Do you ever fly inside a Boeing 737 (let’s read it as 73) before? Those who live in Sarawak should’ve traveled inside a 73 if you wanted to go to the Peninsular. I believe those who ever fly within Peninsular Malaysia will at least flied once with 73, since Malaysia Airlines and AirAsia have a lot of 73. I hope you’d sit at the window seat before, where you’ll be able to see the back part of the wing.
You’d probably guessed that the most interesting time to watch the wing is when during approach (landing phase). That is the time when the aircraft is still descending and you’ll be able to make out houses and shop lots by looking at the surface of the earth. That is the time; the wing is sort of like extending piece of piece at the trailing edge (back side of the wing).
The things extending outward, are called Flaps (Fowler Slotted Flaps, to be more precise). Basically speaking, it is extended to make the wing area larger so that the wing is able to create more lift while slowing down. The plane need to slowdown before touching down on the runway usually at a speed around 130knots for 73 (believe it or not, it’s more than 200Km/h) to reduce tyre stress and wear.
Due to the design of the wing, without the flaps extended, it’ll need to keep a really high nose up pitch to maintain specific lift at specific speed, thus limiting what the pilot can see from the cockpit. So, flaps are used so that the nose can be lowered but still maintaining the same amount of lift at the same speed. This is done by increasing the effective Angle of Attack using the increment of flap angle. Usually the flap comes down in stages, controlled by the pilot (etc. 0°, 2°, 5°, 10°, 15°, 25°, 30°, 40°).
So why aren’t these flaps open all the way from the beginning since they can give so much lift and visual clearance. They create a lot of drag too. And since Jets are built to fly fast, large wing area will not allow that. Lift will be sufficient using small wing area whereby you’ll have high relative airflow (by flying fast). But the simple sweptback wing design will not be effective for low speed flying as it’s very easy for them to stall (wing lose the ability to create life due to turbulent airflow).
When you’re one to two minutes before touchdown, and after maximum (or almost) flap is extended, you’ll see this white smoke coming out near the tip of the wing (which I think unique to the 73). This is actually caused by condensation of the air, cooling the air to its dew point temperature. It is the really high speed of the relative airflow that causes the temperature to go down so much. It’s also due to the enormous amount of lift created by the wing that causes the air to sleep up so fast. You only see this during landing where it’s more humid near the surface of the earth.
Right after touchdown (usually you’ll hear a “tud” or tyre “screak”) you’ll see some part of the wing (in the middle, between the leading edge and closer to the trailing edge) open outward. The surfaces that extended out are called spoilers. During landing, it is used to prevent the wing from making lift by directing the air upward, pushing the wing (and the whole plane) downward, so the wheel (or main gear) brakes can be more effective. It’s also to slow down the plane by creating lots of drags.
If you sit at the window seat where you can only see the leading edge of the wing, you also have your own set of stuff to discover. Flaps on the leading edge are called slats, and they’ll also extent when flaps are extended, but not as much as flaps do. They’re extended to actually make the air traveling on top of the wing to go smoother, so higher angle of attack can be achieve before the wing stall but with the expense of more drag.
Once touchdown, usually after the spoiler is opened and the nose wheel is also down, you’ll notice that the engine’s cover will open (The cylinder shaped thing that’s fix on the wing, 73 got one engine on each wing). That is when the reverse thrust is kicked in. The reverse thrust works by directing the relative airflow sucking into the engine from the front and directing it back to the front (not by sucking the air from behind and push to the front). With that, the reverse thrust is only effective if it’s used at high speed, the point when the airplane just landed and until the plane slows down to 60 knots (~110Km/h). That is the time the breaks will be the only thing stopping the 50,000 Kg (roughly) going as fast as 100Km/h.
For your information, it’s the engines that push the air behind, and make the plane move, accelerate and fly. It’s not the wheels and gears that drive the plane, as it’s only some free moving parts.
I’m not copying from any books for this blog, it’s all from what I know, and I try to be as precise as possible without getting into too much technical details. So if you find anything misleading, please let me know, and I’ll correct it, both the blog and my knowledge.