And now, everything you ever wanted to know (or not) about how aircraft work...

# FIXED WING AIRCRAFT

## The Wings

Wings are absolutely essential to a fixed-wing aircraft: without wings, it won't fly. Period. The cross section of an aircraft wing looks something like this:

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Aircraft wings work thanks to the Coanda effect. Take a look at the wing above. Air moving across the bottom flows in a straight line, whereas air moving across the top has to go over a hump, and thus move more quickly. The increased velocity of the airstream over the wing is matched by an equal and opposite force from below the wing. This is called lift.

To turn, an aircraft uses ailerons. Ailerons work by increasing the lift on one side of the plane while decreasing the lift on the other side. This makes the plane bank to one side and begin to turn. Ailerons are used in concert with the aircraft's rudder, which is located on the tail fin and causes the airplane to yaw from side to side. Neither ailerons nor rudder are really powerful enough to firmly turn the plane by themselves: this is why they are used together.

To nose up and down, an aircraft has a smaller set of wings by the tail fin called stabilizers. These stabilizers have aileron-like flaps called elevators. By altering the lift on the stabilizers, the pilot can make the stabilizers rise or fall in relation to the wings, thus making the plane pitch up and down.

Lift's opposing force is gravity. In order for an aircraft to get off the ground, it has to generate enough lift to overcome its weight. The only way to do this, at least on a fixed-wing airplane, is to push it forward. So we get our next vital component:

## The Engine(s)

There are basically three types of engines used on aircraft: piston engines, jet engines, and rocket engines. All three are used to generate thrust, a force that pushes the aircraft forward. Thrust's counteracting force is called drag, and is mainly caused by wind resistance, which is why aircraft are streamlined.

### Piston Engines

Piston engines are very similar to the internal combustion engine under the hood of your car. The main difference is that instead of driving wheels, piston engines on aircraft drive propellers. Until World War II, this was the only type of engine available for use on aircraft: the Wright Brothers' Flyer used a piston engine, as did the B-29 Superfortress forty years later. The downside to the piston engine is that it is very inefficient, and slow.

### Jet Engines

The first jet aircraft flew in the Luftwaffe of Nazi Germany during World War II, but were still very experimental. Following the war, piston aircraft began to fall out of favor as designers embraced the speed and efficiency of jet-powered aircraft.

Early jet engines were called turbojets, and used compressors to collect air from outside. The compressed air would be injected with fuel and ignited, and the resulting explosion directed out of the back of the engine to create thrust by Newton's Third Law.

In the late 1950's, the turbojet was modified and refined to create the first turbofan. Turbofan engines are turbojet engines with large fans at the front that turn along with the engine's compressor and generate extra thrust. The added fan makes the engine more powerful without demanding extra fuel. Most commercial jet engines nowadays are turbofans.

The turboprop is the logical extension of the turbofan, and uses a low-powered jet engine to drive a full-sized propeller. Most propeller airliners today use turboprop engines. While they can fly faster and more efficiently than pistons, they are still slower than turbojets and turbofans. Turboprop engines are slowly beginning to disappear from all but the smallest aircraft: nowadays, regional jets and business jets are in vogue for light transport applications.

Another type of jet engine is the ramjet, which was first played with in the 1960's. The ramjet has very few moving parts: it is designed to be used at high speeds, where air entering the engine can compress itself without the need for a separate compressor. While ramjets are incredibly efficient and powerful, they can only be used above 350 MPH or so, which makes them impractical to use on most modern aircraft.

### Rocket Engines

Rocket engines use a directed explosion to produce thrust, just like jet engines do, except that they don't use compressed air: they ignite their fuel with liquid oxygen (the most fun chemical on Earth). The Nazis experimented with rocket-powered aircraft during World War II, but eventually confined rocket engines to their ballistic missiles, which is the rocket engine's primary use today (besides the Space Shuttle and other spaceplanes, which have to operate without the luxury of outside oxygen).

# HELICOPTERS

Helicopters look very different from fixed-wing aircraft, but don't let the differences fool you: the same basic ideas apply. A helicopter's rotor is really a set of wings that rotate. By spinning in midair, the rotor blades generate enough air resistance to create lift, just like wings on fixed-wing planes do.

This leaves one major problem: how to keep the helicopter straight in midair while its engine is happily spinning it around. Thus, all helicopters have two rotors. On larger helicopters, there are two full-size rotors that rotate in different directions. On smaller helicopters, there are either two full-sized rotors stacked on top of each other, or one full-sized rotor on top and one smaller propeller on the tail.

To move around in the air, helicopters pitch and bank to change the direction of their rotors' thrust: to move forward, for instance, a helicopter points its nose down. To turn, helicopters change the speeds of their rotors so that one overcomes the other and pulls the helicopter around. The rotors themselves are turned using modified piston or turboprop engines with huge gear differential assemblies bolted on.

The advantage to the helicopter is that it can generate lift without actually moving: the disadvantage is that it has to use much more energy in doing so. There have been several experiments in combining fixed-wing aircraft and helicopters, the most recent one being the V-22 Osprey.

# AIRSHIPS

Airships use engines, rudders, and stabilizers just like fixed-wing aircraft do. Instead of wings, however, they use a large container of lifting gas—the gas can be either hot air, hydrogen, or helium. Hot air is fairly safe, but keeping it hot requires a burner, which uses fuel and quickly becomes inefficient. Most designers of commercial airships choose helium because it is inert. The zeppelins of Germany used flammable hydrogen because of an American helium embargo, and that critical decision eventually led to the Hindenburg disaster of 1937.

Zeppelins are rigid airships—airships where the lifting gas is contained within a metal frame. Blimps, on the other hand, use loose bags of gas held in place by internal pressure.

Vital components common to all aircraft include:

## The Undercarriage

Called the "landing gear"... by everyone except plane people. An undercarriage can be wheels, skids, skates, skis, floats, or the plane's hull, depending on its application.

Most common fixed-wing aircraft use wheels, and today aircraft wheels are usually designed to retract into the fuselage to reduce drag. Some small aircraft can get by with two wheels and a skid on the tail: jumbo jets like the Boeing 747 can have scores of them.

## Flight Controls

Fixed-wing aircraft use a stick or yoke to control pitch and bank, a throttle to control thrust, and rudder pedals to control yaw. Helicopters use a collective to control the main rotor's speed, a cyclic to control its tilt, and pedals to control the secondary rotor.

In days of yore, flight controls were connected directly to the surfaces they controlled, usually by cables. Once airplanes became too large for this to work, hydraulic systems came into use. The current state of the art is fly by wire, where the flight controls are connected to a computer that moves the surfaces with motors.

## Instruments

The four basic instruments on any aircraft are the compass, pitch and bank indicator, airspeed indicator, and altimeter. The latter two are powered by the pitot static system, which uses outside air pressure to determine the speed and altitude of the aircraft. The pitch/bank indicator is a ball that floats in fluid and tells the pilot whether the aircraft is level or not: it's basically the same thing as a carpenter's level, only a thousand times as expensive (it is almost always supplemented by its more modern equivalent, the artificial horizon). The compass, of course, uses the Earth's magnetic field, and is just like any other compass... only, again, a thousand times as expensive.

Other common instruments include fuel flow and RPM indicators, radio altimeters, DME, OBI, and the catch-all horizontal situation indicator. The newest, most expensive airliners and fighter jets integrate hundreds of indicators on a series of well-organized computer screens: these aircraft are said to have "glass cockpits." (And if the multiply redundant computers ever fail, they suddenly become dark rooms hanging in midair...) Fighter jets also project vital information into the pilot's helmet or over their windscreen (the "heads up display").