credit Flight / caption: now we know it was a idle thrust miss setting
Scaled Composites has put a statement on its website about its WhiteKnight Two prototype Virgin Mothership Eve test programme that confirms what Flight has been saying, that the rudder has had to be modified
There are other rather uncharitable comments made by Scaled in the statement but Hyperbola is sure that between the lines of aggressive corporate rhetoric the readers will see that they are getting factually accurate reportage and well informed comment from Flight about this unique aerospace project
Read here Flight's article about the statement and how it fits in with our coverage of the mothership's test programme
on April 24, 2009 4:39 AM | Reply
Well the Hyperbola wording said roll due to crosswind while the scaled document said it was yaw due to asymetric idle thrust setting. The V stabs look huge compared to the ones on the P-38. The P-38 had much more aileron force, however.
on April 24, 2009 9:12 AM | Reply
If you look at the three photos we purchased for publication, and if you were able to see the entire sequence of images that we were offered, you would clearly see that WK2 rolled to port, while we don't deny that yaw was involved too it is not as obvious in the photos as the roll. Eyewitnesses also told us that there was a definite right rudder input so its clear that the pilot was fighting for control on both fronts. Remember that on the P-38 there is a single large horizontal stabiliser between the booms and also I would argue that the wingspan on WK2 is relatvely much larger, because it is going to 50,000ft, that adds to the stability issues.
on April 24, 2009 4:01 PM | Reply
Rob:
Good work. I would like to add a little technical insight to this issue. The fact is that WK2 has relative roll and yaw inertias beyond extreme - probably greater than any previous aircraft, ever. When combined with the potential for extreme yaw asymmetries (with any variation in engine thrust), it is clear why WK2's fins/rudders are so big. No doubt Scaled had this sussed - except for one little thing. They missed the fact that the wake off the main gear would directly hit the fins/rudders at takeoff/approach/landing speeds. Damn.
Let's be honest, now: WK2 will always be a total dog to fly. With aileron input it will respond SLOWLY against those roll inertias, while rudder input has to fight against those yaw inertias. Meanwhile, any little thrust variation will keep your feet very busy (as demonstrated in flight 4). With all of this coupled together, Siebold is to be commended for keeping WK2 in one piece - this guy knows how to fly an airplane.
If the solution was as simple as bigger fins/rudders, Scaled would have them on by now (yes, they work that fast). But they haven't because they know that won't solve the problem. It is the main gear wake combined with the extreme roll and yaw inertias and potential asymmetries that are the problem, and that comes back to the aircraft's configuration. Not so easy to change.
IMHO Scaled made a big mistake placing the engines outboard - obviously for reasons of engine accessibility, wing bending moments and vertical thrustline. The option was to place them inboard, above the wing, which compromises these three issues but does away with the yaw asymmetry problem while greatly reducing roll and yaw inertias. Performance might have been slightly compromised (high thrustline and dirtying up the upper wing surface), but the aircraft would be way easier to handle. (Knowing Rutan, he just might do a retrofit...)
As for the rudder/maingear issue, they might try putting fairings on the gears, but there is probably no room to retract them.
I expect that Rutan and his crew will get this airplane flying adequately, but I predict that they won't let just anybody fly it. It will always be a challenge, and only the most trusted pilots will be let behind the controls.
My 2 cents worth.
Philip (designer, ex LongEZ builder)
on April 24, 2009 6:56 PM | Reply
Rob, you suggest that WK2's handling problems will worsen with SS2 attached. This is not necessarily so, and likely the opposite will be true.
Currently WK2 is being flown very light, probably less than half gross weight. The aircraft is designed aerodynamically to carry heavy payloads at much higher speeds than current weights allow. They may have ballast in the fuselages, though probably not, since that would increase roll and yaw inertias still further. When SS2 is attached at the CG, total mass will increase drastically while roll/yaw inertias will be reduced relative to total vehicle mass. Further, SS2 is stable in itself, and the higher speeds mean higher dynamic pressures, higher control moments, and so on.
Hence, if they can get the airplane to handle adequately without a payload, she will handle just fine with SS2. I can imagine pilots dreading dropping their payload, thus turning WK2 back into a squirrelly dog.
Don't be surprised to see a dummy payload attached at the hardpoint one of these days. Perhaps just a streamlined piece of lead. They will want to know how it flys when heavier, and that's about the only way to make it heavier without further compounding the inertias issue.
Keep up the good work.
Philip
on April 25, 2009 10:28 AM | Reply
Hi Philip, you are right that SS2 may be a stabilising effect. But the aircraft does have to fly from 50,000ft back to the spaceport without that large central mass, so what I was really suggesting was that they have to get its no payload configuration handling right before they move on to attach the payload. The blog post you are referring to was based on all the information we had at the time and now we know more, because our story prompted Scaled to be a bit more open, but at the time we had to interpret the events using what we did know and that was the vert stab mods made by Scaled and the test flights abandoned due to strong winds. Thanks for your very informed comment, it's great when people with such a background contribute to the website I am sure the other readers appreciate getting another informed point of view. Rob.