Lesson 1: Aerodynamics as a Science
Section 2 - Why Study Aerodynamics?
A century ago, man rose for the first time from the surface of the earth in gliders and powered aircraft. This course is to help those curious about how it is possible for things to take flight, including our huge modern airliners. Then, there is the mystery of rockets and spacecraft which along with aircraft are reaction devices to work in accordance with Issac Newton’s Third Law of Motion, stating “For every action, there is an equal and opposite reaction.”
In our next section, we will discuss all three of Newton’s Laws of Motion and their relevance to aerodynamics.
And what specifically makes rockets and aircraft reaction devices? A rocket is similar to a continuously firing machine gun mounted on the rear of a rowboat. As the gun is fired to the rear, the recoil from the stream of bullets moves the boat forward.
A rocket-motor’s “bullets” are minute particles that are thrown out through a nozzle as a propellant is burned in a suitable chamber. The reaction to the discharge of these particles makes the rocket fly in the opposite direction. A rocket contains all the elements it needs to operate, including both fuel and oxidizer.
A jet airplane engine, in contrast, is a reaction device that uses oxygen in the air to support the combustion of the fuel carried on board. Aerodynamics is important to study because it is a science we come across in usage every day. From automobiles to bridge and building structures to aircraft, aerodynamics is at work wherever we go.
Did you ever wonder why a car with more curves and that comes to a point in the front gets better gas mileage than a car that is shaped strictly in the shape of a box? This is due to the aerodynamic term drag. Drag resists the forward motion of an object which means a force is applied against that object as it tries to move.
Simply, a car with less surface area pointing in the direction it is moving will have less drag affecting its performance. There are several forms of drag including friction drag, form drag, and induced drag.
Friction drag occurs in the boundary layer of air while form drag results when the airflow past an object breaks away from that object. This effect produces a swirling air that takes energy from the object. Induced drag effects solely aircraft in that it is a drag due to the lift of that aircraft. It is the difference in pressure found above the wing and the pressure found below. Induced drag causes a vortex which holds the plane back.
Aerodynamics also affects more simpler objects such as a model rocket, a beach ball thrown near the shore, or a kite flying high overhead. When you’re watching your favorite big league pitcher throw a curveball, that curve you see comes from aerodynamics.
BERNOULLI'S THEOREM
In our study of aerodynamics, it is important to note Bernoulli’s theorem and the significance of a wind tunnel. Bernoulli’s theorem, a fluid dynamics term, is the relation among the pressure, velocity, and elevation in a moving fluid which could be liquid or gas, the compressibility and viscosity (internal friction) of which are negligible and the flow of which is steady, or laminar.
It was first derived in 1736 by Daniel Bernoulli, a Swiss mathematician and states "the total mechanical energy of the flowing fluid, comprising the energy associated with fluid pressure, the gravitational potential energy of elevation, and the kinetic energy of fluid motion, remains constant." Bernoulli’s Theorem is the principle of energy conservation for ideal fluids in a steady, or streamline, flow.
This implies that if the fluid flows horizontally so that there is no change in gravitational potential energy, then a decrease in fluid pressure is associated with an increase in fluid velocity. Bernoulli’s Theorem is the basis for many engineering applications, such as aircraft-wing design. The air flowing over the upper curved surface of an aircraft wing moves faster than the air beneath the wing, so that the pressure underneath is greater than that on the top of the wing, causing lift.
WIND TUNNELS
A wind tunnel is an important testing device in aerodynamics. Wind tunnels are ducts in which a controlled, uniform airflow is blown against a scale model.
In a wind tunnel, a controlled stream of air is produced in order to study the effects of movement through air or resistance to moving air on models of aircraft and other machines and objects.
Wind tunnels are classified as low-speed or high-speed. They can be subsonic at 80% of the speed of sound (approximately 760 mph), transonic which is roughly the speed of sound, supersonic - up to 6 times the speed of sound, hypersonic (6 to 12 times the speed of sound, and hypervelocity (over 12 times the speed of sound).
In order to simulate temperatures of flight at speeds of 10,000 miles (16,000 km) per hour and more, the test air must be heated well above the melting point of ordinary structural materials. As a result, these tunnels are operated on an impulse principle and only for extremely short periods on time somewhere in the area of a few thousandths of a second. In addition to aircraft and spacecraft, aerodynamic studies in wind tunnels have been highly profitable and reliable devices for solving design problems in automobiles, boats, trains, bridges, and building structures.
