Everything old is new again with the Osprey. With all the renewed controversy over the V-22, I've had a chance to dig through past files on the aircraft and find some interesting tidbits.
Here's one that makes me feel old - my junior year in college I wrote a "thesis" on what a mess the V-22 Osprey was and how Dick Cheney (at the time, SecDef) was making it worse. Say what you want about my politics (or my truly geeky, lifelong devotion to aviation), but that paper made me realize just how long we've been talking about this platform. And just how long we'll probably keep on talking about it.
I'll spare you the details of my boring college paper and instead tell you about a patent I found - for the "Control system for rotorcraft for preventing the vortex ring state." It's dated February 28, 2006 and is good until March 1, 2025 (when I guess you can start making your own without fear of patent infringement).
The inventor? Ronald Kisor of Bell Helicopter Textron, Inc.
Quick physics breakdown: all helicopters leave a "wake" that behaves in a certain way when the aircraft is in hover mode. Start descending, and all bets are off. Or, as Kisor puts it, "the assumptions of the momentum theory begin to break down." During vortex ring state, the air coming off the rotors is effectively recirculating, meaning the helicopter is ingesting its own wake. The result is total loss of control. It usually happens when an aircraft is in a high rate of descent, but moving slowly on the horizontal plane. In April 2000, 19 people were killed when an Osprey entered vortex ring state landing at Marana Airport near Tucson, Arizona. The pilot was coming in at over 200 percent of the acceptable descent rate and the tiltrotor crashed.
Kisor's patent was for a flight control system that would prevent VRS. Control inputs are sent to the aircraft prior to the onset of VRS, without any input from the pilot and without having any effect on the aircraft's flight path.
At the moment, according to the May 2009 GAO report, VRS in the V-22 is "well-defined and avoidable when the aircraft's forward speed and descent rate stay within prescribed ranges." But the situation can still occur if there's a loss of lift on one of the proprotors, which would cause the aircraft to flip. GAO's recommending further tests, of course.