For height control on a foiling Moth the usual practice to to control the AOA and CL of the main foil with a flap connected to a wand that senses the height of the boat.
One question is whether elevator control, i.e. use the rear foil to change the pitch of the main foil, is viable alternative. This is how airplanes control the AOA and maintain level flight.
I used the stability model (from alans of AUS) to compare elevator control to flap control.
The result is that elevator control seems fundamentally slower than flap control. The graph shows the response of the two different control systems to a artificial step in the water. Step response is the typical way to look at the dynamic response of a control system.
The red curve is the step in the water. The blue and green curves are the response of the elevator control and flap control models as represented by the depth of the main foil.
The flap control model starts dropping as soon as the wand (5 ft in front of the foil) sees the step. On the other hand, the elevator control model rises a little before it starts to drop.
I interpret this result this way: when the wand senses the step, the elevator is pitched up to make the main foil AOA go down. The pitch up of the elevator increases the lift contributed by the elevator which raises the whole boat. Later, the pitch down of the main foil reduces the main foil lift and the boat drops. But the end result is that elevator controlled boat reacts slower than the flap controlled boat.
(note: graph units are feet in both vertical and horizontal axes).
