2012 Winter Project
In winter 2011 Hydrofoil 3 with extended front foils was tested. The configuration proved satisfactory with a stable cruise speed of about 12 knots, top speed was limited to around 14 kn because of the excessive lift generated by the rear foil. The excessive lift resulted in a nosedive.
The objective of Winter Project 2012 was to add flaps to the rear foils and develop a stabilization system to extend the speed range in both extremes. For the control system two waterproof Otterboxes were used. The first contained an Axon MCU and a Razor 9 DOF IMU. In the other Otterbox an Axon MCU, a 2.8" 18-bit color TFT LCD from Adafruit, a 3DR GPS with uBlox LEA-6 module and an OpenLog data logger were to be found. The two Axons were connected via UART. The benefit of having two separate boxes was that the first could be mounted to the hull and the second could be held by the copilot and used to display real time data
For the servos, a circular cut out was performed in the struts. The servos were first covered with heat shrink tube, wound with carbon tows and then glued to a carbon plate; which was then attached with screws to the strut. To create a hinge, one ply of 61g/m^2 kevlar was laminated on the top skin of the foil, the lower skin and core (composed of foamed epoxy resin) and part of the upper skin were then cut. At this stage it was possible to break the upper skin. The kevlar does not break and therefore forms a hinge.
The concept of the controller is to stabilize the craft on the pitch axis and to assist the front V-foils with roll stabilization. Because of the foil geometry, also heave is self stabilizing, no height sensor was therefore needed. As for the surfboard, a model of the system was first developed to test the LQR controller. The plan was to place the control box near the bow. This proved not to be feasible because of the low signal quality attained through the long extension leads to the servos. The PWM output of the axon is only 5V and the high voltage HS-646WP servos are designed for a minimum voltage of 6V. With two servos connected one would consistently receive invalid signals. The control box was therefore mounted at the stern near the motor.
The first day of tests did not yield the results we had hoped for. By analyzing the logs it became clear that the IMU suffered from the vibrations of the outboard motor. After introducing a filter (and the unavoidable phase lag) the peaks in the angular measurements were successfully suppressed. With some careful adjustments on the pitch trim we managed to reach a top speed of 20.3 knots. At these speeds, however, the front V-foils proved to be extremely sensitive to changes of angle of attack and immersion. An increased aspect ratio and angle of the V would help to make the foil less aggressive, a solution could also be to partially ventilate the foils to reduce d_Cl/d_alpha.
The Yamaha 15A started showing signs of fatigue form the beginning of the project. For some to be investigated reason, it did not reach full power, except in some rare occasions. The problem eventually became worse, it started running only on one cylinder intermittently. The usual diagnostic checks were performed on the beach, except that we had an intermittent electrical problem no definitive conclusion was reached. We will now look for a newer, more reliable, power plant and examine the 15A in detail.