September 30, 2004
roll stability
"... The X15'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 verticaltail size increases, and it has presented a multitude of problems to designers of highspeed aircraft. "The solution to the stabilityandcontrol 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 Xprize is aiming for, and same problem, although the X15 had more trouble on reentry (where Rutan's vehicle moves its whole tail offaxis for stability and drag, the X15 came back from space pointy end first (except the one that reentered sideways and didn't make it back intact). The X15 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 wedgeshaped 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 rahrah 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 X15 (in French, sorry) and flight logs.

The (French language) X15 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.