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I've been reading a lot about the Soviet and Russian military aircraft, and it seems a lot of them are built to work on dirt runways, or modified to do so.

How is this accomplished? Is it a simple thing to modify aircraft in this way? Is it just a matter of strengthening the undercarriage and upgrading the tires for more stress? Or is something harder involved, like maybe strengthening the fuselage frame?

NOTE: let's exclude worries about foreign objects going into the engines. Right now I'm only interested in the difference between paved and dirt runways as far as landing gear and strength are concerned.

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  • $\begingroup$ Related and also related. $\endgroup$ – fooot Aug 27 '15 at 18:22
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    $\begingroup$ FOD damage/ingestion is actually the larger concern of jet aircraft designed to land on unimproved strips. Actually touching down and rolling around on packed dirt is fairly academic. $\endgroup$ – KeithS Aug 27 '15 at 18:41
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    $\begingroup$ @KeithS I had a feeling this was true. Nevertheless the undercarriage part is the only part I'm interested in, and it's a completely different type of modification from any engine modifications afaik. I don't really mind if an answer talks about FOD safety too, I just wanted to be clear what I'm interested in. $\endgroup$ – DrZ214 Aug 28 '15 at 2:19
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Just to roll around on "unimproved" airfields (grass/dirt/gravel strips), the biggest thing planes need is simply a decent set of tires:

enter image description here

This MiG-29UN trainer shows off the fairly beefy main tires put on these planes specifically to allow them to more easily roll over minor imperfections in the runway/taxiway surface, including dirt/gravel/grass strips (though here it's landing on a traditional asphalt/concrete surface). Compare these to the relatively small gear wheels and low-aspect rubber on the similarly-sized F-16:

enter image description here
(source: f-16.net)

Now, your question oversimplifies things, because "foreign object debris" or FOD is in fact a primary danger of unpaved airstrips for jets, and this danger has to be accounted for in the design. Here's the main reason the MiG-29 can take off from dirt:

enter image description here

enter image description here

MiG-29s and similarly-minded Su-27s, and their variants, have special doors to close off the lower intakes normally used in flight, instead drawing in air through the gill-like ports over the top of the wing strakes. This prevents the engine from ingesting dust, rocks, birds, rodents, etc on takeoff and landing.

This engine protection system, while effective, adds mechanical complexity and weight which U.S. designers and military brass have generally thought unnecessary given that requirements rarely include the ability to operate from unimproved strips. Most U.S. fighters in the jet age, therefore, have had to operate from airbases with some form of paved runway surface (and the U.S. military actually has higher standards for runway surface quality than most international airports). The U.S. arsenal does, however, have several aircraft that are specifically designed to operate from rudimentary airstrips at forward bases. These designs avoid FOD damage primarily by keeping the engines up as high as the design will allow. For instance, the A-10's two engines are on pods angled up on the fuselage between the wings and tail for a variety of reasons including FOD avoidance:

enter image description here

Most of the other designs in the Western arsenal that can do this are high-wing designs with wing-mounted engines, which keep them out of the dust produced by takeoff and landing in a similar fashion. Here are a C-130 and C-17 landing at an Afghanistan forward base:

enter image description here enter image description here

The Harrier is a notable exception to this general strategy by Western designers of just keeping the engines out of any debris cloud. While it's probably not done routinely, a Harrier can set down just about anywhere, and as this picture shows, it ingests no small amount of dust in the process:

enter image description here

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    $\begingroup$ Yay! Pictures! :) $\endgroup$ – PTwr Aug 28 '15 at 9:58
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    $\begingroup$ Tire pressure is important, you basically want a slightly lower pressure so the tire will flatten out and increase its footprint over uneven ground. But, it still has to roll and to support the aircraft, so the tire itself is made bigger to allow for the "squish" without bottoming out on touchdown. $\endgroup$ – KeithS Aug 28 '15 at 16:37
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    $\begingroup$ Then can I ask what would happen if the F15-E tried to land on dirt? Would the tires blow, the landing gear collapse, and it would require major repairs? Or is it possible to survive with just a few kinks? $\endgroup$ – DrZ214 Aug 28 '15 at 17:03
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    $\begingroup$ If an F-15E tried to land on dirt, what would happen depends on the exact surface. Smooth, hard-packed dry dirt wouldn't be very different from a dusty asphalt runway, which F-15s have been using in the Middle East for 20 years at least. Any larger gravel, stones or dirt clods creating irregularities in or just under the surface will start causing problems, up to and including the gear snapping off on touchdown as the wheel is too small or too rigid to roll over the bump at high ground speeds. $\endgroup$ – KeithS Aug 28 '15 at 17:16
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    $\begingroup$ There is, however, a difference between using a dirt field and operating from one. In an emergency, a pilot's trained to land anywhere he can. In such a situation the plane would then take off again with minimal fuel load and no ordnance to get back to its actual operating base. To actually fly a mission, an F-15 can be loaded down with more than its own weight in fuel and ordnance. The runway surface then has to be smooth enough to minimize shock forces to the gear at max takeoff weight (or the gear must be strong enough to handle the shock forces expected from a dirt strip at MTOW). $\endgroup$ – KeithS Aug 28 '15 at 17:34
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The two main issues faced by aircraft landing on rough runways are:

  1. Ingestion of FOD in the engines, and
  2. Damage to the landing gear.

In case of the rough landing, one of the best ways is to distribute weight using larger and higher number of tyres. For example, compare the landing gear of C-17,

C 17 landing gear
"A U.S. Air Force Airman inspects the landing gear of a C-17 Globemaster III aircraft, July 1, 2014, on the flight line at Joint Base Charleston, S.C. The maintainers performed checks and maintenance around 140701-F-EV310-009" by A1C Clayton Cupit - http://www.defenseimagery.mil/imageRetrieve.action?guid=18df79c5d609dbaa0679ab32d9e6aea6e76200c6&t=2. Licensed under Public Domain via Wikimedia Commons.

with that of Boeing 777, which has about the same maximum takeoff weight (MTOW).

777 Landing gear
Source:

Aircraft like Su-27 use a FOD deflector in their wheels to reduce the damage due to FOD.

Su 27
Source: maybach300c.blogspot.com

One way to keep the engines clear of FOD is to have them mounted high on the wing, where the FOD ingestion is reduced.

C 17
Source: www.defense.gov

Or, the engines can be mounted high on the fuselage.

A 10
Source: theaviationist.com

In case this is not possible in the case of a high-performance combat aircraft, the inlet design has to be modified. The Russians used this extensively as their operational doctrine demanded it. For example, the MiG-29 main engine inlets could be closed for operation from rough runways.

Mig 29 inlets
Source: www.reddit.com

When the main inlets are closed, the air for engine is obtained through inlets on the top of the wings, which are shielded from the debris below.

Mig 29

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    $\begingroup$ While the C17's tires are significantly wider than the 777's (estimated a a 2:1 height:width ratio vs 3:1), both photos show a groups of 6 tires and assuming the persons shown with them are equally tall they're about the same height. (Both are roughly 2.5x the ankle to knee distance.) $\endgroup$ – Dan Neely Aug 28 '15 at 17:34
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There are things called "gravel kits" that allow standard commercial aircraft to land on rough(er) surfaces.

https://en.wikipedia.org/wiki/Gravel_kit

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  • $\begingroup$ Would you please describe the modifications to the Air North plane? $\endgroup$ – rclocher3 Apr 8 at 18:51
  • $\begingroup$ Additional descriptions indeed! The "fender" on the NLG seems fairly obvious (and appears to form part of the NLG door once retracted), however that probe under the engine is a bit confusing. $\endgroup$ – FreeMan Apr 8 at 19:51
  • $\begingroup$ The vortex dissipators are fascinating. If you’ve ever seen a vortex from an intake reach down the ground, you’ll know what does. It gets in the way. m.youtube.com/watch?v=p5l3fD2WIQc $\endgroup$ – MikeY Apr 9 at 0:16

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