Anyone wish to defend bernoulli?

[orionflt]

the big issue I have with the video demonstration is they are stating that Bernoulli principle is based on the air meeting back up at the trailing edge and the increased speed is due to the distanced travailed, where Bernoulli's calculations state a pressure differential is created..... never mind, Hank just said what i was working on, and probably better!

[EricJ]

When they insist on misspelling and mispronouncing "Bernoulli" it doesn't add to their credibility.

 

[bonal]

If Bernoulli effect did not cause lift then it would not matter how much AOA   airflow becomes turbulent and separated from the top of the wing which is now stalled. Being an artist and not a scientist I am more subjective and prefer Bernoulli over newton since its much more elegant as opposed to blunt force

[Guest]

5 hours ago, Yooper Rocketman said:

Hey!!!!

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

Tom

 

Damn, typing on a phone sucks!

Clarence

[Cruiser]

If Bernoulli is responsible for lift then explain how I can get back down to the ground after I take off........

[sleepingsquirrel]

We fly airPLANES not airHAMMERS. Bernoulli and Coanda win in cruise Newton works at high AOA. The wing planes the air and the curl (circulation)  of air provides the lift.

Edited June 27, 2017 by sleepingsquirrel

[Hank]

2 minutes ago, Cruiser said:

If Bernoulli is responsible for lift then explain how I can get back down to the ground after I take off........

Simple--lift is proportional to speed (do I need to look up the equation and define the terms?), so to come back down, just d low down until lift is lesstill than total weight and you'll come down. How fast you will descend depends upon how much less lift you create. Or descend the way I do:  push the yoke to create a nosedown angle, and adjust the trim as speed initially increases and quickly plateaus, so that the nkw, higher lift won't be enough to counteract the negative angle of Attack.

Hey, lookkee, two ways to descend, both of which are compliant with Bernoulli, Newton and empirical evidence!  

[sleepingsquirrel]

Cows do not fly because of the low pressure underneath and higher pressure on their head and rump. I offer this cow in a wind tunnel pressure distribution

 as proof.

[Hank]

3 minutes ago, sleepingsquirrel said:

Cows do not fly because of the low pressure underneath and higher pressure on their head and rump. I offer this cow in a wind tunnel pressure distribution

 as proof.

Kinda makes it hard to jump over the moon, dunnin it?

[Piloto]

None of my balsa gliders had any wing curvature but a flat surface on top and bottom and they glided for long distance. What creates lift is the angle of attack. As you increase the angle of attack the air on the bottom side is more incident into the wing creating a pushing up force. At the same time on the top side the wing surfaces creates a partial barrier to the airflow that reduces the pressure on top. And that is why my balsa planes fly.

José

[MB65E]

4 hours ago, sleepingsquirrel said:

Cows do not fly because of the low pressure underneath and higher pressure on their head and rump. I offer this cow in a wind tunnel pressure distribution

 as proof.

But what's a flying pig look like??

haha,

-Matt

[carusoam]

Lift is important. Without it we don't fly.  Call it a stalled condition...

Excess power,  rules!

The ability to climb...  excess power!

The ability to overcome drag to fly fast ... excess power!

variable lift adds to the utility of the flying machine.  

• Changing AOA increases or decreases lift to meet the requirement of lifting more or less weight.
• adding flaps increases lift during slow flight.
• deploying speed brakes decreases lift, increasing AOA to make back the loss of lift generates a huge braking effect.

Somebody explained it. They call him Bernoulli...

Somebody made it practical.  They called him Al Mooney!  

PP thoughts on mathematics and flying, not a mathmetician...

Best regards,

-a-

[steingar]

10 hours ago, Piloto said:

None of my balsa gliders had any wing curvature but a flat surface on top and bottom and they glided for long distance. 

José

Because of Reynold's numbers, which are orders of magnitude different for your balsa gliders and my paper airplanes than for the real deal (Reynold's number reflects inverse square laws).  Your balsa gliders still fly using the same principles as your Mooney.

[Hank]

Way to go, Steiney--inject some science into an offhand duscussion . . .

[steingar]

2 hours ago, Hank said:

Way to go, Steiney--inject some science into an offhand duscussion . . .

Happy to be of assistance.

[Tom]

The Jetwing was an experimental plane that diverted 100% of the engine thrust rearwards over the top of the wing.  The thing didn't levitate off the ground but it did fly. 

The producers of the YouTube video would probably state that the Jetwing's engine didn't have enough thrust (the engine produced 2200# of thrust and the airplane weighed >3000#). 

I suppose a simple question to the producers of the video is "how can an object that weighs more than the thrust of its thrust source get off of the ground if only simple Newtonian physics are at play..." 

https://en.wikipedia.org/wiki/Ball-Bartoe_Jetwing

(a taildragger no less!)

 

 

 

 

[EricJ]

1 hour ago, Tom said:

The Jetwing was an experimental plane that diverted 100% of the engine thrust rearwards over the top of the wing.  The thing didn't levitate off the ground but it did fly. 

The producers of the YouTube video would probably state that the Jetwing's engine didn't have enough thrust (the engine produced 2200# of thrust and the airplane weighed >3000#). 

I suppose a simple question to the producers of the video is "how can an object that weighs more than the thrust of its thrust source get off of the ground if only simple Newtonian physics are at play..." 

But what if it's on a treadmill?

 

...runs away...

[Hank]

2 hours ago, Tom said:

 

The producers of the YouTube video would probably state that the Jetwing's engine didn't have enough thrust (the engine produced 2200# of thrust and the airplane weighed >3000#). 

I suppose a simple question to the producers of the video is "how can an object that weighs more than the thrust of its thrust source get off of the ground if only simple Newtonian physics are at play..." 

 

Ask your favorite transport pilot. Only a few military fighters have thrust:weight ratios > 1, such as the F-16, and that enables it to accelerate straight up.

The new engines for the 787 each produce 78,000 lb thrust, for a total of 156,000 lb. The old engines produce 64,000 lb each. Variants of the plane have gross weights of 502,000 - 551,000 lb.

Edited June 28, 2017 by Hank

[jaylw314]

Just to be clear, the myths that need to be debunked are these.  They are all seductive because they are partly true:

1. Wings create lift because of the airfoil shape.
2. The airfoil shape makes air flow over the top faster since it is a longer distance than the air over the bottom
3. Bernoulli's principle creates lower pressure over the top since it is moving faster.
4. The lower pressure over the top created by Bernoulli's principle creates a net force upwards

The problems with these arguments are:

1. Wings usually are but do NOT need to be airfoil shaped.  We all understand the barn door analogy, and planes CAN fly inverted.  In fact, if you look at the airfoil shape with the flat bottom in most textbooks, you'll see that the average chord line of the airfoil actually has a positive AOA.  It's AOA that predicts lift, which is why barn doors and upside down planes can fly.  In fact, the airfoil shape is simply cambered, and any cambered shape is more stall resistant in one direction.  The more camber, the lower the stall speed.  You can do that by lowering flaps or by making the wing asymmetric in cross-section.  A plane with a traditional airfoil shaped wing will have a higher stall speed inverted than right-side up.
2. The airfoil shape IS a longer distance over the top, but that has nothing to do with why air moves faster over the top.  In fact, if you fly a symmetric wing or a barn door with positive AOA, air also moves faster over the top.  The reason is that forward stagnation line can be behind the leading edge, which is why your stall warning vane works.  The rear stagnation line is ALWAYS at the trailing edge, though, as long as the trailing edge is relatively sharp.  This creates net circulation which moves air over the top faster for a wing for ANY shape.  Essentially, it is the sharp trailing edge that makes the air flow faster over the top.
3. Bernoulli's equation does predict lower pressure in faster moving air.  However, Bernoulli's equation is only accurate when applied along a single airstream that changes pressure along its path such as in the airstream into and out of a venturi.  It is NOT intended to apply to two separate airstreams of different pressures, such as over and under a wing.
4. There IS lower pressure over the top of the wing, but there is also net airflow downwards at the trailing edge.  They both are caused by faster airflow over the top (net circulation) and result in the same thing, lift.  So those two explanations are equivalent.  The important part to know is that Bernoulli's equation is NOT the reason for the lower pressure over the wing.

Here's the section with pictures:

https://www.av8n.com/how/htm/airfoils.html#sec-airfoils

[mooniac15u]

8 hours ago, jaylw314 said:

1. Bernoulli's equation does predict lower pressure in faster moving air.  However, Bernoulli's equation is only accurate when applied along a single airstream that changes pressure along its path such as in the airstream into and out of a venturi.  It is NOT intended to apply to two separate airstreams of different pressures, such as over and under a wing.

When describing the applicability of Bernoulli's principle to an airfoil it is only being applied to a single airstream; the airstream over the top of the wing.  The lower airstream remains at atmospheric pressure.  It is only the reduction in pressure above the wing that is related to the Bernoulli principle. 

[AH-1 Cobra Pilot]

On 6/27/2017 at 0:29 PM, Hank said:

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 .

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 . . . ].

I made it through 25 seconds of the video before finding an error.  It showed the particles arriving at the trailing edge at the same time, which they do not, if lift is being generated.  In my Aero E department, we had a Smoke Tunnel.  It was a very low-speed wind tunnel that generated streams of smoke along the vertical axis.  You could rapidly turn the smoke on and off to see this effect.  If you cannot get this much right...why should I trust the rest of what you say?

The silly part of the video is pitting Bernoulli against Newton.  Newton's Third Law has to do with Forces.  But what supplies this particular Force?  It is Pressure, which is what Bernoulli explained.  It is not exactly that they cannot be separated, but that one furthers the knowledge of the other.

All dumbassery aside, all explanations require certain Assumptions and prior knowledge.  Without regard for them, you are lost.

[AH-1 Cobra Pilot]

10 hours ago, jaylw314 said:

Just to be clear, the myths that need to be debunked are these.  They are all seductive because they are partly true:

1. Wings create lift because of the airfoil shape.

https://www.av8n.com/how/htm/airfoils.html#sec-airfoils

That is not a myth.  Take a look at a plot of Lift vs. AOA for various airfoil shapes.  Compare a symmetrical airfoil with a NACA 2412 for example.  Since your AOA will not vary by 360 degrees, but normally through less than 90, it is hard to argue that the shape does not create (more) lift.

(Again, back to that Assumptions and prior knowledge thing.)

[Cruiser]

The Wright Bros. proved this way back in 1901/2 with their home built wind tunnel. They tested hundreds of airfoil shapes for their ability to lift.

The airfoil will enhance lift, some more than others but it is not the source of lift.

[jaylw314]

13 hours ago, Ah-1 Cobra Pilot said:

That is not a myth.  Take a look at a plot of Lift vs. AOA for various airfoil shapes.  Compare a symmetrical airfoil with a NACA 2412 for example.  Since your AOA will not vary by 360 degrees, but normally through less than 90, it is hard to argue that the shape does not create (more) lift.

(Again, back to that Assumptions and prior knowledge thing.)

That actually makes my point--ALL wing shapes have increased lift with increasing AOA up to stall AOA.  What changes with shape is how efficiently they produce lift (the amount of lift per unit of AOA) and what the stall AOA is.  But no matter the shape, every wing produces more lift at higher AOA below stall, and every shape has an AOA where they produce no lift.  

14 hours ago, mooniac15u said:

When describing the applicability of Bernoulli's principle to an airfoil it is only being applied to a single airstream; the airstream over the top of the wing.  The lower airstream remains at atmospheric pressure.  It is only the reduction in pressure above the wing that is related to the Bernoulli principle. 

The lower airstream does not remain at atmospheric pressure--it actually increases, which Bernoulli's principle does not predict (see the pressure diagrams in the article).  And the speed increase of the upper airflow is lower than predicted by Bernoulli's principle in the textbook example.  So yes, Bernoulli's principle does exist and does predict the behavior of air under certain circumstances, but not this one.

Edited June 30, 2017 by jaylw314

[carusoam]

@MV Aviation has done some interesting lift calculations /computer solutions/graphics...

Putting this tag here may alert him to this discussion.  Hopefully he can add some details and possibly point to where he stored his discussion...

Best regards,

-a-