Simon Brown wrote:I feel a technical wind turbine discussion is not complete without understanding how the new wind powered cars travel un assisted into a head wind at TWICE the speed of that wind. ... i really wanted to just set the brain matter in motion as I feel that maybe the secrets of wind turbine design can be gleaned from this.
Here is a very brief summary of an originally rather long thought experiment concerning props written in late 2010 a few months after the Black Bird's successful down wind trials. The underlying concept being of looking at fluid dynamics as the sum of all the actions of all of those incessantly active atoms and molecules that make up a fluid mass. This fundamental property of matter was succinctly described by the great William Crookes in a 1879 lecture on Radiant Matter:
"Gases are now considered to be composed of an almost infinite number of small particles or molecules, which are constantly moving in every direction with velocities of all conceivable magnitudes."
It is this incessant motion which is being referred to when mention is made of potential or kinetic energy; these two terms in actuality only describing a change of view not a change in or of energy. Traditionally potential energy is a reference to a mass at rest relative to its immediate surroundings, ignoring the internal energies of the particles making up the mass. Kinetic energy is simply a reference to the change that occurs when a portion of the internal particles begin to move in unison along a common pathway. Looking at wind from this perspective helps provide a somewhat different view of how air molecules interact with prop surfaces and perhaps how prop designs can be improved - or so it is hoped anyway.
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Examining Prop Functions: Conventional thought has it that propellers provide thrust by pushing against the surrounding air, that is why they were initially referred to as aerial screws. However, modern props are also airfoils, which gained dominance over air screws when it became apparent that creating lift, the lowering of pressure along the leading surface of the prop, offset a large amount of required pushing power. This occurs because airfoil props, like wings, are actually environmental energy extraction devices that utilize the energy of air molecules to push the airfoil into the lower pressure zone.
However, an airfoil as a prop is a somewhat more tricky proposition than an airfoil as a wing. For meeting air movement straight on with a narrow leading edge seems to be a much more natural function than progressing in the direction of the whole of ones top surface area.
To deal with this somewhat unnatural situation prop blades have many profiles, they can be symmetrical or asymmetrical, they can be varied in width and depth, twisted or straight, tilted or level, etc. It all depends on the specific application the designer is attempting to achieve but a primary goal is to create a stable low pressure zone ahead of the blade while maximizing push.
For instance, on an asymmetrical blade the front surface may be designed to achieve high lift, that is to create a low pressure area over the front of the blade, while smoothly feeding the air to the back surface of the following blade to minimize turbulence. The back surface may be designed to most effectively push the air away from it maximizing thrust into that frontal low pressure zone. However, a symmetrical blade may have the same profile on both surfaces and depend on the blade being tilted on the rotor to provide an angle of attack to generate thrust.
These two separate functions of the leading surface and the back surface are obviously somewhat in conflict, for the faster the back blade thrusts the whole contraption along the more compressed the forward low pressure zone will become, reducing the lift. Is it possible then to split the front surface from the back surface to create non-traditional designs that are more efficient?
Splitting the Prop: As surprising as it may seem, some of the most innovative and effective concepts along these lines were conceived in the early twentieth century.
Henri Coanda: While the Coanda Effect is well known in wing design and other circles today, his propulsion design based on the same concepts never gained acceptance - fated it seems to live on only in an obscure seldom remembered niche in aviation history. His device was powered by a pressurized gas (red arrows) flowing upwards along the axis and exiting through circumferential slots as a high speed flow over the convex shroud, thus creating a low pressure zone on the front surface.
At the same time, air from the front of the device would be admitted as a low speed flow through a central opening and directed to the back surface. This would generate a higher pressure than that existing on the front surface engendering lift, or in this case: forward thrust without revolving blades. History seems silent on whether a functioning device was demonstrated or not. However, the underlying principles are sound.
Fan Wing: A contemporary device which more clearly demonstrates the splitting of functions between front and back surfaces, can be seen in this device which uses what looks like a stretched version of an old push
lawn mower reel to create lift on a strange type of wing. It is safe to say this wing has a very non-conventional profile, not to mention blade and rotor configuration.
The elongated blade reel sits in a concave trough 'blown' out at the trailing edge. The revolving blades create a high speed flow over the wing's top surface creating lift, while generating thrust by pushing the air to the back.
Getting Specific: Morph Level One or Getting the Morph Out of Your Basic BUFC
Here we will focus on simply Splitting the Prop while keeping the sail-frame-wheel-airfoil direct drive mechanism.
Before Henri Coanda there was Nikola Tesla and his 1913 patent for 'Fluid Propulsion'. This and the later 'Turbine' patent focused on his Bladeless Disc Turbine designs. It could be used to extract energy from a fluid and function as a motor, or it could impart energy to a fluid and function as a pump / fan. Reminiscent of Henri's design, each disc had a central opening through which fluid entered or exited depending on the mode it was in.
If the primary goal of an airfoil's leading surface is to create a frontal low pressure zone, what better way than to use a spinning centrifugal disc to simply move all of those pesky air molecules laterally out of the way. The disc doesn't even need blades since adhesion works wonderfully well. Set the spinning discs in a tight chamber with circumferential exhaust slots designed to use the Coanda Effect to smooth the exhaust flow over a contoured baffle shroud. Form the stationary back surface of the disc chamber so that it is either flat or perhaps somewhat concave, with the baffle shroud extending somewhat past its edge to create a low turbulence zone. This back surface is now a solid stationary sail surface replacing the prop's former revolving back surfaces.
A one yard diameter disc has a surface area of slightly more than 1,000 sq. inches; lowering the pressure above one surface by just 20 percent creates a differential of nearly 3 psi, or almost 3,000 pounds, 1.5 tons, over the entire disc.
To Summarize: Most winds in our Earth's atmosphere move much slower than the molecules which make up the air comprising the flows. In fact mean molecular speeds of up to 500 meters per second are more than a match for the pushing prowess of any prop that has ever been designed or constructed. Therefore, if the molecules that surround us every second of most of our lives will provide more thrust on their own than the best props ever created, why bother with the prop for thrust? There is no point to it, and a stationary solid base will create more thrust per surface area than any prop ever made, as long as a frontal low pressure zone is maintained. And nothing mechanical maintains a low pressure zone better than a spinning bladeless centrifugal disc.
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