Hey Guys – all of this talk about raising flaps to decrease lift and extending flaps to increase lift HAS to be conducted with the understanding that the lift generated (regardless of the flap setting) is due to the airflow over/under the wing. What causes the air to flow over/under the wing? Thrust; pushing the airplane and the wing through the air. Yes, it is true that by extending flaps all the way (for landing) the airplane can fly at a slower speed – but, as I’m sure you all recall, there is a significant increase in thrust required to be able to fly at this slower airspeed. Why? Two primary reasons and they’re both “drag.” The first is the induced drag generated with the increased lift. The second is the form drag from the flaps being further deflected into the otherwise clear air. Those of you who fly will recognize the feeling you get if you trim the airplane for straight and level flight with an approach flap setting and select full flaps, changing nothing else. The airplane slows down almost immediately (directly related to the inertia of the airplane of course) and you either need to add a bunch of power or you need to push the nose over to descend and add some power as well.
The next time you have the opportunity, watch the flaps as they extend and retract. If you’re watching a commercial airplane, the first few increments of flap extension generally only increases the lifting area of the wing. In other words the flaps move rearward, with very little downward deflection. In the intermediate ranges, there is some additional rearward movement but a substantial increase in the downward deflection. And the last increments are almost entirely downward deflection. So, knowing what effect flaps have on the lifting capability of the wing, and keeping the relative movement of the flaps in mind, what happens as the flaps are retracted from fully extended to fully retracted? Yes, lift will be reduced – but that lift is directly related to the amount of “oomph” being used to push that configuration through the air. The other things that get smaller are those drag components; less lift – less induced drag; less deflection into clear air – less form drag.
The other thing you need to be aware of in these discussions is the airspeed that you’re going to be dealing with. Remember that Vref (the speed you should be holding down final – assuming a no-wind condition) is 30% above the stalling speed for that configuration. And the takeoff safety speed (commonly referred to as V2) is about 20% above the stalling speed for that configuration. Sometime, just for grins, you might want to check your airplane manual for these speeds for the same gross weight.