I think I can rephrase Morley's interpretation of the results of the Morley tethered kite sail test reported in Catalyst 50 so that they are a bit more intuitive.
One part of the argument is in this pair of diagrams:
The point of the steepest slope on the left, for 5 degrees angle of attack, corresponds to the best lift to drag ratio on the right. For the Morley kite sail to be be stable with the sheet led to the mast, lift to drag has to increase if the sail swings aft and experiences a larger angle of attack, so that the sail's force points more forward and pulls the sail back in that direction. Lift to drag must also decrease if the sail swings forward and experiences a smaller angle of attack, so that the force points further aft and pulls the sail back there. That stable regime is in the range of angles of attack in which lift to drag increases, so the rising part of the blue curve in thediagram to the right. That means the sail has to be trimmed to an angle of attack below its best lift to drag ratio. The soft and apparently flat cut sail may have already been at its best lift to drag ratio by the time it stopped fulttering, and so fell into the part where the curve is falling, and the sail unstable.
Note that the torque generated by changes in lift to drag ratio depends on how far the sail is from the mast round which the whole sail rotates. In the tested rig, that seems like quite a short distance. Relative to the chord length, it is certainly far less than in a paraglider. So far, this is a rephrasing of Morley's reply in terms that i find more intuitive, not a new argument.
The scond part of the argument depends on something for which I have failed to find quantitative data so far. The centre of pressure of a foil initially moves forward when angle ot attack increases, then back again. With the distance between mast and sail being small, the shift in centre of pressure may well overwhelm even stabilising feedback from changes on lift to drag ratio. I can't do those calculations until I find, for the same foil, both the above diagrams and one relating the location of the centre of pressure to angle of attack.
The most straightforward solution is to lead the sheet not to the mast, but to a point somewhat further aft. That seems to be what David Duncan did with the prototype of his swing sail rig (Catalyst 7). The swing sail rig seems to differ from Morley's tethered kite sail only in that is merely reduces heeling moment, but does not completely cancel it. Alternatively, instead of changing the sheeting point, control the angle of attack by a tailplane, not a sheet. Either modification should restore the intended functionality, and for changing the sheeting point, the swing sail offers a proof of concept.
Sails soft & hard (wingsails), kites, and discussions of aerodynamic theory
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