A force is always an interaction between two material objects.
The airplane exerts a force on the glider causing it to accelerate. The glider in turn exerts a force on the airplane. There are, of course, additional forces present such as the propeller force and air drag on both planes.
Focusing for the moment exclusively on the force between the airplanes.
we can state Newton's 3rd Law.
To every force in nature there is corresponding reaction force.
The reaction force has the same magnitude and acts in the opposite direction. The two forces act on different objects and do not cancel one another. Only forces acting on the same object can be added together.