Anyone wish to defend bernoulli?

[201er]

Video gives a good argument against bernoulli. Let me get out the popcorn as we prepare for a food fight. Or we don't care about aviation related things here anymore?

 

 

[Dream to fly]

I need to go back to school.....        It does bring up some very interesting points.    I'd be interested to see what NASA and Boeing use for their standard.

[Robert C.]

Great video and explanation. I never found the Bernoulli effect intuitive as it applied to wings. But as a High School glider pilot I just accepted what my instructor told me even when it didn't seem clear why those particles of air had to meet up after their trip over/below the wing. Then I got to engineering school and learned that Bernouilli's "law" or "principle" actually states that a change in speed leads to a change in pressure. Also, as the video authors note, the principle specifically is referencing a single flow line.

What makes it so confusing is that the air flowing over the airfoil will be faster than that flowing below. So the formula actually works in that it correctly predicts lower pressure above than below the wing. However, Bernouilli's law does not attribute causality to the shape of the airfoil.

Being a civil, rather than aeronautical, engineer by training, I never gave much thought to Newton's 3rd law as the correct explanation.

Look forward to a proper aeronautical engineer weighing in

Robert

[Robert C.]

Okay, I'm back to Bernouilli being right. I don't get why the downward directed airflow creates a force component that works on the wing.

So why would Bernouilli work?

Atmospheric pressure above and below the wing are for all practical purposes identical. So you can think of there being hard floors above and below the wing, think of it as the wing operating in a wind tunnel. The roof and floor in effect are the stack of air molecules that create the air pressure we know and love in the real world. As the wing moves forward (air blows through the wind tunnel) the air has less space above the wing (distance to the roof) than below the wing (to the floor). hence the air above the wing has to move faster than the air below the wing (same pressure from the front - same number of air molecules - has to go through a smaller space above than below). As the air moves faster the pressure drops and lift is created.

what am i missing?

[Hank]

Newton says that the airflow moving front to back also has a downward component to it. "For every action, there is an equal and opposite reaction" requires an upward force on the wing. The wing directs air slightly down, the moving air directs the wing slightly up. Don't believe it? Draw it to scale, with the correct Angle of Attack from the center of the leading edge to the trailing edge (hint: the leading edge is always higher). Or while driving to work or the store tomorrow, roll down your window, stick your arm out and fly your hand like a wing. Lift the front edge, your whole arm goes up. 

However, for more on Bernoulli, I recommend something like this (but more updated if you wish--the principles have not changed, only the presentation):

I actually started rereading the one on the right when I began flight lessons, but figured out pretty quickly that it was serious overkill . .     The left book explains Bernoulli, the right book applies it to flight (in great detail, with lots of really fun math). We'll talk when you finish reading.

Edited June 27, 2017 by Hank

[Robert C.]

LOL, lying awake rethinking this. I'm back to the Bernouilli explanation being wrong.

If Bernouilli was the only reason then aircraft couldn't fly inverted and symmetrical airfoils wouldn't produce lift. As Hank said, the AoA must be (main part of) the answer.

Robert

[Hank]

It takes two to tango, Robert! Gotta have 'em both.

A symetrical wing maintains level flight by elevating the leading edge. This makes the airflow hit the leading edge below center, shortening the bottom path and lengthening the top path. Thus Bernoulli also affects symmetrical airfoils.

Inverted flight is quite similar, except an even higher upward angle is required  ("nose up" sounds so wrong to describe inverted flight!). Thus the Bernoulli effect is more pronounced when flying a non-symmetrical wing upside down. Look at the airflow lines around a normal airfoil on the cover of the blue book, the top path is much longer; roll that wing inverted, and it will move much further in order to create sufficient lift from an inefficient design (the smooth surface pointing down). Hey, the airflow behind the wing is pointed down, while in front it's pointed UP! The direction changed, creating Newtonian lift (push?) in addition to the Bernoulli flow lift.

Of course, these also create greater Newtonian effects, as the higher the leading edge is placed above the trailing edge, the more downward the airflow is directed . . . . It's the chicken-and-egg tale all over again, you can't have one without the other.  

Edited June 27, 2017 by Hank

[Hank]

Anyone ready to debate if light is a wave or a particle stream???        

[jaylw314]

This was the online book 20 years ago that got me fascinated by aerodynamics and flying:

https://www.av8n.com/how/

Yes it's overkill, but it is the clearest explanation I've seen about how wings generate lift, and how Bernoulli may (or may not) have been wrong.

The Cliffnotes version: The classic Bernoulli explanation in textbooks is wrong.  Wings create lift by moving air down, and that downwash is created by net "circulation" around the wing while moving forward, identical to the Magnus effect that creates lift on spinning ping-pong balls.

And light is neither wave or particle.  If you look carefully, it is actually a herd of fuzzy cats 

Edited June 27, 2017 by jaylw314

[RLCarter]

I know if Bill has 100 apples and gives 12 apples to Kim, Bill still has a crap load of apples.....

[EricJ]

I didn't watch the whole vid, but enough of it to get an impression.   I think the guys that made that vid may also make flat earth vids.   It seems to be popular these days to make a hand-wavy, believable-sounding argument about how established science is wrong about something or other.   

 

 

[ArtVandelay]

It's been said before; Newtons law makes planes fly, Bernoulli laws makes them fly efficiently.

[Guest]

I know that airplanes fly on buckets of cash and barrels of 100 LL. 

Clarence

[Tom]

Lift is really a quantum entanglement issue.....a macro-scale single-slit phenomenon.  The air molecules below the airfoil, once realizing that they've been duped into separating from their entangled twin, push up on the bottom of the airfoil in a vane attempt to reunite, in the process creating lift.

 

Special relativity gets involved when there is a 90-degree difference in airflow against the airfoil, as occurs with a downwind turn. In this circumstance, the entangled molecules below the airfoil become ambivalent regarding the particle separation and cease to apply pressure, causing the airfoil to stall, as always occurs when turning downwind.

 

A more serious take on the coolest thing in all of science:  https://youtu.be/U7Z_TIw9InA 

 

 

 

[Jerry 5TJ]

When I look at the wing during flight nothing seems to be going on out there. 

[Jeff_S]

It would be disconcerting for the general public to realize that after 114 years, the big-brains at Boeing and Airbus (and NASA, I guess) still can't give us an exact reason for what keeps their big birds in the air.  But we humans have lived for millennia not always understanding our world exactly.  That's sort of how I feel about health care politics!

[Danb]

1 hour ago, Jerry 5TJ said:

When I look at the wing during flight nothing seems to be going on out there. 

Jerry I looked out at the wing after I passed behind the water spout yesterday over Lake Erie that rascal was doing a dance while I was seeing stars bouncing off the ceiling. I guess that's newtons fault. 

[Andy95W]

3 hours ago, M20Doc said:

I know that airplanes fly on buckets of cash and barrels of 100 LL. 

Clarence

Wow, Clarence, I knew the Comanche 400 was inefficient but never realized it was this bad!

[M20F]

3 hours ago, M20Doc said:

I know that airplanes fly on buckets of cash and barrels of 100 LL. 

Clarence

Just 400 HP ones.  My Executive sips 1.2 GPH running 300 degrees LOP at 450kts.  

[DonMuncy]

For us simple minded folks, I think Wolfgang Langeweische said it best in Stick and Rudder . A wing goes up by pushing air down; much like a water ski rides on top of the water, rather than sinking. 

[Guest]

2 hours ago, Andy95W said:

Wow, Clarence, I knew the Comanche 400 was inefficient but never realized it was this bad!

Its really that bad.  We dont have a CB's club for Coamnches.

Clarence

[Yooper Rocketman]

31 minutes ago, M20Doc said:

Its really that bad.  We dont have a CB's club for Coamnches.

Clarence

Hey!!!!

You gave me crap a month ago because I couldn't spell Comanche.  I don't even own one!

Tom

 

[Hank]

Wow! Now I've watched the video. They started with a gross oversimplification of Bernoulli's Law, turned it into a straw man, and proved the straw manto be incorrect. They further compounded their error by expanding from an incorrect / incomplete oversimplification not being fully functional to the whole thing being wrong . . . I can do that with your bank account and prove that you are either getting rich beyond your dreams or going rapidly broke, depending on the assumptions I make in my simplified model and how I apply them. Note that they left that part out . . .

Kind of like how so many A&Ps "prove" that running your engine LOP will "burn it up" without presenting any data. 

Show me the math, with all assumptions clearly stated. Once I go through it and don't find any inconsistent assumptions or things I think are inaccurate, and there are no math errors, then I might believe them.

Right now, I stand by my above statement:  Bernoulli and Newton both contribute to lift, and they cannot be separated. Others have held this belief for longer than airplanes have been flying, granting both the title of "Law" because they hold pretty much all of the time [although Einstein did a good job showing that relativistic effects must be taken into account as speeds approach c, but aerodynamics are gone well before then . . . ].

[peevee]

you can show me in the "doesn't care" category

[steingar]

I thought that video was oversimplified at best.  The demonstration of the Coanda effect ignored capillary action, which was responsible for quite a bit shown.  Try pouring some oil over a plastic bottle and you'll see what I mean.

The problem is many things in Physics defy simple explanations, and I suspect airfoils are one of them.  Unfortunately, simple explanations rule youtube.