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 | | From: | Craig Fink | | Subject: | SpaceShipOne roll problem | | Date: | Thu, 16 Dec 2004 08:42:33 -0800 |
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>>> > Adding dihedral really doesn't seem to be the solution to me.
>>>
>>> Space ship One has no dihedral, and a dihedral effect has gravity as a
>>> component. Is it dihedral? No.
>>
>> According to the flight log as given at the Scaled Composites web site:
>>
>> (http://www.scaled.com/projects/tierone/logs-WK-SS1.htm)
>>
>> "SpaceShipOne Flight 16P Anomaly:
>>
>> Highly publicized in the media, the rolls near the end of the motor
>> burn certainly got our attention. Detailed analysis determined that the
>> rolls resulted from a mild thrust asymmetry, which was unable to be
>> offset by pilot inputs at a flight condition of low directional
>> stability. ... The characteristics of excessive dihedral effect and
>> high-Mach low directional stability will be corrected on future
>> spaceship designs."
>>
>> Looking at SS1 doesn't _seem_ to show any dihedral.
>
> Don't confuse geometric dihedral with the dihedral effect (roll due to
> sideslip). Different aircraft without any geometric dihedral can have
> different dihedral responses.
Had to look this one up, yeah your right, someone way back when decided to
call roll due to side slip dihedral effect. Even if it's not associated
with the wing at all. Which includes the two tail fins above the c.g.
The dihedral effect of the swept back leading edge is at least associated
with the wing. The upwind wing gets longer and the downwind wing gets
short wrt the relative wind causing a rolling moment. Also, the upwind
wing is on the high pressure side of the shock wave created by the
fuselage, and the downwind wing would have a lower pressure.
But, in this case it should be at zero or near zero angle of attack on the
wing. Yawing the wing at zero angle of attack shouldn't create a rolling
moment because the changing lengths of the moment arms of the lift on each
wing, because there is on lift. Flying more positive angles of attack, as
they did on the second X-Prize flight would only increase the swept
leading edge's dihedral effect.
The "dihedral effect" of the tail is more interesting. At some Mach number
range, the shock wave from the nose is going to sweep back across the twin
tails. This is ok, if the vehicle is at zero angle of side slip, which it
would not be with a thrust missalignment in yaw. But, at any angle of side
slip, the nose shock wave is going to impinge upon the twin tails
differently. If the angle of side slip is large enough one vertical
stabilizer could be outside the nose shock while the other is inside the
shock. This would mean the two stabilizers would have a different angle of
sideslip. This is probably their yaw and roll stability problem at higher
Mach numbers.
A single vertical stabilizer might be a good idea.
Does anyone in this group like to speculate or not? So far the reponses to
this thread are, the pilot caused, this article says they had a thrust
misalignment, and you don't understand the term "dihedral effect", which
was true.
So what about the large pitch dispersion? My earlier posting on the
horizontal stabilizers is probably all wrong as the vehicle pitched 15 up
degrees not down.
Craig Fink
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