RADIO CONTROLLED HYDROFOIL MODEL ‘EL FOIL’ BY HANS JORGEN HANSEN
The photos in this album show the hydrofoil model named ‘El Foil’ designed and built by International Hydrofoil Society member Hans Jorgen Hansen of Espergaerde near Copenhagen, Denmark.
The model does not represent any real hydrofoil type; rather it is to an original design portraying a small hydrofoil motorboat. The hull is around 400 mm long and 135 mm wide. It is fabricated of light aluminium alloy sheeting. The cabin top and aft deck can easily be removed to gain access to the battery pack and radio control gear. The model has a mass of about 880 grams when complete with battery.
The bow foil is of surface piercing ‘\_/’ configuration with an overall span of 260 mm and chord length varying from 12 mm at the centre to 40 mm at the foil tips. When foilborne, the submerged portion of that foil would have a span of about 180 mm. The bow foil provides the longitudinal and lateral stability for the model. It also incorporates a small fixed rudder projecting vertically up from the foil at its centreline. The stern foil is a fully submerged type without any dihedral and has a rectangular planform of 160 mm span and 14 mm chord length. Both the bow and stern foils are of solid aluminium alloy strips filed back to form streamlined hydrofoil profiles.
The design of the foil units is such that there is an absolute minimum of resistance. The bow foil is connected to the hull only via two thin struts, which in any case are out of the water when the model is running fully foilborne. The aft foil is likewise supported by a pair of thin struts connected to both sides of the hull aft. The single centreline rudder is relatively short and is mounted below the aft foil. The rudder stock is a thin wire running down from the hull and is supported by the aft foil. This arrangement means that there is no inefficient section of the rudder near the water surface. That approach also avoids the tendency to draw down air when in a turn and reduces un-necessary drag when travelling straight ahead. The model has a very rapid turning response with this rudder arrangement.
A single two-blade Graupner propeller of 30 mm diameter and a relatively low pitch propels ‘El Foil’. This is driven by a 12V Buehler type electric motor (sourced from a hair dryer) through a thin inclined drive shaft. Ten 1.2V Sanyo 1 Amp batteries in series supply power. The shaft is only supported as it passes through the hull and at the aft foil. Consequently, there is minimal frictional resistance sapping power from the motor. The shaft extends past the aft foil so that the propeller is well immersed and does not suffer from any ventilation problems when the model is underway. Cooling air is drawn in via a cabin top vent and is forced over the motor by a small propeller which acts as a fan. This is directly attached to the front of the motor shaft.
Control of the model is through a two-channel Futaba radio control unit, one channel for speed control the other for steering. The receiver is a Futaba FP-R102JE type, this being powered by the same battery pack as for propulsion using battery elimination circuitry (BEC). A Tekin TSC 610-G Goldfet electronic speed controller with reverse capability has replaced the Tamiya type mechanical speed controller that was originally fitted. This, combined with the BEC has lead to an overall weight saving through the elimination of one servo and the separate battery pack for the receiver and servos. The remaining servo controlling the single rudder is a Robbe RS50 type.
When the model is hullborne, both the bow and stern foils appear to have a small negative angle of attack. As such, when running the model ahead at slow speeds, the bow tends to want to ‘dig in’ given the relatively short hull. But with more power applied, the hull lifts out and then so too does the bow foil. Likewise, when backing the model up, it would almost be content to become foilborne in that direction! Reversing is therefore done at low power or in small bursts.
‘El Foil’ was built in 1986 but continues to run very well. For a model of its small size and moderate motor rating it has considerable speed and endurance, running for up to an hour on a straight course. Hans Jorgen has estimates the foilborne speed to be about 8 knots (4 m/s). The wide bow foil gives the model very good lateral stability and so it is able to make rapid turns without resulting in any significant heel angle, let alone risk of capsize. Hans Jorgen reports that the model has never had any problems associated with foil ventilation and does not require any fences. This is the smallest of the radio controlled hydrofoil models that Hans Jorgen has built, his other model, ‘Carton Ondule’ based on the PT 150 can run foilborne at a displacement of 12000 grams, more than 13 times the displacement of the little ‘El Foil’.
For more details, Hans Jorgen Hansen can be contacted by posting a message to him on the IHS Bulletin Board at www.foils.org
‘El Foil’ underway. Photo: Hans Jorgen Hansen
‘El Foil’ on stand. Note the propeller shaft projecting aft of the stern foil. The black propeller is hard to see in this photo but is immediately behind the metal cone. The small fixed bow rudder and controllable stern rudder are also visible. Photo: Hans Jorgen Hansen