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 The Ultimative "Why Do Aircraft Fly?" Question...
 Superstring From Austria, joined Jun 2007, 46 posts, RR: 0Posted Wed Aug 8 2007 22:26:45 UTC (8 years 9 months 3 weeks 4 days 18 hours ago) and read 5960 times:

 30 replies: All unread, showing first 25:
 Starlionblue From Greenland, joined Feb 2004, 17661 posts, RR: 65 Reply 1, posted Wed Aug 8 2007 22:32:58 UTC (8 years 9 months 3 weeks 4 days 18 hours ago) and read 5954 times:

 Quoting Superstring (Thread starter):What is the physical reason why an wing generates lift?

"It's complicated."

 "There are no stupid questions, but there are a lot of inquisitive idiots."
 Superstring From Austria, joined Jun 2007, 46 posts, RR: 0 Reply 2, posted Wed Aug 8 2007 22:34:55 UTC (8 years 9 months 3 weeks 4 days 18 hours ago) and read 5950 times:

Oh yes, indeed...

 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 3, posted Wed Aug 8 2007 22:52:18 UTC (8 years 9 months 3 weeks 4 days 18 hours ago) and read 5939 times:

 Quoting Superstring (Thread starter):What is the physical reason why an wing generates lift?

The 35,000' view is that the wing imparts downward momentum to the air. Downward momentum change = upward lift.

 Quoting Superstring (Thread starter):Or it is the same with exp(i x PI) = -1. Many higth school students know this equation, but ask them WHY; you will not get an answer...

That's true, but that's just because most high-school students don't know the power-expansion of e^x. It's pretty straightforward to demonstrate once you do know the math...not the same case with lift.

 Quoting Superstring (Thread starter): What is the thing with the Coanda effect? Does it play a role or not?

Yes. Coanda effect is just another manifestation of the same physical principles that govern airflow over an airfoil.

 Quoting Superstring (Thread starter):Do we need viscosity to explain lift?

No.

 Quoting Superstring (Thread starter):But why does Nature enforce the Kutta conditon?

Because air has mass. Even if you ignore viscosity, inertia prevents air molecules from making finitely tight turns, as would be required to go around a sharp trailing edge.

 Quoting Superstring (Thread starter):Is the pressure difference the result or the cause of the increased speed above the wing,

Neither. Nature is "solving" a Navier-Stokes boundary condition problem. The solution is a steady-state flow-field that defines the pressure and velocity at each point...since both pressure and velocity are proscribed, it doesn't make sense to say that one caused they other. The values are what they are because if they were anything else it wouldn't be a steady-state solution.

 Quoting Superstring (Thread starter):But what is the mechanism by which nature bends the streamlines above the wing to generate the downwash?

Pressure transients. When you first start the airfoil moving you get a dynamic pressure distribution. Imagine you have a stationary airfoil and step-change it forward by a tiny amount. For a very brief instant you will get a vacant region along the top skin and an a pressurized region along the lower skin. Naturally, this will very quickly even out by some molecules moving away from the lower skin and some moving towards the upper skin and, if you stop the airfoil right there, that would be it. If you keep moving though, you'll end up with a steady-state flowfield that curves the streamlines generally downwards. Any other flowfield would result in unbalanced pressures somewhere and the flowfield would dynamically adjust towards the steady state flowfield.

Tom.

 Superstring From Austria, joined Jun 2007, 46 posts, RR: 0 Reply 4, posted Wed Aug 8 2007 23:14:05 UTC (8 years 9 months 3 weeks 4 days 17 hours ago) and read 5926 times:

 Quoting Tdscanuck (Reply 3):Pressure transients. When you first start the airfoil moving...

Well this is of course an analogy, but the best I have seen!

 Quoting Tdscanuck (Reply 3):Or it is the same with exp(i x PI) = -1. Many higth school students know this equation, but ask them WHY; you will not get an answer... That's true, but that's just because most high-school students don't know the power-expansion of e^x. It's pretty straightforward to demonstrate once you do know the math

Yes, but don't tell a real mathematican. He will tell you things like radius of convergence (well which happens to be infinity for exp(x) ) or the convergence of the taylor expansion... However, I am very glad not to be a mathematican...

 Quoting Tdscanuck (Reply 3):Quoting Superstring (Thread starter): Do we need viscosity to explain lift? No.

Well I have already put this question into a thread, but it is never a mistake to ask again. The problem I as a physicist have with "No..." is that if I really forget viscosity how do I consider the effects happening in the boundary layer.
And there is another effect: as I once have asked with my superfluid helium example If We Lived In A World Without Viscosity... (by Superstring Jul 26 2007 in Tech Ops)
I really wonder if an aerofoil would "fly" in superfluid helium. For simplicity lets take an aerofoil which would generate lift at zero geometrical angle of attack. If we put this wing into superfluid helium your analogy you described in

 Quoting Tdscanuck (Reply 3):Pressure transients. When you first...

is not true any more. If there is no viscosity the area being directly behind the wing will have zero velocity, and the area above the projected trailing edge will have a specific speed; and between there will be a sharp velocity "jump".

 KELPkid From United States of America, joined Nov 2005, 6833 posts, RR: 3 Reply 5, posted Thu Aug 9 2007 01:01:43 UTC (8 years 9 months 3 weeks 4 days 16 hours ago) and read 5895 times:

 Well, everyone knows the following about aircraft: 1) No matter how small, they are expensive to operate. 2) They will not fly without some expenditure of fuel, even if they are a glider (and fuel is, as we all know, not getting any cheaper anytime soon). 3) Pilots must pay large sums of money to instructors, flight schools, etc. to retain the privelege of flying unless they are employed as such or are one of the lucky few to be employed as a professional aviator...in which case their employer shells out the money (usually). 4) Mechanics like to eat, so they demand their cut of the money, too... 5) Parts require extra inspection before being certified by the FAA for aircraft usage, so more money goes down that hole... Which leads me to the inescapable conclusion that aircraft fly because someone threw enough money at the problem, and that, furthermore, money keeps aircraft airborne
 Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 Lehpron From United States of America, joined Jul 2001, 7028 posts, RR: 20 Reply 6, posted Thu Aug 9 2007 03:33:37 UTC (8 years 9 months 3 weeks 4 days 13 hours ago) and read 5859 times:

 I just tell people that air changes its properties if forced to change direction, like being forced around something. It doesn't matter if it is a wing, a rock, or a building; the properties of the air (density, temperature, pressure, viscosity, etc) will change. Why does this happen? Well, why does it concern anyone? There hasn't been a situation where something different occured. It is possible for air to change phase while going around something, but that requires ultra high speeds.
 The meaning of life is curiosity; we were put on this planet to explore opportunities.
 Vikkyvik From United States of America, joined Jul 2003, 12139 posts, RR: 24 Reply 7, posted Thu Aug 9 2007 04:27:40 UTC (8 years 9 months 3 weeks 4 days 12 hours ago) and read 5838 times:

 Quoting KELPkid (Reply 5): Which leads me to the inescapable conclusion that aircraft fly because someone threw enough money at the problem, and that, furthermore, money keeps aircraft airborne

And they're airborne for a certain amount of time, generally.

So, following the "Women = Evil" theory:

Aircraft = Money * Time;

Money = SQRT (Evil) ==> Money^2 = Evil;

Time = Money;

Money * Time = Money^2;

Aircraft = Evil.

And for the absolute kicker:

Aircraft = Women.

Which I guess explains why they're generally called "she" and named after women.

Wait, what were we trying to prove again?

 I'm watching Jeopardy. The category is worst Madonna songs. "This one from 1987 is terrible".
 ThirtyEcho From United States of America, joined Dec 2001, 1741 posts, RR: 1 Reply 8, posted Thu Aug 9 2007 10:22:00 UTC (8 years 9 months 3 weeks 4 days 6 hours ago) and read 5786 times:

 I was about to go to bed, so I poured out the remainder of my cup of coffee into the sink. I noticed that the coffee flowed down the side of the cup until I tipped it at such an angle that the flow of coffee separated from the side of the cup. I am with you that the Coanda Effect explains nothing. Where did you get your PhD in aerodynamics, Waziristan?
 Chksix From Sweden, joined Sep 2005, 345 posts, RR: 4 Reply 9, posted Thu Aug 9 2007 10:50:07 UTC (8 years 9 months 3 weeks 4 days 6 hours ago) and read 5769 times:

 "No Viscosity" would be like trying to fly in a vacuum. The aerosurfaces need something to bite to be able to produce a lifting force.
 The conveyor belt plane will fly
 Starglider From Netherlands, joined Sep 2006, 690 posts, RR: 43 Reply 10, posted Thu Aug 9 2007 15:03:49 UTC (8 years 9 months 3 weeks 4 days 2 hours ago) and read 5728 times:

 Quoting Superstring (Thread starter):Why do aircraft fly?

Because the runway is only 10,000 ft. long and the destination is a thousand miles away.

Starglider

 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 11, posted Thu Aug 9 2007 15:48:05 UTC (8 years 9 months 3 weeks 4 days 1 hour ago) and read 5715 times:

 Quoting Superstring (Reply 4):The problem I as a physicist have with "No..." is that if I really forget viscosity how do I consider the effects happening in the boundary layer.

The original question was "Do you need viscocity to explain lift?" and the correct answer is "No." That doesn't mean you'll get the same flowfield if you ignore viscocity...you won't. An invisid fluid won't have a boundary layer. For practical airfoils the Reynolds number is so high that viscocity makes next to no difference, so the answers won't be much different but it will be there.

 Quoting ThirtyEcho (Reply 8): I noticed that the coffee flowed down the side of the cup until I tipped it at such an angle that the flow of coffee separated from the side of the cup. I am with you that the Coanda Effect explains nothing.

The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all.

Tom.

 Superstring From Austria, joined Jun 2007, 46 posts, RR: 0 Reply 12, posted Thu Aug 9 2007 19:11:22 UTC (8 years 9 months 3 weeks 3 days 21 hours ago) and read 5681 times:

 Quoting Tdscanuck (Reply 11):The original question was "Do you need viscocity to explain lift?" and the correct answer is "No."

The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects.

 Quoting Tdscanuck (Reply 3):Because air has mass. Even if you ignore viscosity, inertia prevents air molecules from making finitely tight turns, as would be required to go around a sharp trailing edge.

Well I think that is only the half story. This explains why there won't be tight turns at the trailing edge, but this does not explain why the flowfield stays attached to the airfoil the whole way down to the trailing edge. This can only be explained by the fact that the viscous boundary layer wants to stay attached to the airfoil.

 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 13, posted Thu Aug 9 2007 20:05:26 UTC (8 years 9 months 3 weeks 3 days 21 hours ago) and read 5652 times:

 Quoting Superstring (Reply 12):The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects.

Nope. An inviscid CFD will show you that very quickly.

 Quoting Superstring (Reply 12):This explains why there won't be tight turns at the trailing edge, but this does not explain why the flowfield stays attached to the airfoil the whole way down to the trailing edge. This can only be explained by the fact that the viscous boundary layer wants to stay attached to the airfoil.

Not true. You can do all the simulations you like with an inviscid flow (which has no boundary layer, almost by definition) and the flow will still stick to the upper surface.

Tom.

 ThirtyEcho From United States of America, joined Dec 2001, 1741 posts, RR: 1 Reply 14, posted Fri Aug 10 2007 11:01:44 UTC (8 years 9 months 3 weeks 3 days 6 hours ago) and read 5565 times:

 "The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all." Gee whiz, I guess that all of those wind tunnel smoke tests that show air (a fluid) conforming to the shape of the surfaces of wings were conducted at the same speed at which coffee pours from a cup? I saw a smoke test of the Bell X-1 wing and, sure enough, the air conformed to the shape of the wing. You could even see the shock wave move over the wing. I pour out old coffee fast but not supersonically. Is it your point that the Coanda effect doesn't apply outside of the kitchen sink?
 Superstring From Austria, joined Jun 2007, 46 posts, RR: 0 Reply 15, posted Fri Aug 10 2007 12:12:59 UTC (8 years 9 months 3 weeks 3 days 4 hours ago) and read 5548 times:

 Quoting Tdscanuck (Reply 13):Quoting Superstring (Reply 12): The basic question is whether the Kutta Condition is nothing more than a viscous effect. Isn't the fact that nature "encourages" a flow field to leave smoothly at the trailing edge nothing more than a result from friction effects. Nope. An inviscid CFD will show you that very quickly.

To be honest a CDF-simulation is no proof. I will give you an example:

Lets assume a designer of rollercoasters who is just interested in the speed/acceleration profile of his new design. Well mechanics tells us that he will be able to calculate those speeds/accelelerations just by analyzing the curve which describes the rollercoaster track in 3D space.
But the point is: Our designer of the coaster will be able to calculate the speed/acceleration profile of the coaster simply by using the curve in space ALONE and without haveing to know the constraint forces EXPLICITELY (for experts Lagrange Formalism II vs. Lagrange Formalism I)
However this does not mean that there are no forces which hold the car on track. Of course it is clear that forces are necessary so that the cars are not "flying away" and the engineer being responsible for the structure of the track will definitely know how big those forces can become.
So what's the point? Well simply the principle of constraints. By constraining the cars onto the 3D curve the designer will be able to find his figures. So he uses a "constraining method" but behind the scenes it are the forces which hold the car on track.
In our inviscous problems the Kutta condition plays the role of those "constraints", but the underlaying principle is visosity. Any textbook on aerodynamics (OK at el

It is the same with the inviscous CFD simulations. If the software uses for example simply the Euler equations, well the results are close to reality as the boundary layer often quite thin (I could read that on a 777 for example we have a . But in the beginning I was asking for an explanation and well not for a simplification appearing to be working very well in some low speed regiemes, and I strongly doubt that an CFD which generates a flowfield without using viscosity explicitely is a proof that the Kutta condition is not a viscous effect (as Lagrange formalism II is no proof that there are no constraint forces)

Well finally the thread has run into a small detail (which is on the other hand quite important). However the thing which appears to me is that such powerful tools and so elegant sets of mathematical principles has been developed by engineers/aerodynamicists that thinking about the reason WHY those tools really work is not necessary any more. Well I as a physicist have another quite opposing perspective; in the physicist's guild many are thinking about the WHY and not about HOW (WHY is the speed sound depending on temperature or WHY is an algorithm stable; engineers are often wondering HOW they build/construct something).
Finally it is nothing more than two different paradigms.

 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 16, posted Fri Aug 10 2007 15:47:06 UTC (8 years 9 months 3 weeks 3 days 1 hour ago) and read 5508 times:

 Quoting ThirtyEcho (Reply 14):"The phenomenon with low-speed liquids following a curved surface is largely a surface tension thing...it looks like the Coanda Effect but the physics aren't the same at all." Gee whiz, I guess that all of those wind tunnel smoke tests that show air (a fluid) conforming to the shape of the surfaces of wings were conducted at the same speed at which coffee pours from a cup?

I said liquid, not fluid.

 Quoting ThirtyEcho (Reply 14):Is it your point that the Coanda effect doesn't apply outside of the kitchen sink?

No, my point is that the Coanda-looking effect you get with liquids doesn't have the same physics behind it as the Coanad-effect you see on airfoils in fluids which don't have any free surface or surface tension.

 Quoting Superstring (Reply 15):In our inviscous problems the Kutta condition plays the role of those "constraints", but the underlaying principle is visosity.

I've never seen a CFD program which applies the Kutta condition as part of the solution. I'm not even sure how you'd implement that in a CFD framework.

You don't need to use CFD, a wind tunnel will do. The Reynolds number of real airfoils at practical speeds is so high that the viscous forces are functionally zero compared to the inertial forces, yet you'll still see the flow follow the upper surface.

Tom.

 FLY2HMO From , joined Dec 1969, posts, RR: Reply 17, posted Sat Aug 11 2007 00:32:09 UTC (8 years 9 months 3 weeks 2 days 16 hours ago) and read 5467 times:

 Quoting Vikkyvik (Reply 7):Aircraft = Women

So if all aircraft are women, and all women PMS, then all aircraft PMS

Of course, some more than others, as seen by the large puddles of oil under JT8Ds

They really bleed

 KELPkid From United States of America, joined Nov 2005, 6833 posts, RR: 3 Reply 18, posted Sat Aug 11 2007 01:04:52 UTC (8 years 9 months 3 weeks 2 days 16 hours ago) and read 5458 times:

Congratulations on a mathematical proof of not only my point, but the fact that aircraft are inherently evil (I won't add "and feminine" to that, as my wife would probably make me sleep on the couch...)

 Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 777236ER From , joined Dec 1969, posts, RR: Reply 19, posted Sat Aug 11 2007 02:31:44 UTC (8 years 9 months 3 weeks 2 days 14 hours ago) and read 5441 times:

 You don't need viscosity or Coanda to explain lift (although there's no such thing as an inviscid fluid and you can end up with all sorts of bollocks if you assume no viscous effects). Why don't you want to explain lift in terms of Newton's third law or circulation or Bernoulli? All are equivalent. The airfoil does deflect air through Coanda and via its geometry (and it would continue to do so without Coanda), so there is a reactionary force. The pressure is lower on the suction side than the pressure side and the flow is faster on the suction side than the pressure side, so there's a resultant force. There is a net circulation from the line integral of the velocity around the airfoil, so there is a force. Lift can be explained using Bernoulli and the continuity equation, or Newton, or through circulation. They're all just mathematical models describing what happens in real life. All three have flaws: Bernoulli isn't fundamental, although it's derived from bog-standard Newton, the Newtonian theory doesn't go into the detail of the friction and viscosity that causes the deflection and circulation doesn't explain why the circulation occurs in the first place. If you ignore incorrect assumptions (equal transit time, skipping stone) then all three are valid. Mix and match based on your needs. Edit: The problem you can end up with is that you can get very metaphysical about the nature of causality. Classical mechanics is reversible, so the question of what 'causes' lift makes little sense. The lift causes the circulation/deflection/pressure gradient, and the circulation/deflection/pressure gradient causes the lift.[Edited 2007-08-11 02:38:16]
 ThirtyEcho From United States of America, joined Dec 2001, 1741 posts, RR: 1 Reply 20, posted Sun Aug 12 2007 11:08:41 UTC (8 years 9 months 3 weeks 1 day 6 hours ago) and read 5346 times:

 I said liquid, not fluid. Last time I looked, my transmission "fluid" was a "liquid." Air is a fluid, too. Wherever you got your degree in aerodynamics, sue them to get your money back.
 777236ER From , joined Dec 1969, posts, RR: Reply 21, posted Sun Aug 12 2007 12:07:51 UTC (8 years 9 months 3 weeks 1 day 5 hours ago) and read 5339 times:

 Quoting ThirtyEcho (Reply 20): Last time I looked, my transmission "fluid" was a "liquid." Air is a fluid, too. Wherever you got your degree in aerodynamics, sue them to get your money back.

Tdscanuck was talking about surface tension, something that applies to liquid in ambient gasses, as is the case with coffee flowing out of a cup.

The Coanda effect is caused by different mechanisms if it's a liquid in a gas, or a liquid in liquid/gas in gas. For the liquid in a gas, it can be explained by surface tension. For the liquid in liquid or gas in gas situations, you have to go deeper into the viscosity, friction and momentum.

[Edited 2007-08-12 12:08:13]

 Tdscanuck From Canada, joined Jan 2006, 12710 posts, RR: 78 Reply 22, posted Sun Aug 12 2007 15:34:20 UTC (8 years 9 months 3 weeks 1 day 1 hour ago) and read 5317 times:

 Quoting ThirtyEcho (Reply 20):I said liquid, not fluid. Last time I looked, my transmission "fluid" was a "liquid." Air is a fluid, too.

All liquids are fluids. All fluids are not liquids. Air is not a liquid (at the temperatures we're talking about).

 Quoting ThirtyEcho (Reply 20):Wherever you got your degree in aerodynamics, sue them to get your money back.

Thanks for the tip, but I'll stick with my degree where it came from.

Tom.

 AirTranTUS From , joined Dec 1969, posts, RR: Reply 23, posted Mon Aug 13 2007 06:57:38 UTC (8 years 9 months 3 weeks 10 hours ago) and read 5242 times:

 Quoting Superstring (Thread starter):x Slipping stone theory, or Newtonian theory --> air is being deflected downwards by the lower side of the wing --> Newton's 3rd. law--> lift

This theory could be easy to test (on a model, of course). Make a wing out of a thin, square box and replace the curved wing. If Newton's theory is correct, the plane should lift off, until...

 Quoting Superstring (Thread starter):x Equal transit time theory + Bernoulli --> pressure above the wing is smaller than below --> we have lift.

It has been proven that an airfoil produces an upward force, but the other day I was thinking, why does an increase in angle-of-attack increase lift? Wouldn't the vector in the direction that lift is acting then point up and back of the wing? If a plane is at 150 knots and level to the horizon, let's assume lift is acting straight up. If the pilot were to pull up though and keep a constant airspeed, lift would be acting less in an upward motion, and have a component aft as well. The upward component would be even less than level flight because the same amount of lift is being produced, but its direction is no longer all vertical. So when an airplane rotates to lift off, I don't think lift increases, but thrust and air deflection (Newton's 3rd law) push the plane away from earth. Thrust also no longer acts horizontally either in a climb. It operates horizontally and vertically, and the vertical component acts with the force of air downward to push the plane higher. (Newton's 3rd again)

Would lift be causing the climb, or would engine thrust? I think thrust is. The airplane is increasing its potential energy, so the mechanical energy of thrust would be used to do it. Also, the plane is no longer in unaccelerated flight (gravity is now greater than lift), so to compensate, more power is needed. I'm sure Newton's 3rd law plays a part here. (I think I answered my own question.)

I have noticed while flying C172's, that if I delay rotation by a few seconds, the airplane gets a bit jumpy on the ground, but will not lift off on its own. If I rotate though and immediately level off, the plane can fly at that speed. I think this proves that Newton's 3rd law is a part of flight (ground effect, which I can see now also happens at cruise), especially the transition from ground to air. When commercial jets are in cruise, they have a positive AOA. The wing would be deflecting air at this moment and the engines pushing in both directions.

Further though, we have all seen pictures of wings flexing on jets, especially heavy ones. They would not be flexing unless there was some force applied to them. Is the force lift ala Bernoulli, or is it their deflection of air towards the ground ala Newton?

My belief is that neither Newton nor Bernoulli can completely explain lift by lift themselves. There are definitely elements of both in lift. I see where both are coming from, but neither is completely correct.

 WingedMigrator From United States of America, joined Oct 2005, 2371 posts, RR: 56 Reply 24, posted Mon Aug 13 2007 08:46:42 UTC (8 years 9 months 3 weeks 8 hours ago) and read 5233 times:

 Quoting Tdscanuck (Reply 3):the wing imparts downward momentum to the air. Downward momentum change = upward lift.

That's the best explanation, based simply on Newton's laws. "All" a wing does is deflect the flow downwards with a minimum of drag.

 Quoting Superstring (Thread starter):First of all to make the situation clearer I want to focus on classical airfoils

Why? Airfoils are not strictly necessary for flight... only for efficient flight.

 25 Tdscanuck : You're mixing up the force vector on the wing with the lift vector. The lift vector is, by definition, the component of force on the wing perpendicul
 26 ThirtyEcho : Air is not a liquid (at the temperatures we're talking about). But air IS a fluid. Have your instructor demonstrate a high speed stall; that will teac
 27 Superstring : Well haven't been here for some days... Yes indeed the simulations dont't use the Kutta condition explicitely. However I want to mention an important
 28 WingedMigrator : There is absolutely nothing wrong with this 'theory' of lift, unless you wish to call into question Newton's laws. Pushing air downwards is precisely
 29 Superstring : Thanks thats absolutely right, sKipping stone, not sLipping stone... + Well I thought that I have explained what I mean with the "Newtonian theory";
 30 Theredbaron : Flux capacitor makes it possible...
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