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The Bernoulli Principle

Updated: Feb 13, 2021

The Bernoulli Principle, also known as the Venturi Effect, was developed by the Swiss Mathematician Daniel Bernoulli who was born in the 1800's.

His theory explains fluid dynamics, and is the theory that LIFT, and therefore FLIGHT is based on.

Bernoulli' s Principle states: Total energy in a steady streamline flow remains constant.

He discovered this by placing pressure measuring instruments on, and this is something I heard along the way, on sewerage pipes! (not sure if the sewerage pipe part is true).

There was a total constant pressure running through the pipes. Where there was no blockage, there was a relatively higher pressure. Where there was a partial blockage, there was a relatively lower pressure.

Bernoulli 's Principle is also known as the Venturi Effect - it states that a high velocity flow gives a low static pressure.

DEFINITION: velocity = speed with direction When I was first taught the Bernoulli Principle, it just did not make sense to me. How can fluid / air be pushed through a narrow opening and have the pressure drop? I thought since the opening is narrowed, the pressure MUST increase! That's logical, isn't it? The fact is, that the pressure does drop when the fluid/air speeds up. Try thinking of it this way. The "blockage" creates a surface area that is further / longer than if there was no "blockage", so the air / fluid particles have further to travel. They have to speed up to cover the increased distance in the same time, and because they speed up over this longer distance, less particles pass each point along the way at one time, so less particles passing over a particular point at one time means less pressure.

Fluid flow is not the same as traffic flow. We all know when the road is blocked up ahead, everyone slows down. With fluid, (water, air - below 200kts, fuel, etc), this is not the case. The pressure from behind does not decrease, which means the same volume must get through a smaller gap in the same time, so the fluid has no choice, it has to speed up. Not all the fluid can get through the narrow gap at the same time, but it does not have to, because the particles are moving faster. Less particles pass through at a time, so the pressure becomes less. If that still doesn't work for you, then look at Bernoulli 's Principle mathematically:

Total Constant Energy = Pressure Energy (static) + Kinetic Energy (dynamic)

Static Pressure is the pressure that is essentially the "weight" of the atmosphere, or the Density of the Air as it rests on a body (from Ex 1)

Dynamic pressure / Kinetic (movement) energy is the pressure we feel from moving air, wind, prop wash, hand out of a moving car... etc. (from Ex 1)

In the Bernoulli Principle equation of Total Constant Pressure,(TCP) we can replace "Pressure Energy" with the Greek symbol "Rho" written "p", which represents air density, and "Kinetic / Dynamic Energy" with 1/2 Rho v squared which describes the pressure of moving air, so we can write the same formula like this:

Consider a basic equation: 10 = 5 + 5 10 = 1 + 9 10 = 3 + 7 (The number 10 for Total Constant Energy is just a number I chose randomly. You can use any numbers you like, this is simply to demonstrate the concept). The total energy remains constant. So, if the kinetic/dynamic energy increases, then the static pressure must decrease so the total constant pressure can remain the same, and visa versa.

NOTE: Air behaves like an ideal fluid below 200 knots, above that speed it compresses. For PPL, we only concern ourselves with aircraft that fly below 200 knots. Air pours and flows like an ideal fluid below this speed. EXPERIENCE BERNOULLI'S THEORY FIRST HAND: If you take two pieces of paper and hold them next to and parallel to each other an inch apart, then blow between the two pages from the top, will you blow them apart? No. They will "suck" together. Try it if you don't believe me. This is because the faster air creates a drop in pressure. Things tend to move from a relatively higher pressure to a relatively lower pressure, so the papers move from the relatively higher pressure towards the relatively lower pressure, towards each other.

Have you ever noticed when two trucks pass each other fairly close on the highway, they pull towards each other? Bernoulli's Theory / Venturi Effect in action.

I heard of a tragedy where the parents of a three year old did not know about the Venturi Effect. They let their toddler look out the open train window. Another train passed their train, and the resulting low pressure sucked the child out of the window. I do not know if the child survived.

JUST BELIEVE IT PLEASE If the Bernoulli Principle is still not making sense to you, just accept that when the speed over a surface increases, the pressure above it drops. Then we can move on.

Something doesn't seem quite right?

Now those of you who are awake may be asking how a closed system theory can be applied to an open system like a wing. Good question, one which the NASA scientists have answered beautifully but our Civil Aviation Authority are yet to embrace. The NASA guys say there is a lower pressure above the wing (same as the Bernoulli Theory), but it is because of Newton's Laws and the Coanda Effect, which takes the open system into consideration.

The dynamic pressure portion of the Bernoulli Theory is part of the Lift Formula, - half rho times velocity squared - so I assume the attachment to Bernoulli is because of the paperwork involved to change things. So as far as exams are concerned, please spit out the Bernoulli Theory as the reason behind it all if you want to get the marks. Crazy world we live in, I agree, but I am comfortable with this answering government questions the way they want them answered to get to the desired result... the freedom to fly.

As long as you understand the difference, I am satisfied with that.


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