Wing Sail Foil Testing - Optimum Axis Position

What is the optimum position of the axis of a self-trimming wing sail expressed as a percentage of the chord ?

A self-trimming wing sail must naturally weathervane into the wind. 

It is well documented that the aerodynamic centre of a simple thin foil is at 25% of the chord.

NASA - Aerodynamic Center

This means that 25% is the absolute maximum position along the chord, because beyond that the wing sail won't weathervane into the wind.

As the position of the pivot is moved toward the 25% point, the wing sail becomes more balanced, requiring less effort to increase the angle of attack.

This also means that manufacturing tolerances will have an increasing effect as the pivot point moves toward 25%.

 

As the pivot point moves closer to the leading edge, the effort to increase the angle of attack of the foil increases.

Also manufacturing and assembly issues encourage the pivot point to be a reasonable distance back from the leading edge.

The image below shows the sail from Voyager 2.9. A practical issue is the need to have pivot far enough away from the leading edge to allow for mounting the magnetic encoder ring used for measuring the wing sail angle.

Voyager 2.9 wing sail with mast pivot at 16% of chord

The wing sail with 16% pivot point has been used successfully in lake trials and on the recent ocean passages.

Testing Pivot Position

A series of tests were performed with the test rig by setting the position of pivot at the following percentages of the chord:

15%, 16%, 18%, 20%, 25% and 27%.

Then the trim tab moved in 5° steps from -40° to +40°.

The resultant angle of attack was recorded at each step.

The results are plotted below with the trim tab angle on the horizontal access, and the resulting angle of attack (AOA) on the vertical access.

The Eppler 169 foil has an optimum AOA for lift vs drag, of around 5° to 6°.

The overall trends observed in testing were consistent with expectations.
The sensitivity of the wing’s angle of attack (AOA) to trim-tab deflection increases as the pivot axis moves aft toward the 25% chord point.
At 25% chord the system became too sensitive for practical use: we aim to operate the wing at an AOA below about 10°, yet a trim-tab change of only 1–2° was enough to drive the wing into stall.

It is important that the wing’s response to trim-tab input is not excessively sensitive.
The trim tab must move by a practical amount during normal operation—large enough to be repeatable and to overcome any mechanical backlash or stiction in the linkage—while still giving fine control over the AOA.

The tests revealed an apparent offset of approximately –3° in the trim-tab calibration.
This could be due to small manufacturing or alignment errors, but is more likely caused by flow-field asymmetries in the test setup.

A useful benchmark is the trim-tab deflection needed to achieve a 10° AOA:

• about 7° of tab deflection when the pivot was at 15% chord,

• about 5° when the pivot was at 18% chord.

For completeness, we also explored operating with deliberately large trim-tab deflections that drove the wing well past the stall angle, even though such conditions are outside the intended flight envelope.

Overall, a pivot-axis position in the range of about 15–20% chord appears to be well-suited.
Within this range the wing shows stable weathervaning, and the tab response is strong enough to overcome backlash yet not so strong that small tab motions cause abrupt stalling.

I expect to continue using the 16% chord position for the pivot of self-trimming wing sails.