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Flash
29993.  Mon Nov 07, 2005 5:59 pm Reply with quote

No, I think the point is that nobody missed anything - Sainte-LaguŽ just made the QI observation that if they were built to function like aircraft wings, they wouldn't work - they had to build in the complexities of insect aerodynamics to explain it: continuously changing angles of attack, interactions of opposite wings at the top of the stroke, issues of how many chord lengths of travel are needed for full lift to be developed, vortex shedding and reformation (with opposite sign) at the bottom of the stroke, spanwise flow, etc, etc.

 
JumpingJack
30050.  Mon Nov 07, 2005 7:02 pm Reply with quote

Fair enough.

So wonderful to come across a chap who really understands 'vortex shredding' at last.

We can have hours of fun chewing that over with a Meerschaum and a large Lagavulin in hand.

 
Flash
30056.  Mon Nov 07, 2005 7:08 pm Reply with quote

There's an animation of vortex shedding here. Actually, I thought it was a kind of flat-packed garden hut kit from Scandinavia.

 
JumpingJack
30060.  Mon Nov 07, 2005 7:25 pm Reply with quote

Ha ha ha. Very good.

 
JumpingJack
30061.  Mon Nov 07, 2005 7:26 pm Reply with quote

And what's vortex shredding then?

I'm keen to learn

 
Mostly Harmless
30063.  Mon Nov 07, 2005 7:34 pm Reply with quote

..


Last edited by Mostly Harmless on Mon Jan 09, 2006 10:13 am; edited 1 time in total

 
Flash
30097.  Tue Nov 08, 2005 4:53 am Reply with quote

Mostly - no idea.

Jack, vortex shedding is described thus:
Quote:
The boundary layer will cause the flow across an obstacle to be unstable, and the flow will be unsymmetrical. Vortexes will be created and we have vortex shedding.

and the animation illustrates it. Vortex shredding, I don't know.

 
Icarus
30284.  Wed Nov 09, 2005 9:45 am Reply with quote

Quote:
If you need a soundbite, I believe this question is supposed to make aeronautical engineers giggle nervously: If an aeroplane's lift is explained by the Bernouilli Principle, how come they can fly upside-down?


I've always wondered about that, but never found anyone who I thought was the right person to ask. I thought I'd just completely misunderstood the dynamics of the Bernouilli Principle and it was obvious to everyone else, so....

 
eggshaped
30319.  Wed Nov 09, 2005 2:23 pm Reply with quote

Going back to the original point, timspaceman wrote:

Quote:
it was claimed that although there are many theories, nobody can provide written proof that Jumbo Jets can fly


Now firstly I donít recall the programme, so I donít know exactly what was said, but Iím inclined to agree with the aeronautical engineer.

Fred said:

Quote:
The standard explanation of how we fly is mostly mythóit's just wrong, but it has such a life of its own that you even see it on NASA websites and in physics books.


I believe this statement to be true, however thatís not the same as saying that ďnobody can provide written proofĒ. All this statement says is that most people have the wrong impression, thatís certainly not the same as there being no answer.

Icarus wrote:

Quote:
I thought I'd just completely misunderstood the dynamics of the Bernouilli Principle


On the contrary, you probably understood Bernoulli perfectly, but the problem is that the principal is often misrepresented in science books to explain flight.

The physics behind wings is hard. What is harder is finding a source which is both authoritative and easy to understand. Hopefully this will do, but in my opinion it requires more than a basic knowledge of physics:

http://ej.iop.org/links/q62/,1o67Jn8sNdQB7z3LMJwYA/pe3_6_001.pdf

So to sum up for timspaceman, as far as Iím concerned your aeronautical friend is correct to say that ďnobody can provide written proof that Jumbo Jets can flyĒ is bunk, however if you asked him to explain the principal Iíd wager ten to a penny that heíd give you the wrong explanation.


Disclaimer:
If this was indeed mentioned on the show, then having seen the research process first hand, I would be surprised if Jumping Jack, Gray or Flash (or possibly all three in a red-arrows-style formation) don't come back with something to shut me up.

 
Icarus
30338.  Wed Nov 09, 2005 5:40 pm Reply with quote

Quote:
The physics behind wings is hard. What is harder is finding a source which is both authoritative and easy to understand. Hopefully this will do, but in my opinion it requires more than a basic knowledge of physics:

http://ej.iop.org/links/q62/,1o67Jn8sNdQB7z3LMJwYA/pe3_6_001.pdf


Much more than a basic knowledge of physics, it requires that I make the link work...which I can't, for some reason.


Last edited by Icarus on Wed Nov 09, 2005 6:09 pm; edited 1 time in total

 
JumpingJack
30348.  Wed Nov 09, 2005 6:05 pm Reply with quote

timspaceman is so determined to air this question that he actually started a second thread about it Quite Interestrings.

As I pointed out on his other Jumbo Jets thread we did not say "nobody can provide written proof" (and nor in fact did we mention Jumbo Jets as such).

What was said was:

Quote:
Nobody really understands why aircraft stay up in the air.
There are five leading rival theories of aerodynamics, none of which precisely agree with each other.


Written proof is clearly unnecessary since the proof that Jumbo Jets can fly is that they do. The issue is not whether they can, but why they do...

 
timspaceman
32710.  Sat Nov 19, 2005 4:00 pm Reply with quote

I did say I might not be remembering the episode correctly... The info is ace though, its given me plenty to say back to my engineer friend (now a teacher) and mentioning vortex shredding seemed to help.
He accepts that the forces at work are often misrepresented but insists that everybody knows the one correct theory that shows why planes stay up. But he can't give me a name to look up.
oh, and I started the other string before realising the green room existed and deciding it was a better place to put the query.


Last edited by timspaceman on Sat Nov 19, 2005 4:03 pm; edited 1 time in total

 
timspaceman
32712.  Sat Nov 19, 2005 4:06 pm Reply with quote

Could you post links to the rivalling theories, or is that asking too much? I am defending the shows honour - he likes QI but thinks you twist answers to make them funnier.

 
djgordy
32714.  Sat Nov 19, 2005 4:22 pm Reply with quote

In order to understand how planes fly all you need is an understanding of Newton's Laws of Motion and the Conada effect.

There is a very good explanation here:

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

You will notice that the usual explanation, that planes are almost sucked up into the air by pressure differential, is untrue. The explanation using Newton and the Conada effect shows how planes can fly upside down, though you may not want to try it with a Jumbo Jet.

 
Frederick The Monk
33675.  Wed Nov 23, 2005 4:05 am Reply with quote

djgordy wrote:
...though you may not want to try it with a Jumbo Jet.


Although it has been done - at a Paris airshow if I remember correctly. Can't find a pic on the net though.

 

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