20 Degree Slope
AC makes a valid point. Background: US Army slope landing req't for the UTTAS ( UH-60 result ) was to land at design GW on a slope 0f 12 degrees at any angle to the slope, and at 15 degrees laterally ( L and R ). It was clear while doing those tests that any uneveness of the ground, bushes, rocks would preclude a landing as the rotor tip path was ( eyeball ) 2-3 ft off the ground-at the point of gear touchdown, i.e., with the rotor still level.
Peter mentioned mechanical stability ( ground resonance ). Good point for a lot of rotor designs, as the landing gear design plays an important part of the whole damping picture. Thus the question of how to provide that damping in the collapsible structure suggested. Not impossible, but will come at a fiscal, maintenance, and weight price.
The article also included these statements:
"Reduced risk of damage during hard landings, by as much as a factor of five, compared to conventional landing gear
Ship landings in violent sea states"
The expected reaction to these claims is probably: " Whats not to like about this? ". There is good history regarding both subjects. Reducing damage resultant from hard landings requires increased energy absorption, both in the landing gear and helicopter structure ( recall the number of UH-1 transmissions that wound up in the cabin/cockpit area in Vietnam crashes? ). Serious design aimed at reduced hard landing damage points toward changing or should I say, making universal, existing military crashworthiness standards. But neither the civil user community nor the certification agencies have yet to move in this direction.
As to the ship landings in violent seas, while the RN and USN have taken slightly different design approaches, both design approaches provide a strong mechanical connection between the helicopter and the ship following touchdown. USN testing requires 12 ft/sec landings on a 9 degree slope. Don't know the RN req't, but am sure it is similar.
While there is no question that the technology exists to make a flyable version of the DARPA robotic design, there are some practical issues to resolve as well.