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4 Answers

Flaps for normal takeoff?

Asked by: 2733 views Aerodynamics, General Aviation, Private Pilot


Lately I've been curious about the flap settings for a Cessna 152 for a normal takeoff. The POH states that a normal takeoff may use 0-10 degrees of flaps. For a normal takeoff, is there any benefit from using flaps 10?  In my training, it was always a clean wing. I understand the 10 degrees are used on soft and short field takeoffs because of the extra lift. I'm just curious!


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4 Answers

  1. Best Answer

    John D Collins on Feb 15, 2014

    Flaps will reduce the takeoff roll and permit clearing a 50 foot obstacle closer in to the threshold. The climb angle is often less with flaps, so the main advantage is getting off the ground sooner and starting the climb to clear the obstacle sooner. There is a point of diminishing returns where a flap up climb overtakes a climb with flaps, but usually this is high enough that flaps are an advantage for close in obstacle clearance.

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  2. Brian on Feb 16, 2014

    “I understand the 10 degrees are used on soft and short field takeoffs because of the extra lift.”

    Careful with this statement. Flaps do not add lift, they allow you to fly at a lower speed by increasing lift coefficient. Lift itself will just equal weight, same as without flaps. Flaps lower stall speed, adding drag, and changing the effective camber (shape) of the wing. That is it.

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  3. Jeffrey A. Baylor on Feb 19, 2014

    Brian, I beg to differ. You indeed do change the lift coefficient by adding flaps. But how? At a slower airspeed you are able to “Lift” the same “Weight” by increasing wing surface or “Lifting” area. With the associated Lift comes also the associated drag. If flaps are retracted, one must increase speed to generate an equivalent lift. Lift is added with application of flaps. This is why the Pilots Handbook of Aeronautical Handbook refers to them as “High-Lift” devices.

    The L/D ratio is determined by dividing the CL by the CD, which is the same as dividing the lift equation by the drag equation. All terms except coefficients cancel out.
    L = Lift in pounds
    D = Drag
    Where L is the lift force in pounds, CL is the lift coefficient, . is density expressed in slugs per cubic feet, V is velocity in feet per second, q is dynamic pressure per square feet, and S is the wing area in square feet.
    CD= Ratio of drag pressure to dynamic pressure. Typically at low angles of attack, the drag coefficient is low and small changes in angle of attack create only slight changes in the drag coefficient. At high angles of attack, small changes in the angle of attack cause significant changes in drag.
    The above formulas represent the coefficient of lift (CL) and the coefficient of drag (CD) respectively. The shape of an airfoil and other lift producing devices (i.e., flaps) effect the production of lift and alter with changes in the AOA. The lift/drag ratio is used to express the relation between lift and drag and is determined by dividing the lift coefficient by the drag coefficient, CL/CD.

    Respectfully added

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  4. Brian on Feb 19, 2014

    The term “add lift” translates to either having more lift or better efficiency. Both of which are untrue. Lift will equal weight in any steady condition and your glide distance decreases due to added drag. Flaps allow for a higher lift coefficient, not a greater lift force.

    Keep in mind that induced drag coefficient increases with the square of lift coefficient. This makes it quite clear that anything that increases lift coefficient will increase drag coefficient far more. There is no such thing as lift flaps and drag flaps or flaps that. Flaps increase drag more than lift. If this were not true you’d use flaps to extend your gliding distance.

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