Flaps are essentially similar to ailerons except that they are used to increase the wing lift coefficient and not to generate a roll moment (indeed, the roll caused by a jammed flap can be a serious control issue and must be considered when sizing ailerons). Since flaps are generally only fully deployed during landing, a low drag result is not that important in flap design so that the so-called split flaps can sometimes be fitted to good effect. Since we aim to simplify the wing design where possible, we normally just use a series of movable surfaces along the
Figure 3.11 Fowler flap - note the complex mechanism required to deploy the flap.
full length of the wing trailing edge with the innermost one taking the role of the flap and the outer ones that of aileron, that is, having sized the ailerons, we generally use the remaining real estate on the wing to fit as large a flap as possible - this is what is seen in Figure 3.8. Note also that there is little to gain from extending any flap further forward than the trailing 30% of the wing chord (see, e.g., Hoerner ). Although we commonly used plane flaps, we have tried more complex systems such as Fowler flaps, see Figure 3.11. While traditional flaps will generate section lift coefficients up to 2.0, single-part Fowler flaps will give coefficients as high as 3.0 and two-part systems even more (as long as the flow remains attached to the flap).