Flying – defining terms

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Today I’m going to define some very basic terms for flying. As I work through the Private Pilot test preparation, I’ll be using the terms at times, and if you understand what I’m talking about it will be easier to follow. It also gives me a good review of what I used to remember easily.

First, the airfoil. An airfoil is a structure or body which produces a useful reaction to air movement. This includes airplane wings, rudders, propellers, and helicopter rotor blades. Today we’ll talk about wings on fixed-wing aircraft.

Here is the cross section of a wing:

“A” is the leading edge. That’s the front of the wing. “B” is the trailing edge. The dotted line going through the wing is an imaginary straight line from the leading edge to the trailing edge called the chord line. The chord line does not necessarily go through the wing, depending on the wing’s shape.

If you lower the flaps, the chord line will drop and go from the leading edge to the bottom of the flap.

Before we go any further, I’d like you to wrap your brain around a concept. When we’re talking about the movement of air around an object, the air acts like a fluid. The next term I want to introduce is relative wind. This is the wind felt by an airfoil, but it can be produced by either the airfoil moving or the air moving past the airfoil. Possibly the easiest way to explain that idea is by poking a stick into a stream bed. The water is moving past the stick. That is the relative wind. Even though the stick is not moving, there is a relative wind. Relative wind is parallel and in the opposite direction of the path of the airfoil, so if the wing is pointing up at an angle, the relative wind is going down. Straight and level flight produces wind coming from the front of the airfoil going straight back. If your flight path goes down, the relative wind is upwards.

This explanation is important because it defines one of the very basic terms that will be used when we’re talking about climbing and stalling. The angle of attack is the angle between the chord line of the wing and the relative wind. You can change your angle of attack by changing the control surfaces of the airplane.

The angle of incidence is the angle at which the wing is attached to the rest of the airplane. This doesn’t change.

That’s it for now for the test prep. Next time I write about flying we’ll go through lift and a few explanations of how airplanes fly.

5 thoughts on “Flying – defining terms

  1. One of the things I find interesting about the comparison between a wing in air and a wing in water is that they act the same as long as the relative wind is sufficient enough to make up for the density difference.

    A sub can use small wings at very low speed because water is so dense.

    An airplane must use larger wings at higher speed because air is less dense.

    This also leads nicely to the idea of density altitude. You MUST fly faster at higher altitude because the air is less dense and you have to make that difference up (or have a longer wing). Seems so sensible!

  2. Considering that I live in Nevada, density altitude comes up a lot. I hadn’t thought about the sub comparison – thank you! At some point I’ll talk about taking off at the airport up at Tahoe in the middle of summer and how exciting that can get for people used to denser air.

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