Scaled Composites has admitted to problems with the rudder on its WhiteKnight Two prototype Virgin Mothership Eve, which Flight identified last year and Virgin Galactic denied in January.

In a statement on its website posted on 23 April Scaled admits that it has had to "adjust the rudder forces" and that it has "made three modifications to the rudder aerodynamic balances, along with adding vortex generators".

On the 7 January Flight revealed that Eve had had issues with yaw when taxiing and take-off during its 21 December 2008 maiden flight and commented that the prototype "maye have to have its rudder and yaw stability improved". Changes to the aircraft for these reasons have now been confirmed by Scaled.

In January Virgin Galactic president Will Whitehorn denied that there was a problem telling Flight that: "There are no issues we know of at all at this stage which will affect the programme or the overall design [of the WK2]." Yet it is now known that Whitehorn and his prime contractor were aware of the rudder issue at the time.

Despite Scaled now confirming that Flight's reporting of the rudder issue was correct, the company's 23 April statement goes on to say: "Several recent published articles have been sufficiently inaccurate and negative that we have decided with our customer's approval, to set the record should question the motivations of a publication that reports design or flight test performance that is based only on speculation."

Details of Eve's flight performance have come to Flight from sources within Virgin Galactic. The spaceline's chief test pilot David Mackay told Flight in an interview in March that he had been told that Eve pilot Peter Siebold had experienced vibrations on the first flight.

Scaled now confirms that vibration in its statement when it says, "the first flight, with the gear down, also illuminated that the main landing gear wake signifcantly affected these [rudder] forces." The mechanism for this is the vortices that form within the landing gear wake and impact the rudders, causing the vibration that Mackay described.

The Scaled statement goes on to discuss the 20 April tail strike and the reasons for it. In its 21 April article Flight described the excessive yaw and port roll that Eve underwent upon rotation during one of its 20 April touch and go manoeuvres and provided photos showing the moment of the tail strike, which visibly is only one of the tails, and the rotation and climb inmmediately afterward.

Scaled says: ""we encountered a significant thrust assymetry (not a fault of the engines or the design but of an assymetric idle thrust setting) that resulted in a large yaw". Scaled explains that this can be resolved by a "procedural change". This means that to eliminate this yaw instability factor a change to the idle thrust settings on the prototype's two Pratt & Whitney Canada 308A engines would be made to make sure that, unlike the fourth flight, in future the settings are set correctly.

Source: Flight International