Drag – is the component of the aerodynamic force that is parallel to the relative wind, and acts in the same direction. The coefficient of drag is low and nearly constant at very low angles of attack. As AOA increases, the coefficient rapidly increases. It is never zero and is a combination of parasite drag and induced drag.

Parasite Drag - all drag that is not associated with the production of lift. It is composed of form drag, friction drag and interference drag

• Form Drag – also called pressure or profile drag, is caused by the airflow separation from a surface and the wake that is created in that separation. Primarily based on the shape of the object. It temporarily is significant until the aircraft velocity decreased to match the higher angle of attack. To reduce form drag, surfaces exposed to the airstream are streamlined like a teardrop. This reduces the size of high static pressure leading edge stagnation points and reduces the size of the low static pressure wake
• Friction Drag – is due to viscosity and is a retarding force. Turbulent flow creates more friction drag than laminar flow. It is reduced by making sure the plane’s surfaces are smooth. This is done by painting, cleaning, waxing or polishing, using flush rivets
• Interference Drag – generated by the mixing of streamlines between one or more components. It can be minimized by proper fairing and filleting which allows the streamlines to meet gradually rather than abruptly
• Equivalent Parasite Area (f) – a mathematically computed value equal to the area of a flat plate perpendicular to the relative wind that would produce the same amount of drag as form drag, friction drag and interference drag combined. It is NOT the cross sectional area of the plane. It varies directly with velocity squared

Induced Drag

• Infinite Wing – ideal situation where the wing tip is against a wall and the relative wind is only flowing chordwise, creating lift
• Upwash – the wind hits the leading edge and wants to take the shortest route around the leading edge. This results in some of the air that should of passed under the wing to flow up and over the leading edge. This increases lift because it increases the average angle of attack on the wing
• Downwash – the air on top of the wing that flows down and under the trailing edge. It decreases lift by reducing the average AOA on the wing
• For an infinite wing, the upwash exactly balances the downwash resulting in no net change in lift. Upwash and downwash exist anytime an airfoil produces lift
• Finite Wing – Up and Downwash will not be equal. Once the air reaches the wingtips, it flows around it and the upper surface of the wing. There, it combines with the chordwise flow that has already produced lift and adds to the downwash. Downwash approximately doubles by this process to the spanwise airflow moving around the wingtip
• Induced Drag – the portion of total drag associated with the production of lift.
• Average Relative Wind – because downwash is twice as much as upwash on a finite wing, the relative wind will have a downward slant compared to the free airstream
• Effective Lift – The perpendicular component of total lift. It is less than total lift because total lift is inclined
• In level flight where lift is constant, induced drag varies inversely with velocity, and directly with the AOA

Total Drag – the combined total of parasite and induced drag

Lift to Drag Ratio – an airfoil produces lift, but will always have drag. An airfoil that produces more lift and less drag is desirable. The larger the ratio, the more efficient the air foil

L/DmaxAOA produces the minimum total drag. It is located at the bottom of the total drag curve and any movement away from this will increase drag

L/DmaxAOA parasite drag and induced drag are equal. At velocities below max, the planes is affected primarily by induced drag, while at higher velocities, parasite drag takes over

L/DmaxAOA produces the greatest ratio of lift to drag. This is not the maximum amount of lift!

L/DmaxAOA is the most efficient AOA. L/D is the efficiency of the wing not the engine!

An increase in weight or altitude will increase L/Dmax airspeed, but not affect L/Dmax or L/DmaxAOA. A change in configuration may have a large effect on L/Dmax and L/Dmax Airspeed. The effect of configuration on L/DmaxAOA will depend on what causes the change and how much change is produced