Author Topic: Episode #766  (Read 616 times)

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Offline Belgarath

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Episode #766
« on: March 14, 2020, 03:40:33 PM »
 What's the Word: Moiety
News Items: Shorter Day in Past, Handwashing, CAM for Coronavirus, Lift Follow Up
Who's That Noisy
Science or Fiction
 
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Offline Belgarath

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Re: Episode #766
« Reply #1 on: March 14, 2020, 03:54:04 PM »
To answer Jay's question from this episode:
It works the same way upside down as right side up. Lift is proportional to angle of attack and lift is proportional to speed.  At a given angle of attack increasing speed increases lift and drag.  At a given speed, increasing angle of attack increases lift and drag.  It's all the same four basic forces.
To put it simply, both flying inverted and right side up, the nose of the aircraft will be pointed slightly upwards.
I'm not going to talk to him about symmetrical wings because that will blow his mind.

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Re: Episode #766
« Reply #2 on: March 16, 2020, 02:05:30 PM »
Does Kara know about the whole Monkey/Bird dispute?
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Offline Quetzalcoatl

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Re: Episode #766
« Reply #3 on: March 16, 2020, 06:12:59 PM »
"Don't panic, but use your common sense."

I do appreciate Steve being a moderate voice of reason, on this issue as on so many other issues.
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Offline daniel1948

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Re: Episode #766
« Reply #4 on: March 16, 2020, 07:33:15 PM »
They told us that the ocean bulge is primarily responsible for the tidal locking. The land also plays a part, but I gather from the explanation that it's mostly the oceans. If that's so, it could be part of the reason for the changing rate of decrease in the Earth's rotation: Could changes in the volume of water in the oceans account for at least some of the discrepancy?

It would have been cool to see the moon at an apparent size 15 times larger than it is now. But when that was the case, the Earth would not have been such a nice place for a human to live. Hey, here's an idea for a sci-fi story: Man invents time machine. Man goes back a billion years. Man dies from lack of oxygen. The End. Or how about this one: The Government of Earth sends a crew of people in a space ship to colonize a planet in a different solar system. They step out of their ship and are immediately frozen solid in the liquid-methane rain. The End. This is really a pretty nice place to live, even if the moon is only 1/15 the apparent size it used to be.
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Offline PSXer

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Re: Episode #766
« Reply #5 on: March 18, 2020, 11:01:11 AM »
I've heard of iron rain on brown dwarfs quite a while ago. Is the new bit about the iron rain being detected on a solid surface so you could (in theory) stand on it and get pelted by iron? That one seemed easy to me, because I doubt Steve would say the item was fiction because the ones we know about are brown dwarfs, not exoplanets.

Offline fuzzyMarmot

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Re: Episode #766
« Reply #6 on: March 18, 2020, 11:05:01 AM »
I thought Jay really nailed it on his lift explanation. He explained things in a very accessible way, but still described the depth and nuance of the core concepts. I studied fluid dynamics while getting my applied math PhD, and I always struggled to explain lift to non-technical audiences.

Jay did a great job of describing a key problem with explanations of lift-- people often want an "A causes B causes C...." approach. Instead, the situation with faster, low pressure flow above the wing and slower, high pressure flow below the wing represents a dynamic equilibrium.

A couple of analogies are useful.

Consider the flow of waters in a river with protruding rocks. The obstacles generate a pattern of flow (the "streamlines"; paths that a rubber ducks would follow if dropped in the river). Different locations in the river have different flow directions/velocities and different pressures, depending on the geometry of the obstacles. Some people would say that the pressure differences cause the flow direction/velocity. Others would say that the flow direction/velocity causes the pressure differences. A more useful idea is that the geometry of the obstacles sets up an equilibrium flow (which can be found from the Navier-Stokes equations); at each point in the river there is a relationship between flow direction/velocity and pressure.

Alternatively, consider the distribution of matter in a galaxy. You could say that the gravitational field determines the distribution of mass, or that the distribution of mass determines the gravitational field. The key idea is that the distribution of mass and the gravitational field are related to each other by a simple equation. In the airfoil or river examples, the equations are the Navier-Stokes equations.

Finally, Jay mentioned planes flying upside down. An airfoil with a longer tip-to-tail distance on the top of the wing than the bottom of the wing is not necessary to generate lift. Instead, it is the total geometry of the wing in relation to the fluid flow that generates lift. The geometrical configuration is due to both the shape of the wing and the angle of attack. You can generate lift with a wing that has equal top and bottom lengths, or even with a wing that has a longer bottom length (as in the case with an upside-down normal wing), provided that you change the angle of attack enough. In a river, the relative flow to the left and right of a rock are determined both the the rocks shape, and by how the rock is oriented in the river.

A useful picture is in the third diagram on this site: http://hyperphysics.phy-astr.gsu.edu/hbase/Fluids/airfoil.html

Offline daniel1948

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Re: Episode #766
« Reply #7 on: March 18, 2020, 03:03:45 PM »
^ That's fascinating. Thanks! I presume that the reason for the airfoil shape is not that it's needed to achieve lift, but that it provides lift with the least amount of drag.

I've never been bothered by wings. Even if my understanding was incomplete or incorrect, they always seemed sensible to me. What I cannot wrap my head around are the turbine jet engines. I know they work because I've been in commercial jetliners and I reject the theory that it's actually done with strings. But if I did not have the evidence of experience, I might be inclined to reject the claim that they can actually produce thrust.
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Offline Quetzalcoatl

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Re: Episode #766
« Reply #8 on: March 18, 2020, 05:22:18 PM »
I've heard of iron rain on brown dwarfs quite a while ago. Is the new bit about the iron rain being detected on a solid surface so you could (in theory) stand on it and get pelted by iron? That one seemed easy to me, because I doubt Steve would say the item was fiction because the ones we know about are brown dwarfs, not exoplanets.

It would be a blast to see the iron raining down.
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Re: Episode #766
« Reply #9 on: March 18, 2020, 05:22:52 PM »
I've always felt that the explanation of lift has something to do with the viscosity of air at those speeds. But I don't know the maths for that.
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Offline daniel1948

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Re: Episode #766
« Reply #10 on: March 18, 2020, 05:52:47 PM »
It would be a blast to see the iron raining down.

Since they said iron rain, and not iron hail, I presume we're talking about liquid iron. The heat of the atmosphere would incinerate you. You'd never see a thing. I much prefer water rain. It rained here yesterday and it was very nice to watch (from indoors where I stayed dry). In Roatan the rain came down so warm that it was actually pleasant to go out walking in it. Iron rain would have been less pleasant.
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Re: Episode #766
« Reply #11 on: March 18, 2020, 06:31:21 PM »
It would be a blast to see the iron raining down.

Since they said iron rain, and not iron hail, I presume we're talking about liquid iron. The heat of the atmosphere would incinerate you. You'd never see a thing. I much prefer water rain. It rained here yesterday and it was very nice to watch (from indoors where I stayed dry). In Roatan the rain came down so warm that it was actually pleasant to go out walking in it. Iron rain would have been less pleasant.

When I went to Fiji, the rain was like that.
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Offline Quetzalcoatl

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Re: Episode #766
« Reply #12 on: March 19, 2020, 04:00:22 PM »
It would be a blast to see the iron raining down.

Since they said iron rain, and not iron hail, I presume we're talking about liquid iron. The heat of the atmosphere would incinerate you. You'd never see a thing. I much prefer water rain. It rained here yesterday and it was very nice to watch (from indoors where I stayed dry). In Roatan the rain came down so warm that it was actually pleasant to go out walking in it. Iron rain would have been less pleasant.

I meant of course from within the safety of a super-tough spaceship. :P
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Offline Billzbub

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Re: Episode #766
« Reply #13 on: March 19, 2020, 04:49:48 PM »
My dad and I have had the exact same lift debate that Jay talked about.  He thinks that the faster flow of air over the wing causes lower pressure, and wings are sucked into the sky.  I thought that the mass of the air directed downward by the wing (as a function of its relative geometry and thrust) lifts the wing.  Now I'm trying to frame it in my head as pressure differences.  If the wing is directing air down, that means the air is exerting pressure on the bottom of the wing.  That same pressure is not exerted on the top of the wing.  Therefor, there's a pressure difference causing the lift.
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Offline GodSlayer

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Re: Episode #766
« Reply #14 on: March 26, 2020, 01:55:22 PM »
if turbines and propellers causing thrust forward aren't a problem, and helicopters and jump-jets and rockets with their vertical thrust aren't a problem, why are planes who direct most of their thrust forward in order to feed air to the wing confusing? if the wing had an axle and spun in a circle everyone would be happy, but when it's straight everyone loses their mind?

instead of corkscrewing the air down like a backwards waterwheel and being its own wing like helicopter blades or Harrier turbines*, the whole machine is pushed through the air, which is directed straight down. aren't you just putting in energy to do the work of redirecting atmosphere in all these cases? what makes it any more confusing than swimming to the bottom or the top of a swimming pool? is it really just an argument over the metaphor (what Steve would call scientific truth) that you describe it by? ... why does a submarine make sense while an aeroplane does not? or do they share the same 'problem' discussed?

*obviously Harriers have plane wings, too, but those aren't providing assistance in VTOL like I'm referring to.
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