Wing Flaps Add Interest to Your Indoor Model Airplane
Understand the basics of wing flaps and how they help your model fly
Model airplane Fowler flap
Wing flaps are hinged surfaces, usually at the trailing edge of a wing, that change the shape, or airfoil, of a wing. An airplane design with wing flaps extended will produce the same amount of lift at a slower airspeed. Thus, wing flaps are very helpful with aircraft that need to slow down in order to safely land. In addition, partial flaps can be used to increase wing lift and reduce takeoff distances. Use of wing flaps also increases the total drag of the wing, due to the increase in wing camber. This is typically not a problem, as you will use wing flaps only during the takeoff and landing phases of flight when you wish to slow down anyhow. Excess drag is of much more concern during the cruise portion of flight. All these maneuvers can be easily practiced with a computer RC simulator.
Wing flaps are not typically used on indoor model aircraft, due to their light weight and slow flight speeds. Antique aircraft such as the Demoiselle or Fokker Eindecker flew slow enough that flaps were not required. However, it is still a good idea to understand the purpose of wing flaps and how they are used. Wing flaps can add a lot of character to a scale model, and are not that difficult to incorporate in your model airplane design.
Several different types of flaps
There are several different types of wing flaps, to include plain, split, slotted and Fowler type flaps. Plain flaps are the most common arrangement, and are simply hinged sections of the wing’s inboard trailing edge that deploy down anywhere from 30 to 90 degrees of deflection. Split flaps are a bit less common, and are located on the underside of the wing. A slotted wing flap provides some amount of airflow over the top surface of the flap for better aerodynamic effect. A Fowler flap is the optimum wing flap arrangement, as there is a track arrangement that allows a Fowler flap to actually extend and increase the overall wing area as the flap deploys aft and down.
Four types of wing flaps
Plain flaps are easier to design and add to your model airplane plane than the more complex slotted or Fowler flap variants. Plain wing flaps will also add the least weight to your finished model as compared to the more complex flap designs.
Airplane response to flaps
Each airplane type will respond a bit differently to the use of wing flaps in flight. Usually the best idea is to make your initial flights without the use of flaps. At a safe altitude and airspeed, extend the wing flaps about halfway. Observe how your airplane responds. Usually, there is often some sort of ballooning up (due to the sudden increase in lift) and a pitching down of the nose, depending on the airspeed. It is always best to sort out these aircraft behaviors at altitude before you get lower and slower for the landing phase. Recall also that wing flaps will increase the total drag of your model airplane, thus additional power is always needed when employing wing flaps.
Top view of model airplane flap
For higher performance aircraft, wing flaps can also we used for takeoff. Recall that wing flaps add lift as well as drag. Thus for takeoff, a lower flap setting is used as compared to a landing configuration. As a general rule, no more than 20 degrees of flaps will be used for take off. The increase in lift with this small addition of flaps is considerable, with very little penalty for drag. Once safely airborne, the flaps are retracted, and the wing provides its normal lift at cruise airspeeds.
Flaps and go arounds
One flight safety aspect regarding the use of wing flaps must be kept in mind, and that regards “go arounds”. Anytime you are landing your model airplane, there is the chance you might be required to - with typically very short notice - perform a go around. The runway may be blocked by another aircraft, or some other aircraft may cut in front of you. In a go around, you add full power to abort your landing approach and circle around for anther landing attempt.
The safety issue can arise if you are making the landing approach with full flaps. Full wing flaps can add significant drag, which will affect you model airplane’s ability to climb out properly. In addition, the use of full flaps oftentimes can cause difficult handling of the airplane when adding full power.
Bottom view of the complex triple slotted flaps on a B-747 aircraft
In any case, the pilot actions are the same. When required to perform a go around with full wing flaps, always add power first. Ensure positive aircraft control, and avoid abrupt control inputs to prevent a crash. Slowly raise the airplane’s nose to start a gentle climb. Retract the wing flaps in stages, allowing the airplane to gain speed and control as it adapts to the new wing configuration. It is always a good idea to periodically practice go arounds, so you are fully prepared if you have to perform one for real.