also the danger of sucking up stuff that can destroy the engine as well. as one NASA pilot once told me, "If you sent a dime into a jet engine, the dime will come off a whole lot better then the engine."
The reason is that a slow rotating, large, blade disk can entrain more mass flow (and create more thrust) than a fast moving smaller one. Think hand over waterhose vs. waterfall for instance.
Even when compressed and burned with fuel to accelerate it (creating massive erosion and hot gas reingestion problems I might add), the typical lift engine is producing on the order of 5-8,000lbs and so will also have to be installed in multiples which create their own vulnerability (you can't duct thrust between wingtips if one engine fails) and reliability (start-stop on dedicated lift banks) issues.
As well as sucking up an ENORMOUS amount of fuel in critical (radius-end and mission end) flight evolutions where there may not be much to spare of either thrust or gas.
When you are looking at a loaded operational weight on the V-22 in the range 50,000lbs, you end up with just a HUGE volumetric weight penalties trying to stuff the primary fuselage (fuel and cargo) or wing areas with engines like those on the Mirage 3V or the German Do-31
Of course there is also no less of a problem with lift loss, tilting nacelle complexities (fuel line and control lead coupling seals etc.) and proprotor design efficiencies inherent to spinning large fans at the end of bobweight wings with enormous assymetric torque penalties.
But a better option remains going with the SDLF or Shaft Driven Lift Fan (see F-35B) approach which finally looks like it may be able to provide significant transitional lift without all the 'plumbing' problems of traditional jet VTOL in what are called 'blended wing body' designs.
Such airframes allow for very deep wing roots which may have unswept inner spans that operate like LEX in up and away maneuvering while providing suitable depth to seal large 2-4 meter fan plenums under translating or fenestrating upper skins which may themselves act like either foldaway vertical stabilizers or extendible lift augmentation systems.
SDLF soaks all the torque and rpm through a transmission like a helicopter while allowing the primary (cruise) engine to retain an optimum core design for forward flight efficiencies.
Even as it also allows a malfunctioning STOVL system to perform a conventional landing, something the V-22 would be very hard pressed to do, simply because the transition of the nacelles sucks all their stored energy out while multiplying drag by orders of magnitude.
If it works (namely, if you can find enough residual space in a modified delta wing to put both fuel and fan) the generally higher jet consumption rates will be offset by sufficiently faster cruise speeds to provide greater penetration depths.
It is also hoped that the deep center section will allow for the carriage of preloaded vehicles for drive-on/drive-off warfare that keeps the aircraft far away from any 'assault landing' condition.
If you were to cross a B-2, an F-16XL and a Pelican dropship in your mind, you would probably get a good idea of where we are headed.
