This is a question for the flight school forum.


At low speeds, flaps induce drag, slowing down the aircraft.  At high throttle settings, flaps increase rate of climb, allowing shorter takeoff roles.  My reccomendation is that you go through the lessons to learn about all of the different systems.

Flaps come in three major categories. Simple (the rear part of the wing drops down), Split (the underside at the back of the wing drops down), and Fowler (the underside at the back of the wing drops down and backwards). They are used primarily for taking off and landing. Some aircraft can use them for better slow-speed manoeuvring.  The use of flaps is directly related to the type of flap and the type of installation on each aircraft.  Some flaps produce nearly all drag and are only used for landing e.g. the Supermarine Seafire III which had only two settings closed or all the way open (with brick-like aerodynamics).  Others have settings that produce more lift and are very valuable in a pinch by lowering the stall speed and increasing your ability to turn at slow speeds (the Fowler)

Simply, "to increase the angle of approach without increasing airspeed" - as quoted from something I remember reading a lot during flight training.

It's all covered pretty well here. Nothing to add, maybe just to re-state that there are different types of flaps, and that they can have different primary and secondary uses, depending on the plane.. AND the situation.

I'll stick with a plane I know well; a Cessna 172... and one of the original questions:  "What's the difference between, say, landing with 20 flaps and landing with no flaps at all? "  In a 172, it's quite literally a football field worth of runway. There's an honest 300 more feet of runway needed for a no-flap landing. And though they are NOT speed brakes or spoilers (a speed brake does not increase lift and a spoiler's purpose is to disrupt lift), they will slow you down and get you down quicker (under normal circumstances.. nothing will lose altitude faster than a no-flaps dive, but that gets you into Vno territory). Also.. on the ground and below stall speed, they do function as speed brakes (but there's a fine line between how much they'll slow you, and how much they reduce wheel brake effectiveness by taking weight off of the tires).

Anything I'd say about landing would be redundant. Just remember that they allow for a steeper, slower descent.

Takeoffs are a little more complicated. Flap use on a soft field (where field length and nearest obstacle are not a concern), is purely to get the wheels up off the soft ground ASAP. In a 172, you start your soft-field takeoff roll with 10 degrees of flaps and the yoke ALL the way back, and when the mains lift off, you immediatley pitch down (even slightly nose-down) to build airspeed in ground-effect so that you can start to climb at Vy and retract the flaps. No matter how much lift they generate, you'll gain altitude faster (against time (best rate)), with no flaps. Short field takeoffs (where runway length is your only worry) are similar to soft field; get the wheels off the ground ASAP. Short field takeoffs where there's an obstacle to clear require a Vx climb (most altitude gained for a distance traveled (best angle). This is where it gets tricky. A 172 will  climb better at Vx with no flaps... but since you start the climb WITH flaps and don't have time to build airspeed you must leave them deployed until clear of the obstacle. If you retract the flaps while climbing at Vx, your rate of climb will decrease until a new AoA is established. In other words.. if you're already flying along and need to gain 1000 feet RIGHT now (mountain looming), you'd simply pitch for Vx. Some visualize that the steeper angle that 10 degrees of flaps would allow is what's called for, but that's not the case. You'll get more altitude across the ground at Vx, no-flaps, than you will at Vx with flaps.

beaky wrote on Sep 5^th, 2007 at 8:55pm:


The other thing to say on that point is that flaps allow us to get away with runways which are only a couple of miles long!

DaveSims wrote on Sep 28^th, 2007 at 8:53pm:


Quite. In the type I'll soon be flying a flapless landing is not something to be taken lightly - they are only practiced on touch and goes and overshoots. Landing speed it in the region of 25kts faster and it takes up to a third more runway. Not too mention you'll quite possibly be changing all the tyres and brakes afterwards too (and potentially blocking the runway).

beaky wrote on Sep 5^th, 2007 at 8:55pm:

Depends on what the plane designe had in his mind when drawing the plane. On a Yak-18s and Yak-52s the flaps are just one thing: Airbrakes.