credit Flight/Alan Radecki / caption: see the notch in the bottom of the rudder
In the photo above the new modification to the WhiteKnight Two prototype Eve's rudder, a notch cut into the bottom of it, can be seen and above it, glinting in the sun, are the vortex generators previously added. The photo was taken on Monday 27 July during the mothership's take-off at about 07:37 local time at Mojave air and spaceport as it began its journey to Oshkosh
Scaled Composites founder and chief engineer Burt Rutan told Hyperbola about the latest change to the rudder during a brief chat the aircraft designer had with media after WK2 landed at Wittman regional airport in Oshkosh on Monday 27 July
Rutan explained that originally the only "aerodynamic balance weight," as he puts it was "on the top" of the rudder and what the company discovered during the first flight, back in December 2008, was that due to that "we had an unstable rudder, it was wandering...it was not an optimum first flight because of that."
He described the rudder issue as a situation where when the bottom rudder is "shielded" by the landing gear wake its stablising effects are "taken away". So they decided to add a weight to the bottom of the rudder to increase its stabilising effect - its ability to cope with the wake vortices of the gear essentially
Rutan's engineers' solution then was to add that weight to the bottom to ensure that the gear wake did not, in his words, "change the [rudder's] hinge moments," and ensure that the rudder forces were good
He finished by saying that that was the "major aerodynamic change we made"
This explanation doesn't entirely explain the notch, the most recent change, which actually removes some of the mass of the bottom of the rudder, which Rutan says had to be increased, but it does confirm much of what Hyperbola and Flightglobal has written about
Due to technical problems we can't provide the video of this discussion but hope to in the near future