Now we know that Scaled Composite's explanation for the tail strike and port roll during its WhiteKnight Two (WK2) prototype's fourth test flight on 20 April was thrust asymmetry from its Pratt & Whitney Canada 308A engines caused by a miss setting of the idle thrust settings, apparently giving the starboard engines more power than the port engines
Scaled's statement does not specify which set of engines it was but, just as power was being applied following the touch-and-go the aircraft must have yawed left, because the crew countered with right rudder
If we had not reported the tail strike Scaled and Virgin Galactic would not have broken their silence over the WK2 test programme and we would not have had this explanation
Our eyewitnesses and photographers standing watching the test flight at Mojave air and spaceport were aware of a reported crosswind of about 15kt and also of the rudder issues the prototype had had. And it was this background information on the aircraft's previous performance that certainly informed the discussions that were had after the touch and go photo sequence was circulated
In the sequence of touch and go manoeuvre photos we are seeing an aircraft touch down and then rotate at which point there is a tail strike and an input of right rudder that does not stop the aircraft from yawing left. The left yaw appears to generate left roll, which seems to take some time to control, but eventually it is rectified by a strong input to starboard resulting in level flight, all while the aircraft is climbing
Presumably a part of the recovery to wings level is a manual correction to the power asymmetry. The use of rudder and aileron input as a reaction to yaw and roll is instinctive, but manipulating throttles to correct thrust imbalance while a lot else is going on involves concentrated mental and physical coordination
Taking asymmetry as the explanation a question that can be asked is why the yaw and subsequent roll continued for as long as it did when the rudder input should have corrected for the thrust asymmetry and brought the nose back more quickly?
On a normal certified aircraft there is a requirement that rudder can overcome thrust asymmetry because an engine out situation will create exactly that asymmetry
Does WK2 have a lot of inertia with its yaw and roll?
It is not unrealistic to expect a similar struggle with a gusting crosswind, our first conclusion on 20 April, so how perfect do conditions have to be for WK2? How often will it be able to fly with an eight-person spacecraft slung beneath it?
Flight routinely speculates on the cause for aviation incidents and with this unique twin fuselage aircraft with its long Boeing 757-like wing span there are special aerodynamic characteristics that are of interest to any enthusiast interested in how aircraft perform and how they would be piloted
The story of WK2 continues
on April 24, 2009 6:54 PM | Reply
Rob, I can certainly appreciate your persistence, but if you look at the culture of the folks at SCALED, there's a history of 'learning by doing', so I really don't see this as big an issue as you make it out to be. Remember the wingtip scrapes on Voyager? WK-1 had some teething pains with the wings if I remember correctly. These noteworthy aircraft are made by craftsman, not by computer-driven tape machine, so there's alot of feel involved. And this is not a taxpayer financed operation, so they really only have to answer to Mr. Branson anyway, and I'm guessing he's a man who knows craftsmanship when he sees it.
I think this is why Mr. Rutan and SCALED have always been so secretive (about the unclassified projects): Reporters, even the great ones (sir :) can find a scandal where there truly is none.
I say let these immensely skilled folks do what they do best. Your technical research is truly impressive, but please don't put these folks under a cloud. They are hand-building human-rated spacecraft, after all. Give them the benefit of the doubt, Rob, they've certainly earned it.
on April 25, 2009 2:47 AM | Reply
Mr Coppinger raises the interesting question of what happens when a gusting crosswind strikes the WK2, but, he should also ask what happens when an engine suddenly fails during a high weight takeoff particularly with gusting crosswinds. Perhaps this problem is as simple as the test pilot just let his attention wander and committed pilot error, perhaps the pilots are facing a learning curve because the offset position in the boom causes coupling between roll/yaw and translation which disturbs the pilots instincts and will need to be trained out. Perhaps the tail volume is undersized to the crosswind/engine out condition, or perhaps the control cables to the tail became a little slack during the flight and the rudders became sluggish.
The issues involved, will be determined by careful analysis, I am quite confident the Scaled team is working away at understanding this.
on April 25, 2009 3:26 AM | Reply
On a normal certified aircraft there is a requirement that rudder can overcome thrust asymmetry because an engine out situation will create exactly that asymmetry
Only when above a certain speed (VMC) which is going to be higher than the speed at touchdown during a touch-and-go.
on April 25, 2009 10:34 AM | Reply
Who said anything about scandal? I'm a journalist my job is to question everything. Virgin Galactic has said that the test flights have been flawless, so why do they have to modify the rudder? Surely flawless means there is nothing wrong with the aircraft, nothing has to be changed. But they did. Boeing and Airbus will tell you their programmes do not have government funding, so do we ignore all the problems the 787 and A380 have had? No.
on April 25, 2009 11:16 AM | Reply
The WK2 yaw instability issue IMHO is as follows:
The WK2 (at the present status) is very unstable in full power at takeoff/landing speeds in cross wind and/or in side slips. This is very probably caused by significant unwanted dynamic pressure difference in on the opposite sides of the fuselages and vertical tails generated by the inward engines as these are very close to the fuselages. This would not happen with the engines more far from the craft or on the top of the wing. The instability disappears with engines idling and is unnoticeable in high speeds even with engines in full power. (I will provide larger explanation if anybody is interested).
During the discussed crosswind touch-and-go the perfectly stable landing WK2 with engines idling become immediately very unstable in yaw when the power was applied. It seems to be obvious that the subsequent roll has been induced by the yaw and does not have to do much with the aircraft size and inertia or different idling power.
I don’t like posting here as I know that probably many people tend to think that this fabulous aircraft has a problem (which it obviously does not have), but I’ve contacted Scaled a few months ago with no reply.
I absolutely agree with Scaled that presenting these minor things to general public as “problems” would cause the program more harm than anything else. It is a question of semantics Rob: “these are minor things to work on” as Burt has said and everybody could imagine a Stradivari violin not playing well before tuned. For me all the WK2 test flights were FLAWLESS.
Scaled, wish you good luck!
on April 25, 2009 5:27 PM | Reply
Why didn't they notice the idle miss-setting at takeoff? Rudder effectiveness is nowhere near that of say a touch and go. They would have had to rely on differential braking more than prior taxi tests and previous takeoffs demonstrated as I don't think WK2 (could be wrong) has nose steering. One might ask did they make an adjustment to one or more of the engines and not static test prior to flight? In previous touch and go's this miss-setting had no ill effects or it didn't exist yet or there was no miss-setting on this last flight.
Charles
on April 25, 2009 7:31 PM | Reply
FAR 25.143 specifices that for a 3 or more engine aircraft the vehicle must be controllable with the second critical engine out during all phases, which indicates to me that Vmc needs to be less then Vs or V1.
if a maximum crosswind takeoff is occuring and one or more engines fail, the WK must be able to avoid departing the runway or otherwise losing control.
I hope the issue we saw was one of pilot training and not one of significant design change.