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Why do military aircraft have a lower landing gear than civilian? I have in mind the difference between a Lockheed C-130 Hercules and a Boeing or Airbus.

The first possibility that jumps to mind is that one is more comfortable than the other, which is more robust. But would an aircraft designer reduce robustness to provide more comfort?

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What do you mean by "lower landing gear"? –  casey Mar 3 at 19:36
    
@casey: I mean shorter, the distance aircraft to the floor is shorter. –  Quora Feans Mar 3 at 19:38
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That just ain't true. ATR-42 or BAe146/AvroRJ have similar gear to C-130, not to mention Lockheed L-100. –  Jan Hudec Mar 3 at 23:47

4 Answers 4

Easier to Load

As stated earlier, this is your primary reason. You have to be able to load in cargo from the ground. If you want to get troops and vehicles in fast to somewhere with minimum equipment, you want to be able to roll them of the aircraft, and the lower the aircraft, the easier this is done.

Other considerations:

Shorter gear is stronger

The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.

More Cargo space

The shorter you make the gear, the less trouble you have finding space for it inside the aircraft, where you want to maximise the amount of space you have for cargo, especially for vehicles and the like. The anatomy of a cargo aircraft is that they are typically hollow inside, unlike passenger aircraft where they fold up into the center section.

Landing gear is heavy

Landing gear is very heavy, since its got to support the weight of the entire aircraft above. The shorter you make it- the lighter the aircraft gets.

Better use of ground effect

The low gear may bring the aircraft a little bit closer to the ground, maximising ground effect and lowering your landing speed, again very ideal for rough terrain.


Note: These don't only apply to military aircraft, but to a lot of commericial ones as well, since the benefits are the same. Look at the Dash-8, ATR 42/72, BAe 146 along with the rear-engined planes, and they all try to do this for these reasons, although ease of cargo loading might not come first on the list.

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I am not sure about the rough ground. On rough ground, you need big shock absorbers. –  Jan Hudec Mar 3 at 23:50
    
Countering that: military aircraft that need to operate from unprepared strips may want taller landing gear so they have more ground clearance when operating on rough ground. –  jwenting Mar 12 at 10:00
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@jwenting youtube.com/watch?v=zj0a0wtGQKo shows that you can come a pretty good way with short gear and if you're stuck in the mud you wouldn't want them to be longer than necessary. –  MikeFoxtrot Mar 12 at 12:29

For the aircraft you use as examples the aircraft height above the ground (landing gear height) is mostly a function of use-case and clearances.

For the Boeing and Airbus airplanes you mention, the gear need be long to accommodate the engines hanging under the wings as well as the minimum bank angle needed to avoid wing strikes.

On the other hand, the C-130 has the engines mounted up on a high wing and the aircraft needs to handle big slow heavy things like tanks being driven into the cargo. The need of the tanks specifically probably mandates the interior floor be pretty close to the ground, or you might need extremely long ramps to for the tank to ascend (and these would pose their own problems).

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Latter case is solved in some transport aircraft by having the aircraft "kneel" after landing by use of hydraulic pistons in the landing gear. Effectively you're forcing the shock absorbers to fully compress. –  jwenting Mar 12 at 9:59
    
The tank-specific parts of this answer are actually quite a way off. Tanks are designed to cope with very rough terrain so a short, steep ramp wouldn't be a problem for them (though fitting a large vehicle in at an angle could be an issue). But this is moot anyway because a C-130 can only carry about 20t of cargo, which isn't close to being enough for an M2 Bradley (30t) , let alone a full battle tank (60t for an M1 Abrams). But, still, a low cargo bed is useful for heavy cargo, as you say. –  David Richerby Jun 17 at 9:16

There is no difference between civil and military aircraft. Not the least because the civil and military transport aircraft are actually the same models. Many military planes have civil variants like Lockheed C-130/L-100, other military planes are derived from civil ones, e.g. C-135 is B707.

The difference depends on where the wings and engines are placed. All the aircraft actually have similar clearance from the ground. Just on some the lowest point is fuselage, on others it's the engines hanging down from the wings and on propeller driven ones it is often the propellers.

Cargo planes (like C-130, C-5, An-124 or An-224) are often built with high wings, so the fuselage is lower and therefore easier to load. This is also concern for regional airliners as they are designed to carry their own stairs. That's why BAe146, ATR-42 or Dash-8 are high-wing and other like Bombardier CRJ series have tail-mounted engines so they can also be lower. On the other hand low or mid-wing designs with wing-mounted engines have better aerodynamic properties, which is why that construction prevails for large airlines.

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and don't forget that the 146 was intended as a military transport before being marketed as well as a civil airliner and cargo aircraft. –  jwenting Jun 17 at 6:41

C-130 Addendum

Purpose-designed for cargo - rather novel at the time back then, perhaps. Look at WWII films of the C-47 or Berlin Airlift film of C-54's. How the heck do they get anything in there?

Designed around a "standard truck bed height" floor and a "standard railroad box car" dimensions - 10' x 10'.

The short landing gear derives from the basic design goals. Further, the wheels are in tandem - one behind the other vice side by side. And the tires are fat. These aid operating out of unimproved/dirt fields. The leading tire "plows the way" so to speak for the rear tire. Fat tires lowers the "per square foot" weight on the tires.

The longer you make the gear, the more it will bend. Bad news if you're trying to land on rough ground.

Ironically the C-130 landing gear is wimpy when it comes to side loads. It does not have adequate side-bracing. Straight ahead, we can pretty much plop that puppy on the ground relatively aggressively. But, during taxi, stopping while still in a turn is a write-up and requires maintenance inspection prior to flight.

The first possibility that jumps to mind is that one is more comfortable than the other,

The C-130 is designed for utility, by masochists. It is painfully loud, too cold and too hot at the same time, vibrates like the inside of a jack hammer; and if the the prop synchro-phaser ever fails will drive you insane in short order.

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