FLAPS AND CONTROLS

Highly cambered aerofoils are often referred to as ‘high lift’ sections. It is true, as Figure 7.4 shows, that the highly cambered section has a higher ci max. This is familiar enough and is the reason why full-sized aircraft and some models have landing flaps. The point is not that such surfaces, or their equivalent, highly cambered wings, develop more lift force. In equilibrium, lift equals weight. The wing with flap down has to support only the same aircraft weight, but, operating at a high Cl, it can do this work at a lower airspeed. Hence the value of flaps for landing and take off. Figure 7.6 shows the effect of moving any hinged surface such as an aileron, rudder or elevator, or a wing flap of the plain variety, to different angles. As the flap goes down, the whole ci curve moves to the left on the graph, and upwards. At the same time, if the attitude of the aircraft to the flight path is not altered, the effective angle of attack increases because the chord line of die wing is in a new position. On raising the flap, the converse happens. The effect of such hinged surface movements is a combination of increased camber and increased angle of attack, or vice versa. Split flaps have similar camber-changing effects, but have the advantage for landing of also creating high profile drag. This decreases the L/D ratio and steepens the glide path on the approach, helping the pilot to judge his touch down position accurately. Such flaps have value on models required to carry out precision landings. Under-surface airbrakes, mounted at about 50% of the chord, also change the wing camber and increase ci max. slightiy, with high drag. This type of brake has been used in some full-sized sailplanes but is vulnerable to ground damage. All camber-changing devices change the pitching moment of the aerofoil, as discussed more fully in the last section of this chapter.