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Atmospheric Flight

9-12 Grade Reading

Aerodynamic Lift

Using the buoyancy of a balloon for a flight on Mars, though it seems like a good idea, would actually be rather impractical. The balloon example above demonstrates that it takes so much displaced volume to create useful amount of lift from buoyancy (from the balloon) that a more practical way to create lift needs to be developed.

By moving an object through the air, engineers can take advantage of aerodynamics to achieve more efficient flight. Aerodynamics is the study of changes in the air pressure acting on an object caused by the fluid motion around the object. One way to think of aerodynamic lift is that by moving through the air, it is a way to get more air to help contribute to the lift force. As the wing moves faster through the air, more and more air is used to make lift.

Bernoulli's principle tells us that there is a relationship between the speed of a fluid (in this case a gas) and its pressure. The relationship comes from the conversion of some of the pressure to "dynamic pressure". Dynamic pressure depends on the fluid density and its velocity. The exact relationship is expressed mathematically below.

The dynamic pressure,

q = 1/2rV2

When a fluid is moving, the combination of its pressure and its dynamic pressure must equal the original pressure the fluid had before it was moving.

P + 1/2rV2 = P0

The pressure difference between any two points in the flow is equal and opposite to the difference in dynamic pressure at those points. So as the speed increases, the pressure decreases.

We use Bernoulli's principle for aerodynamic lift when designing the cross section shapes of a wing. The most efficient cross section shape for any kind of flight (subsonic to hypersonic) would be a shape that produces greater velocity and, therefore, lower pressure on the upper surface when it is moved through the air. Which shape will generate the greatest amount of lift and the least amount of drag? A flat sheet makes lift. Take the flat sheet and hold it at a slight angle of attack to the wind and the lift force increases. But if you increase the angle of attack too much, the lift quickly decreases. The problem is the sharp leading edge of the sheet. When the angle of attack becomes too great, the flow cannot turn around the sharp corner of the leading edge. If the flow does not smoothly follow the shape of the sheet it stops generating lift. Take the same flat sheet and curl the front of the sheet down so that the leading edge is lined up with the oncoming flow, and the lift will increase to a greater angle of attack.

 a picture of
two airfoils, the first is straight and is generating drag,
the second is bent with the airflow and is not generating
drag

Now you have a good shape for making lift. The curved front of the sheet helps allow the airflow to more closely follow the surface of the sheet by aligning the leading edge with the flow direction. At the same time, the airflow speeds up significantly as it travels around the curved surface. From Bernoulli's principle, we know that the faster speed generates lower pressure, so there is a lot of lift force created by faster flow over the curved sheet.

three pictures of different
wings

The terminology "properly shaped" includes the requirement that the fluid follows the wing's shape closely. This is referred to as attached flow.

Therefore an improperly shaped design will cause the airflow to separate from the shape of the airfoil. The point at which the fluid stops following the contours of the airfoil is referred to as separation.

a picture of a diagram of
attached
and separated airflow

All aircraft are designed to generate the maximum amount of lift with the least amount of drag. By virtue of its shape alone, an airfoil will generate lift as air flows around it. However, even more lift can be generated by the airfoil if it is tilted with respect to the airflow. This tilt is called an airfoil's angle of attack. As the wing is tilted, the air flowing over the top of the wing flows even faster than the air flowing underneath. As the difference between the speed of the 2 airflows increases, the difference in pressure increases also. So, as its angle of attack increases, the wing generates more lift. Each aircraft has an angle of attack at which the maximum amount of lift can be generated. Increase the angle of attack beyond that and the lift begins to dramatically decrease. At a certain point, the angle of attack is so great that the airflow cannot follow the shape on the upper side of the wing, and it separates from the surface. This point is called the stall angle.

Note: NASA Glenn Research Center's Foil Sim applet provides an interactive experience about generating lift.

To make practical airplane wings, we need to put some structural strength in the wing. There is no room inside the thin sheet to do this, so we need to make the wing thicker. Most wings have a thickness between 10% and 15% of the chord length (the distance from the leading edge to the trailing edge)

a picture of airflow with
physical
attributes pointed out

For the thick wing, we want a streamlined shape that helps the flow follow the surface, so we make a smooth, rounded leading edge, but keep a sharp trailing edge. In the same way that curving the thing sheet made more lift, we can curve the thick wing, too. The curvature is called "camber".

This camber makes the wing shape look more curved on the top surface, and flatter on the bottom surface. Both camber and angle of attack help a wing generate lift, especially when they work together.

a picture of airflow
showing how 
lift is generated

An airfoil shape is used to give the greatest lift possible to an airplane. A flat plate held at the proper angle of attack does generate lift, but also generates a lot of drag. Sir George Cayley and Otto Lilienthal during the 1800's showed that curved surfaces generate more lift and less drag than flat surfaces. Early research also showed that a round leading edge and a sharp, flat trailing edge add to a wing's ability to generate more lift and less drag.

a picture of a wing describing
it's
different sections

Let's construct step-by-step an airfoil section.

A. The length of the airfoil section is determined by placing the leading and trailing edges their desired distance apart. This length is called the chord line.

a picture on how to construct an
airfoil section

B. Add curvature with the camber line. The amount of curvature is determined by the camber line. This curvature greatly helps generate lift.

a picture on how to construct an
airfoil section part B

C. Add thickness above the camber line. The amount of thickness that is added will depend on the amount of strength needed in the wing and the speed the airplane will usually fly.

a picture on how to construct an
 airfoil section part C

D. Add the same amount of thickness below the camber line.

a picture on how to construct an
 airfoil section part D

E. Now you have an airfoil shape.

a picture on how to construct an
 airfoil section part E

How much lift does a wing make?

The amount of lift depends on these things:

  1. the wing's airfoil shape
  2. the angle of attack
  3. the size of the wing (its area)
  4. the dynamic pressure (density and speed)

The shape of a wing greatly influences the performances of an airplane. The speed of an airplane, its maneuverability, its handling qualities, all are very dependent on the shape of the wings. There are, for our purpose here, 3 basic types that are used on modern airplanes: straight, sweep and delta.

The straight wing is found mostly on small, low-speed airplanes. General Aviation airplanes often have straight wings. Sailplanes also use a straight wing design. These wings provide the most efficient lift at low speeds, but are not suited for high speed flight approaching the speed of sound. Scientists measure this lifting efficiency as a ratio of lift to drag or L/D.

a picture that shows the
different
styles of straight wing configurations

The swept wing (forward swept or sweptback) is the wing design of choice for most modern high speed airplanes. The swept wing design creates less compressible drag, but is somewhat more unstable for flight at low speeds. A high sweep wing delays the formation of shock waves on the airplane as it nears the speed of sound. How much sweep a wing design is given depends upon the purpose for which the airplane is designed to be used. A commercial jetliner has a moderate sweep. This results in less drag, while maintaining stability at lower speeds. High speed airplanes (like modern jet fighters) have a greater sweep. These airplanes do not generate much lift at low speeds. These airplanes take off at high speeds and land at high speeds.

a picture that shows the
different
styles of sweep wing configurations

From above, a delta wing looks like a large triangle. It has a high sweep with a straight trailing edge. Because of this high sweep, airplanes with this wing are designed to reach supersonic speeds. The landing speed of these delta-winged aircraft is also fairly fast because their wings do not generate much lift at low speeds. This wing shape is found on the supersonic transport Concorde and the Space Shuttles.

a picture that shows the
different
styles of delta wing configurations

The amount of lift created by a wing naturally depends on its size. The same pressure difference acting on a larger wing area results in more lift. When a wing is moving faster through the air, there is more dynamic pressure in the airflow. The pressure difference between the upper and lower surface is magnified in proportion to the dynamic pressure.


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