Chapter 2 : Why and How RC Model Airplanes Fly

This chapter, we will discuss the aerodynamic forces acting on an rc airplane. Many people just accept the fact that rc airplanes fly because of its wings. Right. But the force that acts on the wings to achieve flight is often overlooked. Yes, we don't have to be a rocket scientist to enjoy this hobby but it pays to learn more about aerodynamics. The wings cross-section is called an airfoil (see Fig. 4a). The forces that acts on the wing of the rc airplane is what makes it fly. Based on the illustration of Figure 4b, the relative wind passes from the leading edge to the trailing edge.

The arrows represent the direction of air molecules and bubble numbers (1) and (2) represents the air molecules. As you can see, the upper camber (Fig. 4a) has a low-pressure area and the lower camber has a high pressure. As the air passes through an airfoil, it separates from the leading edge and travels on the upper camber and the lower camber. The air molecules number (1) and (2) separates from the leading edge and both should meet on the trailing edge at the same point, at the same time. This phenomenon is called the law of continuity.

Without the law of continuity it is impossible for an rc airplane to fly. Because of this, the air molecules on the upper camber travel faster than the air molecules on the lower camber. In Bernoulli's theorem, "as the velocity of air increases, pressure decreases". The pressure on the lower camber is greater than the upper camber; the variance of pressure generates the lift needed by the aircraft.

Figure 4a : Airfoil of a typical rc airplane

Figure 4b : Airfoil of a typical rc airplane

The three basic types of airfoil in our rc model airplanes are: (see Fig 4c)
1) Flat bottom airfoil
2) Semi- symmetrical airfoil
3) Symmetrical airfoil

Of the three, the flat bottom airfoil is recommended for trainer rc airplanes due to its inherent stability. The flat bottom design is stable on the longitudinal axis and self-correcting tendency is very ideal for a beginner. The semi symmetrical type is for intermediate flyers and the symmetrical is for the expert who wants to fly aerobatics (see Fig 4c).

Figure 4c : Basic Types of Airfoils

Now that we learned the mystery of flight, we should also learned something about the weight determination. Eventhough flight is generated because we utilize the wings, but if we disregard the weight consideration, it will not fly properly or will not fly at all. Remember, " a feather flies better than a brick". The area of the wing is very important to know if the aircraft could carry it's own weight. There is a certain limit of the total weight of the aircraft, depending on its wing area, designed speed and type of airfoil. The weight divided by the total area of its wing span is called wing loading (see Fig. 5).

Figure 5 : Wing Span of a Non-Taper Wing

So the next time you but an rc airplane kit and you see the specification (for example: wing loading=800g/sq. cm) you know now that it is the weight carried by the wing per square centimeter. It is advisable to keep the weight down, if the weight of the rc airplane exceeds the recommended weight, the rc airplane needs more airspeed to generate lift (see Fig. 4b). We learned in our previous lesson that as the velocity increases, pressure decreases. The greater the airspeed, the more sensitive the controls become and it will be more difficult to fly the rc airplane. Especially on landing, longer runway is required. The rule of thumb is "keep the rc airplane light". The lighter the aircraft, the more docile the aircraft can be. This is very important to a beginner because the model aircraft should be very easy to fly and very forgiving.

Adding another set of wings can increase wing area. In early days of aviation, there are biplanes and triplanes that fly in the skies. In fact the first rc airplane was a biplane (The Kitty Hawk) flown and built by the Wright brothers. The monoplanes are more efficient than biplanes and triplanes because it does not have wire braces to hold the wings. The only advantage it has is longitudinal stability.

Now let us study the aircraft in flight. There are forces that act on an aircraft in flight, the lift, weight, thrust and drag (see Fig. 6). To achieve a stable straight and level flight, these forces should be balanced, Lift=weight and thust=drag. Lift pulls the aircraft up and weight pulls the aircraft down. If weight is greater than the lift, the aircraft will descend or vice versa. Thrust pulls the aircraft forward and drag pulls it backwards. When the aircraft is taking-off the ground or needs to gain altitude, the thrust should be greater than the drag. As the thrust increases, the lift increases to make the aircraft gain altitude or off the ground. This is also the reason why there is a recommended engine size of the aircraft. The engine to achieve flight must provide the power needed by the aircraft.

Figure 6 : Forces Acting on an Aircraft in Flight

An underpowered aircraft has very terrible consequences. Stalling is loosing the lift and control of the aircraft. Flying below the recommended minimum airspeed causes the aircraft loose lift because the smooth airflow of the wing is distorted or turbulence occurs (see Fig.7). This destroys the lifting force of the wing because as we have leaned before, the low-pressure area on the upper camber creates the lift. This cannot satisfy the law of continuity because of the disturbed airflow. Hence, the control surfaces cannot also do its function because it is depended entirely on the airflow of the wing. If the aircraft has sufficient power, and applied too much angle of attack, the aircraft will also stall because too much drag is present. Increasing the angle of attack, drag will also increase. There are numerous airfoil data from NACA (now NASA) which are used by aircraft designers on designing their aircraft. It contains data about coefficients of lift, drag and angle of attack. But these are beyond the scope of this topic.

Figure 7 : Airfoil Turbulence

For example, the rc airplane is just taking-off the ground with just below the normal take-off speed, pull the elevator up, the next thing is that the rc airplane will climb pointing it's nose up and will dive to the left. The reason why rc airplanes dive when on a stall is to gain airspeed. The propeller torque causes the rc airplane to dive left. That is why rc airplanes are safer at high altitudes and dangerous when close to the ground. Take note, airspeed is the speed of an aircraft relative or against the wind. Ground speed is with referenced to the ground (see Fig. 8). Ground speed is equal to relative wind minus the airspeed of the aircraft.

Figure 8 : Airspeed and Groundspeed of an Aircraft in Flight

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