Setting tweaks to adjust blade size and strength by station

From the Aerodynamics main screen shown below, press the "Parameters" button.

turbine main screen showing  parameters button

The Parameters entry screen will appear as shown below :-

tweaks screen

You can select any tab in any order by clicking on the tab label. Click to ensure the "Tweaks" tab is selected as shown above.

The fields and their meaning are now described. Note that the values that can be entered are constrained to reasonable values and that out-of-range or bad values will be ignored without warning.

The "Tweaks" input tab allows you to make small adjustments at particular stations to fix problems of low blade strength or hydrofoil cavitation or low Reynolds number at a particular station radius. Such problems will have been noticed when looking at detailed caclulation results for a particular station radius (using the "Reports/Show Working" menu item).

The fluid flow thrust on the blade tries to push the blade downwind. This is resisted by the blade being fixed at the hub with the hub pushing back on the blade in the upwind direction but that fixed point is a long way from the the point at which the thrust is acting so there is a "couple", or moment or turning force or beam bending force. Since the blade does not actually rotate in this direction (the plane of the airflow stream lines) the original "couple" must be balanced by an equal couple set up by tension in the upwind surface of the blade beam and compression in the downwind surface of the blade beam.  A size of a couple is determined by a force multiplied by a distance. If the bending couple must be resisted by tension and compression operating over the very small thickness of the blade then the forces for a given couple are very great. If the distances can be made larger then the forces for a given couple become proportionally smaller. There are two ways to increase the distance over which the couple is resisted. One way is to make the blade thicker. Another way is to attach a tension member external to the blade on the upwind face at some reasonable angle to provide an extra resisting couple. Such a tension member can reduce the bending force at the blade roots to zero and much reduce the bending moments elsewhere.

If your blade bends back under the thrust load too much, or if the tension and compression forces are too great for the thin blade to support, then the stiffness and strength requirement can be reduced by a  large factor of more 3 by providing a tension member (like a guy wire) attached between a point near the blade tip and point on the axis well  upwind of the blades (e.g. on a greatly extended and pointed central hub nose cone).  Such a tension member converts much of the bending stress on the blade into a small compression stress in the blade and a pure tension stress in the tension member (which can thus be a wire or rope). Compression stresses may be preferable in materials such as concrete or ceramic. Also compression stresses subtract from the centrifugal tension stresses that dominate small turbines. The problems of streamlining this guy wire and avoiding oscillation remain.

Tension member radius factor
This is the station radius at which the tension member is attached, expressed as a fraction of the tip radius (i.e. 0 means hub and 1.0 means at the blade tip).  The thrust forces are larger nearer the tip since the blade tip sweeps out the largest area of fluid flow, yet the bending force in the blade near the tip are near zero since the distance of the forces from the station radius are small. An optimum point of attachment of the tension member will be between 0.7 and 0.8 of the tip radius.

Tension member attachment angle
This is the angle that the tension member makes with the plane of the rotor expressed in degrees. The larger the angle, the less tension is required for a given couple. 90 degrees would mean the tension member is directed directly upwind. This would be difficult to arrange as there is nothing to which the other end of the tension member could be attached. An angle of zero would mean the tension member lies parallel to and internal to the blade. A practical angle might be between 20 and 45 degrees.

Lift coefficient additional to nominal
This is an addition (or for negative numbers a subtraction) to the nominal design lift coefficient that was specified on the "Airfoil" tab. Since this tweak can be specified at 5 different stations (and is smoothly interpolated for all stations in between) then the effective design lift coefficient can be made fully variable over the stations. So if the nominal lift coefficient was 0.85 and the tweak at station radius factor 0.8 was -0.2 then the effective design lift coefficient for station radius factor 0.8 would be 0.85-0.2 = 0.65. Two side effects follow from this tweak of  negative 0.2. One side effect is that the blade chord width will be automatically designed larger to get the optimum required absolute lift force from the reduced lift coefficient. This increase in size then means a stronger blade and an increased Reynolds number. It also means a reduced negative pressure on the back of the blade (which can be important for hydrofoils). The other side effect is that, since the airfoil is unchanged, the reduced lift coefficient will  be achieved by reducing the design angle of attack for this station from the nominal value (by about 1 degree for each 0.1 of lift coefficient). This in turn will change the shape of  the negative pressure curve which can again be important for hydrofoils. You the user need make no changes to the nominal values for lift coefficient or angle of attack on the "Airfoil" tab since the tweaks are applied automatically during calculations and reports.

Thickening factors
This is a multiplicative factor to the normal thickness of the airfoil shape that was specified on the "Airfoil" tab. For example a "Wortmann  FX 63-137" airfoil is 13.7% thick relative to a unit chord width. At weak stations of the blade (e.g. near the hub) we may want to gain geometric strength by thickening the airfoil section while retaining its normal lift characteristics. So a thickening factor of 2.0 would give a Wortman airfoil which was 27.4% thick relative to unit chord and be twice as strong to resist bending for the same weight. The airfoil might not give such good aerodynamic performance as the original but this may not matter for station radius factors less than 0.5 since high lift to drag ratios are much less important here than they are at the tip.

The other tabs have many more parameters that you can set. You can save your scenario to a file such as "mydesign.zip" (zip filename extension is recommended) by pressing the "Save scenario" button. You can return to the main aerodynamics top level screen by pressing the "Apply parameters" button.

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