Originally Posted by
SplineDrive
The V-22 is an amazing machine, especially considering it’s a 1980’s technology aircraft, but it’s unfortunate that it was the first tiltrotor to go into production because in may ways it represents the worst case for a tiltrotor. High disk loading, far higher than XV-15, 609, V-280, etc. means downwash is high, hover power required is high, autorotation isn’t possible, etc. FLRAA will have lower disk loading, addressing many of these weaknesses. The V-22 was also burdened with blade fold and wing stow, adding weight, cost, and complexity. Its primary operator is the Marines and Navy, which have higher DOC for any aircraft they operate as a result of the seaborne environment and requirements. So the tiltrotor concept gets a poor reputation, when those issues are particular to one implementation, not the concept in general.
A little expansion on the high disk loading issue, which is unique to the V-22. The proprotor diameter in the V-22 was constrained because Navy/USMC wanted it to be able to operate from the two spots abeam the island on the smaller amphib classes of the time. This required reducing said diameter by five feet from optimum, IIRC, which ups the disk loading. But there was also another factor that isn't that well known. During design, Navy was running a competition for the engine to power the JVX (V-22). Bell/Boeing were told to expect an engine of a certain power, in a certain weight range of a certain size, with a given fuel burn. The competitors were Pratt and GE bidding advanced latest technology , with Allison included with a roe conventional engine as a fallback in case Pratt an GE couldn't design an engine that would meet requirements. Both of the advanced engines met the requirements and according to information at the time, the evaluation board recommended award for one of the advanced engines. The story goes that it was GE, but we'll never know for sure because at the last minute the Navy decided not to go with their team's recommendation and awarded the contract to Alison. Allison's engine actually produced more power, but more importantly it was heavier than the other two engines and had a higher fuel burn, which meant more fuel had to be carried, both factors necessitating more structure which further increased weight. So higher weight on top of constrained proprotror size gives you higher disk loading. BTW, a derivative of that GE engine today powers the CH-53K.
Regarding autorotation, the V-22, like all Tiltrotors can do it, but because of the issues mentioned above, does it badly. Of course most large helicopters (the CH-47 is an exception) don't autorotate that well. The decision was made to meet the power off landing survivability requirement by gliding, which gives you more landing area choices.