The four forces

Thrust is the forward force produced by the propeller (or the jet engine); it opposes drag and, as a general rule, acts parallel to the longitudinal axis. Drag is a rearward, retarding force caused by the disruption of airflow by the wing, fuselage, and every other part sticking into the wind; it opposes thrust and acts parallel to the relative wind. Lift is produced by the dynamic effect of the air acting on the wing, and it acts perpendicular to the flight path, through the center of lift; in level flight, lift opposes weight. Weight is the combined load of the airplane, crew, fuel, and baggage, pulled straight down through the center of gravity (CG) by gravity.

In steady, straight, unaccelerated flight, the PHAK is precise about what balances: the sum of all upward force components equals the sum of all downward components, and the sum of all forward components equals the sum of all backward components. In level flight this reduces to lift equals weight and thrust equals drag. But the four vectors are not all equal to each other; lift and weight are typically much larger than thrust and drag, because lift only has to overcome gravity while thrust only has to overcome the airplane's own aerodynamic resistance.

The moment you touch a flight control, you unbalance one pair of forces on purpose. Raise the nose and you increase the wing's angle of attack (AOA), the angle between the wing's chord line and the relative wind; lift increases, and unless you also add thrust, the airplane trades speed for altitude until a new balance is found. Nothing in the airplane "remembers" straight and level; it only responds, instant by instant, to whichever forces are currently unbalanced.

Lift is proportional to the square of the airplane's velocity: an airplane flying at 200 knots produces four times the lift of the same airplane at 100 knots, at the same AOA and configuration. The PHAK expresses this with the lift equation, L = CL x 1/2 rho V^2 x S, where CL is the coefficient of lift (set mainly by AOA and airfoil shape), rho is air density, V is velocity, and S is wing area.

This one equation explains a large share of practical flying. For a given weight, the lift required is fixed, so if V goes down, CL -- and therefore AOA -- must go up to keep producing the same lift; that is why the nose sits higher in slow flight than in cruise. If air density goes down (altitude, heat, humidity), true airspeed must go up to make the same lift at the same AOA, which is the seed of every density-altitude performance problem you will study later in the course.

Drag divides into two basic types. Parasite drag is every retarding force not associated with producing lift: form drag from the shape of the fuselage and cowlings, interference drag where airstreams collide (the wing root is the classic example), and skin friction drag from air dragging against the surface. Parasite drag grows with the SQUARE of airspeed, so it dominates at high cruise speeds.

Induced drag is the direct tax on making lift: it comes from the same spanwise pressure differential that lets the wing lift, and it grows as AOA increases. Since low airspeed requires a high AOA to keep lift equal to weight, induced drag dominates at low speed and shrinks as speed increases.

Because one type of drag rises with speed and the other falls, total drag traces a U-shaped curve with a minimum at one particular airspeed. That speed is L/DMAX, the point of maximum lift-to-drag ratio: the most efficient speed the airfoil can fly, and therefore the speed for maximum glide distance with the engine out.

A wing flying at a positive AOA creates lower pressure above it and higher pressure below it. Air always moves from high pressure to low pressure, and at the wingtip there is nothing stopping it: air spills around the tip from bottom to top, rolling into a fast-spinning vortex that trails behind each wingtip for miles.

The PHAK is direct about the consequence: whenever an airfoil produces lift, induced drag occurs and wingtip vortices are created -- the two are inseparable. The intensity of the vortices is directly proportional to the weight of the airplane and inversely proportional to its wingspan and speed. In short: heavy, slow, and clean (flaps and gear retracted) generates the strongest vortices, which is exactly the airplane configuration you find on departure and on final approach.

The clean thrust-equals-drag, lift-equals-weight picture only holds exactly in level flight. In a stabilized climb, part of the thrust vector points upward and part of the weight vector points backward along the flight path, acting like extra drag. In a glide, part of the weight vector points forward along the flight path and effectively substitutes for thrust -- there is no engine, but the airplane's own weight, resolved along the descent path, is what keeps it moving forward.

This is also where L/DMAX earns its second job. Because it minimizes total drag, L/DMAX is the speed that stretches a glide the farthest over the ground with no power. It is not, however, the best climb speed for a propeller airplane. The PHAK notes that in a propeller airplane, maximum excess thrust -- and therefore best angle of climb, VX -- normally occurs at an airspeed below L/DMAX, often just above stall speed, because propeller thrust itself falls off as airspeed rises.

The four-forces diagram simplifies one more thing: in a real airplane, thrust, drag, lift, and weight rarely act through a single point, so they create small turning moments (couples) that the airplane's designers have to balance. The wing's center of lift typically sits behind the CG, which would pitch the nose down if left alone; the designer compensates by setting the horizontal stabilizer at a slightly negative AOA so it produces a small download that holds the tail down and balances the nose-heavy tendency.

This tail download is not wasted lift -- it is deliberate stability, and it explains why loading the airplane too far forward or too far aft changes handling. You will build on this exact mechanism in the stability lesson later in this module, where it becomes the basis of an airplane's ability to fly "hands off" for a few seconds without diverging.