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I am still trying to understand how the approach speeds for different flap speeds work. I cant find it in the handbook but I want to know the different approach speeds for the following flap settings:

  • 0 degree Flap
  • 10/15 degree Flap
  • 20/25 degree Flap
  • 39/40 degree Flap
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  • $\begingroup$ Do you mean the maximum speeds for flaps? Or do you mean stall speeds? As Jamiec said, flaps don't change your approach speed, but there is a maximum speed at which you can deploy flaps... $\endgroup$
    – Ron Beyer
    Jun 19, 2021 at 12:20
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    $\begingroup$ The appropriate approach speed for an airplane (C 152 and others) certainly does change with the flap setting. The higher the flap setting, the slower the normal approach speed. $\endgroup$
    – user22445
    Jun 19, 2021 at 16:17
  • $\begingroup$ @757toga some folks may be looking at it another way: an assigned approach speed and glide angle (like 3 degrees). In that case (for example a commercial aircraft), flaps (and power) would be determined by weight for a given airspeed and AOA. $\endgroup$
    – Robert DiGiovanni
    Jun 19, 2021 at 23:52

2 Answers 2

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The approach speeds for different flap settings are shown in the Cessna 152 POH. See the 1980 Cessna 152 POH.

Although the POH does not break down the different flap settings referenced in your question (some interpolation is necessary) it does identify the appropriate speeds for flaps up, normal approach (flaps 30) and short field approach (also flaps 30).

The primary benefit of using flaps is to allow for a lower airspeed while still generating the appropriate lift. (this also results in a steeper angle [when a slower speed is used] for landing and takeoff).

1980 Cessna 152 POH enter image description here

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FLAPS 0 can used for a higher approach speed when there is wind. This "clean" configuration will give you the greatest gliding distance (least drag per distance flown). Approach speeds are higher and the glide angle is flatter. This is fine when one has adequate runway to work with. With 0 flaps, the wing has less camber and lower coefficient of lift. It makes up for this by going a bit faster.

Flaps 10 is used for short field approaches. It increases wing camber and coefficient of lift, allowing the aircraft to approach around at a slower speed while still generating adequate lift. Because there is greater drag, the glide angle will be slightly steeper. Better to use this one if the landing area is shorter.

FLAPS 20, 30 (and 40) serve to further steeper the glide angle by increasing drag. These flap settings generally do not increase lift much more and are not used for that purpose because they are excessively draggy.

FLAPS 10 can also be used for short, soft field takeoffs, but the plane climbs more efficiently at Vy, with FLAPS 0.

More reading about flaps here.

Another way of looking at it is: increasing coefficient of lift by increasing camber allows one to fly at the same Angle of Attack at a lower airspeed.

Angle of Attack and Camber form the Lift Coefficient. Too great and angle of attack causes a stall, so it is smarter to increase camber to fly more safely at a lower airspeed.

The variables of lift related to aircraft configuration and velocity are explained in the Lift Equation:

Lift = Density × Area × Lift Coefficient × Velocity$^2$.

Note that velocity is squared.

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