High Lift Devices — Page 212, Lesson 263

Let’s start with the big picture. High lift devices exist for one central reason: to shorten the take-off and landing distances. Why does that matter? Because if an aircraft can get airborne and get back down over a shorter runway, it can operate at greater weights from a given runway length, and that lets it carry greater payloads. So the whole point of these devices is economic and operational — more load, shorter field. Now, why do take-off and landing distances depend on these devices at all? Because those distances depend on the speeds required at the screen. The screen is the obstacle height you must clear at the end of the runway, and the speeds you need there are laid down in the performance regulations. For both take-off and landing, one of the requirements is a safe margin above the stalling speed. Specifically, for take-off you need 1.2 times VS1, and for landing you need 1.3 times VS0. Let me unpack those symbols. VS1 is the stalling speed in a specified configuration — for take-off that’s typically with flaps in the take-off setting. VS0 is the stalling speed in the landing configuration, with full flap and gear down. The 1.2 and 1.3 are the safety margins — you must be flying at least 20% above the take-off stall speed, and 30% above the landing stall speed. Now here’s the key link. The stalling speed is determined by the maximum lift coefficient of the wing, which we call CLMAX. The higher the CLMAX, the lower the stalling speed for a given weight. And if the stalling speed is lower, then the speeds required at the screen are lower, and the distances come down. So to get the lowest possible take-off and landing distances, CLMAX must be as high as possible. So how do we raise CLMAX? One of the main factors that determines the CLMAX of an aerofoil section is the camber. Camber is the curvature of the aerofoil — the asymmetry between the top and bottom surfaces. Increasing the camber increases the lift coefficient CL at a given angle of attack, and it also increases CLMAX itself. So for take-off and landing, a cambered section is desirable. But here’s the problem: if you built the wing with high camber permanently, you’d get high drag at cruising speeds, and you’d need a very nose-down attitude to fly level. That’s unacceptable for cruise. So the design solution is to select a less cambered aerofoil section to optimise cruise, and then modify the section for take-off and landing by using flaps. That’s the whole philosophy — a clean wing for cruise, and flaps to add camber when you need high lift at low speed. Now let’s define the flap itself. A flap is a hinged portion of the trailing edge or the leading edge which can be deflected downwards, and that deflection produces an increase of camber. So the flap is literally a movable part of the wing surface that, when lowered, changes the shape of the aerofoil to a more cambered one. There’s an important distinction here based on aircraft speed. For low speed aerofoils, the flaps will be on the trailing edge only. But on high speed aerofoils, where the leading edge may be symmetrical or may even have a negative camber, there will usually be flaps on both the leading edge and the trailing edge. So a high-speed wing often needs leading-edge devices as well as trailing-edge ones, because its leading edge is shaped for cruise, not for low-speed lift. Now let’s look at the trailing edge flaps, starting with the simplest type — the plain flap. The plain flap has a simple construction and gives a good increase in CLMAX, although with fairly high drag. Because of that drag, it’s used mainly on low speed aircraft, and specifically where very short take-off and landing is not required. So the plain flap is the basic, straightforward hinged surface — effective at raising CLMAX, but not the most efficient in terms of drag, and not the choice for STOL-type performance. Let me show you what this looks like. — that’s Figure 8.1, the plain flap. You can see the hinged trailing edge portion deflected down, increasing the camber of the section. So to tie it together: high lift devices reduce take-off and landing distances, which lets you operate at greater weights and carry greater payloads. The distances depend on the speeds at the screen, which must be a safe margin above stall — 1.2VS1 for take-off, 1.3VS0 for landing. Stall speed depends on CLMAX, so we want CLMAX as high as possible. Camber raises CLMAX, but a permanently cambered wing hurts cruise. So we use flaps — hinged leading or trailing edge surfaces that deflect down to add camber only when needed. And the plain flap is the simplest trailing edge type: good CLMAX increase, fairly high drag, used on low speed aircraft where very short field performance isn’t required.