Anyone wish to defend bernoulli?

[mooniac15u]

8 hours ago, jaylw314 said:

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.  

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.

I was addressing your misleading claim that the Bernoulli Principle doesn't apply because there are two slipstreams:

"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."

The Bernoulli Principle most certainly applies to the slipstream above the wing.  It is irrelevant that there is a second slipstream when considering the top slipstream.  When I referred to atmospheric pressure I was talking about a low angle of attack.  The pressure associated with the lower slipstream is impacted by other factors particularly at high angles of attack where interactions with the bottom of the wing necessarily impact the pressure of the air around it.

[jaylw314]

13 hours ago, mooniac15u said:

I was addressing your misleading claim that the Bernoulli Principle doesn't apply because there are two slipstreams:

 

With all due respect, you misread me.  Bernoulli's Principle is a concept that applies everywhere.  Unfortunately, it is not sufficient to describe every situation accurately or even usefully.  An analogy is the cause of friction drag at high speeds.  Drag from friction is predictably proportional to the square of velocity.  However, at low speeds, this predicts drag should be very low, but in fact drag increases on the back side of the glide curve because of increasing induced drag, which is not accounted for in this equation.  As your speed decreases, friction goes from being the predominant cause of drag to only a minor cause of drag.  Friction still exists but it contributes less and the friction drag equation becomes more and more inaccurate.  Working in reverse, the more poorly an equation predicts the result, the less the underlying principle contributes to the observed result.  So in this case Bernoulli's principle does still apply to some extent, but because its equation fails, it is clearly not the most important contributing factor to lift.

[INA201]

Does a wind tunnel produce the same results on an airfoil as does the same airfoil moving through the air assuming all other conditions are equal.  Is there any unexplainable discrepancies moving from the lab to the real world? Or unexplainable discrepancies with computer modeling?  I hope this doesn't divert the discussion I'm merely trying to keep up with you guys on this subject. 

[carusoam]

INA,

One short coming of the wind tunnel vs. reality...  the wind tunnel doesn't account for the mass of the plane and its momentum...  the plane may stay on a path for a while before stalling.  once stalled the plane will continue towards the ground before recovering. And recovery isn't instantaneous.  It can take some time to restore airflow over the entire wing...

Very few wind tunnels are full scale.  Trying to test new ideas at full scale has been incredibly expensive.  

Have you seen the size of speed brakes? They are the size of pea in an ordinary wind tunnel.  Their real life effect is pretty tremendous. 

 

I get the feeling we are not using the higher level of mathematics that is available to fluid dynamicists...

The three dimensional calculus that includes variations in air density and viscosity and includes time in the calculations.  Somebody mentioned limits and limitations already.

Sure, it is easy to bash somebody's work...

First, you have to explain it, then show where it falls short.

Then build on it, showing where the next level can take you and us...

Destruction is a powerful but empty force.  Construction will get you to the next level.

Simply, burning down a house is powerful. Building a house gets you to the next level.

Jay, you might want to explain your background to help us understand where you are going...?

Why would I want to bash the work of some genius that IS the math behind flight?

How much time do I want to spend on the math behind flight, when my plane's math days were done the day it left the factory?

Did you know Bernoulli was a family of smart people? 

https://en.m.wikipedia.org/wiki/Bernoulli_family

Best regards,

-a-

[MV Aviation]

On 30.6.2017 at 8:28 AM, carusoam said:

@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-

I'm not exactly sure what I can contribute to this discussion. When I started reading this thread, I quickly realized that some are lacking seriousness. This discouraged me to read all comments conscientiously.

What I can say (as an aerospace engineer) is that lift is the sum of a variety of phenomena. The shape of the airfoil (flat plate, symmetrically and non-symmetrically curved) plays a roll as well as the angle of attack. Going to supersonic speeds, wave drag, Prandtl Meyer expansions and so on come into play. Three-dimensional effects further complicate the matter. There is absolutely no doubt that the "equal time argument" is incorrect. AND Bernoulli is only applicable ALONG a STREAMLINE, which is correctly stated in the video.

For those of you who are seriously interested in learning more about that topic, have a look into "Fundamentals of Aerodynamics" by Anderson (The pope of aerodynamics).

Best,

Marco