Wind tunnel (3)

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This data on twist comes from the wind tunnel tests of Rachel Nicholls-Lee.  More background details on these series of tests are on the lift and drag page and the lateral mast bend page.

It was clear from the overhead camera shots of the model in the tunnel that, as wind speed increased, the main leech twisted off quite dramatically.  But what happened to the jib leech?  We have a surprise for you.

Here is data from three of the test runs.  For each of these runs, beta (the angle of the apparent wind) was 30 degrees.  On the left, the main was sheeted at approximately 6 degrees, and the jib at about 14 degrees.  In the centre, the main was at 4, and the jib at 14.  On the right, the main was at 0 and the jib at 16.  These three are selected from a number of other runs at various sheeting angles mainly because they are the ones with the best overhead camera shots.  The apparent wind speed is indicated as W20 for 2.0 m/sec (actually 1.8 m/s as it turned out), W30 for 3.0 m/sec, and W45 for 4.5 m/sec.  In all cases, twist is defined to be zero at boom level.  That is, twist in the sail leech is measured relative to the boom or foot of the sail.

Main twist was measured at each of the three draft stripes, corresponding roughly with each of the batten points at 1/4, 1/2, and 3/4 leech, called "bottom", "middle", and "top".  As the graphs show, mainsail twist increased with wind speed, and (naturally!) it was the head, the top of the mainsail, which showed the greatest increase in twist.

Jib twist was measured at two draft stripes, corresponding roughly with each of the batten points at 1/3 and 2/3 leech, called "lower" and "upper".  Can you see what the graphs show?  Pause right here and figure it, if it hasn't struck you already.  Go ahead, I'll wait.  OK, I'm padding the words out here to make sure you do indeed pause and see what you think, rather than just reading on to see what I think!  This is fun, right?  So, jib twist decreased with wind speed, and (unnaturally!) the head showed the greatest decrease in twist.  Did you see that?  Now that is not what I expected to see, and I spent an anxious few hours checking.

While watching the tests from the observation window at the wind tunnel, it was clear that, at the highest wind speed, the jib topping lift just released.  In fact the model was set up with rig tensions to achieve exactly that, to have the topping lift just beginning to float by itself and thus have the jib leech taking all of the wind pressure.

To continue in the same "self-quiz" vein, here is Brian Hard's AutoCAD wire-frame of the test model at the lower and higher wind speeds from the lateral mast bend page.  The twist-off of the main leech is clear.  Can you see why the jib twist has in fact decreased at higher wind speed and not increased as we would have naively expected?

Go ahead, study the diagram, pause right here and figure it, if it hasn't struck you already.  Yup, I'll wait.  OK, I'm padding the words out here to make sure you do indeed pause and see what you think, rather than just reading on to see what I think!  This is fun, right?  So, as far as I can see, we had jibstay sag, coupled with the mast head sagging to leeward, that meant the jib luff moved further away from the centre plane than did the jib leech.  This greater movement, greater sag, of the jib luff relative to the jib leech effectively meant that twist was taken out of the jib.  And, because the luff sagged to leeward most at the head of the jib, that is where most twist was taken out.  Now that is something I certainly had not realised.

2005-12-18


2011 Lester Gilbert