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Drag is the aerodynamic resistance acting parallel to the relative airflow and opposite to the direction of motion. Thrust acts forward to overcome it, while lift and weight act roughly perpendicular to the flight path. In steady level flight thrust equals drag.
With no vertical acceleration, the sum of vertical forces must be zero, so lift balances weight. Thrust balances drag in the horizontal direction and is unrelated in magnitude to lift. This balance is the basis for calculating the lift required at any weight.
For steady, incompressible flow along a streamline, an increase in velocity is accompanied by a decrease in static pressure so that total pressure stays constant. This is the basis of the venturi explanation of lift generation. Total pressure does not fall in ideal flow.
Within the normal operating range, lift coefficient rises almost linearly with angle of attack. Beyond the critical angle the airflow separates and the coefficient falls abruptly, producing a stall. This is why angle of attack, not airspeed alone, determines stall.
Angle of attack is measured between the wing chord line and the direction of the relative airflow. It is distinct from pitch attitude, which is measured relative to the horizontal. The two coincide only in specific flight conditions.
A stall occurs when the angle of attack exceeds the critical value and the airflow separates from the upper surface. It can occur at any airspeed or attitude if that angle is reached. Airspeed and power changes only matter because they alter the angle of attack required.
On a conventional cambered wing the centre of pressure lies behind the leading edge and, in normal flight, generally ahead of the centre of gravity. Its position moves forward as angle of attack increases. This movement contributes to static longitudinal stability considerations.
Induced drag arises because the pressure difference between the lower and upper surfaces generates wingtip vortices and downwash. It is therefore a by-product of lift and increases as the coefficient of lift rises. It is greatest at low speed and high angle of attack.
In a level turn the load factor equals one divided by the cosine of the bank angle. The cosine of 60 degrees is 0.5, so the load factor is 2.0. The stalling speed therefore increases by the square root of two, about 41 percent.
Stalling speed increases with the square root of the load factor. At 60 degrees of bank the load factor is 2.0, so the stalling speed becomes 50 times 1.414, about 71 kt. This is why steep turns demand higher approach speeds.
Indicated airspeed reflects dynamic pressure, which depends on density. At higher altitude the lower density means a greater true airspeed is needed to produce the same dynamic pressure. True airspeed therefore rises for a constant indicated airspeed.
In a steady climb there is no acceleration, so excess power is converted into the rate of gaining potential energy, that is, the rate of climb. Rate of climb equals excess power divided by weight. Excess thrust, by contrast, gives the climb angle.
Climb angle depends on the ratio of excess thrust to weight, so the steepest climb occurs where excess thrust is greatest. This speed is lower than the best rate-of-climb speed, which depends on excess power. The distinction matters for obstacle clearance.
Glide range depends on the lift-drag ratio: the flattest glide path is obtained at the speed giving maximum lift over drag. Minimum sink rate gives maximum endurance instead. Wind alters the achieved ground range but not the best-glide airspeed.
Lift equals the lift coefficient times dynamic pressure times wing area. If area doubles with weight and speed fixed, the required coefficient must halve to keep lift equal to weight. This explains why heavily loaded aircraft with small wings need high coefficients and high angles of attack.
Induced drag is inversely proportional to aspect ratio for a given lift coefficient and span loading. A long, narrow wing therefore produces weaker tip vortices and less induced drag. This is why gliders use very high aspect ratios.
Constant indicated airspeed means constant dynamic pressure only if density is unchanged. In thinner air the true airspeed for the same indicated value is higher, so the lift coefficient needed falls and the angle of attack decreases. The reverse applies when descending into denser air.
In any steady unaccelerated flight, forces along the flight path must balance. The weight component along the flight path contributes to forward motion, but thrust still balances the remaining drag so that net force is zero. If thrust exceeded drag the aircraft would accelerate.
A symmetrical aerofoil has identical upper and lower surfaces, so at zero angle of attack the pressure distribution is the same on both sides. No net pressure difference exists and therefore no lift is generated. Lift appears only when the angle of attack becomes non-zero.
Extending flaps increases camber and effective wing area, raising the lift coefficient at a given angle of attack. The same deflection also increases drag, which steepens the approach path and permits a slower approach speed. The critical angle of attack is generally reduced rather than increased.
Vortex strength depends on the lift coefficient and weight: a heavy aircraft flying slowly in a clean configuration needs a high lift coefficient. This produces strong downwash and vigorous tip vortices. Flap extension spreads the vortex and reduces its peak strength.
In a level turn lift must balance both weight and the centripetal requirement, so lift equals weight divided by the cosine of the bank angle. The cosine of 45 degrees is 0.707, giving about 16,970 N. The load factor is therefore about 1.41.
At the absolute ceiling the engine and propeller can produce just enough power to maintain level flight, leaving no excess power for climbing. Any attempt to climb further results in a descent. This is why service ceilings are defined by a small residual climb rate.
A forward centre of gravity increases the nose-down moment that the tailplane must counter, so it must produce more download. The extra tail download adds to effective weight and reduces the maximum attainable lift coefficient, raising the stalling speed. Longitudinal stability also improves at the cost of higher trim drag.
Sweep reduces the chordwise component of the airflow, so the effective Mach number normal to the isobars is lower than the freestream value. This delays the onset of shock waves and raises the critical Mach number. Sweep does, however, bring penalties such as poorer low-speed handling and tip stall tendencies.
In steady level flight the vertical forces balance, so lift must equal weight. If weight rises, the wing must generate more lift, either by increasing angle of attack or speed. Thrust, drag and side force act in other axes and do not balance weight directly.
Angle of attack is defined between the wing chord line and the direction of the relative airflow. It changes as the aircraft pitches or as the flight path changes. Angle of incidence is a fixed rigging angle between chord and fuselage reference line.
Bernoulli's theorem states that in a streamline flow, an increase in velocity is accompanied by a decrease in static pressure when energy is conserved. Faster air over the upper surface therefore produces lower local pressure. This pressure difference contributes to the total lift.
The lift equation shows lift proportional to the square of true airspeed, along with density, wing area and lift coefficient. Doubling the speed therefore quadruples the lift if other factors stay constant. This is why speed has such a strong effect on performance.
For steady, unaccelerated level flight there is no net force in either the vertical or horizontal axis. Lift balances weight vertically, and thrust balances drag along the flight path. Any imbalance would produce acceleration.
Load factor is the total lift produced divided by the aircraft's weight. In a level turn, lift must exceed weight to provide the horizontal centripetal component, so load factor is greater than one. It increases with bank angle.
The boundary layer is the thin region adjacent to the surface where viscous friction slows the air, from zero at the surface to free-stream velocity at its outer edge. Its behaviour determines skin-friction drag and whether the flow separates. It exists over both upper and lower surfaces.
Dihedral gives lateral (roll) stability because a sideslip causes the lower wing to meet the airflow at a greater effective angle of attack, creating a restoring roll. This returns the aircraft toward wings-level flight. Longitudinal stability is provided mainly by the tailplane.
Lift per unit area equals dynamic pressure multiplied by lift coefficient: wing loading = q × CL. Rearranging gives q = 1,200 / 0.5 = 2,400 Pa. This shows dynamic pressure must double when the lift coefficient is halved at fixed wing loading.
Stalling speed varies with the square root of the weight ratio when the maximum lift coefficient is unchanged. The ratio is 1.6, whose square root is about 1.265, so 60 kt × 1.265 ≈ 76 kt, closest to 75 kt. Heavier aircraft therefore stall at higher speeds.
Load factor in a level turn equals one divided by the cosine of the bank angle. The cosine of 60 degrees is 0.5, so the load factor is 2.0. The wing must therefore produce twice the aircraft weight in lift.
The critical angle of attack at which the wing stalls is a characteristic of the aerofoil and its configuration, not of weight or speed. Weight and altitude change the speed at which that angle is reached, not the angle itself. This is why angle of attack is the reliable stall indicator.
The flattest glide, giving maximum range, occurs at the speed where lift-to-drag ratio is greatest, because the glide angle depends on the cotangent of that ratio. Minimum sink gives the longest time aloft but a steeper angle. Maximum thrust is irrelevant with the engine at idle.
Induced drag coefficient is proportional to the square of the lift coefficient divided by aspect ratio. A higher aspect ratio therefore reduces induced drag for the same lift. This is why gliders use long, slender wings.
Lift is proportional to air density, so at the same true airspeed and angle of attack a reduction in density reduces lift. To maintain level flight the aircraft would need a higher angle of attack or higher true airspeed. This is why true airspeed must increase with altitude for a given indicated airspeed.
The critical Mach number is reached when the local airflow over the most favourable point of the airframe first accelerates to sonic speed, even though the aircraft itself is still subsonic. Beyond this, shock waves begin to form and drag rises. It is always lower than the drag divergence Mach number.
Parasite drag is proportional to the square of the true airspeed at a fixed configuration. Doubling speed therefore multiplies parasite drag by four. Induced drag, by contrast, falls as speed rises for a given weight.
The lift curve slope is the change in lift coefficient divided by the change in angle of attack. The change is 0.3 over 5 degrees, giving about 0.06 per degree. This slope is roughly constant in the linear portion of the lift curve.
In a steady climb the lift vector is perpendicular to the flight path, which is inclined upward, so only a component of lift opposes weight. Lift therefore equals weight multiplied by the cosine of the climb angle, which is slightly less than weight for normal climb angles. Thrust provides the component of weight along the flight path.
Induced drag coefficient is inversely proportional to aspect ratio at constant lift coefficient. Doubling aspect ratio from 8 to 16 therefore halves induced drag from 0.02 to 0.01. This is why high aspect ratio wings are more efficient.
Equivalent airspeed equals true airspeed multiplied by the square root of the density ratio to sea level. The density ratio is about 0.738, whose square root is about 0.859, so equivalent airspeed is about 129 kt. Rechecking the standard values, the closest option reflects a reduction below true airspeed.
For a fixed weight, induced drag falls as speed rises because a smaller lift coefficient is needed, while parasite drag rises with the square of speed. Minimum total drag occurs where the two are equal. This trade-off explains the U-shaped total drag curve.
A forward centre of gravity increases the tail download needed for balance, adding trim drag and increasing effective wing loading. The higher wing loading raises the stalling speed. The stalling angle of attack itself is unchanged, and take-off distance usually increases.
Rate of climb depends on excess power, the difference between power available and power required for level flight. As altitude rises, engine power available decreases while the power required at the high true airspeed for level flight increases, so excess power shrinks to zero at the absolute ceiling. Lift is still produced, but no surplus power remains.
Laminar sections keep the boundary layer smooth over a greater portion of the chord, reducing skin-friction drag in the cruise condition. This benefit is easily lost through surface roughness or contamination. They do not primarily raise maximum lift or the stalling angle.