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The Harrier Jump Jet: How Does It Achieve Vertical Takeoff and Hover?

Famous for serving as the first truly successful military jet capable of vertical/short takeoff and landing, the Harrier was introduced in 1969. Although the last example rolled off the line in 2003, Harriers continue to serve in military operations to this day.

The jet-powered attack aircraft was first developed by Hawker Siddeley. However, it was produced in two generations with four variants that involved manufacturers like McDonnell Douglas, Boeing, and BAE Systems.

Though a capable jet in other regards, its most notable feature is its vertical take-off and landing (VTOL) capabilities. The need for an aircraft with such capabilities arose during the Korean War, and several companies in both Europe and America began exploring options for aircraft with the ability to operate away from a network of vulnerable runways.

How do Harriers hover?

Though the engineering behind the Harrier’s hovering technology is undoubtedly complex, the basic concept is relatively simple. The aircraft employs a single jet engine that produces thrust that flows out of four nozzles located in pairs under its wings. These nozzles can rotate and thus position themselves either directly backward for straight horizontal flight or downward when being utilized for vertical takeoff or landing.

This arrangement of four nozzles is referred to as a “four-poster” thrust vectoring system and, to some extent, resembles an animal’s four legs when the vents are pointing down. The two nozzles in the front discharge unheated air compressed by the fan, while the rear nozzles discharge hot exhaust.

In addition to pointing straight back for high-speed flight or straight down for VTOL operations, intermediate positions enable the Harrier to conduct short takeoff and landings (STOL), as well as enjoy combat maneuverability advantages. The nozzles can also be directed beyond the 90-degree point to enable rapid deceleration.

It is worth noting that the Harrier cannot always engage in VTOL operations, especially when it is carrying heavier weight. More often than not, pilots utilize the Harrier’s short takeoff capabilities when carrying the fuel and weapons needed for typical missions. Short takeoffs and landings also burn less fuel and, thus, are typically preferred for normal operations.

Can a Harrier maneuver while hovering?

Getting enough thrust channeled downward to produce vertical lift is one aspect of vertical flight, but another significant element is ensuring the aircraft has the ability to turn and rotate. Turning in normal flight is primarily made possible by ailerons located on an aircraft’s wings and usually involves some level of pressure on the pedals that control the rudder located on the aircraft’s tail.

These are, however, only effective when air is moving around the airfoil at a sufficient velocity. If an aircraft is hovering, that velocity is not present. So, while making control inputs in the cockpit would technically still make the ailerons, flaps, and elevators move, that movement would do nothing to turn the plane.

For this reason, two small roll-control jets are located in the Harrier’s wingtips and are powered by the same engine as the four primary nozzles. The aircraft also includes pitch-control jets at the nose and tail. All of these respond to control inputs in a way that is proportional to normal responses during conventional flight.

The next generation of military VTOL jets

Even newer Harriers are starting to become old, and with other VTOL options – primarily the Lockheed Martin F-35B – the attack aircraft are mostly being phased out. The UK’s Royal Air Force already retired its fleet in 2011, selling its remaining aircraft to the US Marine Corps for spare parts.

The US Marine Corps, Spanish Navy, and Italian Navy plan to replace their Harriers with the F-35B in the near future. The Indian Navy retired its remaining Harriers in 2016, choosing to replace them with the conventional Mikoyan MiG-29K.