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Farnborough Air Sciences Museum, Farnborough, England

51° 16′ 56.28″ N, 0° 45′ 10.8″ W

Making Aircraft Move

The Farnborough Air Sciences Museum has a collection of early aviation equipment focusing on the science of flying. It’s a small and concentrated dose of aircraft and aviation equipment, based on an almost discarded collection of historical artifacts from the Royal Aircraft Establishment at Farnborough.

The most important exhibit is an original jet engine built from Sir Frank Whittle’s design. Whittle patented the jet engine in 1930, and in 1941 the first British jet-powered aircraft, the Gloster E.28/39, was flying. The W2/700 jet engine (see Figure 46-1) on display is the ancestor of all modern jet engines and was the first production jet engine in Britain.

Figure 46-1. Whittle W2/700 jet engine

There’s an early afterburner (called at the time an “augmenter”) that was designed to fit behind the Whittle engine and generate extra thrust by injecting fuel into the hot gases escaping from the jet engine.

There’s also a display of a helicopter rotor, which explains how the blades rotate and pitch to provide lift and motion.

The museum has a significant collection of wind tunnel models, including models that illustrate the performance of the Concorde at low speeds. There’s a collection of shock cones used for wind tunnel research of air intake shapes for supersonic aircraft. There’s also a working wind tunnel demonstration.

And of course, the museum has a collection of aircraft. There’s the cockpit of a 1971 Hawker Siddeley Trident passenger jet—the Trident was a pioneering aircraft because of its speed (Mach 0.88) and its ability to land “blind” using automatic landing equipment. The cockpit has been restored, with all the instruments working, and is set up for visitors.

There’s also a Sea Harrier “jump jet” capable of vertical takeoff and landing, a British Lightning T5 jet fighter from the 1960s that’s capable of Mach 2, and a Puma Helicopter.

The museum is entirely run by expert volunteers, and many of them are on hand to explain the exhibits.

Practical Information

The museum is open on the weekends, and entrance is free of charge. You can find full details from the Farnborough Air Sciences Trust website at http://www.airsciences.org.uk/.

The Jet Engine and Newton’s Laws of Motion

A jet engine, also often called a turbojet, is a form of reaction engine, which is an engine that makes a vehicle move by exploiting Newton’s Third Law: every action has an equal and opposite reaction (for more on Newton, see Chapter 69). At the back of a jet engine is a nozzle through which hot, pressurized air is expelled at very high speed. This creates an equal and opposite force acting on the engine and pushing the vehicle forward.

In a turbojet, the high-speed air is created by sucking in air from the atmosphere and using it to burn fuel. The core of a turbojet has three essential parts: a compressor, a combustion chamber, and a turbine (see Figure 46-2). These parts are all found in the gas turbine engines commonly used as power for tanks, hovercraft (see Chapter 50), and helicopters, but in a turbojet, it’s the hot gases leaving the fourth part, the jet nozzle, and not the rotation of the turbine itself, that provides power.

Figure 46-2. Turbojet engine parts

The compressor and turbine are linked by a shaft: when the turbine rotates, it turns the compressor. As air enters the jet engine, it first encounters the compressor, which increases the air pressure up to 40 times. The compressed air then enters the combustion chamber where jet fuel is injected and ignited.

The burning fuel increases the temperature of the compressed air, and a high-temperature, high-pressure flow of air hits the turbine, causing it to turn (which turns the compressor). The air and other gases then leave the engine through the nozzle, and as the air expands (and its pressure drops to normal atmospheric pressure), it accelerates. This high-speed air produces the thrust by Newton’s Third Law.

Newton’s Second Law (that the force, F, required to accelerate a mass, m, is proportional to the mass and the acceleration, a: F = ma) can be used to calculate the thrust generated by a jet engine. The mass being accelerated in a jet engine is the air entering it.

If a mass m of air enters the engine with velocity V and exits with (greater) velocity v and takes time t to pass through, the resulting thrust F is proportional to the mass of air sucked in and the difference in the air’s speed between the intake and nozzle. This equation is shown in Equation 46-1.

Equation 46-1. Calculating the thrust of a jet engine

The large jet engines used in commercial aircraft actually employ two different thrust-generating techniques: they consist of a standard turbojet engine as just described, but also have a very large fan in front of the compressor. These engines are known as turbofans.

The turbine drives the compressor as before, and compressed air and fuel is burnt and ejected to create thrust as in a turbojet. But the turbine also drives a large fan, which accelerates the air entering the engine; the air passes around the engine and exits without passing through the turbojet (see Figure 46-3). Nevertheless, Equation 46-1 applies, as this bypass air (so called because it bypasses the turbojet) is moving faster than when it entered and creates thrust.

Figure 46-3. A turbofan engine

The fan accelerates a large amount of air by a small amount, producing additional thrust. This acceleration is provided almost for free—it requires very little extra fuel to power the fan—and increases engine efficiency.

When you look into a jet engine on a commercial flight, you’ll see the huge blades of the fan; the turbojet is hidden from view.