Mechanics of an airplane in flight. Photo Credit: av8n.com
Update on technical issues: issues have been fixed, regular posting will resume tomorrow (Sunday).
Turning is also tricky business. On airplanes, aerodynamic forces can be broken down into two components: drag and lift. Lift is a two dimensional vector in the plane (geometrically speaking, not to be confused with a physical airplane) perpendicular to the direction of relative wind, however in “normal flight” we can make the assumption that lift is in the direction of perpendicular to the relative wind and wing span as seen above. Basically we can assume all these forces are happening in a single plane. Weight is mass times local gravitational acceleration (which can/will change during flight). Thrust is a force produced by the engine that is in direction along the axis of the engine. This is especially important if your engine is right smack dab in the middle of your vehicle…
Only when you’re flying straight, level, at a constant speed, constant altitude, and through still air can you assume thrust+drag=0 and lift+weight=0. So pretty much in any other case there are forces going on all kinds of crazy directions in three dimensional space.
Non-intuitively, lift is less then weight in two situations: high power nose climb and low power down decent. Sounds paradoxical but in fact thrust takes up some weight in climb and drag takes up some weight in decent. Force balance, it helps.
Any who, why do we even care about these forces? Well during normal flying (sounds weird to say because I don’t consider flying normal) we don’t, except during a turn. During a banked turn the lift vector is inclined out of the plane (geometric) of the image above, so in order to pull the airplane around the turn and keep it at that altitude, the lift vector must be significantly greater than the weight. So the lift vector can be summarized into two components during a turn: a vertical component to oppose lift and a horizontal component to change direction in which the airplane is flying.
Let’s start simple. Boat turns are relatively simple because they neither have to fly or dive. A flying boat would be pretty cool though…but I digress. Say you want to make a right turn, in the figure below, turning the rudder to the right will cause the boat to yaw starboard. Relative wind will hit the port side of the boat and create tremendous drag which will incidentally turn the boat to the right. The force on the rudder by the wind is smaller than the force of the wind of the portside of the boat which means turning. Now apply all this to airplanes and you have a simple boat turn on an airplane. This sounds great and all, however, turning the airplane properly by using the wings is way more effective and efficient than boat turning. There is also a lot more going on during banked turning as well.
Boat turn. Photo Credit: av8n.com
Airplane turn. Photo Credit: av8n.com
When using your wings to turn, there is a tendency to overbank which basically means overturning. As seen in the figure below, the distance the inside wingtip has to travel is smaller than the outside wingtip and since the outside wing is traveling farther in the same amount time, it must be moving faster.
Airplane planar simple circular motion. Photo Credit: av8n.com
The lift generated by airfoils depends on the square of airspeed, so the outside wing will be producing more lift which leads to overbanking which leads to a spiral dive into oblivion. Luckily for us, this effect depends on the ratio of wingspan to the radius of turn. For stubby wings, high airspeed, and shallow bank angle, you’ll never notice the effect. For a glider however, you need to deflect the ailerons against the turn to create an opposing force. And to make things even more convoluted, to counteract something called long-tail slip effect, you will be holding your rudder into the turn. So your controls seem like a paradox at this point but this will mean a much better turn then a simple boat turn. Long-tail slip effect is a topic for another day, but if you’re an aspiring pilot or you just want to learn more, please visit http://www.av8n.com/how/#contents for more of your aerodynamic needs.