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When reaching critical AOA,

Asked by: 454 views Aerodynamics, Aircraft Systems, Commercial Pilot, General Aviation, Private Pilot, Student Pilot

As you reach critical AOA, is it induced drag or form drag that is aggravating production of lift ?

I understand that the form drag arises from the shape of the airplane and induced drag, as you pitch up more and more, boundary layer forms and air is allowed to move straight forward...

 

Connor.

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2 Answers



  1. Skyfox on Nov 03, 2016

    Induced drag is a direct byproduct of lift. Basically, it’s the rearward-acting component of the lift vector. The lift vector is typically perpendicular to the chord line of the airfoil. As you increase the AOA, that vector gets tilted backwards and more of what was a vertical component of lift is transferred into a horizontal component of lift acting rearward. Your maximum coefficient of lift is at the critical AOA, which would have the most rearward component of lift and so maximum induced drag.

    Parasite drag is the other type of drag, not related to the production of lift, and includes form drag, skin friction drag, and interference drag. Form drag is easily described as whatever shape the aircraft punches through the air. In a typical cruise configuration the aircraft is clean (flaps up, gear up if retractable) and nose level to make it as aerodynamic as possible for that higher cruise speed. That’s because at higher speed the wings don’t need a high AOA to produce the same amount of lift. But while the form is minimized in that flight attitude, drag due to the presence of the air the aircraft has to push through increases significantly with speed; every time the speed doubles, wind drag quadruples. At low speed as in the slow flight configuration, even though the aircraft is nose high and presenting a lot of form to the relative wind, the lower relative wind has a much lower drag on the form as well as on the aircraft skin and surface intersections (ie. lower skin friction drag and interference drag), making total parasite drag lower.

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  2. Brian on Nov 09, 2016

    The boundary layer is always there. It’s a thin layer of air that covers the entire airplane. On the top of the wing the boundary layer near the back of the wing will reverse flow as the airplane’s AOA increases. The area just in front of that reversed flow will be stagnant and anything forward of that will still be moving in the direction of the free stream (the rest of the air passing by the A/C). The stagnant area in the boundary layer, called the stagnation point, is the point where the airflow on top of the wing separates from the airfoil. As your AOA is increased this stagnation point, which starts near the trailing edge in high speed flight, creeps farther and farther forward toward the leading edge.

    Watch this video of an aircraft that has Tuffs across it’s wing as the pilot demonstrates a stall. It will demonstrate what I explained above: https://www.youtube.com/watch?v=WFcW5-1NP60

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