September 30, 2004

roll stability "... The X-15's maximum altitude was extended to 354 200 feet, but not until after much trial and error..."

"... though the tail surface provides stability in pitch and yaw, no purely aerodynamic means has been found to achieve roll stability, since the airflow remains symmetrical about the axis of rotation. The coupling between roll and yaw becomes more severe as vertical-tail size increases, and it has presented a multitude of problems to designers of high-speed aircraft. "The solution to the stability-and-control analysis is the development of an adequate mathematical model. But such an analysis also requires a mathematical model for the pilot..."

  • Timely!
  • Yep -- same altitude the X-prize is aiming for, and same problem, although the X-15 had more trouble on reentry (where Rutan's vehicle moves its whole tail off-axis for stability and drag, the X-15 came back from space pointy end first (except the one that re-entered sideways and didn't make it back intact). The X-15 addressed the roll problem with an active roll controler that could exert twice the force the pilot could manage, and time the response better -- and by using huge blunt wedge-shaped vertical tail pieces, the lower one so large that half of it had to be dropped off before the vehicle could be landed.
  • I'm serious, hank, this is a great link. Hopefully it'll get picked up. I was reading the rah-rah coverage and noticed that there have been three distinct explanations given for the roll, which I take to be a sign of concern, expressed in laconic testpilotese. They must be really struggling with the decision about flying next week. Whatever happens, I bet Paul and Burt stay on the ground next flight.
  • More on the X-15 (in French, sorry) and flight logs.
  • The (French language) X-15 site looks better than anything I've yet found in English. The flight logs show up as an index page, but I get a "not authorized" screen when I try to look into them. Arrrrrgh.