Flaps are a misunderstood feature. Sometimes even the people who are using flaps don’t understand them. I’m no expert, but I will address flaps as a model airplane builder and casual user of flaps. Maybe I can help you decide whether or not the benefits of flaps justify the added complication of adding them to your airplane.
First, I want to make an esoteric point about the way that people think about RC planes. It’s very common to think in terms of simple scalar values. For instance, it’s common to think of planes as either easy or hard to fly, thus there is a scale with gliders and trainers on one end, and WWII models on the other. All other planes fit somewhere on this scale, and once you master one plane you have to move forward to the next one or risk boredom, so you can’t go backwards and start flying a trainer again after you have flown a pattern plane. This really isn’t accurate, but it is a common way of thinking. There are lots of design features that get included in these erroneous scalar definitions. For instance, flat bottom airfoils are for beginners or people who are getting old, and symmetrical airfoils are for cool people who like to have fun. Semi symmetrical airfoils fall in the middle, and of course an undercambered airfoil is beyond flat bottom, enough so that most people don’t want to have anything to do with them. Another common thought is that taildraggers are cool, but tricycle landing gear is nerdy or goofy looking. Similarly, high wing planes are nerdy and goofy, but low wingers are cool and of course biplanes are extra cool. Going slow is for beginners, and going fast is for experts and those who like to have fun. Once you go fast, you won’t ever want to go slow again. It sounds stupid, but a lot of people think this way.
As far as I can tell, flaps are one of those features that most RC pilots want to place on an arbitrary scale of good vs bad (usually toward the bad end) without really thinking about what they are for… so maybe they should go at the good end because smart people use them….or something like that. For most people flaps are a mystery. In fact, this entire notion of beginner to expert, or good vs bad, etc, does not address the true significance of RC aviation. This hobby is about exploring engineering principles and developing skills as a pilot. Each airplane is a unique design, and a pilot can challenge himself to master it and its idiosyncrasies. Flaps modify the flight performance, and as such are a fascinating option for any RC pilot who really wants to think about flight parameters and how to use them.
What a lot of people think flaps are for
Most people seem to think they are for increasing lift and reducing stall speed. These are real effects, but they are not the main reason to want flaps.
Given this misconception, plus the fact that it requires effort to install flaps on an airplane, most RC pilots don’t fool with them. If flaps are mentioned in a conversation about Telemasters, Kadets, or other slow planes with high lift and a flat glide, the thing you usually hear is “You don’t need them, because your plane is too slow already”.
What flaps really are for
Flaps are a high-lift device that have two major effects, plus some other side effects. The major effects are to decrease airspeed and to increase glide slope angle when landing. The side effects include lowering the stall speed, reducing the likelihood of tip stall, and possibly changing the angle of attack and thus the required elevator trim setting.
Now imagine that you have a Kadet, or a Telemaster, or a primary trainer, or an old timer. Your runway is not very long, it may be bordered by tall trees, or maybe there is a power line nearby or a street that you don’t want to fly close to. Your plane has a long, flat glide, and it just doesn’t want to come down. Instead it just glides along a few feet off the ground and uses the entire runway. Flaps will decrease the airspeed, decrease the stall speed, and greatly increase the angle of descent, which will allow an accurate landing on a short runway.
Let’s get back to the common notions of scalar qualities that I mentioned in the second paragraph above. Generally if you say that you’re going to fly a slow plane with a flat bottom wing, some folks will say that you’re going backwards as a hobbyist. Then if you say you are going to equip it with flaps, some folks will say that the plane is already slow enough. “It doesn’t need flaps.” I wish I had a nickel for every time I heard that. In a philosophical sense, you need flaps about as much as you need a model airplane, which is not at all because it’s just a toy. But if you think of model aviation as an exploration of engineering combined with the pursuit of skill to execute various maneuvers, flaps are quite a nice addition to any plane that would benefit from a steeper landing approach or a reduced landing speed.
Should you include flaps in your next building project?
You might want to try flaps some time if you like to fly and you want to explore new possibilities, or if you like to build new things, or both. If you have a heavy wing loading, or if you have a very light wing loading, there is a possibility of running out of runway. In both cases, flaps will slow the plane down, make it come in at a steeper angle, and allow you to use a shorter runway.
Different types of flaps
You can use basic trailing edge stock to make plain flaps. On any plane with strip ailerons you can cut the aileron in half, and use the inboard half as a flap. This is good, but there are better designs.
If you want to get a bit more involved, check out the Guppy plan on Outerzone. This design requires you to create a separate trailing edge in the vicinity of the flap. It’s more complicated, but with a little bit of foresight it doesn’t have to be difficult. This type of flap is quite effective and would satisfy most people’s curiosity. I’ve had a lot of fun with plain flaps.
Split flaps are easier to build. Add a 1/16″ ply panel to the bottom of the wing, hinged at the front with covering film. Check out the Cloud Niner plan and article on outerzone to see how these are built and used. This is the draggiest flap, and it will really slow your plane down.
For a bit more lift and less drag, build a slotted flap. This requires an offset hinge. It’s a bit more difficult, but if you’re an observant pilot who likes to try different equipment, it’s worth it. These are especially good for planes with high wing loading.
If you’re really enthusiastic, try building some variation of the Fowler flap. Fowler flaps are the best choice for planes with high wing loading because they produce more lift and less drag than the other types. If you’re good enough to build these, I’ll just shut up because I’m mostly here for basic information.
So, the big question is “Do you need flaps?” The answer is yes. Build them on at least one plane, and not just flaperon mixing on your transmitter. Build a real, separate flap on each wing, and give it a try. They’re a lot of fun. Then if you don’t like them, you can accurately say that you don’t need them. But before you knew you don’t need them, you did need them so you could find out.