## Principles of aerodynamics

The forces and counter forces of flight

The forces and counter forces of flight

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Say 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).

Lets stop bickering and get to the point of this thread!exactlyaerospace stuff, but we still need to know it (well, some of it).Anyways, as long as I'm posting, in level flight Thrust=Drag and Lift=Weight.

I'm just confused as to what the point of it is.Me too :P