My curiosity derives from thinking, based on some hand-done CT/sigma ( blade loading ) calculations that the prop rotor is already at fairly high Ct/sigma values already when coming to a hover.
It seems to me that Nick Lappos, during some of the early discussions on the Tilt Rotor design (particularly the MV-22), discussed the Prop Loading issues and talked about Ct/sigma values. Maybe we can search back through the thread or related threads and see if we can resurrect those posts.
Am I correct to assume those values are very high on the Osprey.....and by high mean they are approaching the level where blade stall might be encountered?
A few seconds of googling found this......
Aerodynamic Blade Loading: [CT/σ] [Ct/sigma]
is a simple concept that lets us see how far from stall the rotor is operating, similar to the coefficient of lift for an airplane's wing. It is the coefficient of thrust divided by the solidity.
For most modern airfoil sections, deep rotor stall is experienced at a Ct/sigma of about .20 to .21 at hover. As retreating blade stall takes over, the Ct/sigma for stall at 160 knots is about 0.10 and by 200 knots, it is about 0.06.
For a tilt rotor, the blades are purposely made with less chord than a helicopter, because the thinner blades are then operating at a higher angle of attack in a hover, and are more efficient. This means that they can save power in a hover by operating at a high Ct/sigma. The downside is that there is little margin left over for maneuvering at low speed. For helos, the blade chord is sized up to allow flight at high speed, so it is way oversized for a hover. Tilt rotors don't need the extra chord for high speed because they are on the wing by then, and the rotors are props!
The V-22 has a hover Ct/sigma of 0.175, which means that it will stall at only about 1.2 to 1.3 g's in helo mode (.21/.175). A typical helo has a hover Ct/sigma of about 0.09, so it never gets close to stall at low speed (ever pull 2.4 g's at 40 knots? It's a wild ride!)
~ Nick Lappos
The Sikorsky ABC has demonstrated blade loadings up to 0.185, but the full lift capacity of the rotor system has yet to be demonstrated ~ (1976)
From Prouty "Helicopter Aerodynamics - p.68" "Most rotors are at the peak of their efficiency in forward flight when CT/σ is about .08.
A second source of information....a research paper done by a Naval Officer.
http://www.dtic.mil/cgi-bin/GetTRDoc...f&AD=ADA204856
The other issue that is important is Disk Loading and its effect upon Autorotational capability which has had a very major effect upon the Osprey performance.