The mysteries of aerodynamic lift • Hacker News Board

Re: The mysteries of aerodynamic lift

#171

Post by mike_ivanov » Wed, Feb 05, 2020, 3:06 PM UTC

mike_ivanov wrote: post #156

glofish wrote: post #147

Earlier quoted context omitted.

> I beg your pardon? Not sure what could be unclear there at all. Nuclear energy can be turned into any other energy and vice versa. As long as something has mass it has energy - whether or not we can readily transform that is beside the point. Your recurring yo-yo example only demonstrates that you don't understand the physical phenomena in the first place. The linear momentum is conserved when the yo-yo pulls on yo…

It feels that you've got some idea that I'm not understanding the difference between energy and momentum because it's a common misconception, and hold on it. I do understand that they are different. My objection is your insistence on them being "unrelated". As of the yo-yo, let's remove the muscle power and the wobbling earth out of the picture and consider a it a closed system. We've got a fully wound-up yo-yo, not…

Correction: it will go up given an ignorably small jerk on the line at the lowest point. Not the best example, right... What I was trying to say is that one can trade rotational momentum to linear (and vice versa) as long as the kinetic energy of the system stays the same.

Re: The mysteries of aerodynamic lift

#172

Post by rtkwe » Wed, Feb 05, 2020, 3:38 PM UTC

starpilot wrote: post #133

rtkwe wrote: post #129

Earlier quoted context omitted.

I've always wondered since learning slightly more in college physics. What is supposed to make the air going over the top of the wing suddenly go faster than the air going over the bottom of the wing in the Bernoulli model?

Lower pressure region sucks the flow down to attach to the upper surface. As the pressure is reduced, speed must increase to conserve momentum.

The way I was always taught the Bernoulli explanation was that the speed difference created the lower pressure which causes the lift. So it seems a little circular, what's causing the low pressure in your version of it? Is it a mix because any time the flow separates from the wing it leaves a low pressure area that attaches the flow to the wing?

Re: The mysteries of aerodynamic lift

#173

Post by baq » Wed, Feb 05, 2020, 3:53 PM UTC

o_nate wrote: post #83

baq wrote: post #69

Earlier quoted context omitted.

Depends. Newton’s third law is scientific enough to explain what’s going on. Bernoulli and Navier-Stokes and all the hard equations are needed when you need to optimize the system or understand the micro scale, but they’ll simplify into conservation of energy or momentum in the macro limit.

Newton's law doesn't do a good job of explaining why airplane wings have the shape they do. Why not just use a flat wing at a 45 degree angle?

Exactly what I had in mind when I said Navier-Stokes.

It isn't the job of Newton's third law to explain that. It's job is to say that if you have 200 ton aircraft and want to keep it in the air, it has to push 200 tons of air down no matter if it does that with a carbon-fiber CFD-optimized wing or a barn door nailed to the airframe.

Re: The mysteries of aerodynamic lift

#174

Post by scooble » Wed, Feb 05, 2020, 10:48 PM UTC

BerislavLopac wrote: post #122

scooble wrote: post #97

Earlier quoted context omitted.

> preferred because it requires far less crew I think that only really applies when the sails were set from aloft. I'm familiar with square-rigged cargo vessels of around 60ft in the UK that were crewed by 2.

Yes, more modern versions of square rigs (using motors, or designed without the yards -- again, spinnakers) do have vastly reduced need for the crew. But I was focusing on traditional rigs.

The vessels I was thinking of, Humberkeels, are traditional.

Re: The mysteries of aerodynamic lift

#175

Post by nimish » Thu, Feb 06, 2020, 5:43 PM UTC

mannykannot wrote: post #164

nimish wrote: post #154

Earlier quoted context omitted.

Not quite. You can't just derive the equation of state that's equivalent to Bernoulli's principle from Newton's laws. Only for incompressible flow is it a consequence of Newton's second law (NS) See https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Deriva... -- the broader (applies to compressible flow too) derivation is via conservation of energy and mass

That is a fair point, but to be clear, it does nothing to rehabilitate the notion that Newton and Bernoulli provide independent components of lift that have to be added (or, for that matter, that one is right and therefore the other is wrong, which is another common misunderstanding that has shown up elsewhere.)

Great because I maintain that Newton and Bernoulli are emphatically not independent perspectives and any explanation that ignores one or the other is incomplete :)

Glad we agree!

Re: The mysteries of aerodynamic lift

#176

Post by james-imitative » Thu, Feb 06, 2020, 6:00 PM UTC

Anyway, the point is that aeroplanes _do_ fly. I have been in an aeroplane that flew, and it flew using wings that had engines attached to them. I have also seen a variety of planes fly through the sky, and they have all had either engines or propellors propelling them.

Wings alone are not enough. An aeroplane that only has wings does not have enough "thrust" to continue flying. It's as simple as that.

Re: The mysteries of aerodynamic lift

#177

Post by mannykannot » Thu, Feb 06, 2020, 9:27 PM UTC

nimish wrote: post #175

mannykannot wrote: post #164

Earlier quoted context omitted.

That is a fair point, but to be clear, it does nothing to rehabilitate the notion that Newton and Bernoulli provide independent components of lift that have to be added (or, for that matter, that one is right and therefore the other is wrong, which is another common misunderstanding that has shown up elsewhere.)

Great because I maintain that Newton and Bernoulli are emphatically not independent perspectives and any explanation that ignores one or the other is incomplete :) Glad we agree!

We have been here before :( You can have a complete explanation without Bernoulli, or including it. It is not a necessary component of a rigorous explanation.

Re: The mysteries of aerodynamic lift

#178

Post by luckyscs » Fri, Feb 07, 2020, 2:46 AM UTC

taneq wrote: post #149

luckyscs wrote: post #52

Earlier quoted context omitted.

It's like our brains aren't meant to handle resolving that not only is it pushed through, it's sucked into a thin and ever moving void. It's pushed and pulled at the same time, in otherwords, part of the continuum.

Suction isn't a real thing, though. It's just our description of when lower pressure on one side of a thing allows the thing to be moved by the higher pressure on the other side. The pressure that the air above the wing applies to the top of the wing is lower than the pressure that the air below the wing applies to the bottom of the wing. The net force is upwards.

I'm not sure it's that simple, and my physics knowledge isn't so strong but if it's a gradient, the force of the push can't exist without being the same thing as the force of the pull. They seem to be one in the same, the force isn't onna particular side, it's the effect of the delta between points on the gradient.