Re: V22 Osprey fails OPEVAL
3 top,
That plot shows purely the effects of the disk loading, where the smaller disk eats more power to create a given amount of lift, and so needs more engine power. If it were plotting total efficiency of the configuration, it might show the tilt wing and tilt rotor closer, because the large vertical drag of the tilt rotor wing (about 10% of the total aircraft weight) is much reduced in a tilt wing (where the wing is rotated around so its leading edge is aligned with the downwash.) Nonetheless, the smaller props of the tilt wing usually make its disk loading so high the reduction in vertical drag is only just cancelled.
the concepts of disk loading are so poorly understood that I made it a primary thrust in the discussion, because it is the biggest headache for VTOLs to overcome as they try to compete with helicopters. Other factors are important, but not so easily handled:
1) Stiffness of the wing requiring much heavier wing structure and lost payload. The tilt rotor has to have a very stiff wing, aeroelastic response of the wing is the biggest design effort to solving the tilt rotor problem, since a wiggly wing quickly gets excited by the rotors and a major structural failure can result. The success of the V22 is greatly due to the ability to predict and quell these vibrations, and the wing depth, chord and stiffness are not designed for cruise efficiency, they are designed to assure that it all is stiff enough to stay together without giving itself a case of the flutters. Similarly, a tilt wing must have this isolation, or it will fail. The tilt wing also has to have a very strong, slop-free pivot joint if it is to succeed, and this is a major design problem. Rotating primary structures in flight may seem easy, but the resultant designs are heavy, complex and often unsuccessful.
2) Vertical drag - the wing sits in the downwash, and "weighs" more as a result, for a tilt rotor this is at least 8% of the weight of the aircraft and can be as high as 12%. For a conventional cabin=class helo, it can be 4 to 5% of the gross weight. That means a V22 could carry 5,000 lbs more payload in a hover if that vertical drag disappears. Big factor
3) Dual controls and a tilt mechanism. The stuff that drives a twin rotor helo ina hover, and an airplane in cruise and a tilt mechanism in conversion must all be carried around all the time, and it is heavy, expensive and high maintenance stuff. The V-22 has all the parts of an F-111 and a Chinook at the same time! It has three times the number of flight critical actualtors as a helicopter.
4) Horsepower systems - The cost and weight of a flying machine is driven by the power it needs and the fuel it burns, this is what the design starts with. Since a tilt rotor needs about 50% more power for the same payload, it must spend quite a bit more on engines, and on all the power transmissions, as well as the extra weight of this stuff.
None of this makes the tilt rotor impossible, but all of it makes the explanation of why the tilt rotor carries so little with so much power. How much the customer wants to spend for the job is the real issue, of course.