Vortex Cell Wing

kiwinjuneer,

I'm not an aerodynamicist, but I've worked with quite a few. And it's been my personal experience that they usually tend to exaggerate the efficiency gains of their aero "tweaks". Either that, or their analytical and wind tunnel work doesn't translate into predicted gains in actual practice.

As an example, Boeing claims that their new 787 will be up to 20 percent more fuel efficient with it's swoopy tail surfaces, wings and fuselage. The reality is that most of their predicted efficiency gains will come from the engines, not from the aero configuration.

I looked at the "Vortex Cell Wing" concept, and to my uneducated, untrained eye, it looks like it will only perform as advertised under very limited conditions.

Just my 2 cents.

What's the theoretical advantage? does the red bit circulate more slowly so it reduces the skin friction?

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

Thanks for your comments. My query arose from a discussion I was having with some colleagues where I asked them why a wing hadn't been developed that worked on the stepped surface hydroplane principle. About a week later, one of them showed me the article referenced including the diagram that tbuelna has posted.

This idea definitely works on boats, including offshore powerboats but then water is rather denser than air. Will be interesting to see how VortexCell the thinktank established to take this further, fares for thick wing aero applications.

Greg, a sportscar is a "thick wing". Now you know why the original Ferrari 246 Dino looks like it does?!

"What's the theoretical advantage? does the red bit circulate more slowly so it reduces the skin friction? "

Greg, note the vector component of skin friction, if the recirculation zone is as indicated, is in the forward, or "thrust", direction.

The drawback is that there is now a recirculation zone which creates turbulent flow downstream from the step, and resultant pressure drag behind the step. This idea has been around for years. Some model airplane enthusiasts were in to the U of Wash. when I was in grad school some (ahem) 20+ years ago, and got time in the wind tunnel to test their "recirc cell" wings. IIRC, the measured benefit, if any, was very small, very difficult to measure, and very dependent on Reynold's number (only works in a fairly narrow range) and free-stream turbulence. One problem they noted is that the span-wise cell seemed to cause stalled airflow to spread more rapidly across the wing, resulting in a sharper stall characteristic. This persisted even when they tested "fences" placed at various intervals across the span.

The analogy to a stepped hull is misleading. A stepped hull works by reducing a hull's surface area exposed to water flow, and instead exposing that area elevated out of the water to lower-drag air flow. The same principle does not apply here.

Thanks btrueblood. I didn't think through the stepped hull analogy too well.

What I find interesting though, is that the EU has committed $2.2M to a 3 year study so it seems that they think there is something in it. Maybe you could pop over to Europe to enlighten them and save a couple of million...

This has been examined and patented by two amateur aerodynamicists:

I have no idea what its advantage is, or if they tested it rigorously.

[]



Steven Fahey, CET

I think this is a way to reduce vortex shedding. It has been used mostly on airfoils that operate at very low reynolds number.

Remember the Aerobee flying ring toy? It had a little lip spoiler that worked this way.

In the 90's some of the 18 foot Skiff sailboats were using square backed wing masts to help reduce vortex shedding near the leading edge. (think this was Frank Bethwaite?)It was said to help prevent stall in light wind.

On a windsurfer sail you can hear this kind of vortex shedding near the leading edge start - the sail humms just before stall. (It is a good way to keep from oversheeting in light wind) Windsurfer sails are very high tension like the head of a drum, so the sound is amplified.

For an airplane maybe the hope is to reduce wetted surface by having wings with a shorter chord for the same span and thickness?

Hey Steven,

Thanks for the very interesting link. I was about to write a note to the effect that the stepped profile may reduce drag but couldn't possibly generate lift because it contravenes the Bernoulli Principle. But then I went and read the link...

As a Aerospace guy, I would be interested to hear what you and your colleagues think about it. Interestingly, I recently had a conversation with the son of a friend who has just graduated with a BE (Mech) and he informed me that the Bernoulli Principle of flight is no longer considered to be correct. Now that I have read your link, I am going to have another chat to him and see they are teaching in uni these days.

I read recently that scientists figured out (finally, to the chagrin of ID proponents) how a bumble bee flies. Bernoulli's principle is not used by bees - I don't remember the technique those little guys use.

Maybe there is more than one theory of flight?

Here's one interesting account I read about why bumble bees can fly.

It is obvious to any scientist that the bumble bee can fly as experiment proves it. So what is this business about proving bees cannot fly? And who started it?

First let's look at the physics behind the story. If you are asked about flight the first thing you do is to use the equations which describe how much lift an object has. You compare the lift to the weight of the object. If the lift is greater than weight then the thing can fly. Bumble bees are pretty big, weighing almost a gram, and have a wing area of about a square centimetre. Tot up all the figures and you find that it cannot generate enough lift at its typical flying speed of about one meter per second. But that doesn't prove bees cannot fly. It proves that bees with smooth, rigid wings cannot glide. Experiment has proven this too. With the aid of dead bees and a little lacquer it is easy to show that they really cannot glide.

So how do they fly? Actually that turns out to be a very interesting question and one that reveals a lot of physics. Why do bees flap while jumbo jets have fixed wings? It is a question of size and this is revealed in a figure called the Reynolds Number. Osborne Reynolds was a Victorian engineer who was interested in what happens when you place an object in a stream of liquid or gas. The number named after him is a ratio which tells us, for a particular object, how much lift you get compared to how much drag or resistance you get. A low Reynolds number means little lift for a lot of drag and a large Reynolds number means a lot of lift.

The Reynolds Number depends on the size of the wing. Bigger wings give bigger Reynolds numbers. Now if, again, you put in all the numbers you find that bees work at very low Reynolds Numbers (1000 or so for a honeybee, as little as 15 for the aphid-eating chalcid wasp). This means that their flight is very inefficient because as a wing starts to move to create lift the drag holds it back. It is fairly straightforward to show that birds can generate enough lift to fly once they are in motion with air flowing smoothly over their wings, but many of them would have great difficulty taking off. Small insects, according to this model, cannot fly at all. Of course, all this proves is that the model is incomplete.

Some brilliant work by Torkel Weis-Fogh has shown us how small insects do fly and it has led to some rather neat insights into nature's cunning. If you are small and want to fly you have a problem. The Reynolds Number is against you so you cannot glide and flapping is very hard work. A wing is a device which encourages the air to flow over it so that when it leaves the rear wing edge, the air moves downwards. That produces a thrust upwards on the wing. A smoke-filled wind tunnel shows this beautifully with curling eddies of smoke flicking off the wing edges. Unfortunately to make a good eddy takes time. The wing has to move a few times its own length to get things started. This makes it tricky if you are going to flap as the maximum travel of a wing is about its own length and very little lift is generated for most of the stroke. Nature has come up with a number of interesting solutions to this problem of which the "clap-fling" is a good example. When a small bird or insect wants to take off it needs a lot of lift. What it does is bring its wings together above its back so they clap, expelling air from between them. As the wings are separated, air is drawn quickly in to fill the void. The wings are flung apart and lift is generated immediately as the air is already in motion in the correct way. You can hear the clap. The characteristic whirring of a pheasant taking off is caused by its wings clapping. Almost 2000 years ago Virgil recorded in The Aeneid that a rock dove claps its wings as it takes off - a passage he stole from Homer but he added the bit about the clapping.

So in asking how bees fly we find that they are remarkably clever about it. Aircraft can generate enough lift that they do not need such tricks, but they do need long runways. Birds get enough lift to fly but for take-off need a boost. Just the poor old bee and about a million different species of winged insect need some extra trickery to stay aloft.

But how did it all start? Where does the story date back to? J.H.Mcmasters states that the story was prevalent in the German technical universities in the 1930's, starting with the students of the aerodynamicist Ludwig Prandtl at Göttingen. The story he tells is that a noted Swiss aerodynamicist, whom he does not name, was talking to a biologist at dinner. The biologist asked about the flight of bees and the Swiss gentleman did a back-of-the-napkin calculation of the kind I described. Assume a rigid, smooth wing and so on. Of course, he found that there was insufficient lift and went away to find out the correct answer.

In the meantime the biologist put the word around, presumably to show that nature was greater than engineering, and the media picked it up. The truth, as now, wasn't newsworthy so a correction has never been publicised. The man on the Clapham omnibus, therefore, continues to tell me that science is a load of crock because it once proved that bumble bees cannot fly.

No claims about an airfoil profile have any value until a L/D curve has been developed.
It is a rather simple series of tests to perform.
Test it before spouting off about the supposed advantages of anything.


Steven Fahey, CET

tbuelna made mention that most of the efficiency gains with the 787 come from the engines.

This being the case, what became of the golf ball surfacing I read about a few years back, that was claimed to reduce high speed drag by 13%? Another unfounded "spouting off" that never came to fruition or killed by economics?

No, that one is real. You can buy these (somewhere), but can't use them in tournament play, and I think most golfers frown on their use as unsportsmanlike.

that seems a bit odd, considering that they use optimised clubs which improve their stroke performance ...
why not optimised balls ?

Apparently there are very strict regulations about how many dimples, and what shape dimples, are to be on a golf ball used in tournament play. The addition of just a few more dimples than the standard has been shown to make a measureable difference in the distance a ball will fly, as does using hexagonal (vs. standard circular) dimples. Also, the coefficient of restitution of the ball must fall within very precisely defined limits; there was a case awhile back of a player who had some of his scores nullified because he used a new "liquid core" ball that bounced too well.

Again, you can find these improved balls available commercially, and whether you tell your boss that you used them to beat him at the game is up to you. And, if you google around, you'll find a number of balls that are advertised as being banned - though this may just be a marketing ploy in some instances.

from this site:


"Millions have since been spent researching the properties of various formations of dimples. Many manufacturers have produced balls of new standards but most have been rejected for tournament usage. A few years ago, the USGA banned the Polara ball, claiming that it undermined the integrity of the game."

and this site lists an article from Golf Digest discussing a proposed USGA limit on club head size:


from the website,

"Today we still hit each brand of ball with the mechanical golfer, but instead of hitting the balls outside onto the range, we hit them into a net and measure their launch conditions off the clubhead (velocity, direction, and spin). We then use our Indoor Test Range (ITR) to precisely determine how each ball flies. The Indoor Test Range is a 70-foot long "tunnel" through which the balls are launched using a golf ball launcher that is similar to a pitching machine. The ITR allows the USGA to accurately measure the aerodynamics of a golf ball in flight. This information is used in a sophisticated computer program to accurately calculate driving distance of an actual drive. This "virtual" distance data is highly repeatable and not subject to weather variations.



Other Ball Tests

Each ball is carefully measured for size and weight. The balls are then tested to determine their initial velocity. All of this is carried out in a climate- controlled laboratory to make certain that all balls are evaluated at the same temperature and humidity.

Balls that pass all of the tests for conformance are listed in the "List of Conforming Golf Balls" that is published at the beginning of each month on the USGA's Web site.



Golf Club Testing

All components of a golf club are subject to evaluation by the USGA to determine a club's conformance. Heads, grips, and shafts all have specific specifications that must be met. Some of these are objective; like the width and depth of grooves. And like the ball, the tools range from simple devices such as a ruler, to complex test instruments such as contour readers for measuring groove sizes, and to a USGA-developed pendulum test to determine the flexibility of the golf club face.

Some of the standards for golf clubs and other golf equipment, such as tees and gloves are less objective and require a detailed examination of their intended use and consideration of past precedent to make a determination of their conformance. Nevertheless, conformance determinations are made on more than two thousand clubs, club components, tees, gloves, etc., each year.



Facts and Figures...


A golf ball remains in contact with the club face for only about 450 microseconds (0.00045 s), much less time than it takes to blink your eye.
During impact the clubhead exerts an average force in excess of 2,000 pounds on the ball, compressing it about one-fourth of its diameter.
All properly struck golf shots are hit with backspin, making the golf ball fly just as the wings provide lift to an airplane.



Research

Equipment conformance testing isn't all that goes on at the USGA Research and Test Center. The Technical Staff constantly monitors the game and how equipment advances are affecting its evolution. This is accomplished by closely studying the performance statistics; conducting scientific studies of professional golfers and recreational golfers; and through detailed research about why and how golf equipment works the way it does.

All of this to protect the world's greatest game."

Thanks for a really interesting post. Unfortunately my post wasn't at all clear because the "golf ball type" surfacing I referred to was applied to and claimed to achieve a 13% drag reduction on a high speed AIRFOIL. I can only assume that as with a golf ball, the dimples helped reduce wake turbulence at very high reynolds numbers.

I also seem to recall Cathay Pacific(?) experimenting with some sort of stick-on drag reducing film but the claim for this was "only" 8%.

The performance of Denis Connor's Stars & Stripes America Cup match racing yacht was claimed to have taken a quantum leap in the finals due to a secret drag reducing film having been applied.

Perhaps this technology didn't make it because of adhesion problems or something even more mundane like cleaning? Appearance might have been an issue to; Acne Airlines doesn't sound quite right.

Maybe you aerospace guys can tell us the real reason why planes are smooth and not pockmarked...

Try this paper for a description of some of the drag-reducing devices discussed here. The bottom line is that drag can be reduced significantly by riblets and laminar flow devices, but the limitations in retention of adhesive riblet sheets and blockage of riblets and laminar flow devices with general dirt limit their effectivity at the moment.

Also look up laminar-flow airfoils (the P51 fighter used one). They work great until the surface is fouled.

Rough surfaces can help at certain limited (not high) Reynold's numbers, typically around 10^4. The idea is to prevent gross seperation behind a body by "tripping" the laminar boundary layer. A turbulent b.l. can stay "attached" (avoid gross seperation) better, limiting the amount of seperated flow and limiting the resultant pressure drag it causes.

A roughened high speed airfoil seeing a 13% reduction in drag? Hmm, maybe, but only a a certain limited condition (e.g. at high angle of attack, or with flaps extended). Doubtful that a 13% reduction would occur at a "cruise" condition or you'd certainly see more press on it.

Topics on this thread are bouncing all over the place.

On the 787 claims for 20% fuel savings:

Boeing Vice President Walter B. Gillette, who heads 7E7 engineering, manufacturing and supplier teaming, counts the fuel savings as 8% from better engines, 3% from aerodynamic improvements, 3% from systems efficiency, 3% from lighter materials and 3% from the combined effects of the first four, for a total of 20%.

Aviation Week December 6, 2004 p.62.

We are 50+ years into the commercial jet age. A 20% gain would be huge for such a mature technology!

On laminar flow, check Honda's claims for their new light jet. They exerted a lot of effort to achieve laminar flow over the nose section of the fuselage (at least in the wind tunnel) for which they are very proud. But did they factor in a skin joint for the radome, pitot tubes, static ports, angle of attack vanes, landing gear doors plus the dirt and bugs common for aircraft in everyday service?