The tubes are essential to prevent flexing of the control rod when its pushed. When the plane is in flight, there is a lot of pressure on the control surfaces. Heavier control rods can go for longer runs (most aileron push rods don't have tubes), but for the tail surface, without them you'll get flex. This flex can be VERY bad, especially if the elevator or rudder starts fluttering, which will cause the rod to bend (ask me how I know). The tube will prevent this. A little glue goes a long way to ensure a stiff, reliable push rod.

The easy way to glue them in is to sand the tube lightly where its in contact with the glue point (normally a drilled hole through a fuselage former). Put the tube in place, then drip thin CA around it. It will wick in and hold the tube in place.

On my GP CAP 232, where the tubes penetrate the fuselage, they are mounted through a slot with about 1 inch outside the fuselage. They are then fixed in place with a mixture of microballoons and 30 minute epoxy. When the epoxy is fully cured, the tube is cut and sanded flush with the fuselage. Makes a very clean installation.

Brad