sentences:  The use of a foam-plastic rubbing strip enables the fan to maintain the optimum blade-tip-to-shroud gap, said to be one-thousandth of the fan's diameter according to RFB; the ring slot ensures smooth air flow even at high power and low airspeed, and can also open asymmetrically to cope with offset inflow when the Fantrainer is flown at a relatively high angle of attack.Structurally, the Fantrainer's forward fuselage and centre section is formed around a metal box keel; the cockpit section, which is mainly composed of moulded plastic, is fixed upon this keel.This approach enables the cockpit shape to be readily modified, allowing it to be adapted to imitate various different aircraft.