Aeronautics

Parts of an Airplane and Their Functions

The modern aircraft has five basic structural components: fuselage, wings, empennage (tail structures), power plant (propulsion system) and the undercarriage.

Structure

Four Forces

The fuselage is the main body structure to which all other components are attached. The fuselage contains the cockpit or flight deck, passenger compartment and cargo compartment. While wings produce most of the lift, the fuselage also produces a little lift. A bulky fuselage can also produce a lot of drag. For this reason, a fuselage is streamlined to decrease the drag. We usually think of a streamlined sports car as being sleek and compact - it does not present a bulky obstacle to the oncoming wind. A streamlined fuselage has the same attributes. It has a sharp or rounded nose with sleek, tapered body so that the air can flow smoothly around it.

Wing Placement

 

The wings are the most important lift-producing part of the aircraft. Wings vary in design depending upon the aircraft type and its purpose. Most airplanes are designed so that the outer tips of the wings are higher than where the wings are attached to the fuselage. This upward angle is called the dihedral and helps keep the airplane from rolling unexpectedly during flight. Wings also carry the fuel for the airplane.

Dihedral

 

 

Tail Section Types

The empennage or tail assembly provides stability and control for the aircraft. The empennage is composed of two main parts: the vertical stabilizer (fin) to which the rudder is attached; and the horizontal stabilizer to which the elevators are attached. These stabilizers of the airplane help to keep the airplane pointed into the wind. When the tail end of the airplane tries to swing to either side, the wind pushes against the tail surfaces, returning it to its proper place. The rudder and elevators allow the pilot to control the yaw and pitch motion of the airplane, respectively.

 

Various Propeller Configurations

Within a piston engine, the pistons can be arranged in four ways: radial, in-line, oppositional and "V." The radial engine has pistons arranged in a circle with the spinning shaft in the middle. These engines were once the most widely used aircraft engine. They never found much favor outside of aviation and are not used in modern aviation.

The pistons on an in-line engine are lined up one behind the other along the length of the shaft that turns the propeller. These have been used in many applications including cars. They are not used a great deal in aircraft, as they tend to be long and heavy. Aircraft engines must be as lightweight and compact as possible.

The oppositional piston engine is much like the in-line, except that the pistons are mounted in pairs on opposite sides of the shaft. This makes for a much shorter and lighter engine. In-line engines have become very popular in the small airplane market.

The "V" engine is much like the oppositional engine, except that the pistons are not parallel to each other. Instead they are slanted in a "V" arrangement. The V8 engine is perhaps the most well known engine as it has been used to power millions of automobiles. The V8 is rarely used in airplanes as they tend to be heavier than the oppositional engines.

 

Tail Types

2-bladed prop
3-Bladed Prop
4-Bladed Prop
An 8-bladed contrarotating propeller
A 2-bladed propeller from a Beagle Pup A 3-bladed propeller from a Me-109G A 4-bladed propeller from a B-29 An 8-bladed contrarotating propeller from an Antonov AN-22

 

Piston engines drive a spinning shaft. The propeller is attached to that shaft. At least two (but usually three or four) blades make up the propeller. The more blades, the more air that can be moved by the propeller. A blade has an airfoil shape which generates lift as the blade slices through the air. Because the propeller is pointed forward the force generated is in a forward direction - that is, it thrusts the airplane forward.

The undercarriage or landing gear consists of struts, wheels and brakes. The landing gear can be fixed in place or retractable. Many small airplanes have fixed landing gear which increases drag, but keeps the airplane lightweight. Larger, faster and more complex aircraft have retractable landing gear that can accommodate the increased weight. The advantage to retractable landing gear is that the drag is greatly reduced when the gear is retracted. When flying on a commercial airliner you will notice that the pilot retracts the landing gear very soon after the airplane leaves the ground. This helps to decrease drag as the airplane ascends.

The power plant is simply the propulsion system and consists of the engines. The sole purpose of the engines is to provide thrust for the airplane. There are many different types of aircraft engines including: piston, turboprop, turbojet and turbofan. Turbojet and turbofan are jet engines. Some aircraft, notably gliders, do not have an engine. To take off they must have another source of thrust - that is, the tow-plane which pulls them into the air.

Jet Propulsion and Jet Engines

Jet propulsion is similar to the release of an inflated balloon. The pressure inside the balloon is pushing in all directions. It is also "jetting" out from the mouth of the balloon. The end of the balloon opposite to the mouth is not open. This creates an imbalance and causes the balloon to move in the direction away from the open mouth. Jet engines work in a similar fashion.

There are several types of jet engine: ramjet, pulsejet, turbojet, turbofan. The last two are the most widely used.

Propulsion

The ramjet is as simple a jet engine as can be found. Air enters the inlet and is compressed. This raises the pressure of the air. As the air arrives at the combustion chamber, fuel is added and an electric spark is generated. This causes a controlled explosion that raises the temperature and the pressure of the air tremendously. The hot, high-pressure air "jets" out the nozzle of the engine providing the forward thrust. This seems so simple, why would anyone want a more complex engine? The weakness of this engine is that the air coming in the inlet must be traveling at a very high speed (supersonic) for good efficiency. A ramjet does not work well at low speeds. This is simply not practical for most flying situations.

The pulse jet solves the problem of requiring supersonic speeds. It works well at a lower speed and with a little help, can get started when it is standing still. It is much like the ramjet, except that it has doors that close the inlet. When the doors are open, the air flows in and is compressed. The doors then close, forming a chamber in which the fuel is ignited. The hot, high-pressure gas then "jets" out the exhaust nozzle. The cycle of air in, doors closed, air out, then repeat, is where this engine gets its name. Pulse jets are not widely used for two reasons. They are very noisy and inefficient. They are the gas guzzlers of the aviation world.

The turbojet was the first really useful jet engine to be built. The air flows into the engine through the inlet. The design of the inlet makes the air slow down and also raises the pressure. The air then goes through the compressor where sets of blades compress the air even more, greatly raising the pressure. The air then enters the combustion chamber where the fuel is added and ignited. The very hot, high-pressure air rushes past the turbine blades making them spin very fast. The turbine blades are connected back to the compressor blades by a shaft. The turbine blades take some of the energy from the air and returns it to the compressor. The hot, high pressure air that gets past the turbine, "jets" out the exhaust nozzle thrusting the engine forward.

To increase the thrust available, a device called an afterburner is sometimes built into the engine. Fuel is dumped into the hot exhaust gas exiting the nozzle causing another controlled explosion. This makes the air even hotter which adds more energy to it, thereby increasing the thrust. This is not an efficient thing to do however, and is only done for brief periods when extra thrust is needed, for example, on takeoff or when a burst of speed is needed during a dog fight, or when an extra push is needed to reach supersonic speed. You may have seen movies with high performance jets, like the F-14. If you watch one of these jets from the back, and the pilot turns on the afterburners you will hear a burst of noise and see an orange glow around the outlet of the engines. The airplane will then shoot up into the sky.

The turbofan is a refinement to the turbojet that results in a more efficient engine. A large set of fan blades is set right in the front of the inlet. The fan works much like a propeller, thrusting the engine forward, pushing a large amount of air backwards. As the air is pushed back by the fan some of it goes into the engine and some bypasses the engine. The engine that sits behind the fan is basically a turbojet. The air that goes into this engine receives the same treatment as air that goes through the turbojet. The turbine in a turbofan drives the fan as well as the compressor. The air that "jets" out the back of this engine has less thrust than air that exits a turbojet, but that decrease in thrust is made up for by the thrust generated by the fan. A turbofan engine actually is more efficient than a turbojet and is quieter as well. Afterburners can be fitted to a turbofan if required.

The turboprop engine is essentially a turbofan engine where the fan is replaced by a propeller. The propeller is placed outside of the inlet. A gearbox is introduced which controls the spinning of the shaft, enabling speed control for the propeller. This is the most efficient means of propulsion, however it is limited in forward speed. Because the propeller is out in the free stream air, not mounted in the inlet (where the air speed is reduced) the propeller has to rotate at faster speeds. The speed of the propeller approaches the speed of sound well before the airplane itself. As the airplane approaches the speed of sound, drag greatly increases. So the speed of the airplane must be kept well below the speed of sound to prevent the tips of the propeller from going too fast.

If you want to fly at moderate speeds efficiently, then turboprops are a good choice. If you want to fly fast, but subsonically, then turbofans are a good choice. If you want to fly supersonically then a turbofan with afterburner is a good idea. If you want to fly slowly and only have a small budget or a small airplane, then a piston engine is a good choice.

On March 23, 2004, NASA's X-43A streaked through the sky above the Pacific Ocean at Mach 6.83. This test flight earned the unmanned test aircraft the world record for the fastest speed attained by an aircraft with an air-breathing engine. It flew more than twice as fast as the SR-71, the fastest air-breathing manned vehicle to date. In the past, only aircraft powered by rocket engines could reach these kinds of high velocities. The X-43A achieved its record through the first free-flight use of a scramjet engine. Conventional jet engines combine air and fuel in a combustion chamber featuring subsonic airflow. But the scramjet engine features supersonic airflow throughout the engine. While scramjets can deliver rocket-like speeds, the fact that they use oxygen from the atmosphere rather than carrying a heavy oxygen tank gives scramjets potential advantages over rockets in terms of payload capacity and flight duration. The scramjet aboard the X-43A is designed to take the aircraft to or beyond Mach 10.

X-43a Prototype

The X-43A is the prototype for what could be a new generation of ultra-fast jet aircraft. It was part of the Hyper-X hypersonic flight research program that was conducted by NASA’s Langley Research Center and Dryden Flight Research Center. In the test configuration, the X-43A was released from a Pegasus rocket at an altitude of 29,000 meters.

Lessons learned with the X-43 have been incorporated into the U.S. Air Force Research Laboratory’s new X-51A. The X-51A is being built by Boeing with a Pratt & Whitney supplied engine, and is slated for launch in December 2010.