by An Aeronautical Engineer (with Jim Fetzer)
“This article explains why high-speed flight in relatively close proximity to the ground [as for Flight 77] is virtually impossible – in any fixed-wing aircraft”–An Aeronautical Engineer
Among the most important proofs that no Boeing 757 hit the Pentagon is that the official trajectory — barely skimming the ground at over 500 mph and taking out a series of lampposts — is both aerodynamically and physically impossible.
I have argued in the past that it is aerodynamically impossible because ground effect would not allow a 757 flying at that speed to get closer than 60 or even 80 feet to the ground, where 80 feet is higher than the Pentagon at 71 feet is tall.
And it is physically impossible because the effects of a plane flying over 500 mph hitting a series of stationary lampposts would be the same if the plane had been stationary and hit by lampposts flying at over 500 mph: they would have ripped the wings open, where the jet fuel stored in them would have burst into flames and the plane could have cartwheeled across the lawn.
While I regarded my arguments as impeccable, I was faulted on the ground that “ground effect” would not make it impossible for a Boeing 757 to barely skim the ground, which led me to look further into the matter and discover that I had the right concept but the wrong name, where it is better described as “downwash”, as the technical study that follows explains in detail.
That was a benefit from participating in the thread on Truth and Shadows, “Gage concedes his entry into 9/11 Pentagon quagmire has been divisive”. But some of those who were participating displayed scant familiarity with the evidence, which Dennis Cimino and I have recently surveyed in our study, “Reflections on the Pentagon: A 9/11 Photographic Review“.
It also became clear that some students of 9/11 do not know the basics of “No Plane Theory” (or NPT, as it is sometimes called), which maintains that none of the official 9/11 aircraft–for Flights 11, 77, 93 and 175–actually crashed on 9/11. It does not mean that no planes at all were involved on 9/11, where the trucker buddy of a friend of mine from JFK research had told him that he had been in front of the Pentagon and watched as a commercial carrier flew toward it and then swerved over it, as I have explained in many places.
Indeed, the work of CIT and others supports the inference that a plane had flown toward the Pentagon, but that its trajectory took it north of the nearby Citgo station, which became a bone of contention; because it the plane was north of the Citgo station rather than south, it cannot have been Flight 77. The presence of that plane, which CIT has documented, therefore, does not defeat NPT and the following study strengthens it. But there should be no more doubt that the official story of a Boeing 757 hitting the Pentagon is not even possible.
GROUND EFFECT vs INDUCED DRAG
(Or, the difference between Downwash and Wake Turbulence)
by An Aeronautical Engineer
PREFACE
Many online debates about the “Pentagon 757” continue to confuse an aerodynamic phenomenon known as Wake Turbulence (caused mainly by Induced Drag) with Ground Effect (caused as a reaction to downwash).
In a nutshell:Ground Effect is a reaction to the downwash sheet produced by the wing; it is a function of Lift.Wake Turbulence (i.e., wingtip vortex) is produced by wingtip ‘spillage’; it is a function of Induced Drag.
Numerous illustrations of ‘tip vortices’ have been circulated online for years with intent to “explain” ground effect. Such arguments are based on a misunderstanding of some of the fundamental principles of aerodynamics (I.e., the pure scienceof aerodynamics practiced by degreed engineers, as opposed to the few hours of “Theory of Flight” taught to pilots at flight school).
In discussions on flight in close proximity to the ground, proponents of this argument confuse the effects of tip vortices with those of ground effect. They argue that since tip vortex energy rapidly dissipates during flare, so would “ground effect” be mitigated, thereby permitting an aircraft to be flown at high speed, in straight-and-level flight close to the ground. This assumption is false.
[NB: Ground effect, too, obviously, is greatly diminished (in fact, reduced to zero) during the landing flare, which is performed at or near stall speed. Indeed, it is this reduction in ground effect that allows an aircraft to “land”! This discussion, however, concerns the forces produced by ground effect while the craft is in high-speed flight near to the ground.]
This article explains why high-speed flight in relatively close proximity to the ground is virtually impossible – in any fixed-wing aircraft*.
[* An exception would be cruise-type missiles, whose flying surfaces are more akin to horizontal fins than wings, and thus have extremely high wing loadings (lbs/sf) — much higher even than of ground-attack fighter-bombers. NB: Commonly invoked photos of aircraft in flight close to the ground do not depict flight at anywhere near maximum speed; they are photos taken of craft flying at greatly reduced throttle settings – which is what actually happens during any landing!]
This discussion will delineate why the highly energized ground effect region beneath the wings, whose intensity is directly proportional to the lift generated by the aircraft, would prevent the craft from getting any closer to the ground than approximately one half its wingspan while flying at high speed.
In the case of a Boeing 757, given its wingspan of 124 feet, this would equate to roughly 62 feet AGL.
[NB: Sixty-two feet is a theoretical minimum; a practical minimum would actually be considerably higher. In a real-world situation (such as allegedly at the Pentagon), a pilot — especially one as demonstrably inept as Hani Hanjour who could barely fly a trainer — probably could not have got within 100 feet of the ground in a Boeing 757 flying at 400 mph.]
THE FUNDAMENTALS
In order to gain a clear understanding of what constitutes Ground Effect, one must first gain a grasp of how Lift is generated. Only then can one understand the dynamics of Downwash, the main contributor to ground effect, and how this differs from the factors that contribute to wingtip vortices.
In order to properly understand Lift, one must first let go of the common Bernoullian theory we’re all taught in school (and flight school), and approach it from a Newtonian viewpoint. As I shall show, the Bernoullian explanation — embraced by students and most pilots alike — is an illustration of convenience that is fundamentally flawed.
While Circulation Theory constitutes the most technically accurate method of computing Lift, the Newtonian explanation can also be employed to graphically describe Lift without introducing the misconceptions and errors inherent in the Bernoullian version.
In order to really understand Lift, one needs to abandon explanations based on Bernoulli’s theorem, and clearly comprehend the following:
An aircraft’s wing, basically, is an AIR DEFLECTOR. As an aircraft moves forward, the wing essentially deflects the resultant airstream downward. This downward deflection of air causes an equal and opposite reaction upward. This upward reaction (equal to the weight of the aircraft when in level flight) is what is termed ‘Lift.’
This downward deflection of air, which occurs along the entire span of the wing, is also what constitutes downwash. This downwash ‘sheet’ has a vertical component that is normal (I.e., at a right angles) to the direction of flight.
When an aircraft flies in close proximity to the ground, it is this vertical component of Downwash that causes the ‘cushion’ of air between the wing and the ground.
This “cushion,” when sufficiently energized (by an aircraft’s high speed), behaves much like a pneumatic “spring,” and tends to resist any action against it — such as that caused by a pilot trying to force the aircraft down against it.
This is what is known as Ground Effect. To summarize:
Ground Effect:
- Is directly proportional to the speed of the aircraft (i.e., faster the speed, stronger the ground effect)
- Becomes discernible when an aircraft descends to within approximately ONE wingspan of the ground
- Becomes progressively more pronounced as the craft descends further
- Reaches its maximum value when the aircraft is at a height of approximately one HALF wingspan, at which point it begins to demonstrate characteristics of incompressibility (i.e., offers resistance, or “pushes back”)
This is a Law. This Law applies to ALL fixed wing aircraft.
The faster the airplane flies, the stronger the downwash, more pronounced the ground effect. When speed is reduced, downwash is reduced, ground effect is diminished, and the aircraft is able to get closer to the ground. This is what happens during landing.
WINGTIP VORTICES/WAKE TURBULENCE
Wingtip vortices, on the other hand, are the principal source of ‘induced drag’, and the main contributors to wake turbulence.
These tip vortices are a result of the ‘spillage’ of high-pressure air from beneath the wing to the low-pressure region above it. Obviously, this natural tendency for pressures to equalize can occur only at the tips (it would be impossible elsewhere along the wing since it would be prevented by the intervening wing structure). When this tendency to equalize pressures is satisfied at the tips, it results in powerful counter-rotating conical vortices that are carried rearward by the slipstream.
[NB: These wingtip vortices, the biggest contributor to what’s loosely referred to as “wake turbulence”, can, when generated by a large aircraft, present a serious hazard to light aircraft flying in its wake. The effects of wake turbulence are also notoriously problematic at airports with parallel runways, because they tend to traverse the ground outward when aircraft approach the ground and flare to land. This is the reason parallel runways are designed with ample separation between them.It also explains why air traffic controllers ensure sufficient separation between aircraft queued to land — especially at airports with minimal runway separation and frequented by “heavies.” The energy inherent in these vortices is directly proportional to the weight of the aircraft, its speed, and the wing’s angle-of-attack. This ‘wake turbulence’ decays relatively slowly at altitude, where their energy is greatest due to high cruising speed. This is the reason light aircraft pilots go to great lengths to avoid crossing wakes created by heavies.]
To gain a clearer understanding of the distinction between tip vortices and downwash, let’s do a quick thought experiment. Let’s imagine an aircraft with inordinately large wingtip “fences,” i.e., vertical plates at the tips. This would, in theory, reduce any pressure-induced spillage at the tips to zero, and thereby preclude the formation of vortices.
But this would have virtually no effect on the spanwise downwash sheet!
As will be explained further below, a wing’s downwash is what reactively produces Lift. Wingtip vortices, on the other hand, are the primary contributors to induced drag. The fancy Winglets evident on many modern airliners and executive jets represent a design effort to minimize spillage at the tips, thereby reduce induced drag, and as a consequence, improve operational efficiencies (E.g., fuel savings).
To better understand downwash, here’s another interesting thought experiment: If one were to place a very long weighing scale — as long as, say, the wingspan of a Boeing 757 — on the ground, and then fly a 757 just a few feet above it, the scale would momentarily register the total weight of the aircraft! It is the force of the downwash that causes this reading. In the case of a Boeing 757, the scale would fleetingly read over 200,000 lbs — the actual weight of the air deflected downwards (which is equal to the weight of the aircraft).
During straight-and-level flight, the force of the vertical component of the downwash will always be equal to the gross weight of the aircraft (according to Newton’s Third Law, as will be discussed later).
The Three Descriptions of Lift
Let’s begin by defining three descriptions of lift, only one of which is commonly used in flight schools and pilot training manuals.
1. Circulation Theory
Generally called the ‘Mathematical Aerodynamic Description of lift,’ it is used principally by aerodynamicists. This description uses complex mathematics and/or computer (CFD) simulations to calculate the lift of a wing. It often uses a mathematical concept called “Circulation“ to calculate the acceleration of the air over the wing. Circulation is a measure of the apparent rotation of the air around the wing. While useful for calculations of lift, this description is rather abstruse, and does not lend itself to an intuitive understanding of flight, which is why it is rarely used (or known) outside the realm of aerodynamicists.
2. Newtonian Theory
This description (also far more accurate than the Bernoullian explanation, as shall be shown) is presented here in order to provide a clearer understanding of downwash (and therefore, ground effect). We shall call this the Newtonian Description of lift. This description of lift is based primarily on Newtonian laws and a phenomenon called the Coanda Effect. It is uniquely useful for understanding the phenomena associated with flight, and useful for an accurate understanding of the relationships in flight, such as how power increases with load, or how stall speed increases with altitude. It is also a useful tool for making rough estimates (“napkin calculations”) of lift. The Newtonian Description of lift is also useful to pilots who desire to gain an intuitive understanding of how to fly the airplane.
3. Bernoullian Theory
This we shall call the Popular Description, which is based on Bernoulli’s theorem. This is the explanation we’ve all been taught in High School. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift even in flight schools and flight training manuals.Its major disadvantage is that it relies on the “principle of equal transit times”, or at least on the assumption that because the air must travel farther over the top of the wing it must go faster. This description focuses on the shape of the wing, and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle-of-attack of the wing.In the following discussion, in order to keep things as simple as possible, we shall discuss only the latter two concepts: the Newtonian Description, and the Popular Description.
BERNOULLI – The Popular Description of Lift
Students are usually taught that an airplane flies as a result of the Bernoulli principle, which says that if air speeds up, the pressure is decreased. (In fact this is not always true. For instance, the air flows fast over the airplane’s static port—a little orifice on the side of the fuselage—but it still measures the correct static pressure that the altimeter, and other instruments, use as their reference.) This theory attempts to show that a wing generates lift because the air flows faster over the top, thereby creating a region of low pressure. This explanation usually satisfies the curious, and few challenge its conclusions.
Some may wonder why the air goes faster over the top of the wing — and this is where the ‘popular description’ begins to fall apart.
In order to explain why air travels faster over the top of the wing, many have resorted to the geometric argument that the distance the air must travel is directly related to its speed. The usual claim is that when the air separates at the leading edge, the part that goes over the top must converge at the trailing edge with the part that goes under the bottom. This is the so-called “principle of equal transit times”.
It would be reasonable to ask if the numbers calculated by the Popular Description really work. To that end, let’s look at a relatively simple example. Take the case of a Cessna 172, a popular, high-winged, four-seat airplane (and the one Hani Hanjour could barely fly). Its wings must lift 2,300 lb at its maximum flying weight (i.e., its Gross Take-Off Weight, or GTOW). Given the particular airfoil shape of the Cessna’s wing, the path for the air over the top of the wing is only about 1.5% greater than under the wing. Using the Popular Description of lift, the wing would develop only about 2% of the needed lift at 65 mph, which is about the slowest possible flying speed (pre-stall) for this airplane.
In fact, calculations based on the Popular Description would suggest that the minimum speed for this wing to develop 2,300 lbs of lift is over 400 mph — which, of course, is absurd.
If one reverses the problem and asks what the difference in the path of the air would have to be for the Popular Description to account for 2,300 lbs of lift at that same airspeed (65 MPH), the answer would be 50%. I.e., The thickness of the wing would be almost the same as the chord length! [Chord is the straight-line distance from a wing’s leading edge to its trailing edge.] In other words, the profile of the wing would look much like that of a whale, with a huge bulbous upper surface!
But here’s the rub: Who says the air that separates at the leading edge of the wing must meet at the trailing edge at the same time?
This is key.
If you conducted an experiment in a simple wind tunnel using smoke streams, you would see that the air that goes over the top of the wing gets to the trailing edge considerably before the air that goes under the wing. In fact, the air is accelerated much faster than would be predicted by the “principle of equal transit times”. Also, on close inspection one would see that the air going under the wing is slowed down from the “free-stream” velocity of the air.
The principle of equal transit times holds only for a wing with zero lift.
The Popular Description also implies that inverted flight is impossible (the air would then have to travel a greater distance along the underside). It certainly does not address aerobatic airplanes, which have symmetrical wings (the top and bottom surfaces have identical camber, I.e., the same ‘shape’), or how a wing adjusts for the great changes in load such as when pulling out of a dive or in a steep turn.
So, why has the Popular Description prevailed for so long? One answer is that the Bernoulli principle is easy to understand (remember the classroom example of the bottle of perfume with the squeeze bulb?). There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, as the above discussion elucidates, our understanding is not complete with this explanation. The problem is that we are missing a vital piece when we apply Bernoulli’s principle.
We can calculate the pressures around the wing if we know the speed of the air over and under the wing, but how do we determine the speed? As we will soon see, the air accelerates over the wing because the pressure is lower, not the other way around!
Another fundamental shortcoming of the Popular Description is that it ignores the ‘work’ that is done. Lift requires power (I.e., work/time). An understanding of power is key to the understanding of many of the interesting phenomena of lift. It would behoove readers who are pilots to pay particular attention.
A Napkin Calculation
Let us do a napkin calculation to see how much air a wing might divert. Again, let’s take as an example the Cessna 172, a trainer with which this writer is familiar.
A Cessna 172 weighs about 2,300 lbs. Traveling at a speed of 140 mph, and assuming an effective angle of attack of 5 degrees, we get a vertical velocity for the air of about 11.5 mph right at the wing. If we assume that the average vertical velocity of the air diverted is half that value, we calculate from Newton’s second law that the amount of air diverted is on the order of 5 tons.Thus, a Cessna 172 at cruise is diverting about five times its own weight in air per second to produce lift.Now, imagine how much air is diverted by a 250-ton Boeing 747 on takeoff!
Diverting so much air downwards is a strong argument against lift being just a surface effect as described through the common Bernoulli effect; in reality, only a small amount of air traveling around the wing actually accounts for the lift. In fact, if we were to attribute sustainable lift to the Bernoulli principle, in order to divert 5 tons/sec the wing of the Cessna 172must accelerate all of the air within 18 feet above the wing! One should remember the density of air at sea level is about 2 lb per cubic yard.
So how does a thin wing divert so much air? When the air is ‘bent’ around the top of the wing, it pulls on the air above it and accelerates that air downward. Otherwise there would be voids in the air above the wing. Air is pulled from above. This pulling causes the pressure to become lower above the wing. It is the acceleration of the air above the wing in thedownward direction that produces lift. As to why the wing bends the air with enough force to generate lift will be discussed in the following section, “Coanda Effect.”
Also, as the wing moves along, air is not only diverted down at the rear of the wing, but air is pulled upwards at the leading edge. This “upwash” actually contributes to negative lift, and more air must be diverted down to compensate for it. This is a rather complex area, which falls under the above-mentioned “Circulation Theory,” and in the interests of simplicity would be best left out of this discussion. Let it suffice to say that ahead of the leading edge, air is moving upwards (upwash), and at the trailing edge, air is diverted downwards (downwash). Over the top the air is accelerated towards the trailing edge. Underneath, the air is accelerated slightly forward. This should give the reader a sense of why aerodynamicists refer to this phenomenon as “circulation.”
So, why does the air follow this pattern? First, we have to bear in mind that air is considered an incompressible fluid for low-speed flight. That means that it cannot change its volume and that there is a resistance to the formation of voids. Now the air has been accelerated over the top of the wing by of the reduction in pressure. This draws air from in front of the wing and expels if back and down behind the wing. This air must be compensated for, so the air shifts around the wing to ‘fill in.’ If one is able to determine the “circulation” around a wing, the lift of the wing can be calculated. Lift and circulation are proportional to each other, and are calculated mathematically.
It is a fact the top surface of the wing does much more to move the air than the bottom. So, the top is the more critical surface. Thus, airplanes can carry external stores, such as drop tanks, ordnance, etc., under the wings but not on top where they would normally interfere with lift. That is also why wing struts under the wing are common but struts on the top of the wing have been historically rare. A strut, or any obstruction, on the top of the wing would usually interfere with its lift. [NB: There some aircraft, such as the new HondaJet, that have their engines mounted on top of the wings, but these are exceptions with special design considerations.]
COANDA EFFECT
A natural question is, “how does the wing divert the air downwards?”
When a moving fluid, such as air or water, comes into contact with a curved surface it will try to follow that surface.
To demonstrate this effect, hold a water glass horizontally under a faucet such that a small stream of water just touches the side of the glass. Instead of flowing straight down, the presence of the glass causes the water to wrap around the glass. This tendency of fluids to follow a curved surface is known as the Coanda Effect. From Newton’s first law we know that for the fluid to bend there must be a force acting upon it. From Newton’s third law we know that the fluid must exert an equal and opposite force on the glass.
So why should a fluid follow a curved surface? The answer is viscosity, the resistance to flow, which also gives air (which is a fluid) a kind of “stickiness”. Viscosity in air is very small but it is enough for the air molecules to want to stick to the surface. At the surface the relative velocity between the surface and the nearest air molecules is exactly zero. (That is why one cannot hose the dust off of a car.) Just above the surface the fluid has some small velocity component. The farther one goes from the surface the faster the fluid is moving until the external full stream velocity is reached. Because the fluid near the surface has a change in velocity, the fluid flow is bent towards the surface by shear forces. Unless the bend is too tight, the fluid will follow the surface. This volume of air around the wing that appears to be partially ‘stuck’ to the wing is called the “boundary layer” and is less than one inch thick, even for a large wing.
The magnitude of the forces on the air (and on the wing) is proportional to the “tightness” of the bend (called “camber” in an airfoil). The tighter the air bends, the greater the force on it. This is why most of the lift is concentrated at the forward portion of the wing, at its region of maximum curvature. In fact, half of the total lift on a wing is typically produced in the first 1/4 of the chord length.
Lift (and therefore Downwash and Ground Effect) as a function of angle of attack
There are many types of wing: conventional, symmetric, conventional in inverted flight, early biplane wings that looked like warped boards, and even the proverbial “barn door”. In all cases, the wing is forcing the air downwards, or more accurately, pulling air down from above (though the early wings did have a significant contribution from the bottom). What each of these wings has in common is an angle of attack with respect to the oncoming air.
Angle-of-attack is the primary parameter in determining lift.
To better understand the role of the angle-of-attack (AOA) it is useful to introduce an “effective” AOA, defined such that the angle of the wing to the oncoming air that gives zero lift is defined to be zero degrees. If one then changes the AOA both up (positive) and down (negative), one finds that the lift is proportional to the angle. A similar lift versus AOA relationship is found for all wings, independent of their design. This is true for the wing of a 757, an inverted wing, or your hand out the car window. The inverted wing can be explained by its AOA, despite the apparent contradiction with the popular Bernoullian explanation of lift!
A pilot adjusts the AOA to adjust the lift for the speed and load.The role of the AOA is more important than the details of the wing’s shape in understanding lift.The shape comes into play in the understanding of stall characteristics and drag at high speed.
Typically, the lift begins to decrease at a “critical” AOA of about 15 degrees. The forces necessary to bend the air to such a steep angle are greater than the viscosity of the air will support, and the air begins to separate from the wing.
This separation of the airflow from the top of the wing is called a “stall”.
NEWTONIAN (“PHYSICAL”) DESCRIPTION OF LIFT
So, how, then, does a wing generate lift? To begin to understand lift we must review Newton’s first and third laws. (Newton’s second law will be introduced a little later.) Newton’s first law states a body at rest will remain at rest, or a body in motion will continue in straight-line motion unless subjected to an external applied force. That means, if one sees a bend in the flow of air, or if air originally at rest is accelerated into motion, a force is acting upon it. Newton’s third law states that for every action there is an equal and opposite reaction. As an example, an object sitting on a table exerts a force on the table (its weight) and the table exerts an equal and opposite force on the object to hold it up.
Similarly, in order to generate lift a wing must do something to the air.
What the wing does to the air is the action, while lift is the reaction.
In the “classic” diagram that everyone is familiar with, the air comes straight at the wing, bends around its shape and then leaves in a straight ‘slipstream’ behind the wing. We see these kinds of pictures even in flight manuals. The air leaves the wing exactly as it appeared ahead of the wing. In such a situation, there is no net action on the air. Therefore, there can be no lift!
In reality, the air passes over the wing and is deflected downwards. Newton’s first law says there must be a force on the air to bend it downwards (the action). Newton’s third law says that there must be an equal and opposite (upward) force on the wing (the reaction).
In order to generate lift, a wing must divert a large mass of air downwards.
The lift that a wing generates is equal to the change in momentum of the air it diverts downwards. Momentum is the product of mass and velocity (mv). The most common form of Newton’s second law is F= ma, or force equal mass times acceleration. The law in this form gives the force necessary to accelerate an object of a certain mass. An alternate form of Newton’s second law can be written thus:
The lift a wing produces is directly proportional to the amount of air diverted downward, times the vertical velocity of that air.
It’s that simple.
For more lift the wing can either divert more air (mass), or increase the vertical velocity of a given mass of air. This vertical velocity behind the wing is the vertical component of the “downwash”.
The greater the AOA of the wing, the greater the vertical velocity of the air. Likewise, for a given AOA, the greater the speedof the wing the greater the vertical velocity of the air. Both the increase in the speed and the increase of the AOA increase the vertical velocity of the deflected air. It is this vertical velocity imparted to the air mass that gives the wing lift.
When the aircraft is near to the ground, it is this deflected sheet of air that is the principal cause of ground effect.Wingtip vortices (induced drag) and their own energy systems do not directly enter into this equation.
If one could actually “see” air, an observer on the ground would see the air traveling almost straight down behind the aircraft! This can be demonstrated by observing the tight column of air behind a propeller, a household fan, or under the rotors of a helicopter; all of which are “rotating wings.” If the air were coming off the blades at an angle the air would produce an extremely wide cone rather than a tight column!
The wing develops lift by transferring momentum to the air. For straight and level flight very near to the ground (E.g., the alleged Boeing 757 at the Pentagon) this momentum eventually strikes the earth. That is why, as mentioned earlier, if an airplane were to fly over a very large weighing scale, the scale would momentarily “weigh” the airplane.
And this is why when you stand under a ceiling fan, the air blows DOWN on you — even though its blades are almost horizontal (small angle of attack), the generated airflow certainly does not scatter radially sideways. It is this same “downwash” that exists beneath Air Cushion Vehicles (ACVs, or hovercraft, as they’re commonly called). This highly energized cushion of air is what keeps a hovercraft aloft.
Consider the following little thought experiment: Imagine a small radio-controlled model hovercraft. Let’s imagine you have the craft hovering right in front of you, a few inches above the ground. Now, imagine placing your hand atop the craft, attempting it to push it down, against the cushion of air beneath it. What would happen? The craft would react by resistingthe applied force, pushing back against your hand — behaving much like a compressed air “spring.”
If you continued to push it even further, the resisting force would increase to where, eventually, the craft would begin to struggle, all on its own, fighting to revert to its original position. Continue to push downwards, and the craft will actually thwart your efforts and escape the constraints of your hand by slipping sideways and away from your hands!
This is precisely what happens when a pilot attempts to ‘push’ an airplane too close to the ground while flying at high speed. It is the same downwash present in the hovercraft that also prevents fixed-wing aircraft from getting too close to the ground at high speed.
Ground Effect becomes appreciable at a height above the ground of about one half the aircraft’s wingspan, and increases in intensity in inverse proportion to that distance.
To summarize:To make a hovercraft settle down to earth, the operator must reduce power, which in turn reduces the force of the down-blast (downwash). Similarly, to bring an airplane down to earth, the pilot must reduce power, which reduces airspeed, which in turn reduces lift, and thereby reduces the downwash, which allows the craft to settle upon the ground during landing.
- The amount of air diverted by the wing is proportional to the speed of the wing and the air density;
- The vertical velocity of the diverted air (downwash) is proportional to the speed of the wing and the angle of attack;
- The lift is proportional to the amount of air diverted (downwash) times the vertical velocity of the air;
- The vertical component of the diverted air, when in close proximity to the ground, is the primary cause of “ground effect”, and occurs along the entire wingspan. Ground effect has nothing to do with, induced drag, tip vortices, or wake turbulence.
- The energy within the ground effect layer (‘cushion’) is directly proportional to the aircraft’s weight, its airspeed, and angle-of-attack.
CONCLUSION
In order to better understand the forces inherent in the downwash sheet, and the effect these forces have on ground effect, it is important that Lift be understood in terms of the physical (Newtonian) description as opposed to the more common Bernoullian explanation.
When these spanwise downwash forces and effects are considered separately from the forces inherent in wingtip vortices (wake turbulence), it becomes clear why the alleged Boeing 757 could not have flown in a flat trajectory at 400 MPH for a distance of nearly a mile at a height lower that about half its wingspan, or approximately 60 feet.
The energy generated by its downwash would have prevented the aircraft from getting any closer to the ground.
The only way the craft could have got any closer to the ground would have been through a reduction in power, which, according to all available data, did not occur in the instance of “Flight 77”.
EPILOGUE: “FLIGHT 77”
The US Department of Defense could easily end all conjecture, speculation, and rumors about this highly controversial event by releasing one clear photograph of a Boeing 757 approaching the Pentagon.
Since there were 96 — ninety-six — outwardly aimed CCTV cameras ringing the Pentagon, it is entirely reasonable to assume there must exist at least one clear frame of this world-changing event. Pentagon officials claim all ninety-six cameras mysteriously malfunctioned simultaneously just prior to the incident, and as a result no photographs are available.
The ONLY photograph released by the Pentagon to date is a solitary video frame captured from a security camera located in a parking lot.
The release of one clear photo of an inbound AA Flight 77 in a ground-hugging approach to the Pentagon would summarily end this interminable argument once and for all, and render all further debate on the subject moot.
However, given that in thirteen years the government has produced no such evidence, it can be safely assumed none shall be forthcoming in the future. That being the case, independent investigators seeking conclusive proof of the real events of that day are left with few, if any, avenues of recourse.
That being said, the following two possibilities bear potential to effectively nullify the official narrative and offer an alternative hypothesis; they are offered for the reader’s consideration:
- Extend a challenge to any pilot in the world (any pilot — fighter ace, test pilot, astronaut…) to replicate the alleged maneuver in an actual airliner. For that matter, replicate it in any powered fixed-wing airplane regardless of size — from a fabric-covered Ultralight to a multi-ton jumbo jet. A videotaped account of the feat (recorded from inside the cockpit as well as by a ground-based observer) would conclusively prove everything presented in this paper, and summarily end all argument.
- Watch the Italian production Zero: An Investigation into 9/11
- NB: The writer has actually attempted this maneuver in three different types of aircraft, and proved beyond any doubt that it is practically impossible to force an aircraft any closer to the ground than a height of approximately half its wingspan when operating at full power. All three aircraft demonstrated a surprisingly powerful tendency to want to climb — even under forced application of full forward stick – thereby confirming the tests performed in a flight simulator in the above-mentioned video. Obviously, this is an extremely unusual flight domain, and thus one that is rarely — if ever — experienced by any civilian pilot (and very few, if any, military pilots), and explains why such counter-intuitive aircraft behavior under these abnormal conditions remains unknown to most aviators.
The author is a degreed professional aerodynamicist, now retired. The paper has been vetted by the head of the department of aerodynamics at a prominent European university. All names withheld by request. The reader is encouraged to have its accuracy verified by any graduate aerodynamicist. [NOTE: This is one in a series of articles being republished since veterans today.com deleted them in a dispute with its Senior Editor, Gordon Duff, about which I have since written several articles.]
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