As per your diagram , the aircraft experiences T (I guess its forward propelling/throtelling force)and D (backward drag force) in the horizontal direction (x). You need the velocity to apply this equation :-
A possible solution maybe :-
(T-D)/m = A = Gives you the acceleration of aircraft in positive x direction.
If T and D are not time varying forces then velocity calculation will be pretty straight forward :-
v = u + A*t
where t is time lapsed from the initial point till now and u the velocity at initial time
If these forces are not constant you have to go for integration:-