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Powered aircraft must produce forward "thrust" to overcome rearward "drag" and then also be able to accelerate forward. The forward thrust also enables aerodynamic lift to occur, which balances the weight of the aircraft. Much of the drag that exists that is associated with airfoils (wings) is due to turbulence that develops along the top of airfoil and behind the airfoil surface. In many situations regarding Bernoulli Lift, around 6/7 of the total drag is due to this turbulence, with only 1/7 actually being unavoidable.
Until now, very sleek airfoil shapes and relatively narrow wings have been the standard ways of trying to minimize this turbulence effect. However, these sleek shapes also necessarily have less Bernoulli lift effect, for standard physics and aerodynamic reasons.
Another trend in modern aviation is due to the enormous power now available in aircraft engines. It is the reliance on a second manner of lift, "reaction lift". This type of lift is entirely due to the wing surface being tilted, so that oncoming air is deflected down by impact against the bottom of the wing. This is simple Newtonian action-and-reaction. Unfortunately, that type of lift creates even more drag due to massive turbulence behind and above the wing surface. With plenty of power available, that is not a problem, but it causes much more consumption of jet fuel.
Modern airliners use both methods of lift, with the majority of lift due to reaction lift, because that enables aircraft to carry greater payload weight, but at the cost of much less stability. Economics drives the world, and the ease of adding extremely heavy payloads, and therefore greater income, has generally kept up with moderately increasing fuel costs. That situation is changing, now that petroleum costs are greatly increasing, which means that aviation fuel is also much more expensive. Modern airlines rarely seem to make actual profits any more!
Find some book or some wind tunnel movies to carefully look at that turbulence that is above an airfoil. It is very well established that it is NOT random turbulence! (That's IMPORTANT!) It begins near the front above the upper surface of the airfoil, and it has a very periodic (but unpredictable) motion, a pattern, it is NOT random.
All Aerodynamicists use one specific book, "Theory of Wing Sections" (1949, McGraw Hill/Dover), as the central reference book regarding wing designs. (It says something that a 1949 book is still so dominant, but it indicates how little actual advance has occurred, except in supersonic flight, in the last 50 years!) Page 84 discusses laminar flow starting at the front of the wing, at low and moderate lift coefficients, as long as the wing surface is relatively smooth. At a minimum-pressure point, complicated effects occur where the flow generally changes to a turbulent flow. This commonly occurs less than 1/4 along the chord (width) of the wing, so turbulent flow is the predominant situation for most of the area of the wing. This results in a large amount of Aerodynamic Drag, meaning that a lot of extra fuel must get burned to fly.
From the reference system of the wing structure, air in front of the airfoil approaches the front (leading) edge, where the molecules are split apart into two separate flows, one above the airfoil and one below it. The upper path is the one on which we will concentrate here, although this same reasoning could be applied to the under surfaces of the airfoil as well.
The airflow therefore begins as what is called laminar flow, a very smooth and orderly motion of the air past the airfoil. At some distance along the width (chord) of the airfoil, small turbulences start to develop, in what is called a transition zone. Soon after, all laminar flow is gone, and the third stage of the airflow is called turbulent flow
In practical aircraft, laminar flow rarely continues beyond 1/4 of the wing chord, and well before the halfway point, fully turbulent flow exists. We will look carefully at each of the three phases in conventional airfoil design.:
Since the 1930s, we've been told by the popularizers of science that a
technique called "laminar flow control" would enable airplanes to
sip fuel and shrug off drag, slipping through the air like dolphins
in a ship's bow wave. In the realm of large subsonic airliners--the mass
transit of aviation--the attainment of practical laminar flow may well
represent the final breakthrough to which pure aerodynamics can lead us.|
With every experimental demonstration of the concept--and there have been many--we seem to reconfirm that, yup, this stuff works. If anything, the application of laminar flow control seems closer today than ever, which may be why it is the object of extensive NASA study, serious airframe-industry attention, constant university research, and considerable rivalry between the United States and Europe. ... It has been estimated that a 10 percent improvement in airliner performance would increase the winner's market share by $80 billion a year.
"Every molecule of air takes the path of least resistance," explains aerodynamicist John Roncz, designer of wings for such airplanes as the globe-circling Voyager (see "Wing Man," Dec. 1990/Jan. 1991). "Imagine you're an air molecule, sitting there floating along in space, and an airplane comes toward you. Unless you're pushed out of the way by the fuselage, you end up flowing either above or below the wing, and there's a single molecule of difference between those two paths. There's a traffic cop at the leading edge, called the stagnation point, and he decides who goes over the top of the wing and who goes under the bottom." The molecule that takes the low road, under the wing, need only go with the flow. High pressure helps to hold it against the airfoil as it slides aft, just like the air that presses against your palm when you stick your hand out the car window and angle it slightly to make a "wing."
But the little guy that the stagnation cop sends over the top of the wing has a harder job: there's low pressure up there trying to tear the air molecule away from the wing surface and set it to bouncing and burbling. A laminar flow air molecule travels like a surfer sliding smoothly down the crest of a wave, always an instant ahead of disaster. A turbulent molecule is a wiped-out dude plunging toward the beach ass over teakettle.
Ideally, the boundary layer is only an inch thick at most, and the effect dissipates quickly away from the wing's surface. But if you could somehow eliminate most of that nearby layer of air, say by sucking it away with a million tiny vacuum cleaners embedded in the upper wing, you'd eliminate a lot of the wing's turbulent flow. In fact, this is what engineers refer to as "active laminar flow control"--any system that deals with the turbulent layer on the wing's surface.
Turbulence creates drag. It'll also make lift, unless the turbulence gets so extreme the wing stalls. The airflow over an ordinary wing remains laminar for only the first 20 percent or so of chord (the distance from the wing's leading edge to its trailing edge). But the greater the turbulence, the greater the drag. And the greater the drag, the greater the amount of fuel that has to be burned to achieve a given speed. Or, to put it another way, the shorter the distance the airplane can fly on a given amount of fuel. Get rid of that drag and the airplane will fly either farther or faster--or it can be built with a smaller, lighter wing and do both.
Figuring that natural laminar flow produced entirely by the shape and smoothness of the wing was a hopeless phantasm, the British initiated the first active laminar flow control project. In 1955 three de Havilland Vampire jet fighters were fitted with several kinds of porous wing surfaces through which the turbulent air closest to the wing was vacuumed away to create laminar flow. NASA was at the same time trying a similar strategy on a Lockheed F-94 interceptor, another straight-wing jet.
Both tests worked, after a fashion, but the airplanes were encumbered with complex extra systems, requiring considerable power to run the suction pumps. The tiny holes in the Vampires' wings--which actually weren't all that tiny--set up their own airflow disturbances and weakened the wing skin enough to cause it to deform in flight.
In 1966, Northrop and the U.S. Air Force ... built two of the largest X-planes ever flown: the X-21As. The experimental twin-jets started life as weather-reconaissance Douglas WB-66 jets, electronic warfare versions of which saw service over Vietnam. Under a distinctive humped back, each X-21 sported a swept laminar flow control wing lined with thousands of spanwise razor-thin slits that were in turn perforated with over 815,000 minuscule holes, each of which sucked away turbulent air into a vast internal network of nearly 68,000 ducts, all leading to a pair of high-pressure pumps under the wings. The B-66's main engines were moved from their under-wing pylons to aft shoulder mounts like those on a typical business jet.
The X-21s were meant to prove not only that active laminar flow was achievable but that such a system could be manufactured, maintained, and operated in an everyday environment. "The X-21As proved conclusively that...[laminar flow control] is both effective and viable," experimental-aircraft authority Jay Miller writes in his book The X-Planes. "However, they also demonstrated that LFC incurred certain maintenance penalties that were not easily overcome...[and] that production technology for manufacturing LFC surfaces and related components was...prohibitively expensive for all but experimental aircraft."
The X-21A program had demonstrated that active laminar flow could be achieved using a hand-built wing that required constant maintenance--much of it devoted to keeping the pinholes from clogging with dust, dirt, and bugs--and enough power on board to run the hungry pumps. Active laminar flow control seemed to be a laboratory oddity with no hope of practical application. Unfortunately, that may be nearly as true today as it was in 1966.
The size and shape of the pinholes--and tuning the exact amount of suction applied through them--are the keys to the success or failure of any active laminar flow control system. In the 1940s and '50s, the trick was drilling the holes small enough or finding a porous wing material strong enough. In the '60s and '70s, the holes got smaller and more precise, but the problem became one of keeping them from clogging with dust and dirt.
Originally published in Air & Space/Smithsonian Magazine, JUN/JUL 1995. Copyright 1995, Smithsonian Institution. All rights reserved.
In considering the new deck of cards example, one can see why laminar flow allows extremely easy and free passage of the fluid (air) to go past with extremely low friction (drag). A chart on page 100 of the book referenced above shows that the Skin Friction Coefficient for Laminar Flow, at speeds that conventional airliners fly at, is around 1/7 as great as the same coefficient for Turbulent Flow.
There are some characteristics of air which affect just how smoothly
the air can go by an airfoil, particularly the density of the air
and the dynamic viscosity of the air. The (relative) velocity of the air is
also very important, as is the length of time (distance) that the
air is moving along that boundary layer. There is a defined number,
called the Reynold's number which is generally used in calculations.
If we consider an airliner wing at 30,000 feet altitude, the temperature
is very cold and the air pressure is rather low, and the density and
dynamic viscosity are both well known, the Reynold's number is given
Re = 4100 * V(mph) * L(feet).
For an airliner flying at 500 mph, by the time that the air has gone (L = 1) one foot along the airfoil surface, the Reynold's number is already 4100 * 500 * 1 or around 2,000,000. A key usefulness of the Reynold's number is in determining whether laminar or turbulent flow exists. A common guideline is that if the Reynold's number is above about 500,000, laminar flow ceases. Therefore, we would be considering a situation where laminar flow had already disappeared and fully turbulent flow was acting. This situation would then apply for the remaining 12 feet of wing width, where fully turbulent flow exists.
Lower on that same page, there is a discussion regarding the amplification factor of disturbances (turbulences) of various frequencies. References to assorted theoretical and experimental research efforts regarding various frequencies are cited there. The specific point here is that there ARE distinct frequencies involved. An extensive section, from page 143 through 182, discusses "Effect of Surface Condition on Lift Characteristics". This section primarily identifies three primary sources of disturbances: (a) surface irregularities; (b) surface waviness; and (c) engine or aircraft vibration.
Regarding surface irregularities, page 157 mentions that dust particles on the wing surface, near the leading edge, tend to cause the transition from laminar to turbulent flow to occur. The frequency of such disturbances tend to depend on the relative airspeed and certain dimensions of the object and the wing. A reference is made that dust particles which adhere to the oil from human fingerprints may be expected to cause transition to turbulent flow! Extensive discussion is about polishing and waxing the wing surfaces and various types of paint finishes, to minimize aerodynamic drag. However, the general conclusion reached is that wings that are in actual situations will have dust, ice, insects, and possibly battle damage, and that attempts to provide super smooth surfaces might not be practical, because of the amazing sensitivity regarding really tiny particles initiating the onset of turbulence.
Regarding surface waviness, page 164 mentions that standard construction techniques cause more difficulty in limiting chord-wise surface waviness than in maintaining the required surface smoothness. The specific point here is that surface waviness always has a characteristic length, and between that length and the airspeed, a specific frequency of created turbulence would develop from such surface defects.
Page 173 begins the discussion of airframe and motor vibrations. Again, the only important concern here is that such vibrations have natural frequencies, which depend on structural characteristics of the airframe and the engines.
In either my approach or the MEMS approach, the net effect will be as though there had been essentially no incipient turbulence in the first place! This then has the effect of shifting the transition from laminar to turbulent flow rearward along the airfoil surface. In my system, repeated identical modules behind them sense any newer incipient turbulence and repeatedly cancel it out. If the entire airfoil surface is covered with such modules, then ANY turbulences, from any source causes, would always be promptly cancelled out. The result would be a nearly laminar flow along the entire chord of the airfoil. The effect cannot be perfect, because additional small turbulences constantly keep beginning during processing, so pure laminar flow would probably not occur. However, those new small turbulences would then be cancelled out by the next module rearward. Large-scale turbulence is therefore impossible with this system.
In July 1998, I built some fluid-flow test apparatus and I confirmed that virtually all turbulent flow was eliminated and nearly all laminar flow was present.
Considering that a modern wing has so much turbulence as to cause around seven times the amount of aerodynamic drag that a purely laminar flow would cause, this improvement could theoretically reduce aerodynamic drag by 86%, down to 1/7 of what is now considered unavoidable. That situation would result in a fuel consumption of around 1/7 as much as occurs today. That theoretical possibility is unrealistic, I think, but if this TURCAN (TURbulence CANcellation) system can reduce fuel consumption by even half, 50%, enormous economic benefits would result.
As I am a Theoretical Physicist and not in the Aeronautical industry, I do not happen to have access to a wind tunnel to test any prototypes! Therefore, my progress regarding developing this invention is somewhat stalled!
In early 2003, after initiating US Patent procedures, some contacts were made with individuals at Boeing, McDonald-Douglass and other aircraft designers (who each have their own wind tunnels!) In each case, I was told that they employed the world's most brilliant Engineers and Designers and if any idea was worthwhile, THEY would have thought of it! Interesting! (They haven't!) I have some really sad (and funny) e-mails from several of those "hot-shot" Engineers who, without actually understanding the concept at all, felt the need to try to humiliate me in an assortment of ways. Fortunately, having gotten my College Degree from the University of Chicago in Nuclear Physics, I am not usually intimidated by the pseudo-intellect of people who believe they know it all! Particularly when they make it clear that they didn't even examine what it is that they are criticizing!
After assorted frustrating communications like that, it seemed clear that I was expected to "jump through hoops" for them, to somehow "earn" their valuable time. That's not going to happen! In Physics, a hypothesis must be very well researched in order to have any potential merit; I believe I have already done that with this Turcan concept. The kindest response I received was from a lower-ranked Engineer who informed me that I should develop the technology, build a substantial-sized prototype and wind tunnel test it, and THEN they MIGHT consider looking at it! So, apparently, rather than even bothering to read the descriptions that I sent to them, they expected me to personally spend maybe $1,000,000 or more first? Well, that's not going to happen either!
It seems to me that they should have been VERY interested in "wasting" at least a few minutes with me! The US Government is currently (2003) financing some extensive research which HOPES to reduce aerodynamic drag by 3% but actually expects the benefit to be around 1.5% improvement. That research is taking many years and many millions of dollars. I am proposing a far simpler and more reliable system which, if it only performs TWENTY TIMES that well (30% reduction of aerodynamic drag), I will consider it somewhat of a disappointment! And the technology is already well developed and seems capable of being incorporated into both existing and new aircraft within a year.
Wouldn't it seem worth wasting a few minutes of someone's time to patiently listen about such a possibility? After all, if an aircraft manufacturer was able to offer a design that had such tremendous improvements regarding fuel consumption (the largest individual expense of all airlines), wouldn't it seem that that manufacturer would then sell new aircraft to replace nearly every airliner in the world? But, I guess, as a Physicist, I couldn't know such things!
No, I will NOT fill out endless paperwork in order to qualify to apply for permission to present such an invention, either to an aircraft manufacturer or government agencies. So, it may be a long time that airlines each spend millions of extra dollars every day for Jet fuel. If some airline executive some day sees this and sees value in potentially saving a few billion dollars every year for his company, maybe some progress can happen. Otherwise, I have tried my best to enable that savings to happen. I have also tried my best to enable American aircraft manufacturers to sell a LOT of new aircraft. I find all this both peculiar and interesting!
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago