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

enter image description here

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. $\endgroup$ 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. $\endgroup$ 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. $\endgroup$ 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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This was the lowest mass solution at the time of P1127 development.

Remember that weight is at a premium for vertical take off. The first hover tests of the P1127 prototype were only possible with fuel for only 3 minutes in the tanks and 700 lbs of equipment stripped from the airframe. A conventional landing gear would had made hovering impossible in 1960.

But be warned that this gear layout will not be easy to get right. It took until 1967 to get it to behave well. What went wrong?

  • In the first tests, the main gear oleo strut had more travel than the outriggers, so the airplane would tilt over to one side as soon as one of the outriggers would be lifted from the ground. The sideways component of thrust thus set free would let the plane skid sideways, which in turn resulted in a rotation around the nose gear. With thrust being equal to weight, even 6° of tilt will produce a sideways acceleration of 0.1 g!
  • So for the next test the outriggers were placed on platforms. While lift off was possible this way, landing inevitably took place without those platforms, and the same dramatic sequence unfolded at all subsequent takeoffs. Bleed air was at a premium, so roll control was insufficient to keep the airplane level.
  • The first successful hovers were possible after the outriggers were fitted with temporary extensions. Now it was time to try a conventional takeoff, starting with taxi tests. The landing gear fought back hard by exhibiting a large deadband in the nosewheel steering mechanism, followed by far too great sensitivity once control inputs exceeded this deadband. The resulting weaving made takeoffs impossible.
  • When that was fixed, the main wheel brakes took over in prohibiting takeoffs. During the next taxi tests, a brake induced torsional oscillation developed and broke a main leg.
  • After that it was the turn of the outriggers to act up. They were initially castoring, but showed a tendency to shimmy and finally had to be locked for flight.

Only in May 1961 was the first conventional take-off possible. The nosewheel steering system was barely useable, the brake judder was severe, the anti-skid often failed and the pilot could not feel that the wheels were skidding at any time. Several tyres were burst.

Source: John Farley's Harrier Development lecture.

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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. $\endgroup$ 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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