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General Information about Aerodynamics

This section of the Foil Sim package gives an overview of aerodynamics and explains how the simulator should and should not be used.

It is remarkable to note that humans did not learn to fly until the last century. Despite the daily example of flight from birds and insects, humans did not unravel the mystery of flight for thousands of years. Why do you suppose that is true?

The physics of controlled flight is a very complex problem. It took many years to develop the underlying mathematics, the understanding of aerodynamics, the understanding of thermodynamics for engines, the understanding of materials processing for lightweight structures, and the theories of stability and control. All of these had to be mastered before human flight was possible.

The Study of Aerodynamics

When solving a very complex problem, the process can be simplified by breaking it up into many smaller, less difficult problems. In order to simplify the study of aerodynamics, the aerodynamicist must decide which effects are important and must be considered and which effects can be neglected. The following areas must be considered:

1. Airflow is more easily analyzed when it becomes steady and constant with time. It is difficult to describe or visualize the movement of a body through three-dimensional space where air can move in three directions. In special circumstances, air can move in only two directions, which is much easier to describe and visualize. But because problems in aerodynamics change quickly with time, it is even easier to analyze airflow when it becomes steady and appears to be constant with time.

2. Air is assumed to be a single gas with uniform properties in most aerodynamic studies (even though real air is made up of many gasses).

3. The compressibility of air is usually ignored because its effect is important only at flight speeds near and above the speed of sound.

4. The viscosity (stickiness) of air, which causes air to stick to surfaces moving through it, resulting in a frictional drag and the heating of the air at high speeds, is usually ignored. The viscosity of air is hardly noticeable and has been neglected for many years.

After determining which effects can be neglected, the flow problem can be solved by modeling it in a wind tunnel, on paper, or with a computer.

Flow Modeling Problems

There can be several possible problems involved with flow modeling, and the model limits must be considered when drawing conclusions. Several problems are given below:

1. The model may not accurately represent the real vehicle.

2. There may be math errors in a mathematical model.

3. Limitations on the model may be ignored when drawing conclusions. For example, if compressibility is neglected, the model should not be used near the speed of sound. Also, a two-dimensional model will not properly describe the flow over a swept, delta wing because the flow is three dimensional.

Limitations to FoilSim Airflow Simulator

The simulator, which models the steady, two-dimensional, non-sticky, incompressible flow around several airfoil shapes, has several limitations:

1. The simulator will not model any brief changes that could occur as flow conditions change. It assumes that the airfoil has been at the prescribed flow conditions for a long time and that the incoming flow is steady.

2. The model is two dimensional and will model only very long wings.

3. The model will not properly model drag or the stall of the wing, a condition where the boundary layer separates from the wing surface, disrupts external flow, and kills lift.

4. The simulator can be used only at low airspeeds (typically less than 250 mph).

Even though there are limitations, the mathematical model used in this simulator was developed many years ago and has been used as the basis for undergraduate airfoil theory at many universities for many years.

Analysis of Fixed Airfoil Model

The View Panel in Foil Sim is a drawing of the airfoil with air flow moving past it from left to right, which is the view of a wing being tested in a wind tunnel. Airplanes fly by moving through almost still air. Yet when wind tunnels are used to test wings, the airplane model is fixed and the air moves past the model.

The idea of keeping the airfoil fixed and having the air flow move past the airfoil can be confusing. Yet you experience the same thing every day. When you stand on the corner and watch a car go by, the air around you is relatively still . As the car moves through the air, there are aerodynamic forces present. If you were in the car and held your hand out the window, you could feel the aerodynamic force pushing on your hand. It feels as if the air is moving past your hand as fast as the car is moving; the forces on the car are the same whether the car drives past you or whether you are riding in the car.

The same thing is true of airplanes. Whether the airplane moves through the air or the air is pushed past the airplane, the forces are the same. It is usually easier, less expensive, and less dangerous to test airplanes in a wind tunnel before trying to fly them.

Airspeed vs Ground Speed

As you use the simulator, you will see that airspeed gives only the speed of the air far upstream and far downstream of the airfoil. However, air can also speed up or slow down in the immediate vicinity of the airfoil.

It is important to distinguish between airspeed and ground speed. The important factor in the generation of lift is the speed of the air relative to the wing (airspeed), not the speed of the wing over the ground (ground speed). This becomes important on a windy day.

If the wind is blowing 20 mph out of the west, and an aircraft requires 100 mph of airspeed to generate enough lift to get off the ground, the aircraft must reach 120 mph across the runway to lift off if headed east with the wind (120 - 20 = 100). But if headed west against the wind, the aircraft must reach only 80 mph ground speed (80 + 20 = 100) to lift off. Pilots always want to head into the wind when taking off. This explains why airplanes take off from different runways on different days. It is also why aircraft carriers (which can travel in excess of 35 mph) turn into the wind to launch aircraft. Just as the plane falls off the end of the flight deck, its wings are going at the speed of the wind, plus the speed of the carrier, plus the speed of the catapult, plus the speed generated by its engines. No wonder airplanes can take off in such a short distance on an aircraft carrier!

Now that you have read the General Information about Aerodynamics, let's begin the Airfoil Lessons.

Click here to go to the Lessons.