The internet is a wonderful thing, enabling communication and dialogue with people you wouldn't otherwise get to know. Through this blog, a gentleman called Ramapriya who is working in Khartoum in the Sudan has contacted me to ask a question about why flaps are used when landing.
Ramapriya's question is perfectly natural: if flaps increase "lift" then why use them when you are trying to land - surely this is when you want less lift to "stick" to the runway! And what is the difference between spoilers and flaps? Here is Ramapriya's original question:
I've always seen spoilers and flaps deployed fully upon landing. While the reason for spoilers is straightforward, I haven't yet figured why flaps are deployed too. Isn't the landing roll the time when you'd be wanting all the load of the craft to be on the main wheels, which is where the brakes are, instead of creating lift (which is what I presume flaps do) whereby the load gets transferred onto the wings and possibly lessening the braking effect? I know the plane would be decelerating all the time with the engines throttled back fully and even the forward thrust probably depolyed, yet why create any lift possibility at all? Wouldn't braking be more effective with no flaps deployed? Or does the drag produced by the flaps compensate for the lift?
Now I'm not an expert on airliners, but I am happy to expose my ignorance (best way to learn!) and so here, for those interested, is my answer:
The purpose of flaps is to enable the aircraft to fly more slowly without
stalling. Deploying flaps increases both lift and drag. Use of flaps when landing thus enables a steeper approach path, better visibility over the nose and a slower touch-down speed. The slower the touch-down speed (perhaps 120~160kts for an airliner, or around 60~70kts for the smaller aircraft I fly), then the less braking and runway length are required to complete the landing roll before safely exiting the runway at taxi speed. Generally speaking, full-flap will be used for landing as this provides maximum benefit in terms of reduced landing speed and extra drag during the landing roll. (Flaps are also commonly used for take-off in order to decrease the speed at which the wing starts to provide lift - but only partial flap is used for take-off to avoid having too much drag whilst trying to accelerate to take-off speed). A simple definition of aircraft flaps is here: http://en.wikipedia.org/wiki/Flap_(aircraft)
The purpose of spoilers is to decrease lift, while the purpose of air-brakes is to increase drag without altering lift. Sometimes the two effects are combined. One of the most difficult things to do in an aircraft is to "go down and slow down" at the same time - spoilers and air-brakes help with this. In flight, air-brakes are used in the approach phase to slow the aircraft down (it is common for example to have speed restrictions in busy terminal control areas e.g. below 250kts when 30 miles out say, or max holding speed of 180kts below 10,000 feet perhaps). The second use of air-brakes/spoilers is provide additional drag and dump any residual lift during the landing roll, as you have noticed. See simple definition here: http://en.wikipedia.org/wiki/Air_brake_(aircraft).
When landing an airliner, they need to combine the effects of flap (primarily to reduce touch-down speed, secondarily to add some drag) with the effects of spoilers/air-brakes (primarily to add drag, secondarily to dump lift once safely on the ground) as well as reverse thrust, and wheel braking. The shorter the runway / heavier the aircraft / faster the touchdown then the more of these forces will be required at once.
Confusion here seems to stem from the idea that flaps add lift, but during the landing roll we don't want any more lift: the answer to this is that once the aircraft has touched down, it is quickly decelerated (by combined effects of drag, reverse thrust and wheel brakes) to less than the the stall-speed of the fully-flapped wing; therefore the wing (by definition) is no longer producing enough lift to be able to fly. Airline pilots have to calculate in advance a target touch-down speed to be just above the stall speed as they reach the landing threshold - for a given aircraft, this speed will vary considerably depending on the weight (load, fuel, etc), runway length, surface type (e.g. grass has less friction than tarmac) and external conditions (headwind, crosswind, gusts, windshear, runway condition e.g. wet/dry etc).
Landing small aircraft is far simpler of course, but we still have to make take-off and landing performance calculations - see GASCO (General Aviation Safety Council) article here on this: http://www.gasco.org.uk/upload/docs/2005_1_Flight%20Safety16-18.pdf.
Regarding landing without flaps: this is something that we practice because we have to be prepared for situations where e.g. the flaps malfunction for some reason. When landing without flaps, you have to make a flatter approach, you have less forward visibility over the nose therefore, and you touchdown say 10kts faster - which significantly increases the landing run (i.e. runway length required); which is as you would expect given the explanatiom above perhaps!
I hope this is as interesting to others as it is to Ramapriya and me!
Andy
Mum, Ken, Nam at Palm Beach
