Tweaking Aerodynamic balance

The general concept of using a canards to control the angle of attack ( AOA ) of the main aerofoils remains but the control is slightly different to that which was previously described:

Aerodynamic balance to control AOA of main aerofoil

The pivot of the main aerofoil is slightly forward of the nominal centre of pressure ( 1/4 chord position ).

The Canard is mounted on the support connecting it to the main aerofoil. It pivots forward of its centre of pressure and is connected to the shaft about which the main aerofoil pivots such that its chord remains constantly tangential to the giant circle about which the aerofoils rotate. In a similar manner to the original concept, this connection has an elastic ( spring )section which allows the canard to rotate to reduce its AOA to the apparent wind when the wind velocity at the AOA exceeds a design limit allowing the canard to feather in incremental steps as the apparent wind velocity increases.

For winds within the design rating the AOA on the canard is the angle of the apparent wind to the tangential velocity. The aerodynamic load on the canards results in the main aerofoils rotating to create a balancing torque with an AOA that can be greater than the AOA on the canard. Previously the AOA on the main aerofoil was smaller than that of the canard, but it was independent of the angle of the apparent wind to the tangential velocity.

Twin tail aerofoils are also mounted on the rear of the main aerofoil. They are also pivoted forward of their centre of pressures. They can rotate freely ( only damped ) up to a design angle each way but do not rotate beyond that angle. The AOA on these tail aerofoils will be”zero” or the AOA on the main aerofoil less the associated design angle of freedom of rotation. The twin tail aerofoils thus limit the AOA of the main aerofoil

The advantage of this approach is that construction for marine use can be simplified: A neutrally buoyant submerged ring structure is connected to the central floating generating hub, which is tethered to secure its location. Each main aerofoil can be joined to the submerged ring structure, via its own large floating platform and large universal joint, such that they can each freely rotate and rock with waves about these joints. Non rotating shafts up from these joints enables simple links to the canards. These links can include dog clutches for complete feathering for any maintenance. Because the platforms can rotate the stays can be secured to fixed locations, removing the need for tracks, or turn tables.

The output from a VAWT based on this approach will be greater than that of fixed aerofoils due to the increased AOA of the apparent wind on the main aerofoils at design wind speeds. The system also retains the capability of feathering to safely harness energy from extreme winds

Alternative Design concepts for VAWTs

The simplicity of Vertical Axis Wind Turbines has major advantages but historically VAWTs have not been as efficient as Horizontal Axis Wind Turbines ( HAWTs). As previously pointed out in the section “ look at the limitations of and how vertical axis wind turbines work,” The design constraints of existing VAWTs limits the potential for scaling up. It was also pointed out that HAWTs are becoming bigger but with a reduction in efficiency.

Improvement of the efficiency of VAWTs has been demonstrated by using pitch control and also by the tweaking of substantial symmetrical aerofoil profiles

Pitch control (via the control of a canard) is an important feature of the new concept previously outlined on this site.

The efficiency of any size VAWT could be improved by using aerofoils of variable geometry.

The deployment of flaps alters the lift and drag coefficients of an aerofoil very significantly. Ailerons are flaps that can be deployed in both directions. The deployment of ailerons, that are almost full length, on VAWT aerofoils is a means of optimising the lift force while in both the windward and leeward sides of a revolution.

Aileron Controlled by movement of aerofoil tip without altering the angle of attack of the apparent wind.
Variable geometry by aileron control.

By mounting the tip of an aerofoil on parallel arms of equal length the tip can move away from an apparent wind with no alteration to the angle of attack. The rotation of those arms can then be used to drive the position of the aileron. There are many ways in which that can be accomplished, but for long aerofoils it is envisaged that hydraulics could be advantageous. In the example shown the aerofoil’s chord is tangential to the circle of rotation and the angle of attack of apparent wind will vary but it will always be on one side when the aerofoil is in the windward part of its rotation and on the opposite side while in the leeward part of its rotation. The tip will move dependent only on the direction ( windward or leeward ) of the apparent wind. This drives hydraulic pistons, which act as pumps, The pump pistons being of a larger bore are linked to a number of pistons to move the ailerons. For large aerofoils, which would have large ailerons the force on the ailerons due to stronger winds needs to be limited. A spring link within the system would limit such forces, by limiting the angle of deployment of the ailerons. To ensure that the aileron deflection is limited its centre of pressure must be to the rear of the pivot.

Large aerofoils must be able to feather into wind to reduce the angle of attack during extremely strong winds.

The novel system with pitch control using a canard , previously discussed, incorporates this feature but normally VAWTs with fixed aerofoils do not. ( There are no really large VAWTs yet.) Large aerofoils could be mounted such that they could rotate about an axis forward of their centre of pressure, with the rotation against a preset torque, which would return them to their normal position. For example multiple cams with pressurised followers could apply a returning torque. The bottom of the aerofoil could be connected to such a shaft / cam arrangement.

A lesson from sailing: When the wind is stronger the boat heels over more. Not that it is practical to heel an individual aerofoil, inclining such an aerofoil reduces the angle of attack of the apparent wind on it, reducing the load on it.

How to incline an aerofoil : Split the aerofoil into two sections with a pivot between them, with the pivot parallel to the chords. The frame that supports the pivot ensures the pivot is tangential to the circle of rotation. Each frame could be mounted on a giant flywheel or as in the illustration below onto a float connected to a submerged framework that connects to a generating hub.

incline aerofoil in strong winds

Manufacturing the aerofoil in 2 identical parts is the most economical. A tip can be added to each one such that the top one has a bigger area but the bottom one is slightly heavier. In the centre there is torque resistance / return system. For example cable fixed to a drum can be fixed to a weight via a hydraulic damper. When a combination of the difference in area of the tips, the angle of attack ( windward or leeward) and the wind speed causes the torque ( of either direction) to pick up the guided weight, the aerofoil inclines, which reduces the angle of attack. This will only happen when the design limits for normal operation are reached. The torque to increase the inclination depends on the mass difference between the tips and the offset of the tip to the vertical. If the aerofoil is one of variable geometry controlled by a moving tip, a counter balance and arm ( not shown) can be added to prevent G forces or inclination influencing the position of the tip . Note as the inclination approaches the horizontal the angle of attack approaches zero and the lift forces on the tip decrease to zero resulting in no deflection of the aileron with no lift force on the aerofoil. this is equivalent to being fully feathered into wind.

The general principle can be applied to units which have aerofoils sections of different lengths and a lower pivot point, but it may be important to maintain the pivot point at the aerodynamic centre of the sections. It is complicated slightly by the effects of wind shear. But note the effect of shear in strong winds is reduced due to any inclination of the aerofoil.

The illustration shows sealed ballast tanks within “catamaran ” floats. These floats with their support frames for the aerofoils allow them to pivot on a submerged frame. This frame forms a full ring round and is connected to a floating generating hub via spoke / cables. The pivoting of the floats removes any twisting stress that might otherwise be imposed on the “ring” frame but allows for tangential and radial loads. The torque from the radial load on the aerofoil times the height of the pivot is absorbed by additional displacement of one hull and the raising of the other.

The water will rotate with the frame and the Couette flow / fluid flywheel effect are the same as previously outlined.

Hello world!

Hope you find this site interesting . If you have come straight in to this post click on ” Back to Home” at the top right hand side ….to see why better wind turbines are needed and how that can be achieved.

While it is possible to strengthen aerofoils and use generators of larger capacity within wind turbines to capture the energy at higher wind speeds. It would increase the cost and carbon footprint of energy production.

Energy can be stored independently to the source of production but that implies an oversupply in some periods to boost supply during periods of peak demand or for short periods when the wind speed drops. Such energy storage systems are not cheap and also have a carbon footprint, increasing the overall carbon footprint of energy production.

The approach outlined for the novel VAWT enabling wind energy to be stored kinetically dramatically increases energy production pro rata to generator capacity, reducing the need for external energy storage systems. The system could for example run at reduced generation capacity, storing energy kinetically ensuring higher demand can be met for a period even if the wind speed dropped.

Combining the above with the potential to extract a lot more energy per unit area is a means of significantly reducing the carbon footprint of energy production. I hope you will support the concept.

This planet is our home and our childrens future. what legacy do you wish to leave?