WING MISCON. |
GOOD STUFF |
NEW STUFF |
A ring-vortex (or a cylindrical vortex-pair) acts like a single object. The air "thinks" that a ring-vortex is a sphere. The moving ring carries the air in the ring along, but it also carries along the air in the "donut hole," and the combination forms a moving air-ball. Yet since this "sphere" contains internal flow, the surface drag of the sphere surrounding a vortex-ring is insignificant when compared to the drag on a real sphere. Throw a ball and you get air friction, but throw a vortex-ring and you get almost none. BRAINSTORM! Make a motor-spun flettner wing aircraft. Replace each normal wing with a pair of cylinders, one above and one below. WHen this airplane moves forward, the cylinders will counterrotate, and the drag will be extremely small. This resembles the "smoke-ring blimp" discussed elsewhere. OK now tilt the two cylinders so they have an "angle of attack." They will spin differently, with the upper cylinder going faster. Add the rotations together and you don't get zero anymore: the wing has become a flettner rotor. The main difference is that the drag is tiny. DOUBLE-ROTOR FLETTNER CRAFT. They need no motors to drive the spinning cylinders. The air alone does it.
Water or Air: no important differenceAnalyze wings under water or in air. The results, the shape of the flow patterns, will be the same. According to the physics of aerodynamics, air is not "special," and underwater wings are just like normal aircraft wings. The physics applies to all fluids including both gases and liquids. So, if a theory of flight relies on the significant compressibilty of air, then that theory is wrong, since wings work just fine when flying under water. A proper theory must explain all wings, rudders, propellors, etc, and not just the ones flying in air.
Pressure differences linked to flow deflectionsThe solid parts of wings, rudders, and propellors deflect the moving air so it flows at an angle. The deflected air tries to leave a pocket to one side and an excess buildup on the other side. Instead of creating a vaccum and a high-densiy zone, the gas or liquid moves. It rushes into the vacuum in just such a way that the fluid density never changes. The fluid slows down and avoids the "buildup" in just such a way that the fluid density never changes. But the pressures certainly change! The fast flow is associated with a low pressure, and the slowed flow is associated with a high pressure. No "vacuum pockets" in the air, but there certainly are regions of different flows and different pressure.
Bluff Airfoils are MisleadingMoving ball: air moves backwards while ball moves forwards. Moving air across ball: air slows down ahead, speeds up behind. slow air ahead: high pressure retards ball slow air behind: high pressure drives ball ahead pressures cancel! fast air above: low pressure lifts ball fast air behind: low pressure drives ball down pressures cancel! Streamline the ball, and the pressures above and below it remain low. Tilt the streamlined ball or "airfoil", and the pressures change. But the several different pressures are extremely misleading! The streamlined ball supports a complicated pressure pattern even when NOT tilted. Conclusion: to understand lift, analyze a tilted flat plate instead of an airfoil. That will teach you about the lifting force alone because it strips away the crazy pressure fields which appear around airfoils.
Why use 'airfoil' shape at all?Why are airfoils bulbous in front and pointy in back? Well, we should ask what happens if the airfoil is reversed? It turn out to work just fine, but only as long as no flow-detachment or discontinuous flows appear. The front of a train can be sharp and pointy. The same goes for rockets. However, the tiniest tilt will cause a massive "stall" and a huge drag force. When a pointy airfoil is tilted, the air isn't able to make it around the sharp leading edge. Even a tiny tilt will trigger a massive stall. But put the point in the back and make the front edge bulbous, and things work great. The bulbous front edge prevents stalls. Therefore we must conclude that airfoil "streamlining" isn't there to prevent viscous drag. Instead it's there to prevent the 2nd-order drag whcih comes from "stall" and turbulent stirring of the air.
Trailing edge MUST be sharpNow lets look at the rear half of the reversed airfoil. The bulbous rear has a very interesting effect: when the airfoil is tilted, it DOESN'T DEFLECT THE AIR. Instead the air flows straight back from the bulbous part. This makes some sense: when the airfoil had a sharp trailing edge, inertia forced the upper/lower flows to leave the edge in whatever direction the edge was pointing. But without that sharp oint, the upper/lower flows collide head on, and they can decide on their own the anle of departure. WIthout that sharp edge to guide them, the air flows ignore the tilt angle. (Well, if the air was to flow MUCH faster, then the upper/lower flows would again obey the tilt angle, but only because the flows "detach" and the bulbous part becomes shrouded in dead air which mimics a sharp trailing edge.)
Why is this not in textbooks?Aerodynamicists are misled by a widespread textbook error: the classic analysis of inviscid irrotational flow around an infinite cylinder and around an infinite cylinder which is spinning. This analysis tells much about the lifting force, but it hs an unfortunate feature: an infinite flow field which applies forces to distant surfaces. In the real world we have no infinite cylinders, and the flow field around a finite cylinder creates pressures which drop off differently than those around an infinite one. A finite cylinder only applies forces to distant surfaces if those surfaces are within a few cylinder-lengths distance. Maybe an electrical analogy will help. Look at a single electron. It's field extends to infinity, and it attracts distant charges. But look at a DIPOLE (an electron and positron adjacent.) The dipole field extends to infinitey, but it drops off so fast that the dipole only interacts with charges which are within a few dipole-separations distant. Another: AC in an infinite wire will induce the same current in an infinite metal plate no matter how far that plate is from the wire, but a WIRE RING is different. Alternating Current in a wire ring will induce smaller and smaller currents in an infinite plate as the ring is moved away from the plate. In other words, aerodynamics undergrads have been studying the "Ground effect" without realizing it. They thought they were explaining how wings work, but instead they were explaining how wings interact with the nearby ground during very low altitude flight. This is a major mistake, and it has repercussions which resonate through all of aerodynamics science. The mistake is perfectly sensible: it is easier to analyze an infinite wire than to analyze a coil. An infinite wire gives a 2D field distribution which is instantly grasped by students, while coils give 3D fields which are m uch more difficult to understand. But the force-pair between an infinite wire and an infinite plane is constant regardless of distance betwen them. Flows around infinite wings explain "venturi effect" and ground-effect flight. They do not explain how airplanes work. Hey, the smoketrail pattern around an infinite cylinder displays viscous interaction, and the speed gain of air passing by the cylinder does not equal the speed loss of air approaching and receding:
(animation from http://www.av8n.com/irro
I need an animation of TWO COUNTERROTATING CYLINDERS so I can see how the fluid outside the cylinders behaves.
Created and maintained by Bill Beaty. Mail me at: .
Best if viewed with ANY Browser.