In a steady (unaccelerated) and level flight, an airplane is in a total balance i.e. four major forces cancel each other L=W means levelled flight ( balance in vertical direction) , T=D means no acceleration or deceleration i.e. uniform speed (balance of forces in horizontal direction) where L is the lift force generated by lifting devices i.e. wings mainly, and/or canards and/or fore-wing and also the tail but that is normally aimed to counter or provide the moment for pitching motion. The lift force balances the weight W = m*g L=CL* q *Sref, Lift is an aerodynamic force i.e. force due to the flow of air q= 0.5*rho*U^2 dynamic pressure CL is coefficient of the lift CL= CL0+CLa*a CLa basically the slope of the cl curve and CL0 is the intercept... and for flate plate CLa =2*pi and it is dCL/da as i said it is just the slope. Now the question comes to mind is how does the wing produces lift, so the simple explanation is considering a 2-D sections of the airplane wing (it is a 3D object/surface) called airfoil as shown in the figure blow ( taken fron internet) So when the air flows past an airfoil it divides into two paths, above and below the airfoil. If there is angle between the chord of the airfoil and incoming wind, the lengths of paths are different and that causes pressure difference between the lower and upper surfaces of the airfoil. The upper surface normally has lower pressure and thus known as suction side while the lower surface has a higher pressure or positive pressure. This pressure difference generates the lift View attachment 315681 In figure below you can see the chord, leading edge and trailing of an airfoil. View attachment 315684 Here is the airfoil with forces View attachment 315678 Now, in D=T, is an aerod. force in horizontal direction ( or parallel to the wind speed while the lift is at right-angles as shown in the figure above) for a levelled flight and is known as drag that simply means air resistenace and T is the thrust...which provided by the engine or specifically by the jet D =Cd* q *Sref D = T holds only when the airplane is flying with uniform or constant speed but if thrust goes up, it will accelerate and vice versa. So you can see drag as a negative effect/force and normally the designers try to minimize it or they try to optimise the lift to drag ratio. As we discussed before, the angle between the direction of airflow i.e wind speed and the chord is called angle of attack (denoted by greek letter alpha or briefly AoA)...so the aerodyn. forces in the wind coordinates are life and drag while in body fixed (airfoil) coordinates are axial and normal forces with cofficients Cx and Cz as showin the figure above and these can be transformed into each other by simple vector or trigonometric analysis i.e. drawing the components of the forces one coordinate and equating them with the forces in the other coordinate and vice-versa and the AoA is only angle needed in this simplified case. L = N*cos(a) - T*sin(a) D = N*sin(a) + T*cos(a) A resultant force R is just obtained by the vector addition of axial and normal forces. Now as soon as the aircraft will depart from this levelled and steady flight the balance will be disturbed and net force won't zero and components of the forces will appear and that's where trigonometric ratios come into play. However a wing is a 3D object so the flow over it is 3D...which is not necessarily a good thing as it results in the 3D effect and can reduce the flight efficiency of an airplane. What happens is that the streamlines on the suctions side of the wings get contracted while the on pressure side reverse happens and this differential resultsi n vortex shedding and vortex is rotating flow structure in the wake and effect anything that it interacts. We will see it in more detail when we discuss the wing, canard, wing-tip devices (i.e. wing lets and wing tip mounted weapons). Now lets move on to more flight mechanics. So look at the picture below most probably inspired by our favorite F-16 . Here it is necessary to understand that how the body forces and the aerodynamic act on the body of the airplane...Due to a complex configuration of an airplane the pressure distribution around it changes however the resultant of the aerodynamic forces seems to act through a point called center of pressure while weight of the airplane acts through center of the gravity (cg) and normall there is a distance between these two points and thus it gives rise to a couple (moment) i.e. tendency to rotate about its spanwise or pitching axis. View attachment 315707 Thus the nose of the airplane in the picture about will go up and would keep going up if not arrested by some measure so this configuration is unstable and needs active input by the pilot or the flight computer to keep it flying but it gives an advantange to the airplane i.e. it is ready to maneouvre more easily... a pretty important characteristics for the fighterjets especially during the kutta-fight . However the passenger and cargo airplanes are designed to be inherently stabl and that means the Cp is moved aft of the c.g.