Devil 49 said:
Skid gear can allow you to find a level landing point if the surface is really rough.
Nick sez:
Nope! On rough ground, it is usually hard to find stability with skids, unless the ground is also soft, where the skids can sink in a bit. That is because the skids form two parallel lines that are in one flat plane, and they must retain that relationship even on rough ground, where such planes don't exist. Usually, skids end up with one or both balancing or teetering on firm rough ground, because the long skid's straight line can't conform to the rough or rocky ground, so much of the skid doesn't touch. A wheeled helo can almost always find stability with the three wheels all touching firmly even on the roughest ground. Three wheels simply find their own plane and allow stable touchdowns anywhere, albeit with a sloped aircraft.
I have heard of marketing hype that discusses how much better skids are on rough terrain, and that is simply untrue. The US Army, with 7,000 helicopters in its inventory, and missions all over the world no longer will buy a skidded helo.
Devil 49 is right, skids don't conform to the surface, and the touchdown pressure on the ground is actually higher than wheels (more tendency to sink in and create dynamic rollover problems in skids). The touchdown pressure of the typical Bell 212/412 is about 170 psi. The typical tire pressure of a wheeled helo is 125 to 140 psi (the S-76 is the exception, at 165 psi for its tiny wheels). The psi of the landing gear directly measures the sinkability on soft terrain. That's why the HUMMER has a tire pressure adjustment system, and can go down to 10 psi if needed on mus or sand.