Almost every day, even in this age in which air travel is so routine, I hear people say they are apprenhensive about flying. Apprehensive is too mild to describe some, they are afraid.

What's the reason for this fear and apprehension? Most people who express a fear of flying say that it is because they don't understand why something so large as a jetliner can leave the ground and remain in the air. The fact that the atmosphere, the air, in which a Boeing 757, for example, operates is invisible certainly contributes to this lack of understanding. And rightly so.

The best way to understand how an airplane works is to build one. So, that's what we're going to do, verbally build an airplane.

Let's start with the most important component, the fuselage. The fuselage is the most important component because it's in this capsule that you and I are going to ride.

The American Heritage Dictionary defines the word fuselage as, "The central body of an airplane that accomidates passengers, cargo, and crew, (the payload, no pun intended) and to which the wings are attached." The word fuselage comes from the french word fuseler which means, to shape like a spindle.

The fuselage of our imaginary jetliner will be a tube ("...shaped like a spindle.") approximately 13 feet in diameter and 150 feet long. About the same size as three Grayhound buses parked end to end. This will allow for 210 passengers in 35 rows of six seats each; three seats on each side of an aisle that runs the full length of the tube. Also included in this space are galleys fore and aft, toilets, an empty row, or two, where the door(s) is located, and a small amount of storage space.

This tube, or, more precisely the structure of this tube, must be strong enough to do what we want it to do but at the same time it must be as light in weight as possible. Everything that goes into our airplane, must be as strong and as light as possible, because eventually we are going to want to lift the airplane up and propel it through the air, and the lighter the airplane is the easier it will be to do this. Engineers work very hard to make aircraft structures as efficient as possible, and ultimately the success of any airplane depends on how good they are at designing structures that are light and strong.

Efficient airplane structures are the result of a number of simple basic design principles, selecting the right material, keeping the amount of a given material used to a minimum, and the design of structures that can be used for more than one purpose.

All modern airplanes are made mostly of aluminum. Aluminum is the best material yet that aircraft engineers have found with which to build airplanes because it is strong, and light in weight. There are other materials, like titanium, that are as light, or lighter, than aluminum and stronger but these materials are much more expensive and are harder to work with than aluminum so they are used sparingly. Steel is also used where higher strength is required such as in landing gear components or engine mounts. Steel is stronger than aluminum but much heavier.

The newest materials are the so-called composite materials. These materials consist of a strong woven cloth matrix imbedded in, or, held together by, a resin compound. Entire airplanes such as the Beechcraft Starship have been built of these materials. However, thus far these materials have not replaced aluminum for the major structural components in airliners. Composites are used for control surfaces, fairings, and some of the skin panels.

Selecting the right amount of the right material and putting it in the right place to make the airplane only as strong as it needs to be, selecting the most efficient shape where you can, and designing structures that can be used for more than one purpose is the essence of good aircraft structural design.

Using the right material in the right place and in the right amount requires that engineers know precisely where the forces (loads) on the structure are and how strong these loads are. Only then can they know what material to use to design a given component. This phase in the design of an airplane takes a lot of analysis and testing.

Selecting shapes that are not only efficient from a structural standpoint but shapes that can be propelled through the air using the least amount of power is extremly important. The tubular shape chosen for our fuselage is the best example of this concept. To see how efficient (light and strong) a tube is for this purpose experiment with a simple paper tube like the one on which your paper towels come.

The fuselage is designed as a tube because a so called "body of revolution", like a tube, can be propelled through the air with a minimum of resistance (drag).

However, there is another very good reason why the cross-sectional shape of the fuselage is round. It is round because it must hold pressure, like a balloon. Here's the reason:We as humans are used to living on the bottom of an ocean. An ocean of air. Air is a mixture of gases, 78% nitrogen, 21% oxygen, and the rest a variety of other gases. The top, or surface of the "ocean", is at a height of approximately 250,000 feet above sea level.

Because of gravity the ocean of air, called the atmosphere, has weight. A column of air one inch square and 250,000 feet high (to the top of the ocean of air) weighs 14.7 pounds at sea level. That is, the sea level atmospheric pressure is 14.7 pounds per square inch (psi). As we go higher in the atmosphere this pressure becomes less and less because there is less air above us the higher we go. At 10,000 feet the atmospheric pressure is 10.1 psi, at 18,000 feet 7.3 psi, one half that at sea level, at 30,000 feet 4.4 psi, and at 41,000 feet the atmospheric pressure is a mere 2.6 psi, only 18 percent of the sea level pressure. Once we exceed 250,000 feet for all practical purposes we are in space. There is no air in space and, therefore, no atmospheric pressure.

Oxygen is the fuel that the human body runs on. As we ascend into the atmoshere, at some height the pressure of the oxygen in the air mixture falls below that required to force oxygen molecules, in sufficient quantities, through the tiny membranes in the lungs and into the bloodstream. Above this height an oxygen deficiency (called hypoxia) will exist. The height at which we as humans enter this door to a hypoxic state is 10,000 feet.

To maintain a comfortable state above 10,000 feet, and one in which we can operate normally as human beings, a state in which we are not hypoxic and therefore impaired, we have two choices; (1) we can add more oxygen to the air we breath, or (2) we can increase the pressure of the air we breath.

Since the pressure of the oxygen in the air mixture we breath is directly proportional to the amount of oxygen in the mix (Dalton's Law), if we increase the amount of oxygen we will increase the (partial) pressure of the oxygen in the mix. This is the solution the military uses in fighter aircraft. However, to do this the crew must wear an oxygen mask at all times, something not acceptable in commercial operations.

Complete protection, however, not only from hypoxia, but from all the other physiologic effects of low barometric pressure as well, is best achieved by pressurizing the aircraft cabin itself so as to maintain as normal a pressure environment as possible within the aircraft regardless of the actual flight altitude. This method allows the people on board the aircraft to move freely in a comfortable environment unencombered by oxygen masks or other specialized high altitude equipment, and during descents from altitude protection against pain in the middle ear and nasal sinuses can be provided by permitting the pressure in the cabin to increase slowly in a controlled manner.

Using this method also goes right along with using the components of our virtual airliner for more than one purpose. The compressed air used to pressurize the cabin as we climb into the atmosphere will come from from the engine(s) which, of course, are also used to propel the airplane to 30 or 40,000 feet, and speeds of 8 tenths the speed of sound.

In a typical airliner or corporate jet, pressurizing the cabin easily allows the passengers to remain at a sea level environment as the airplane climbs to approximately 24,000 feet. From there up to the maximum certified altitude of the particular airplane the cabin cannot exceed 8000 feet. It's the law.

Finally, the most productive way to minimize the amount of material used is to design aircraft strutures to serve more than one function. The fuselage for instance is not only the capsule that provides space and environmental protection for the required amount of passengers and/or cargo but it is the structure to which the aerodynamic surfaces are attached.

The wing structure of most modern day airplanes serves as the largest fuel storage tank on the airplane. Even the horizontal tail of the Boeing 747-400 stores 3100 gallons of jet fuel.

What does all of this mean to you and I as passengers? It means that as long as the airplane in which we are flying is operated in accordance with the paramenters under which it was designed it will be strong enough to take anything that it encounters. Not only that but by design all airplanes that carry passengers are at least 100 percent stronger than they need to be as a safety factor. When the engineers find out what a particular load will be on a component of the airplane they multiply it by two.

So, our fuselage is now equipped to accomodate passengers, but it is about as far from being an airplane as a Greyhound bus. We must literally give it wings for it to be able to fly. In the next episode we will do just that, so keep watching this page.