Flaps are the simplest and most effective control surfaces, but at the same time mostly misunderstood. In theory flaps are used to increase the wing’s lifting capability by changing the curve of the airfoil but keeping the angle of attack unchanged. With the use of flaps we achieve to change the shape of an airfoil i.e semi symmetrical to a high curve & high lifting capability airfoil. Even if their functionality is similar to that of the airbrakes, they should not be confused with them as found in WWII birds such as the Junkers Ju-87 Stuka, the Aichi D3A Type 99 Carrier Bomber or Val and the Douglas SBD Dauntless.
Junkers Ju-87 ”Stuka”
Aichi D3A Type 99 Carrier Bomber or Val
Douglas SBD Dauntless
Something that confuses the Rc pilots is the fact that FLAPS have two aerodynamic effects at the same time. They change the LIFT in conjunction with DRAG. The amount of each one’s differentiation depends on the amount of the curvature that is achieved and the critical point where DRAG becomes the prominent result of the FLAPS lowering. A wing’s high lifting capability with increased curvature is understood via the noticeable increase of DRAG.
Almost all kinds of airfoils are prone to airflow separation near the wing’s trailing edge. In Rc models the low Reynold’s number tend to create vortexes a little bit earlier so the airflow separation happens earlier explaining why our models fly with an angle of attack greater than their big brothers.
When the FLAPS are lowering, the surface being protruded into the air stream makes the airflow to stick again to the airfoil thus generating a greater amount of lift.
It has to be noticed that the angle of attack and the airplane’s inclination remain unchanged. LIFT increase occurs to the most semi symmetrical airfoils when FLAPS are lowered up to 15-20 degrees. Those Rc pilots who are using FLAPS tend to overlook that FLAPS are used in different inclination positions depending on the need and the flight status. So at take-off typically an airplane needs 10 degrees, while during landing 50 degrees or more are necessary if high rate of descent is required.
Basically the FLAP’S role during take-off is to increase the wing’s LIFT thus making the model to take-off at a shorter distance and to achieve greater climbing rate. During landing the enhanced DRAG that is being produced, succeeds to minimize the landing speed and distance. When FLAPS are lowered up to 15 degrees, the model tends to gain height as the redundant LIFT steps in. The model’s nose should not go UP or DOWN if the FLAPS are properly designed and the model will move as an elevator to a higher altimeter.
On the other hand, if they are lowered more than 20-25 degrees they tend to act as airbrakes and because of the DRAG generated aft from the CENTER of GRAVITY, the model’s nose tends to rise. Some airplanes tend to lower their nose when FLAPS are lowered. A high wing or a low wing model affects this phenomenon, contributed by the type of the model’s airfoil and the proportion of the air flow’s disturbance to the horizontal stabilizer by the lowered FLAPS. Below are some pictures of the basic categories of FLAPS and their individual characteristics.
Plain flaps lower the wing’s trailing edge increasing its curvature and, therefore, its lift.
Split flaps generate a lot of drag by disturbing the airflow on the underside of the wing.
Fowler flaps move rearward and downward increasing the wing area and curvature.
Slotted flaps allow high-energy air to flow from underneath the wing up and over the flap to help prevent airflow separation.
Basically is easy enough to add separate FLAPS to your model if you haven’t already activated the FLAPERON MIX if it is available from your radio. Pay attention to the linkages and connections to avoid the catastrophic FLUTTER.
It is wise to use a heavy duty servo especially when the model is fast and relatively heavy. Take special care when the FLAPS are not extended, to retain both of them at the same level in order not to affect the model’s trimming.
Try them in flight, get to know your model’s behavior, enjoy low speed landings and minimize the stalls!