Macroscien said..

sotired said..

Well, I guess I want to determine something first; when I breathe in, am I sucking a breath in or is the universe pushing a breathe into me?

Excellent example, in our case sucking doesn't exits in physical sense...
or do exist ?
One may claim that breathing air IN is the active act the require muscles to "suck" the air in, others may claim that you do exhale air actively then air refill your lungs automatically.
We could test this idea experimentally when you stop breathing for a longer period , say for an hour and we check if your chest in the state "inflated" or "deflated"

I did that test, and for an hour it was definitely inflated. I don't have any witnesses, but you can trust me. If I could have captured it at 1000 frames a second it would still prove the same thing.

So, you would argue they are the same thing. Your lungs are trying to expand and as a result any lower pressure area is drawing in air, or the rest of the world is blowing it in.

.. and as the old saying goes, is there such a thing as gravity or does the earth just suck?

Macroscien said..

NotWal said..

Macroscien said..

...

if that means that model air plane on International Space Station doesn't have a lift ( in absence of gravity but still plenty of air around ?)

...

Yay, something new :)
I reckon if you threw a model glider in the ISS it would do a back flip and stop.

Remember that my questions are designed not to clarify but confused you ( assuming that you thought that you know already what is going on with sails and airfoils in general) .I guess that my example above points to some weakness in arguments in article ( requirement for gravity to achieve lift)

Where does it say that gravity is required?

Chris 249 said..

Where does it say that gravity is required?

Who ? I don't know . You could check authors name on the link above

Macroscien said..

Chris 249 said..

Where does it say that gravity is required?

Who ? I don't know . You could check authors name on the link above

It just seems to be related to the fact that the air in our atmosphere has pressure because of gravity. Without gravity, our air would wander off into space.

However, air doesn't just try to equalise pressures because of gravity (although as said above, the pressure in our atmosphere is created by gravity that stops air from wandering into space). If you flapped a piece of paper inside a space station under zero gravity it won't leave a little hole without air behind it - the air pressure will equalise itself.

Take an inflatable boat or bicycle tire. Push down on the top to displace some air. When you stop pushing, the air will rush back UP from lower down the tire or boat to the top, AGAINST the force of gravity. This is clear evidence (not that it is required) that air will seek to equalise its pressure even against gravity - that's what gives us most of our weather too.

There is no big mystery behind the way a sail works. Many thousands of very smart people have spent many years combining their brainpower and lots of high-tech machinery to work it out.

Chris 249 said..
However, air doesn't just try to equalise pressures because of gravity

You have this right because once you mentioned gravity- our attention is instantly to Newtonian - gravity law,
and once you talk about gases mechanics that will be more like Robert Boyle domain

This seems fairly philosophical, but I'll have a crack
To me it's about suction, or flow induced by the potential difference between an area of high pressure and low pressure. Air is drawn towards the low pressure zone - suction.

to go off on a tangent, we have SPAC, the soil-plant-air-continuum. Water potential of a fully wetted soil (field capacity) is about -10kpa. as soils dry, the soils water potential becomes more negative. At -1500kpa ( a dry soil), the suction a plant has to apply to extract water from the soil is so great that many plants will die. But the plant isn't really applying suction, it is the atmosphere, acting on the plant leaf. the plant is just a conduit between the soil and the atmosphere. The water potential in the atmosphere is far more negative, -100 Mpa - there's nothing pushing the water vapour there, it's all about suction! Not the same, or is it?

Stretchy said..
This seems fairly philosophical, but I'll have a crack

I love this plant comparison very much
I do already imagine that I could purchase bag of genetically modified seeds . get into soil, water a bit and in month or so
I have fully grown leaf race sail - 5.0 m

wait a bit and grow to 7.8m2
I am not botanic but I am guessing that cross between banana leaf and bamboo stick ( serving as a mast and battens) could do very well.

TristanF said..
Air isn't drawn or pushed from an area of high to low pressure, it just moves to equalise the pressure.

That is very clever explanation but
think about that
If I was that air pressure - instead of just pushing on the sail ( which is un-penetrable ) - I will simply go around sail and equalize the pressure difference in instant
(?)

Stretchy said..
Water potential of a fully wetted soil (field capacity) is about -10kpa. as soils dry, the soils water potential becomes more negative. At -1500kpa ( a dry soil), the suction a plant has to apply to extract water from the soil is so great

Something absolutely new !
That is absolutely nobody mentioned before.If the wet sail have the same speed potential as dry sail ? We drop sails into water very often and once is dry once is wet .

Maybe super wet sail is faster then dry ? or one side dry other wet ?? Can we do one side hydrophobic and another hydrophilic for speed racing competition ? Definitely if you add evaporation equation to our already complex lift calculation , things are getting very complicated. I love it

To answer this question, one needs to understand the following:

(1). Pressure, and
(2). A vacuum.

(1). Pressure
It is defined as Force/Area.

(2). A vacuum
It is defined as a void free of anything. Well, maybe not quite as there is no such thing as a "Pure vacuum".

A vacuum, as the definition suggest, has nothing within it. So if there is nothing, how could there be any force to "suck" the sail forward?

So the only logical explanation remained has to be the positive pressure that pushes the sail forward.

Remember that Cadbury chocolate advert where the fuzzy hair professor Julius Sumner Miller showed us kids how to suck an egg? Well, not quite. His experiment depicts a hard-boiled egg being sucked/pushed into a glass jar, when a piece of paper was burnt within it.

Was it sucked or pushed? It was pushed. The vacuum created by the burning paper that facilitated the egg to be pushed in by air pressure.

Macroscien said..

Mark _australia said..
the molecules are zipping around in a random way and when there is a pressure differential they go mostly in that direction unti

I like this molecules analogue, Mark.
Did you played table pool ever Mark . Obviously you did.

So now lets imagine for a moment that our air molecules as big as table pool balls,
Half a kilo each (?)
Forget about all this pressure stuff and imagine that our wind is just a stream of high speed balls hitting our sail.

If our sail is perefectly align along this rain of speeding balls - none hit , none lift

If we tilt our sail and balls start hitting our internal sail ( leward ?? never can remember this lee thing ) they do transfer kinetic energy to our sail and bounce sideways same time.

That will be pure pushing +1, Mark .We just don't need to care about ball flying above, missing our sail.

Actually, that is one of the explanations that is shown to be untrue on the NASA site linked to earlier. It is also shown to be untrue by the Einstein "dog leg" upper aerofoil failure mentioned in the previous post. It's also shown to be untrue by anyone who wants to try. Just take a sail and put something to leeward to block the airflow. You will go slower - a lot slower - which shows that we DO need to care about the "balls missing our sail".

NASA shows your theory to be wrong here;

www.grc.nasa.gov/www/k-12/airplane/wrong2.html

The information is out there, and you may as well just read it since you are NOT going to discover something that Einstein was too dumb to work out.

Chris 249 said..

your theory ...

correction required. It is not mine theory. It is just mine interpretation of Mark suggestion.
Because in a minute later I will trying to defending another SBreezer claiming opposite view.
Beside you forger that my goal here is not to explain, but confuse
If anybody believe that know all physics behind lift, all I am trying to do is seed some doubts, throw you off that post.
Glory for those that keep standing hard.The is illustration of my position here:



All I am doing is trying to know you off of your certainty post. sometimes I win, sometimes I loose

Now there is very hard nut to crack for all our pushers and suckers here.

Lets imagine that we take a simple glass pane and water hose.
Lets perform following experiment.
1.- we keep the glass pane at the angle and pour water from hose on to it.
Water obviously spread on the glass and goes down.

2.-Now we till our glass pane to 90 degree, completely vertical.
Most of the water still "stick' to the glass and fall down

3.Now we tilt glass even further and now it is like 270 degree, almost upside down and water stream is still following this glass surface. Upside down!
Should fall off, one may say. What is causing this water to stick to the glass

? Is it glass sucking the water ?, air pressure pushing onto it? vacuum or pressure ? or something else?

Is it related to our sail example or not at all ?

Macroscien said..

Chris 249 said.

Actually, that is one of the explanations that is shown to be untrue on the NASA site linked to earlier. It is also shown to be untrue by the Einstein "dog leg" upper aerofoil failure mentioned in the previous post. It's also shown to be untrue by anyone who wants to try. Just take a sail and put something to leeward to block the airflow. You will go slower - a lot slower - which shows that we DO need to care about the "balls missing our sail".

ok you enlighten us what is untrue.
What is your truth then ? We are happy to see your interpretation (if you got any )

sucking , blowing, pushing or pulling, altogether or none. ?

When it comes to choosing we always have limited option , and need to decide on something, then defend it, till you can , then give up and change mind. I guess that will be mine strategy.

I gave my interpretation on the first page - it's often called the Newtonian or reaction theory. The best defence is that it fits reality, as demonstrated in anything from aircraft to sails and wind tunnel tests.

Chris 249 said..
Let me ask you a question - do you seriously think ...

You may have this wrong ( again).
I am not here to explain anything, clarify,take sides. I don't present my opinion at that matter at any second.
I am all ears, listening you yours.

Even more. I am simply forbidden to express my personal view, because, it becomes instantly subject to personal attack ( regardless of merit behind) .
What is a bit unfair , you just punching neutral arbiter on the scene.