Wing Sail Foil Testing - The Setup

The designs of the wing sails used with the Voyager sailing drones have been established by studying other designs, by using a lot of intuition and by judging whether it looks right.

It is time to perform some more rigorous testing to determine the optimum values of some key parameters of a self-trimming wing sail.

Design Questions

These are some questions to be answered to assist in designing and operating the next wing sails.

What is the optimum pivot point for a self-trimming wing sail ?
What is the relationship between Trim Tab angle and Angle of Attack, and hence what size should a trim tab be ?
What is the optimum trim tab angle when running ? Should the trim tab be reversed when running ?

Wing Sail Test Rig

I developed a test rig to allow a series of relative measurements to be performed indoors.

The airflow is provided by a large domestic fan.

But tests quickly showed that the airflow from a fan is too turbulent to be useful for performing measurements. So a columnator or flow straightener was developed to improve the quality of the airflow.

This was constructed primarily from rolled up sheets of A4 paper, contained within a wooden frame. It wasn't great, but it was good enough to get useful results.

Fan, flow straightener and the test article.

Fan and flow straightener

The airflow in the vicinity of the test article was around 2.8m/s.
This was measured using an Air Velocity Sensor Module, the Renesas FS3000-1015.

Test Article

The test foil is an Eppler 169 (E169) with a 400mm chord. This chord size represents the approximate size to suit a wing sail for Voyager 3.

The 400mm chord yields a Reynold's number of around 76,000 for the 2.8m/s airflow.

The test foil has been designed to support testing with the following characteristics and adjustments:

The position of the axis may be varied from 15% of the chord to well over 35%.
The trim tab is adjustable with a scale to easily set a desired angle.
The trim tab is split in two, to allow for measuring authority versus size.
The test rig includes a scale to measure angle of attack.
The wing sail mast bearings are supported by load cells to provide a relative measure of the load in 2 dimensions. The load cells are rated at 1kg max, and include digital readouts in tenths of grams as a relative measure of force.