Aerodynamics & Flight
Before building your aircraft, we think it’s important to learn a little bit about how they work.
The following contains brief summaries of key aerodynamic concepts. For more comprehensive information, you can refer to this guide.
Wings
Lift:
Most RC planes have airfoils that have curvature on top and a flat bottom surface. As the wing moves through the air, it leaves gaps (or low-pressure zones) in the air behind. Imagine pushing an oar through water, as you try to move the oar, it leaves gaps where it just was which the water then rushes in to fill. The curvature of the top of the airfoil means that the area behind it isn't exposed to oncoming airflow. This means that a gap (or low-pressure zone) forms on the rear of the top of the airfoil.
As you can see, the area behind the wing is not exposed to oncoming airflow. To compensate for this pressure difference, the air flowing above the wing will rush downwards to fill this gap, this speeds up the air and therefore, decreases its pressure. (Since faster-moving particles are heated up and take up more space). In fact, the air above the wing flows much faster than the air beneath even after it leaves the airfoil.
The air flowing above the wing is less dense and has lower pressure. The air flowing below the wing will be slowed down by the bottom face of the airfoil and, therefore, compressed, creating a dense, high-pressure zone. The higher pressure below the wing pushes on the wing from beneath while the low pressure above the wing pulls on it from above. This helps generate lift.
Another important part of lift is that air beneath the wing is deflected downward by the bottom face of the wing and the air above the wing is being pulled downward by the low pressure generated by the wing, both of these actions push the wing up because every action has an equal and opposite reaction.
These two factors combined help generate lift and counteract gravity keeping the plane in the air. Lift can be increased by either going faster (as more air travels over the wings generating more lift in a specific period of time) or increasing angle of attack.
Angle of attack: When increasing the speed of a plane isn't enough or isn't an option, you can increase the angle of attack to increase lift. Increasing the angle of attack of a wing will cause it to deflect more air and create a larger pressure gradient between the top and bottom of the wing leading to more lift. However, once the angle of attack and pressure gradient get too large stall will occur. Additionally, by increasing your angle of attack you sacrifice thrust since the wings generate more drag.
Boundary layer: The boundary layer is a layer of stationary air surrounding the wing. This is how it forms: In front of the wing, the air is at about atmospheric pressure, and as it passes over the wing it speeds up and loses pressure. However, once the fast-moving air hits regular higher-pressure air behind the wing, it slows down. For the fast-moving air, the higher pressure zone is like a brick wall. The boundary layer air is held in place by this difference in pressure, along with friction with the surface of the wing, both preventing the boundary layer from moving along the wing. All wings have a boundary layer to some extent but well-engineered wings will have thinner ones. Larger pressure gradients will allow less air to pass behind the wing and enlarge the boundary layer. The boundary layer hinders lift since it acts as an extension of the wing and, therefore, simulates a lower angle of attack at the top of the wing.
How stall is caused: As I explained above, the boundary layer is a layer of stationary air surrounding the wing. As you increase angle of attack, the pressure difference between the wing and the air increases. This enlarges your boundary layer. When the boundary layer becomes too large, airflow separation occurs. Refer back to the first sketch of an airfoil above and how the air is not flowing around the wing. That's airflow separation. Airflow separation greatly reduces lift (because now only the bottom of the airfoil is generating lift) and also causes loss of control of the ailerons on the wing. This is because instead of pushing on oncoming air, the ailerons are only affecting the boundary layer (if pointing upwards).
Control Surfaces
How control surfaces maneuver the plane: Control surfaces allow the plane to pitch up or down (elevator), yaw left or right (rudder), and roll clockwise or counterclockwise (ailerons). When control surfaces are moved they deflect air in a certain direction and in doing so, move in the opposite direction. For example, if you move your elevator up, the air that would've been flowing straight is now deflected upward by the elevator. As an equal and opposite reaction, the elevator, along with the entire rear of the plane then moves down. This causes the plane to pitch up.
Ailerons: You may be wondering: How does rolling the plane with the ailerons allow you to turn? When the plane rolls, the lift vector (of the wings) is no longer perpendicular to the ground. The lift vector can be redrawn as two component vectors, one pointing horizontally and the other vertically (shown below). The horizontal vector is equivalent to a turn.
Drag
There are many factors contributing to drag on planes (such as induced drag and pressure drag), but in RC planes, you don't really have to worry about drag and efficiency. The more important thing is that your plane is structurally sound. A general tip though is to try and reduce the amount of surface area on your plane and minimize the amount of surface area perpendicular to the travel direction.
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