Re: Buoyancy is Imaginary
- From: Goofball_star_dot_etal <who@xxxxxxxxxxxxxxxxxx>
- Date: Thu, 01 Oct 2009 16:36:25 +0100
Roger Long wrote:
On Sep 29, 7:32 am, Goofball_star_dot_etal <w...@xxxxxxxxxxxxxxxxxx>
wrote:
There are numerous examples of equating inconsistant units. Here is oneThe reaction to these presentations on the web is always the same.
example of gobeldygook:
The professionals, especially teachers, like them and they gather all
sorts of nit picks from others. That particular bit of gobeldygook
came from an article published in a leading aviation Emagazine and,
last I heard, was being used as an introduction to the subject in at
least one college course.
These are not intended to be physics texts. There are plenty of
those. The intent is to provide a plain language viceral
understanding of the basic principles. Units and terms most
recognizable to the reader with little prior knowledge are preferable
in a quick and light treatment.
Why this kind of thing worthwhile? I've had a whole career (I'm
hardly "budding") to watch people with naval architectural degrees and
complete understanding of the math and unit consistency come to
really bone headed conclusions that have greatly hampered the
commercial and educational sail industries because they didn't start
with a gut understanding of the physics and let numbers and anal
attention to unit consistency lead them to absurd conclusions. If
they had first understood the subject on this kind of level, they
might have made better use of the mathematical tools. Most college
courses and texts start right off with the math.
These articles are just starting points and not intended to be much
above the level of Sunday newpaper supplement stuff. Professionals
tend to see them for what they are and their limited value and net
posters as opportunities to show how smart they are. Happy to have
provided the opportunity.
--
Roger Long
The point is not that I am a "clever clogs" but that you publish stuff as an "expert" and get it plain wrong. As it happens I am just "an oily rag".
The vertcal force on an airfoil (lift) is equal to vertcal *rate of increase of momentum* of the surrounding air. Not displacement, work or anything else.
Force has the dimensions (MLT-2) the same as Mass (M) times acceleration (L-2) (from F=ma)or the same as mass flow rate (MT-1) times velocity (LT-1). M represents Mass, L length and T time. 1/T^2 is represented as T-2 etc.
In the case of airfoils, turbines etc. there is not a fixed mass but a mass flow. It is a lot easier to measure the pressures over an airfoil than the increasing momentum of the air but they are two sides of the same coin.
No work is done on a plane in level fight at constant speed. Its potential energy is not increasing with height and its kinetic energy is not increasing with velocity. The lift is equal to its weight and its drag is equal to the thrust. All the power ends up heating the air, although initially some goes into increasing the kinetic energy of the air you cannot get at this number by looking at the lift, as you suggest, since kinetic energy and momentum are not the same thing.
Context re-inserted:
"Note the net downwards displacement of the air. The essence of all Newtonian physics is the symmetry of energy conservation (the equal and opposite reaction business). The work done by accelerating the mass of air downwards is exactly equal to the work required to keep the aircraft aloft. The work required to shift it from left to right in the animations is an important aspect of the drag that the engine must overcome."
http://www.rogerlongboats.com/Circulation.htm
.
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