1. Introduction
Aerodynamics is the science of air in motion and explains how and why an aircraft flies. It can be compared to, and contrasted with, aerostatics.
2. Aerostatics
Contrasted with aerodynamics, aerostatics is the science which explains how and why a hot air balloon, or similar craft, attains lift. These aerial vehicles do so by means of the buoyancy principle.
Air is compressible…that is, its own weight compresses it. The lower its location in the atmosphere, the more air…and therefore weight…is above it, rendering it densest at or near the ground. Conversely, as it rises, it becomes thinner.
Hot air balloons utilize these varying conditions to attain lift. Heated air, or lighter-than-air gas, within a balloon’s envelope causes the balloon itself to rise, because its internal air is less dense than the surrounding air. When it reaches the altitude where the density of its internal air equals that of the surrounding air, it ceases to rise and attains a state of internal and external equilibrium…that is,
Internal gas density = external gas density
At this point, the downward pressure exerted on the balloon equals the upward pressure on the balloon.
Balloons are designated aerostats because their lift is attained in a static air mass…that is, an air mass which does not move. An aerostat, such as a balloon, moves vertically, but relies on existing wind direction and speed for its horizontal motion. As a result, it cannot be relied on for specific-direction transportation.
Aerostats with controlled movement employ one or more propellers for velocity and direction, and are designated airships, but these propellers do not provide or augment lift.
There are two types of airships:
1. Non-rigid airships, such as the Goodyear blimp.
2. Rigid airships, such as the Hindenberg, which had contained an internal framework.
3. Aerodynamics
Unlike an aerostat, an aircraft is heavier than the air it displaces and hence cannot be buoyed up in a static air mass. Its lift can only be achieved by the science of aerodynamics.
Aircraft are subjected to four forces in flight: thrust, drag, lift, and weight.
Thrust opposes drag, while lift opposes weight. It is these latter two, however, which play the greatest role in aerodynamics.
In order to overcome weight, itself the result of gravity, an aircraft must create a force at least equal to it to produce straight-and-level flight. That force, of course, is lift, but how is it created?
Part of the answer lies with a Swiss mathematician, Daniel Bernoulli, who had sought to determine pressure differentials of water streams flowing at varying speeds. Because he had died in 1783, or one year before the Montgolifier Brothers of France had successfully made the world’s first aerial balloon ascent, his experiments had no connection, or intended connection, with aviation, although they ultimately did.
Many of these experiments had been conducted with the aid of a venturi tube, which, because of its varying diameters, had been able to restrict the flow of liquid through it at certain points. It had been at these smaller diameters that flow speeds of liquid had increased, but, surprisingly, their associated pressures had decreased, resulting in an inverse correlation and the law of physics which states:
As speed increases, pressure decreases.
This law partially aids us in determining how lift can be generated when its principle is transferred to a wing. Before we discuss how this occurs, we first need to review a few wing-related parameters.
The wing itself, of course, is that surface which creates lift on heavier-than-air craft and can provide the platform to which the engines are attached. Its leading edge is its forward, geometric edge, while its trailing edge diametrically opposes it. An airfoil is a cross-section of a wing and its camber is the characteristic bulge or hump on its upper surface. That camber is tantamount to its ability to produce lift.
When a free stream of air, which has a uniform velocity, intercepts a wing’s leading edge, it does so at what is known as the stagnation point, whereafter it divides and either flows over the wing or under it. This, however, is where its uniform velocity ends, because the distance it must travel over the wing, as a result of that bulge-like camber, is greater than the distance it must travel below it, yet both flows reach the trailing edge at the precise second. The only way the upper surface flow can achieve this is to increase its speed. Imagine a student leaving his house and walking one mile to school. His brother, who lives in the same house, leaves at exactly the same time, but takes a route which covers two miles. Yet he arrives at the school at the exact same time as his brother. The only way he had been able to achieve this had been to travel twice as fast.
With this increase in speed, the Bernoulli Principle comes into play. As you recall, as speed increases, pressure decreases. And this is exactly what occurs on the wing’s upper surface.
With pressure now diminished, the wing is free to rise, producing lift. This lift can also be partially explained by another physics principle, which states that an object always takes the path of least resistance. If you had been locked in a room with two doors and one of them could not be budged open with all your strength, yet the other opened with minimal effort, which path would you take?
As pressure decreases above a wing’s upper surface, the path of least resistance is up! An imbalance of pressure forces, as had occurred with the aerostat before it had been disconnected from its tether on the ground, now exists between the upper and lower wing surfaces. Unlike the aerostat, however, the wing’s lift is only produced by the motion of the air over and under it. As a result, the air is said to be dynamic, and the contraction of these two words results in the term “aerodynamics.”
Whether air movement over an airfoil is produced by the wing itself traveling through it, or the air blown over a stationary wing, mounted in a wind tunnel, the same result is obtained. Since few people have wind tunnels in their homes in which to test these principles, it cannot be demonstrated to any scientific degree to the amateur. Nevertheless, two informal methods exist with which to do so.
The first, intrinsically attempted by most children, occurs when they extend their arms out of a car traveling at significant speed. Seeming to ride a cushion of invisible air below, and no longer obstructed by equally invisible, but greatly reduced pressure above, they react by rising up and it usually requires concerted effort to keep them from doing so.
The second method entails blowing air over a slender, wing-representative strip of paper. Logic may tell you that, as you blow over the top of the paper, that it will be forced downward, but instead, verifying the Bernoulli Principle and the forces of aerodynamics, it rises. Lift has therefore been created it. Try it!
A very small amount of lift is also created when an aircraft is close to the ground, such as shortly after take off or shortly before touchdown. The force of the aircraft is exerted downward, while the ground itself reacts and sends a counterforce upward, to the aircraft, echoing Sir Isaac Newton’s Third Law of Motion. Every action has an equal, and opposite, reaction.
Because the amount of camber is integral to the creation of lift, wings can artificially increase this parameter by two methods:
1. An aircraft can increase its angle-of-attack, pitching upward so that the upper wing surface offers a greater curve to the air’s path.
2. Wing leading and trailing edge slats and/or flaps increase the wing area and curved path the air must follow, particularly during slower, post-take off and pre-landing flight realms.
The next time you fly, you will know a little more why.
