Learning to fly RC airplanes is a step by step process. First you learn one control and then you move on to the next. Before you know it, you’ve masters the four basic controls: throttle, elevator, rudder, and ailerons. Once you try your hand at flying a scale subject, you’ll be introduced to another set of controls in the form of the flaps. Since most full-size aircraft use then, many scale models include them for true scale flight performance. If you have never flown a model equipped with flaps, there are a few things you’ll need to know. There are right ways and wrong ways to use them and this article should help you understand the fundamentals.
Overall, when flaps are lowered they change the wing’s lift and drag characteristics and in so doing, lower the stall speed. By changing the camber of the wing, both lift and drag are increased for a given airspeed. As a result the speed at which an aircraft can land is affected. If you have never flown with flaps before, don’t worry. Flaps add flexibility to your model’s flight envelope, and they offer a fun new experience.
Though there are four basic types of flaps: plain, split, Fowler, and slotted, the plain flap is the most common and is simply a hinged portion of the trailing edge. Usually hinged at the top of the control surface, plain flaps deflect in a downward direction. Super Cubs and other sport scale models use plain flaps to keep construction and function simple.
The major advantage of flaps is that they shorten (and steepen) your landing approach by allowing your model to fly more slowly while in a nose-down attitude. Here are some helpful hints.
Deploying flaps may result in the plane pitching up or pitching down. You must use elevator to compensate and keep the plane on the desired flight path. Another characteristic of flaps is that the first half of the flap’s deflection results in a greater increase in lift while the second half results in a greater increase in drag. Flaps also impart a large structural load on the plane’s wing, so they should only be used at lower airspeeds. Often, full-size planes will have their airspeed indicators marked for safe flap operating range.
Since flaps provide more lift at slower airspeeds, you must be aware that when you retract them in-flight you will lose that additional lift and the plane will want to sink. For this reason, if you must do a go-around, make sure you increase power before retracting the flaps. Failure to do so could place your plane very close to stall speed before you can accelerate to a safe speed. This also applies during takeoffs with flaps. In most cases it is safer to take off with the flaps retracted or deflected no more than about 20 degrees. Larger deflections add more drag and can cause the plane to become airborne at too low of an airspeed.
Flying a scale model with operational flaps is a very rewarding experience. Not only do they look neat, but they also provide the same benefits as the full-size version.
Flaps impart increased loads on the wing and require attention during their installation. Make sure you use enough heavy-duty hinges on each flap and a heavy-duty control horn. There are many ways to actuate the flaps, including torque tubes and bell cranks. For large, fast or heavily-loaded models, the best way is to use a servo for each flap. These planes will also benefit from the flaps being locked in the down position preventing the airstream from blowing the flap back to the up position. This basically means that the servo arm is directly in line with the flap horn at full deflection and this takes the strain away from the servo. This is accomplished by turning on the radio and selecting full down flaps and choosing a servo horn position that is in line with the horn. Now, retract the flaps and make up the linkage from the servo to the horn. The amount of flap deflection is determined by the length of the servo arm; for more flap deflection, place the linkage farther out on the arm. The use of ball link connectors will smooth out the control action and help to eliminate binding.
There are several options for transmitter flap actuation methods. The least desirable is to use a two-way switch, which results in only flaps up and full down. This is not very scale-like and can result in large pitch changes when the flaps are abruptly deployed. A three-position switch will allow the use of a half-flaps setting for more scale-like flight. A knob or slider switch is another way to go and allows an infinite number of flap settings. The only drawback here however is that it is somewhat difficult to tell how much flap deflection you are selecting.
Servo Speed Reducer
Another way to minimize the trim changes associated with flap deployment is to slow down the speed of your flap servos. Many programmable radios have the ability for you to adjust the response of specific servos. But most pilots will find that simply adding a speed reducer, like the Go Slo III from Sonic-Tronics, is a quick and easier way to deal with the situation. When the flaps take several seconds to lower, it minimizes the abrupt change in lift and gives the plane more time to settle down. Simply plug the unit in between the receiver and the flap servo(s) and you can adjust the speed from two to 10 seconds of travel in either direction with no loss of servo torque. Dual trim pots allow you to adjust both the up and down speeds independently.
Types of Flaps
Here are the four types of flap configuration. Each has its own advantages.
These move rearward and downward at the same time increasing the wing area and curvature.
These flaps allow high-energy air to flow from underneath the wing up and over the flap to help prevent airflow separation.
Hinged at the top, these flaps lower the wing’s trailing edge increasing its curvature and, therefore, its lift.
Recessed into the bottom of the wing when in the up position, these flaps generate a lot of drag when deployed by disturbing the airflow on the underside of the wing.
Lowering the flaps induces a twisting load to the airplanes’ wing. The type of flap as well as the wing’s design will determine the amount of twisting action, with the split flap generating the least amount.
With molded foam airplanes, some of the simpler, less expensive ones come with flap function as an option. Many times the flaps are actually molded into the wing and only need to cut free and then installed with the needed hardware. Here’s how to do it yourself.
To make your foam fighter’s flaps functional, you will need hinge-point-style hinges (three for each flap), a servo, and control linkage for each flap as well as two control horns.
Use a straight edge and cut through the top and bottom of the flap leading edge.
Using a razor saw, cut the root end of the flap surface free from the wing’s trailing edge.
Using medium sandpaper, sand the leading edge and the root end of the flap smooth.
Glue the control horn to the flap so it aligns with the servo well in the wing.
Tape the flap in place on the wing, make the hinge locations and then use a 1/8-inch-diameter brass tube with the end sharpened and form the hinge pockets for each hinge location. The hinges should be inserted so the pilot pins are below the bottom of the wing and all aligned with one another. When you’re satisfied with the installations, remove each hinge and glue it back in place with slow-setting epoxy.
Install the servo and adjust the linkage so that with the flap in the up position, the servo is at the end of its travel.
Use your radio’s end-point adjustment so that the servo moves the flap to its full down position when at the opposite end travel point. Here a dial switch is used to control the flap position.
Whether you are flying molded foam backyard fliers or giant-scale, gas-powered warbirds, flying flap-equipped airplanes is a great experience. Flaps allow you to operate your model from smaller flying areas and come in to land at a steeper angle without building up excess airspeed. This enhanced flight performance provides you with additional flight options while improving the scale operation of your model. Give flaps a try.