Podded engine

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Podded engines on a Boeing 707.

A podded engine is a jet engine in a pod, typically attached below the wing or to the tail of the aircraft. The pod itself is called a nacelle.

Placing engines on the wing provides beneficial wing bending relief in flight. Early British jet aircraft had their engines buried in the wing root. See the de Havilland Comet, the V bombers and the Hawker Siddeley Nimrod. The further the engines are away from the fuselage the greater the wing bending relief so engines buried in the wing root provide little relief. Almost all modern large jet airplanes use engines in pods located a significant distance from the wing root for substantial wing bending relief. The pods are in front of the wing to help avoid flutter of the wing which, in turn, allows a much lighter wing structure. Locating the pod below the wing provides each engine with air undisturbed by the fuselage or wing. The wing also blocks some engine noise from reaching the aircraft cabin.^{[citation needed]}

Fuselage-mounted podded engines

The Cessna Citation uses two engines podded in the rear.

Smaller jet airplanes like the Cessna Citation are generally not suited to podded engines below the wing because they would be too close to the ground. This is also the case with aircraft designed to operate from unimproved grass or gravel runways. Instead, in these cases the convention is to mount two (or occasionally four) podded engines located at the rear of the fuselage, where they are less likely to be damaged by ingesting foreign objects from the ground.

This mounting location provides no wing bending relief but, following an engine failure, does offer much less yaw due to asymmetric thrust than would wing-mounted engines. Careful examination of such engines will show them typically mounted nose-high. These engines are mounted to face the local flow of air, and the local airflow at the airplane's tail is typically descending with respect to the centerline of the aircraft's fuselage.

The VFW-Fokker 614's overwing podded Rolls-Royce/Snecma M45H Mk. 501 turbofan

Overwing podded engines

As unusual counter-examples, the VFW-614 and the Hondajet place the podded engines above the wings. The Antonov An-72 and the Boeing YC-14 also place their engines above the wings, mounted very close to the wing. This placement utilizes the Coandă effect allowing a lower minimum flight speed and decreasing the amount of runway needed for takeoff and landing (i.e. STOL).

Military aircraft

Modern jet fighters almost never use podded engines (the US Navy used the S-3 Viking but these are not designated fighters and are now retired) instead typically embedding the engines within the fuselage. This was not true of earlier designs, however. The Messerschmitt Me 262, for example, used under-wing pod mounted engines. The Bell P-59 Airacomet used engines mounted in pods tucked up against the fuselage.

Many military bombers, transports, and tankers however do use podded engines. Examples include the Boeing B-52 Stratofortress strategic bomber, the Lockheed C-5 Galaxy transport, and the McDonnell-Douglas KC-10 Extender tanker.

An exception to the general rule is the Fairchild-Republic A-10 Thunderbolt II, which uses fuselage-mounted podded turbofan engines despite its role in combat as a ground-attack aircraft.

Advantages

A Boeing 787's engine pod. The chevrons on it are to reduce noise.

• Podded engines on the wings can act as vortex generators.
• Podded engines can be located significantly outboard on the wing, where the wing thickness is too small to accommodate a buried engine. The further outboard, the greater the wing bending relief.
• Podded engines in front of the wing provide maximum resistance to flutter of the wing. This is why almost all of the engine pod is located ahead of the leading edge of the wing.
• Podded engines can ease maintenance access.
• Podded engines can reduce noise within the cabin (since the engines are farther from the cabin, and for engines mounted below the wing, the wing itself acts as a noise shield).
• It is often easier to change engine models when the engine is in a pod. For example, the Boeing 747 uses engines from GE, Pratt and Whitney, and Rolls Royce, the changes being mostly isolated to the pods themselves.
• If a podded engine explodes, catches fire, or breaks free from its mounts, it is less likely to critically damage the aircraft than an engine embedded within the airframe. Although such events seldom happen to modern jet engines, this possibility helps explain why podded engines are commonly used on commercial and general aviation aircraft that may carry fare-paying passengers. Military combat jets are often occupied only by crewmembers who can bail out of the craft in an emergency, making this safety factor less crucial.^{[citation needed]}

Disadvantages

• Podded engines can increase drag.
• A podded engine hanging low from a wing can suffer more damage from foreign objects (i.e., objects can get sucked into a low-hanging jet).
• In a ditching or water landing, podded engines hanging from the wing increase the stress on the wing by increasing the amount of drag caused by the water. This can cause the wings to shear off or flip the aircraft and destroy the fuselage as happened to Ethiopian Airlines Flight 961.

References