What "exactly" is Lift? (or Downforce)

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J.A.W.
J.A.W.
109
Joined: Mon Sep 01, 2014 4:10 am
Location: Altair IV.

Re: What "exactly" is Lift? (or Downforce)

Post

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.
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

Hutchie.91
Hutchie.91
3
Joined: Tue Feb 15, 2022 3:25 pm

Re: What "exactly" is Lift? (or Downforce)

Post

J.A.W. wrote:
Sun May 01, 2022 1:52 am
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.
J.A.W.
109
Joined: Mon Sep 01, 2014 4:10 am
Location: Altair IV.

Re: What "exactly" is Lift? (or Downforce)

Post

Hutchie.91 wrote:
Sun May 01, 2022 1:04 pm
J.A.W. wrote:
Sun May 01, 2022 1:52 am
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?
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

J.A.W.
J.A.W.
109
Joined: Mon Sep 01, 2014 4:10 am
Location: Altair IV.

Re: What "exactly" is Lift? (or Downforce)

Post

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

https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

Acalidien
Acalidien
0
Joined: Mon May 02, 2022 1:44 pm

Re: What "exactly" is Lift? (or Downforce)

Post

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
Last edited by Acalidien on Mon May 02, 2022 1:51 pm, edited 1 time in total.

johnny comelately
johnny comelately
79
Joined: Thu Apr 09, 2015 11:55 pm
Location: Australia

Re: What "exactly" is Lift? (or Downforce)

Post

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
Just_a_fan
561
Joined: Sun Jan 31, 2010 7:37 pm

Re: What "exactly" is Lift? (or Downforce)

Post

Hutchie.91 wrote:
Sun May 01, 2022 1:04 pm
J.A.W. wrote:
Sun May 01, 2022 1:52 am
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)
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