Flaps are popularly used for full-scale aircraft as a means of
altering the lift-to-drag ratio of a fixed wing, especially at low
speeds. Small flap extensions, say 10 , can increase the lift of an
airfoil, allowing shorter takeoffs and much slower landings. Large
flap extensions, typically up to 40
, markedly increase the drag of
the airfoil, allowing for slow landings with steep, predictable glide
slopes. In fact, many aerodynamically-clean aircraft practically require
some extension of flaps to land easily at all! Even small general
aviation planes like the venerable single-engine Cessnas will float along
seemingly forever without flaps, but have a markedly steep glide path
with full application of flaps.
Radio-controlled models can often benefit from the same application of principles. Many of us have had one model or another that was very fast, very ``clean'' - and almost impossible to slow down to land predictably or reliably. Scale models are notorious for this kind of behavior, especially models with high wing loadings such as WWII warbirds.
Most modelers don't add flaps to their planes, however. One reason is the popularity of simple 4-channel radios; however 6-channel radios are arguably even more common nowadays, and most of those pilots still never use the 5th or 6th channels. Increased complexity in wing construction is certainly an issue, especially since most kits don't include plans or materials for flap construction. So what is one to do?
One neat way to easily retrofit flaps on your plane is by the use of flaperons - a term implying use of ailerons as flaps. To use flaperons, you will need a radio with mixing capabilities (i.e. just about any ``computer radio'') or will need to install a mechanical mixer. The structure of the wing remains intact, since the ailerons were presumably there in the first place!
There are, however, some issues that must be addressed when using flaperons, namely 1) Adverse yaw while the ``flaps'' are down, and 2) Pitch trim changes when lowering the ``flaps''. How shall we deal with these issues?