This is not a typical post, but as I’m sitting in my balcony writing, I hear this roar – not too unusual for the area, given that Back Bay is one of Boston’s most frequented helicopter zones. But living in Cambridge, so near to MIT has significant advantages; in addition to the views of downtown Boston, MIT is the site of many unusual visits by all manner of equipment. Today it was a Marine Corps Osprey, which flew 250 feet over my head in a banked turn for a landing approach at MIT’s field and a demonstration.
On the photograph I show below, the white bubble in the field is the cover for a full-size racquetball court, which provides some proportion of the aircraft’s size, tip to tip, its width being nearly 85 feet. Its tilt rotor design looked simultaneously powerful and fragile, and since there is no stabilization power in the rear of the airframe, one wonders how it avoids tipping over something that has happened several catastrophic times. From what I could see, the solution seems to be to move the craft, which is much larger than a normal helicopter, incredibly slowly during landing maneuvers. With its disproportionally enormous blades, one challenge in the design of the Osprey centered on evading the critical vortex ring state, in which the air forced out from under the blades is perpetually sucked back around to the top, resulting in zero lift. The short cut was to keep the craft moving forward slightly as it landed, thus using mostly “clean air” with each cycle, since high descents with low airspeed were found to be the major cause of airlift loss disasters and thus the Osprey is not very good for stationary hover.
Osprey landing at MIT field (click for larger image)
Click above for a close up of its subsequent takeoff. The seemingly slow blades are a scan line phenomenon, but the silence immediately after the craft takes off is earnestly uncanny. As mentioned before, in its need to minimize vortex ring state, the Osprey does not execute true vertical takeoff, but banks forward imperceptibly and moves ahead ten feet during ascent.