# Myth of Bernoulli's Principle

http://www.npr.org/templates/story/story.php?storyId=3875411

http://www.aa.washington.edu/faculty/eberhardt/lift.htm

To anyone in the know, does this explain the lift gained from concaves?

don’t try to confuse things with the truth…

I’ve seen top fin designers explain how fins work to the public for years using only the Bernoulli effect explanation, and been chastised for using the Physical Explanation instead. It is so bad I’ve had laymen tell me that side fins at enormous negative angles of attack are still generating lift because they are assymetrically foiled!! In my view, to a first approximation, it is easy. Fins re-direct water flow. The change in water flow is balanced by equal and opposite forces on the fin. If you know how the water flow is changed by the fin, you know how the fin is working. And it mostly follows in a straightforward manner from there and you can go on to minimizing induced drag and what flex is doing.

If Roy was still allowed to post, I suspect he’d have something to say on this topic. Probably something controversial, but educational none the less. Roy, if you are reading and have a comment, just email it to me and I’ll post it. Unless of course Mike thinks that is a violation of forum rules.

Concaves and lift they supposedly generate are not directly explained by either of the two articles, partly because concaves don’t (directly) generate lift. Concaves, to some extent, trap air under the board and decrease friction. Less friction, less of the surface of the board is needed, at an appreciable angle of attack, to produce lift. Less angle of attack and less wetted surface, less drag, too.

The bonzers are supposed to act as a “venturi”. They are wider further fore, and narrower further aft. This compression of the water flow forces the water to flow faster (here’s Bernoulli), and so the water coming through the board center is moving faster, and you get better fin action off the center fin.

Another line of thought is that the turbulent layer is made deeper in the middle (under the concave) and thinner outside the concave, where the rail fins are.

Another line of thought is the concave acts in turning, so that water crossing the rail is re-directed downward, and lifts the rail upward. The decreased depth of the rail contributes to the increased speed.

It is clear that at the speed surfboards travel and IF a surfboard is going straight, a concave is substantially slower than a flat board. However, what matters is speed off the bottom turn, and most riders agree that concave under the front foot is faster than flat or Vee, and that the rail lifts a little with a well made concave.

I remember a guy in Oz was making “jet” bottoms in the early '90’s. Rather than channels, there were interweaving mini-channels from about mid-board back to the tail. If turbulence was the design goal, the jet bottom seems like a winner.

There’s this guy in NZ who has made heaps of interesting boards over the years and we were talking about this topic and basically he agrees with Blakestah and mogwai: the Bernoulli principle is not as useful for analysing lift as the simple theory of fins or wings as water (or air) flow direction units. This guy has these tunnel fins and he finds that it is much easier to describe their behaviour as water flow direction units. People who try to predict the behaviour of a tunnel fin always seem to assume that the tunnel will be constantly lifting in a vertical direction (at 90 degrees to the water flow), regardless of the angle at which the wing is set up in relation to the bottom. This is not what happens. As MTB once pointed out, even aircraft with no foil on their wings (or symmetrical foils) can fly, which is a shot in the foot for Bernoulli’s theory.

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As MTB once pointed out, even aircraft with no foil on their wings (or symmetrical foils) can fly, which is a shot in the foot for Bernoulli’s theory.

On the contrary, the bernoulli’s principle doesnt state anything about foils or wings but about changes of pressure and velocity which are directly related to the angle of attack of a surface (fins, wings…etc). The incompressible flow of a flat surface, a wedge surface or a curved can be studied using many mathematical and physical equations including bernoullis.

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I remember a guy in Oz was making “jet” bottoms in the early '90’s. Rather than channels, there were interweaving mini-channels from about mid-board back to the tail. If turbulence was the design goal, the jet bottom seems like a winner.

Erle Pedersen has been making jet bottoms since about 1975…

my brother has 3 of them. His comment was that they only work at their best in the glassy queensland point waves they were designed for. They’re shown in the video ‘ripping down walls’, from memory . Single fins originally…tricky setting THREE fins with the criss cross patterns !

I want to give one a ride one day…[ any victorians reading this ? simon is at ‘Island’ ]

Bernoulli’s Principle WRT aircraft is applicable to lift producing airfoils, which are not necessary to fly IF you have sufficient thrust to overcome gravity and drag. The F4 Phantom gained negligible lift from its wings, it flew only because of the massive amounts of thrust its engines developed

JTS

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The F4 Phantom gained negligible lift from its wings, it flew only because of the massive amounts of thrust its engines developed

JTS

And even if they had never pushed the F4 Phantoms into the air, those J79 engines have provided Craig Breedlove plenty of fun driving around on dry lakebeds at 600+mph. (in the Spirit of American land speed record “cars”. they even run on regular shell pump gasoline)

Another urban myth regarding the F-4 Phantom!..I used to be a mechanic for F-4s and they were commonly named flying bricks and the previous explanation was used routinely. Once I got into college and graduated from aerospace engineering I realized the fallacy of that concept. If the lift is practically negligible why are there procedures for engine failure landings and the wings were designed with critical airfoil configuration instead of a basic wedge shape? Primarily because the wings DO create plenty of lift, enough to carry three times the aircraft weight. Additionally, the horizontal stabilizer has a reverse airfoil to generate lift downwards instead of relying on angle of attack which creates a bigger percentage of drag.

If we were to believe urban legends blindly I would be dead by now…I got bitten by a daddy longlegs spider which according to the myth has one of the deadliest venoms, but the mouth is so small that can penetrate human skin…FALSE, false, false!

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As MTB once pointed out, even aircraft with no foil on their wings (or symmetrical foils) can fly, which is a shot in the foot for Bernoulli’s theory.

On the contrary, the bernoulli’s principle doesnt state anything about foils or wings but about changes of pressure and velocity which are directly related to the angle of attack of a surface (fins, wings…etc). The incompressible flow of a flat surface, a wedge surface or a curved can be studied using many mathematical and physical equations including bernoullis.

A direct calculation of lift is quite possible by solving closed-form Bernoulli’s equations. The contribution to lift by the “increases in velocity leads to decreases in pressure” generates only a tiny fraction of the lift experienced by a wing.

Using the change in the momentum of the air as your guideline comes a lot closer. Wings deflect air down so you can go up. And, if you go to school to be a pilot, you would be taught both, and also taught to think primarily in terms of angle of attack and changes in momentum.

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Concaves, to some extent, trap air under the board and decrease friction. Less friction, less of the surface of the board is needed, at an appreciable angle of attack, to produce lift. Less angle of attack and less wetted surface, less drag, too.

Does anyone else agree with this hypothesis? I do not.

This is simple. Water in a wave is moving up the face. Concave creates lift by capturing and redirecting the water toward the tail rather than letting it slip off the rail, like a vee. When in a bottom turn the bottom of the board is presented to the water traviling up the face. Your pressing on the deck and the water is pressing against the bottom. If you sqeezee a bar of soap the bar travels perpendicular to the force. Same thing happens here, and we call it drive. Concave captures more force and pushes this force towards the tail more effectivly than any other bottom shape. Therefore concaves have the best drive and accelleration. The only way performance surfboards could have ended up as small (in volume) as they are is by being very effective at accelleration. Concaves are one of the main aspects that have allowed this progression.

Add a couple Bonzer side fins near the rail to help redirect the bottom spray water and create a little more thrust… : )

Only a (Bonzer) surfer knows the feeling…

blakestah , im not sure if ive interpreted you correctly …

but ill just make some statements , and then see how they sit with you …

if it was as simple as redirecting water , then we could use a thin sheet of something stiff and it would work fine …

the bernouli principle , has a major role to play ,

i read your statement yesterday about negative angles of attack still producing lift , how you questioned that concept …

as it is contrary to what ive found , i thought i would do some simple experiments…

using my air gun , place a flat sided assymetrical foil on the table , blow air over it , it lifts up , you can place your fingers just above it so it presses up against your fingers …

i did the same thing with 3 different side fins , there was one that was an early plactic fcs , it didnt even lift up , not much lift there ,

then i started lifting the trailing edge up by placing , nails , screws or bolts under the trailing edge so there was a reverse angle of attack , the thickest foil was still lifting at the highest reverse angles of attack …

that is a simple experiment that shows just how real the bernouli law is and how crucial pressure difference is to the equation…

if you have no foil , you get no pressure difference , angle of attack then makes the difference in pressure , but because the stall angle will be greatly reduced with no foil , you have a redirection with a component of drag added …

so overall you have half of the power of your redirection converted to drag , which means a board that wont hook as fast into a turn and then loses speed through the turn …

when in reality a turn should increase your speed …

if a fish doesnt pump and just coasts with a straight body itll slow to a stop …

it can only accelate with a pumping motion using pressure differences on either side of its body …

when i was a kid my grandparents had a farm which had a fairly brisk flowing river bordering two sides of the property , i would go and play in the dam all day during school holidays , there was sections with rapids , fast flowing areas still with smoothe flow free of turbulence ,

i would watch fish trying to swim upstream , sometimes you could see a fish moving upstream not even pumping its tail , but just holding its body curved and moving in a zigzag pattern across the river , effectivly swimming upstream without any effort ,

using nothing else but pressure difference …

we can do all the calculations in the world , use fancy programs to work out lift to drag ratios …we can get numbers and say there is your answer , but will they give us an understanding of whats really going on ???

regards

BERT

Bert,

I have some comments with regard to your post, but they are too extensive to post here (for technical reasons, as well as just plain boredom–I suspect–for many readers). If you would like, please PM me, and I can elaborate on them and my comments below via e-mail or PMs.

Some questions to ponder:

1. Why does everyone (implicitly) assume that the flow over the more convex surface of an asymmetrical wing section speeds up? Why can’t the flow just move at the same speed over that surface–but just arrive at the trailing edge later? Consider a river splitting into two smaller rivers. One continues straight ahead; the other meanders around (constant cross section) over 10 times the distance of the first before rejoining with the first branch. Does the flow in this meandering river go 10 times as fast as in the first branch? …somewhat faster? …the same speed? …slower? If it goes faster, why? …and by how much?

2. For Reynolds Numbers within the range of 42,000 to 420,000 (and maybe other values as well…this range is the only data I have available) a thin cambered foil (similar to the cross-section of a Venetian blind leaf) produces greater lift per unit angle of angle-of-attack (relative to the zero lift angle) than typical foiled sections (e.g. NACA). The drag is also less than, or no greater than, that of the foil sections when all are operating at a lift coefficient of unity. Since the flow path lengths are identical adjacent to both the upper and lower surfaces of the foil, are not the flow speeds virtually the same on both sides, and hence the pressure difference near zero? …if not, why not (keeping in mind that the area available for flow on each side of the foil is essentially unbounded)? If they are the same, how is the lift that is observed generated? (Note: this lift follows trivially from Newton’s 2nd and 3rd Laws of Motion, plus the Coanda Effect.)

3. If, as you appear to claim, a foiled fin produces a gain in speed when executing a turn (without an input of energy into the system via the surfer transferring some of his internal energy to the board/surfer system via his muscles, as when pumping/skating/etc.), then it should be possible to tow a surfer and board up to some representative speed (say 17-19 mph) on flat water (i.e. no other source of energy), then have him let go of the tow rope and continue on at the same speed (or even accelerate) by executing a series of turns. Do you believe this?

4. Studies have shown that the fish are moving as you observed by taking advantage of the flows associated with transient and/or quasi-steady eddies present in the stream flow (in fact this energy extraction from the eddies is being considered as a possible replacement for “fish ladders”).

5. In my opinion, your air hose/table/fin experiment(s) are flawed by the presence of the table. This can drastically alter the flow field, and hence the presence/absence/magnitude/direction of lift generated, in a manner that would not occur were that boundary to flow not be present. A probably lesser, but possibly also contributing factor to artifacts in the observations, is the divergence of the flow from the air hose nozzle.

MTB

I didnt have time to read/respond to bernoilli (still recovering from fukcn’ Jeanne) but I do have a comment about concaves.

If you look carefully at a shortboard with a deep concave, the concave rocker is flatter than the rail rocker.

Some/most of the speed benefits may simply be from a flatter center rocker…speed…and curvy rail rocker…control and turnability. But I also feel that flow redirect/concentration is a key factor based on some tests that I’ve run.

Finally, I’ve realized that boards with more rail rocker benefit most from deeper concaves…more rocker, more concave (flatter center), less rocker, less concave needed.

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Some questions to ponder:

1. Why does everyone (implicitly) assume that the flow over the more convex surface of an asymmetrical wing section speeds up? Why can’t the flow just move at the same speed over that surface–but just arrive at the trailing edge later? Consider a river splitting into two smaller rivers. One continues straight ahead; the other meanders around (constant cross section) over 10 times the distance of the first before rejoining with the first branch. Does the flow in this meandering river go 10 times as fast as in the first branch? …somewhat faster? …the same speed? …slower? If it goes faster, why? …and by how much?

Good question. I think the traditional explanation comes from air foil physics where the medium (air) has very little movement compared to the foil through it. Surfboards are more complicated since they are moving in many directions while the medium is moving in many directions sometimes with different densities (ie. foam).

However the simpler environment (air foil physics) is nice for isolating & examining individual phenomena.

Anchor Steam anyone?