I have been thinking about this one for a while, because it has always bothered me a little. S-Foils seem rather pointless by WEG's rules. They have no effect on the ship's performace, and, since you can't see them, they aren't even a cool effect. I decided to come up with some rules on the use of S-Foils in games.
POSITIONS: Cruise (Retracted) and Attack (Extended)
TIME TAKEN: 1 round for S-Foils to extend or retract
LANDING - A ship cannot land normally with its S-Foils in Attack Position, and they must be retracted to Cruise Position. If the S-Foils are stuck in Attack, then the ship must be caught via a tractor beam and then placed on a landing cradle of some sort (or it can crash land, or the pilot can just eject and abandon his fighter)
HYPERSPACE - When S-Foils are extended in hyperspace it increases the size of the ship's drive field from anywhere from 50 to 100%, with little or no increase in available energy. As such, if a ship travels in hyperspace with S-Foils in Attack, its multiplier is increased by 1 level (Example: An X-Wing, with a Multiplier of x1, makes a Hyperspace jump that is one day in length. However, the piloted neglected to retract his S-Foils, reducing his Multiplier to x2. As a result, the trip is doubled in length). Some S-Foil equipped vessels are naturally designed to travel with their S-Foils extended, so that this rule does not effect them.
WEAPON ACCURACY - Given Fire Control Dice for weapons mounted on S-Foils is the rating for the weapons when the S-Foils are in Attack Position. If the pilot attempts to fire said weapons while S-Foils are in Cruise Position, he receives a penalty of -2 pips from his Fire Control.
MANEUVERABILITY - If a ship's engines are mounted on its S-Foils (ala X-Wing), then the vessel's maneuverability is also affected by the S-Foils. Given Maneuverability Dice is the rating for S-Foils in Attack Position. If the vessel's S-Foils are in Cruise Position, it receivevs a penalty of -1 pip from Maneuverability.
These rules can be applied to any other type of vessel that is equipped with S-Foils. However, application of these rules MUST be based on available information of the design of the ship.