An aircraft with positive dihedral (or lateral)
stability will roll AWAY from the sideslip. Note
that we flight scientists ALSO consider this
positive stability - the DERIVATIVE is negative,
but the STABILITY is positive.
That is, with aircraft nose to the right, sideslip
from the left (negative sideslip), yaw string to
the right, the aircraft will roll RIGHT (positive
roll). This is because of:
(1) the forward going/forward swept wing generating
more lift - in this case the left wing. This is
the important mechanism for a swept wing aircraft.
(2) The fuselage crossflow induces upwash on the
inboard upwind wing, and downwash on the inboard
downwind wing, causing again more lift on the
upwind or forward wing. This is the mechanism for
a high wing aircraft, principally. It also
applies to a low wing aircraft with any
significant dihedral (which is added in order to
create this effect).
This is considered 'positive' stability (i.e. a
good thing) because it means that following a
right rudder input, generating sideslip from the
left, the aircraft will roll right and so turn
right. If that didn't work it would be near
impossible to fly with rudder alone, as the
rudder would turn the aircraft in the 'wrong' direction.
So, to consider the door opening mechanism for
turning the Cessna. In order for the aircraft to
roll left with no other disturbances, the
aircraft must have a rolling moment to the left.
If we discount a direct moment due to the door
(which would necessarily be small) then the most
likely effect is indeed the dihedral effect (roll
due to sideslip). However, for the aircraft to
roll left there must be sideslip from the right,
and the nose must have been forced left. This is
not consistent with the operation of the drag
from the door. And as noted, any effect on the
vertical tail would appear most likely to disturb
flow on the right hand side, which would cause a
side force to the left on the tail, yawing the
aircraft the wrong way.
Both these mechanisms are likely to be of small impact, because of
(1) the door is very close to the aircraft
centreline, in terms of lateral offset. So the
yawing moment will be small unless the drag is
large - likely so large as to either cause a
noticeable speed change (not mentioned) or make
it hard to open the door?
(2) the flow over the fuselage of the aircraft is
already disturbed by the propowash and all the
other untidiness on the aircraft. The downwash
from the wing will also cause any disturbance
from the open door to drop down and so miss the
tail anyway.
I postulate (and this is only a guess, unless I
sneak across the airport and find Cessna's wind
tunnel data for open doors on light singles
):
The open door generates a sideforce on the
fuselage (acting as a flat plate). A door on the
right side will generate a sideforce to the left.
If acting forward of the c.g. (as I suspect it
is) then this will yaw the aircraft slightly
left. This will create the necessary sideslip
conditions for a left wing down roll, and so a
turn to the left
Perhaps a controlled experiment should be arranged?