If we go back to aviation’s infancy, airplanes were seldom equipped with flaps. Thanks to Newton’s Law, however, we have never had an airplane stranded in the sky because its flaps didn’t work. Eventually all airplanes will land. Flaps serve several functions, which we will explore now.
First, flaps usually have multiple settings. Often they are labeled by degrees and frequently referred to by notches or position. Frequently, production airplanes from the 1950s through 1970s avoided the weight, maintenance and expense of a flap motor by the “armstrong method” or the Johnson bar. Simply designed handles in the cockpits lower flaps with the pull of an “emergency brake-like” handle. Usually they have 3 or 4 detents or stops along the way to full flap deployment. Most often these notches represent 10º increments of deployment, so 3 notches is 30º of flaps.
Studying aeronautical engineering, it is noted that everything which protrudes into the slipstream creates drag. Drag created by a willing act, like lowering the flaps, is known as induced drag. So there is a penalty that nature exacts from the deployment of flaps in the form of slowing the airplane down because of the willingness of the pilot to get some benefit from this act. In the case of the first notch or 10º setting, the majority of the benefit gained is lift. The first notch has the effect of extending the chord of the wing, thereby creating a larger lifting surface and permits the airplane to fly at a slower airspeed. The stall speed is decreased significantly allowing a landing at a slower speed. This makes it easier on the landing gear, brakes, tires and occupants.
The next notches of flaps are generally an increase to drag. They help the plane slow down and will usually increase your rate of descent. They get you out of the sky faster without an increase in airspeed. With an increased sink rate that additional notches of flaps create, care must be given to round-out and flair above the runway to ensure you don’t pounce down too hard. Again, this can be tough on your mechanical systems and the humans inside.
Other uses of flaps include short field landings and slow flight. Frequently, you will see glider tow planes deploy a notch of flaps to be able to lift off the runway at slower speeds as well as fly slower during the tow. Gliders generally are much slower machines than the powered planes that tow them aloft, so slower flight with a greater margin of safety above stall speed is accomplished with flaps. Short field take-offs are frequently accomplished by deploying the first notch or 10ºof flaps. This allows the pilot to take off at a slower (safer) airspeed and climb at a greater angle to avoid obstacles. Often, soft field take-offs are assisted by a notch of flaps to get the airplane flying into ground effect faster in order to get off the surface and avoid friction created by soft or sloppy conditions.
Finally, in mountain flying courses, you will find that the deployment of the first notch of flaps, although justified by either soft-field or short-field conditions (or a combination of both) is delayed. It is the practice in high density altitude and marginal conditions at high elevations to get the airplane rolling down the runway first without flaps. At a few knots before take-off speed, the first notch of flaps is then deployed and the airplane will usually take off into ground effect like an Otis elevator. The delay in lowering the flaps is done to allow the airplane to get to flying speed quicker without the drag of flaps in the slipstream. Try it sometime, it really works well.
Remember, the electric flap handle knob on modern airplanes is designed to look like a flap section. This is designed this way on purpose for a number of reasons. It can easily be identified by touch, it cannot be mistaken for a gear handle, which is shaped like a tire and if the cockpit fills with smoke or your vision is somehow otherwise impaired, you can still positively identify this appendage, whether you need to raise or lower the flaps.