What "exactly" is Lift? (or Downforce)

[J.A.W.]

Cold is the 'natural' steady-state, & its the input of heat-energy that causes perturbations.

If you've ever stood before the ice-face of a glacier, you can literally feel the cold radiating out,
(as well as dragging chilled air downstream fluid-wise - just as gravity draws down the melt water).

Oddly enough, its still common for a catastrophic cabin depressurisation in flight to be described
in terms of unrestrained objects being 'sucked out', whereas they're actually 'blown out' - as noted
earlier in his post above - by 'mrluke'.

Like you'd hardly describe the final 'pop-shot' of a Champagne cork exiting a bottle under pressure,
as being an example of - the comparatively low-pressure atmosphere - 'sucking' the cork out.

[Hutchie.91]

If you've ever stood before the ice-face of a glacier, you can literally feel the cold radiating out,

I thought heat cannot flow from cold to hot without the input of external work into the system?

[J.A.W.]

If you've ever stood before the ice-face of a glacier, you can literally feel the cold radiating out,

I thought heat cannot flow from cold to hot without the input of external work into the system?

Try the effect for yourself sometime, perhaps it is more evident in higher air-density, like the pair
of glaciers that descend accessibly to near sea-level on the West Coast of South Is, New Zealand?

[J.A.W.]

Here's the official take on the O.P. subject according to NASA:

https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html

[Acalidien]

Hi, Just wondering if this forum post was written by OP as well? As this post seems like a more expanded upon version of the linked one. Thanks! I've been looking for an explanation of what causes the lower pressure on the upper surface of an airfoil and cant seem to figure it out.

https://aviation.stackexchange.com/ques ... 6202#16202

[johnny comelately]

not an aero person but found this quite instructive (coming from a low base)
i suppose the difference between negative and positive ground effect
The second half is maybe more interesting, talk is about changes in CofG vs CofP,
the oscillations (300mm) and they seem to try and control that with engine speed

[Just_a_fan]

If you've ever stood before the ice-face of a glacier, you can literally feel the cold radiating out,

I thought heat cannot flow from cold to hot without the input of external work into the system?

Quite right. Cold doesn't radiate (because cold is just an absence of heat, just as dark is an absence of light)(*). The effect being described in the glacier example is just the air close to the ice being as cold as the ice and thus conducting heat away from the skin and there is no gain from heat radiated from the glacier and thus the skin is cooled.

(* to be accurate, all objects radiate heat but those that are "colder" than their environments will absorb more than they radiate and thus will warm up until they are in equilibrium with their environment. Those that are "hotter" than their environment will radiate more than they absorb and thus will cool to equilibrium with their environment. If one puts in work, one can make this process work in reverse - e.g. fridges)

[venkyhere]

Sorry to be bumping up an old thread.
Very interesting read, felt I was back in uni.

If I may, want to point to tiny disagreement (semantics, not the concept) :

▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Flow over the Upper Surface of the Wing
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬

When a wing moves at subsonic speeds, the low pressure area on the upper surface kind of "sucks" in air ahead of it. If we pick some random packet of air for a moment, then as it's travelling towards this wing, then the other air packets above itself, and downstream of itself, there will be "less bouncing around of molecules" because they are being drawn towards this region of low pressure,

How did the 'low pressure area on the upper surface' get created in the first place ? Asking because, this is the root cause that leads to a series of events (which you have further described) , that ultimately results in low-pressure on top of the wing. Isn't this a chicken-egg problem ?

Probably you should add - "as the wing moves forward at a slow speed, it disproportionately vacates molecules that were erstwhile occupying an area (around where the top of the wing is now) into a completely different area several inches below (that now falls below the bottom of the wing). This creates a 'partial vacuum' like zone, which entices the molecules in front of the moving wing to rush into this vacated spot (enticed fast flow). But since the wing keeps moving, more erstwhile still molecules are shunted down, more enticement-flow caused and so on. The faster the wing is made to move forward, the greater this vacating rate and enticement rate ; thus leading to lower and lower pressure in a local zone just atop the forward half of wing."

Just a suggestion, to prevent someone else like me pointing out a chicken-egg problem.