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I was looking at pictures of Harrier jets and I noticed something.

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They have four landing gears. That seems pretty unusual for a fighter jet - most have only 3 in a triangle formation.

What is the reason for this 4 point configuration? It seems to just add complexity and cost without really increasing stability.

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    $\begingroup$ I'm going to go out on a limb and say that a standard tricycle gear setup would have put the main gear directly in the blast zone of the vectorable exhaust. Having the 2 main gear on the center line keeps them out of all exhaust zones, and the little outriggers keep it from tipping over. $\endgroup$ – FreeMan May 19 '15 at 3:47
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    $\begingroup$ As well, there is a difference between standard airplanes and harrier, it lands vertically so we can't compare both. $\endgroup$ – Trebia Project. May 19 '15 at 10:14
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    $\begingroup$ @TrebiaProject. it can also take-off and land conventionally so the comparison remains valid. $\endgroup$ – ratchet freak May 19 '15 at 10:50
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    $\begingroup$ @ratchetfreak maybe we can compare as well with Piaggio p136 and CL415 they also take-off and land convetionally. The point I am making is that there is an stronger requirement for design (vertical land) changing the concept. Applicable as well to amphibious planes. $\endgroup$ – Trebia Project. May 19 '15 at 12:05
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The main load bearing landing gear is in a bicycle setup, with 2 outriggers for stability.

This allows the gear to be lighter, stay close to the center and out of the way for the jet blast.

The Lockheed U-2 also has its gear in the bicycle configuration and takes off and taxies with detachable "pogo" outriggers for stability.

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The Harrier is designed around its rather large centerline engine with its 4 non-standard exhaust ports. In order to balance properly, the engine's nozzles have to be in just the right place and we can't have annoyances like landing gear doors opening just before landing and deflecting the thrust stream that is keeping the thing in the air.

Fighter jet wings are typically too thin to fold the gear into them like on some airliners (and the bombs / missiles get in the way too).

So, the gear gets to fold up into the space just behind the aft nozzles where it doesn't disturb anything important. In almost every other jet fighter the engine occupies this space instead. Yes, tricycle gear makes for easier landings but as this plane is designed to land straight down that isn't a big issue. And we tend not to put green pilots into the things either so if they do need a regular runway the driver should be fairly good.

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And if the Harrier had been a conventional terrestrial fighter, it probably would have used a ubiquitous tricycle landing gear. The driver for Harrier's unusual landing gear configuration is the requirement to hover via an unconventional single engine vs available fuselage space. This necessitated that the Rolls Royce Pegasus engine to be located directly over the center of gravity for the aircraft. Consequently, this dictates the placement of the engine right on a/c centerline about midway along the fuselage and the engine now occupies much of the fuselage space which could normally be used for stowage of a tricycle landing gear when retracted. The rotating engine nozzles and their associated exhaust also would interfere with a tricycle main gear located on the fuselage. Designers both at Hawker Siddley in the 1960s and McDonnell Douglas in the 1980s looked at the problem and concluded that a bicycle type landing gear mounted fore and aft of the engine with retractable outrigger gears on each wing represented the best solution in regards to weight and balance, structure, and drag for the aircraft.

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The Harrier's wings have large anhedral, the wingtips are close to the ground. Without the outriggers there is a risk that payload carried back to base on the outer pylons gets damaged if the pilot sets down the aircraft with a roll angle or a roll rate causing a roll angle to build up.

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  • $\begingroup$ The pylons are kind of necessary to stop the plane falling over when it's stopped! So this doesn't really answer the question, which is why the plane doesn't use a conventional three-wheel set-up. $\endgroup$ – David Richerby May 24 '15 at 13:29
  • $\begingroup$ The answer as to why the landing gear layout is as it is requires you to know the design decisions made, and in which succession they were made. The design decision (or combination thereof) that lead to the landing gear layout is the answer to the question. I would think the landing gear decision was made after other, more important things were decided. $\endgroup$ – user7241 May 24 '15 at 13:38

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