Sorry to be pedantic, and at risk of being flamed, but the "obviously only one single reaction force" is not, ISTM, correct. As pointed out by an earlier posting, there are really zillions of little aerodynamic forces acting all over the aeroplane - but for many purposes they can be treated mathematically as though they were a single reaction force, i.e. their "resultant", or they can resolved into two forces at rightangles, by convention called lift and drag. Neither is more right or wrong than the other, it is more a question (as yet another pointed out) of which one suits a particular purpose at the time.
Incidentally, there are also couples (twisting or turning forces) acting upon an aeroplane. Couples cannot be resolved into a single force, if I recall my maths. and engineering training correctly. For basic explanations of P of F these are usually ignored. (Any unbelievers, consider e.g. trailing edge vortices - created by apply a couple to the airflow - Newton's third law leads to the deduction that there is an equal and opposite couple experienced by the aircraft. A single prop creates another couple, in a different plane.)