## Step 10: Practical Applications : Race Car

A Race Car, in fact almost all cars are designed in a way to avoid lift and stick to the ground at all times. We wouldn't want cars just suddenly rising up due to high velocity. So keeping in mind the Bernoulli Principle scientists designed cars in a way completely opposite to that of an airplane. A race car employs Bernoulli's principle to keep its rear wheels on the ground at all times while travelling at high speeds. A race car's spoiler—shaped like an upside-down wing, with the curved surface at the bottom—produces a net downward force, thus keeping the car to the ground.
<p>Hi, my name is Integza and i have a education channel on youtube.</p><p>I have a video explaining the Bernoulli's principle, is very graphical and easy to understand because doesn't use much technical terminology:</p><p>https://www.youtube.com/channel/UC2avWDLN1EI3r1RZ_dlSxCw?sub_confirmation=1&amp;feature=iv&amp;src_vid=t7MsYZgtXXo&amp;annotation_id=annotation_4127173749 </p>
<p>As I commented under your Bernoulli video, you work too hard to justify Bernoulli. The pressure is higher in the large section because there is a constriction where the diameter reduces. The pressure in the smaller section is lower because it is free to flow away from the higher pressure region. If you view the smaller section as simply a hole in the pressurized large section (view it as a pressurized tank), the fluid easily escapes out this &quot;hole&quot; to the lower pressure region. It is pressure differences that cause accelerations in a fluid. A higher pressure region represents a force (F) that pushes (accelerates A) the mass (M) of air toward a lower pressure region. F=MA.If you have a gradual change in diameter, the momentum description is not valid.</p><p>-- </p><p>Cheers</p>
<p>The majority of these demos are classical misinterpretations of the applicable science as well as Bernoulli's actual Principle. Some of the other comments bring-in many things including math which DOES NOT explain the physics/science.</p><p>The true physics is not difficult to understand if you can get all these misconceptions out of your mind.</p><p>Speed of a fluid DOES NOT create a lower pressure. </p><p>Speed past a surface (or visa versa) does not create a lower pressure. </p><p>Bernoulli mentions neither of these. </p><p>If a stream of fast air did have a lower pressure than surrounding air, then, the fast stream would be squeezed narrower and narrower by the higher pressure around it as long as it moved and this simply does not happen.</p><p> Pressure at a point in a fluid acts in all directions; can't violate that. Therefore, the pressure in the stream (at its outer boundary) pushes out with the same pressure as the atmospheric pressure outside it pushes inward.</p><p>Paper airplanes (flat balsa wings and Wright Bros wings) have no curved, 'LONGER' upper surface, so this well known bad explanation has a serious problem.</p><p>The lifting paper and tissue paper/leaf blower are a curved airflow followed by entrainment (from viscosity).</p><p>This floating ball explanation is messed up. If the pressure was lower UNDER the ball it would not be held up. Then, It is centered by curved airflow.</p><p>The atomizer (insectide spray) is due to a curved airflow (commonly called &quot;turning&quot; of the air) over the vertical tube's end.</p><p>The Bunsen burner is due to entrainment (viscosity).</p><p>The envelope opens because you blow air into it, thus increasing the internal pressure.</p><p>Air has mass and a force is required to accelerate mass (even air). That force is pressure (difference) and pressure alone. </p><p>A CURVED AIRFLOW requires a force because this is an acceleration. This force can only be a pressure difference. That pressure difference is from the relative motion of fluid and object.</p><p>Pressure differences are created DIRECTLY by the relative motion between fluid and object. [no Bernoulli &quot;fast&quot; air, no half venturi pinching, no other false science]. Walk through water and feel how you must push some water around! You create a higher pressure in front of you by pushing on the water (and lower pressure behind by moving away from water). THAT'S WHAT CREATES THE PRESSURE CHANGES.</p><p>That pressure difference (cause) accelerates water (effect) around you. A higher pressure region accelerates (pushes) air toward any (and all) lower pressure regions. The same thing applies to wings and air: except, to our advantage, the higher pressure is under the wing; lower above. </p><p>These very same pressure differences then cause all of the the fluid accelerations we see around a wing. Anything else is bad science and violates Newton.</p><p>This simple concept explains all the air flows around the wing (including the up-wash and down-wash). </p><p>The air above and below a wing are unrelated and Bernoulli can not be used to compare velocities and pressures. Therefore, saying the upper air is lower pressure because of the difference in speed to the lower air is completely false science and terrible math. Bernoulli deals with accelerations and pressures that occur along a single path (streamline).</p><p>Again: A higher pressure region accelerates (pushes) air toward any (and all) near-by lower pressure regions. </p><p>Also, it is the path the air is forced to take by the wing that is what you should focus in, not simply the wing's shape. </p><p>Finally, (once you fully understand it, it'll be no surprise that) from the siationary air's frame of reference, the fastest moving air around a wing is UNDER it ! This is easily shown on real wings. This blows the &quot;fast-air-has-lower-pressure-than-some-nearby-slower-air&quot; out of the water. That is a serious and long standing misunderstanding by amateurs, but not those in the field.</p><p>Don't believe me. </p><p>For a better understanding of the true physics, see these authoritative references:</p><p>Krzysztof Fidkowski How Planes Fly</p><p>https://m.youtube.com/watch?v=aa2kBZAoXg0 </p><p>David Anderson:</p><p>https://m.youtube.com/watch?v=hQ99JkaOwEk </p><p>Babinsky;</p><p>https://m.youtube.com/watch?v=XWdNEGr53Gw </p><p>McLean (though he gets rather heavy later in the video):</p><p>https://m.youtube.com/watch?v=QKCK4lJLQHU</p><p>Regards, Steve Noskowicz</p><p>Science &amp; Technical Advisor</p><p>challengerillinois.org</p>
The discussion between valhals_end and lemonie/eplunket was AWESOME!!! way to go Valhalas_end.
<p>Thank you, this really helped with my homework. 8/8!</p>
<p>Very Informative! (y)</p>
The curves :)
If&nbsp; airplanes relied upon Bernoulli's Principle, they wouldn't be able to fly upside-down would they?<br> <br> L
If they only flew in an incompressible (and ideal) flow regime, then technically yes, depending on the airfoil shape - there are symmetric wing designs that allow low-speed airfoils to fly upside down (but you can't neglect the fuselage and tail influences on aerodynamics when you extend from infinite-wing studies - see Prandtl Lifting Line theorems on airfoils - to fully 3D flow over airplanes). Most wings see compressible flow, though (if not fully spread across the wing, like in high transonic or supersonic flow, then in pockets of influence, which can be forced to occur on special airfoil designs down to Mach 0.3), where Bernouilli's Principle is superseded by compressible gas dynamics.<br><br>Most of these practical examples would operate at high enough speeds so compressibility would be an issue, but for simple understanding, Bernouilli's Principle can be applied.
It isn't right.<br> If you watch aeroplanes taking-off; they run along the ground then <u>lift the nose</u>, Bernouilli's Principle doesn't slowly get 'em higher the faster they go.<br> <br> L
Ahh, but you are missing a crucial point there - the aircraft's weight and center of gravity / centroid / center of inertia. Howtowithmanish is correct that during take-off, Bernouill's Principle does apply (the aircraft is moving at a low enough speed / Reynold's number for compressibility to not be an issue). The bottom of the wing is developing increasing pressure while the air above the top drops in pressure, developing a pressure and suction surface on the bottom and top, respectively. This generates lift (upward force normal to the wing's span). That's all Bernouilli's Principle is for - creating lifting force.<br><br>As the aircraft continues in it's ground roll, increasing in speed, the lifting force increases until it matches and then exceeds the weight of the airplane. If you have a long enough runway, you can take off without ever lifting the nose (flat takeoff) because you are generating enough positive lift. But to expedite this, the planes take advantage of both ground-effect (which generates an upwash of air that helps lift the plane) and use their flaps or tail to force a rotation, lifting the nose (although some planes are designed with a center of gravity (CG) such that upward lift will automatically force the nose up - planes with canards are great examples). When the nose lifts, you again rely on lift to continue off the ground and stay aloft.
It's not best explained by Bernouill's Principle; if you look at landing-flaps and how hydrofoils work in exactly the same way, then you realise that it's an old explanation that still kicks-around in text-books and word-of-mouth...<br> <br> L
Is sad that Bernoulli's principle is still being used to explain flight. Especially when the explanation is even easier. The sum of the forces equal zero. That's it. If the air is holding the plane up, then the plane must be pushing the air down. The figures that show airfoils with the air coming straight off the back are misleading. The air has to come off pointed down.
They also say to never argue with an engineer. I tend to agree with that - that entire argument took me only 5 minutes to formulate, and I never even had to reference the 30 or so aerodynamics, fluid mechanics/dynamics, and CFD texts I have sitting on my shelves next to my computer. Believe me, I argued many of the points you and eplunkett raised way back in intro classes - my professor's must have enjoyed my naturally competitive and argumentative nature - and was always shot down by simple explanations drawn in minutes.
Its been almost 7 year since I graduated with my aero degree. Since then I've been exclusively working on aircraft structures. So knowledge as to whether or not the N-S equations ultimately boil down into some form of bernoulli's principle was dumped from my brain years ago. What I do know if that if the air imparts a force on the plane (lift), the plane has to impart a force on the air (downward motion). I realize the photo you included shows the streamlines (or pathlines... not sure of the difference in terminology) at the start and finish at approximately the same angle. That figure was not accompanied by the lift data, so it could be at zero lift. NASA has <a href="http://http://www.grc.nasa.gov/WWW/k-12/airplane/right2.html" rel="nofollow">a website and applet</a> that give a simplified explanation for why airplanes fly. I haven't ready the whole site, but the applet is a nice visual. I was also not saying that some form of bernoulli's was not at play at all. Sure, the upper surface does measure a lower pressure. My problem is that the principle is often misrepresented, for example, the idea of equal transit time over the upper and lower surface, or a venturi effect, both of which can be easily disproved.&nbsp;
I agree fully with your last point - the transit time is not the same, and can not be, as velocity of the air stream over the top has to increase to create a suction surface. But the argument that flow turning is what generates lift is not entirely valid for commercial aviation for one main reason (by the way, the image shows a cambered airfoil at zero angle of attack, which by definition of not being symmetric generates lift - I do not know the relative magnitude, but a NACA 2412 has positive lift at zero angle of attack...if I remember the charts right, I believe the CL value to be around 0.1 to 0.3 for low Re flow). If you study turbomachinery (aircraft compressors, fans, or propellers), you will often see very large air turning angles (sometimes reaching 90 degrees in impulse turbines, or between 30 and 50 degrees for lower power axial turbines), but these are achieved by inputting (compressor) or extracting (turbine) massive amounts of power from high-speed (in a rotating frame) flows, regardless of the flow type. For external aerodynamics, there is no large energy change - gravitational forces are minute compared to high-speed turbine shafts - so the relative amount of turning compared to the airfoil camber is necessarily small. This is also partially due to two phenomena: <br>1) For a cambered airfoil, the airfoil generates an upwash effect at the trailing edge for positive attack and a downwash for negative, due to vortices that grow over the suction surface being shed (outside of regimes of realizable vortex structures like Von Karman sheets).<br>2) One of the main principles of potential flow lies in the inability of particles to traverse across streamlines without an energy (force) input. As I mentioned before, if the air at the trailing edge deflects downward considerably, eventually it must meet the &quot;freestream&quot; air that is unaffected by the airfoil, which is modeled as a uniform potential flow (ignoring wind and wash). If you separate the streams of freestream and airfoil-traversing air, as is done in the &quot;patching&quot; methods of boundary layer + Bernouilli's + potential freestream, when the airfoil-traversing air reaches the freestream, it cannot cross the streamlines in the freestream, because no external force is acting upon the airfoil stream at that point to continue their downward deflection. Thus, as it reaches the freestream streamlines, the airfoil streams must turn to match the freestream.<br><br>Here's where both of our points meet halfway, though - my explanation refers to far-field effects, before and after the airfoil has passed, while your deflection phenomena is a near-field effect, directly at the trailing edge. In my regime, Bernoulli's Principle is applicable because of the potential flow assumption (which holds very well for uniform freestream flow), but in your regime, unless the aircraft is flying at a very low Reynolds number (or the airfoil is a of very special laminar design class), the trailing edge and wake are turbulent, and Bernoulli's Principle can not be applied without liberal corrections. So yes, directly at the trailing edge of the airfoil, you will see a downward deflection, but this region can not be considered in my argument - this would be implicit in the wake models and boundary layer models that need to be added to potential flow / Bernoulli's Principle equations for the far-field. Additionally, as air is never truly incompressible, the effects of any downward deflection can be naturally damped by compressibility effects, reducing the visibility of these deflections - similarly, the wake of the wing will damp out far behind the trailing edge (although the length scale there can be several airframe lengths) and collapse back into the freestream flow.<br><br>If you've been out of aerodynamics for a while, take a look back at the Lanchester-Prandtl Lifting Line theorem derivations and compare against thin-airfoil and thin-plate solutions. Most of these derivations include a boil-down to the methods I described above, since this chain can be used to predict many airfoil solutions to within 80% or so accuracy of numerical solutions, depending on the assumptions, flow structures, and how the solutions are patched - not a bad basis, especially since in the first half of the 1900's, these were the only tools available in closed-form. By the way, it's not that the N-S equations boil down to Bernoulli's Principle per se, but rather the N-S equations inherently hold Bernoulli's Equations in their analytical form. Yes, algebraically, you can reduce N-S to Bernoulli if you apply the right substitutions, boundary conditions, and assumptions, but it's more important that under these restrictions, N-S will always give the exact same solution as Bernoulli, because both are derived from fundamental equations.
Yes! thanks for the contribution.<br> <br> L
Back in the late 80's there was a computer storage solution that relied on Bernoulli's Principle.<br><br>http://en.wikipedia.org/wiki/Iomega_Bernoulli_Box