## Principles of aerodynamics

The forces and counter forces of flight

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active| newest | oldestSay you have an object falling through the atmosphere, if that object is falling less than gravity, it's producing some drag and/or lift. If the object is falling slower than terminal velocity then there *has* to be lift being produced.

Let's consider a free body diagram in the vertical direction for a glider: in the -Y direction you have gravity or the weight. In the positive Y direction you have a component of the drag and a component of the lift. Sum all of these and you get a force which is equal to m*a. In level flight a=0 so the forces sum. Now, the force of gravity is equal to m*g where g is the gravitational acceleration. If m*a is less than m*g then there has to be some positive upward force. This comes from a component of the drag and the lift.

As I posted earlier, a symetric airfoil at 0 degrees angle of attack (AoA) produces 0 lift, but at positive AoA it produces positive lift. If AoA is negative, it will produce negative lift, that is a force downward.

You might be confused about airfoils as they are conventionally drawn. The typical airfoil is drawn with a rounded top and flat bottom. This is known as a "cambered airfoil". Basically a cambered airfoil produces lift at 0 AoA (the trade off is that they often stall earlier), but there is a negative AoA where there is 0 lift produced, which we sometimes use to compare the lift-curve slope with other airfoils. (Lift-curve slope is the graph of lift vs. AoA, the slope of which is often 2*pi [don't ask me why]. To explain further, in aero we often use C_l; coefficient of lift. Now C_l=C_l_alpha*alpha where alpha is the AoA, so that C_l_alpha is the lift curve slope (this was for a symetric airfoil, for a cambered airfoil, there's a constant in there)). That was longer and probably more confusing than I intended, if you have a question, just ask.

I also forgot to mention that a flat plate *can* generate lift when it sees a positive AoA.

I mean this type: http://www.instructables.com/id/EV3GW00JZ4EYF8LZG3/ -- not the kind that you spin (for boundary layer effects).