cdn.streamlike.com/secure/Covers/68/17853.jpg

dingaling 11y ago

They make an horrific noise when deployed, the blade tips are highly supersonic. I've heard one once as a groud observer and I can only imagine the racket they make in the cabin. For that reason they're not tested in routine service but there are ground-testing rigs that spin them up to a fraction of their maximum rotation speed. The A320 series have RATs but the 737s ( all series ) don't since they remain manually-flyable without electrical power.

unwind 11y ago

I'm confused. It looks like a turbine powered by the movement of ambient air, in this case due to the aircraft's movement through said air. But the article says: The RAT generates power from the airstream by ram pressure due to the speed of the aircraft. How is this dependent on the pressure, rather than just the speed of the air? Would a RAT from an aircraft not work on a stationary object subject to wind? Or is such an object also experiencing ram pressure (just the other way around due to reference frame shifting), i.e. all (wind) turbines work due to ram pressure?

jonsen 11y ago
"ram pressure" is a link to its own explaining wiki-page.
- unwind 11y ago
  Yes of course, I read that and it didn't help. It talks about the drag experienced by an object moving in a fluid. I guess I'm just dense.
  - jonsen 11y ago
    You are right, not really informative. Maybe Ram Air Turbine is a bad name. A turbine driven by the airflow from "ramming" through the air?!
JorgeGT 11y ago
Aerospace eng. here, you are right in that is a confusing terminology. Ram pressurein this context is a way to refer to the available kinetic energy due to airflow: 1/2 x density x speed^2 (also called dynamic pressure). A point to stress is that the energy delivered to the plane systems by this auxiliary turbine is thus varying with the square of the speed of the plane. That means that as you slow down for landing, the RAT delivers less and less energy... and the commanding authority of the hydraulic-actuated control surface lowers, making the plane less and less controllable. Not fun!
- toomuchtodo 11y ago
  You'd think the RAT would charge a battery acting as buffer. But who am I to armchair quarterback an entire aerospace industry.
  - astrodust 11y ago
    Having a sufficiently large battery introduces another point of failure, something undesirable in a situation where you're already in bad shape. You don't deploy the RAT unless shit is going sideways.

PhantomGremlin 11y ago

Be sure to follow the link to Pinnacle Airlines Flight 3701. That's a double Darwin Award, well deserved.

goldenkey 11y ago
Wow, that was quite a legendary chain of fuckups. Link for others: https://en.m.wikipedia.org/wiki/Pinnacle_Airlines_Flight_3701 https://en.m.wikipedia.org/wiki/Pinnacle_Airlines_Flight_370...
- astrodust 11y ago
  It takes a lot of gall to double down on your mistakes in a situation like that. These two should've been stripped of their license had they survived that stunt.
  - goldenkey 11y ago
    It seemed like they were hoping the 2nd engine wasn't totally cashed. So they lied to the ground about having only 1 engine failure. I would imagine being alive is worth embarrassment but given the prestige of a pilot, I really can't say more than, bad choice.
blackflame7000 11y ago
The one above it is also pretty dumb.

chinathrow 11y ago

Here's one of those RATs in motion on a 777: https://www.youtube.com/watch?v=FfB2EP71hqY https://www.youtube.com/watch?v=FfB2EP71hqY

boxctim 11y ago

Hooke was this by any chance inspired by a reddit discussion?