Introduction to Flying Fixed-Wing Aircraft

Alex P -

By learning how a plane flies you’ll get better control over your Aero.

The Physics

Four aerodynamic forces work together to make a plane fly: thrustdraglift, and gravity. Thrust is the forward speed of the plane. Drag is a backward pull on the plane, a result of the friction that the wing generates as it moves through air molecules. Lift is the force that pulls the plane up. Gravity is the weight of the plane. We can group these four forces into pairs of opposing forces: lift and gravity; thrust and drag. If you want the plane to go faster, the level of thrust must exceed the level of drag; if you want to go up, lift must exceed gravity.


Four Aerodynamic Forces


The Controls

To understand how the Aero controls work, we need to understand the plane’s control surfaces: the ailerons; the rudder; and the elevator. These work with the throttle to control the plane’s speed and direction. Let’s look at how flight controls work one by one.

Throttle: Left stick up and down; the propeller’s motor power, which controls the Aero's forward speed (thrust) as well as the rate of climb or descent. The throttle is important, because thrust generates lift. When your Aero moves forward, air flows both above and below the wings. But check out the wings, viewed from the side: The top has a curve, but the bottom is flat. This shape is called an airfoil. When the plane moves forward the airfoil deflects air in different ways, so that the air that passes above the foil has less pressure than the air that passes below it. The heavier air below the foil pushes the wing up into the lighter air—or, you could say the lighter air pulls the wing up out of the heavier air. (The air passing around the wing also generates drag, the slowing force, which the throttle helps to overcome.) If your Aero moves too slowly, or if it’s at a high altitude where there’s not much air pressure, then the wings will lose lift and gravity wins: the Aero will fall. This is called going into a stall. You can recognize a stall in a few ways: the plane will slow down and fly at an unusual “nose-up” attitude; the flight controls will be less responsive; or lastly, the plane will suddenly lose altitude. To overcome a stall, you need to increase your speed. You can gain speed either by increasing the throttle (left stick forward) or by angling the nose down (right stick forward) and descending into a glide. Typically, recovering from a stall requires a combination of the two: more throttle plus downward glide.

Elevator: Right stick up and down; moves the horizontal hinged section on the tail of the Aero. The elevator is the most important control surface. The position of the elevator determines the pitch of the Aero—whether its nose points up, down or level. If you pull back on the right stick the elevator fin angles upward, and the air now hitting it pushes the tail down, which causes the nose to lift, and the Aero climbs. When you push the right stick forward, the elevators angle downward and the Aero descends. It’s also possible to push the elevator up and remain flying straight and level, with a “nose-up” attitude. In order to climb, the plane also needs more thrust. This means that the elevators, in combination with gravity, play a major role in determining the plane’s speed.



Ailerons: Right stick left and right; moves the hinged sections located on the back edge of the Aero's wings, near the wing tip. The ailerons work together to control roll. Roll is the rotation of the plane on its longitudinal axis. Hold the plane out straight in front of you, and imagine a line running through the plane from nose to tail. Rotate the plane on that axis, and you can see why it’s called roll. When you’re flying and you move the right control stick to the right, the Aero's right aileron angles upward and the left aileron angles downward. This changes the way the air flows over the wings. The air pushes the right wing down and the left wing up, and this causes the Aero to roll to the right. When you push the right stick to the left, the left aileron angles down and the right one angles up, and the Aero rolls left. When you combine the ailerons with the elevator, the plane will make a banked turn. 

Lift, it turns out, plays a big part here. As the Aero moves forward, its wings generate lift, no matter what position the plane is in. You can be turning, sideways or upside down—there’s always lift pulling on the top (curved part) of the wing. So if you roll the Aero to the right and simultaneously apply up elevator—by pushing the right stick forward—lift pulls on the wings and the Aero will enter into a banked turn in that direction.



  Rudder: Left stick left and right; moves the vertical hinged section on the Aero's tail. The rudder controls the Aero's rotation on its vertical axis, also called yaw. If you hold the plane straight out in front of you, imagine a line running through the middle of the plane from top to bottom. The rudder makes the plane spin on this axis. The rudder’s essentially a vertical wing. The movement of this fin to the left or the right changes the way that the way the air flows over it, just like the other control surfaces. When you move the left stick to the left, the rudder angles to the left, which causes the air pressure to increase on the left side of the tail. This pressure pushes the tail of the plane to the right, which in turn causes the nose to rotate, or yaw, to the left, and the Aero will turn in that direction.




To learn more about autonomous planes, visit the APM:Plane documentation project here. Happy flying!