Towing a glider at Pulborough Brooks

Somewhere in the picture is an aeroplane towing a glider. The driving force on the aeroplane is from the engine. This will have to overcome air resistance on itself as well as that of the glider. At constant speed the engine force must equal the counter forces on the aeroplane. The tension in the tow rope would then merely need to be equal to the size of the drag forces on the glider. I was wondering whether the rope would go slack at constant speed but since the glider experiences drag, that can't be true.