Chris OCallaghan skrev den 20 Feb 2004 05:53:14
-0800:

Where I'm having trouble is seeing why there is any difference between
sinking, level, and climbing. From the point of view of raf, these
should all be same. I choose to set my longitudinal reference based on
the fuselage, not the horizon. If I do this, the aircraft always has a
sink rate, not relative to the ground, but relative to the projection
of the fuselage centerline at a given time (T=0) and proportional to
the angle of attack -- that is, the rate at which the aircraft "falls"
away from this projection (dT). Since it shouldn't matter which
reference frame I use to make my observations, my confusion arises
with the suggestion that the pseudo-rolling moment reverses beetween
sink and climb.

When turning, the only rotation is about the vertical (earth-fixed) axis.
Then take it to the extreme case of diving straight down (here defined as
the longitudinal axis vertical). In that situation, all of the rotation
will be around the longitudinal axis of the aircraft - i e roll.

In turning flight with the fuselage level (longitudinal axis horizontal),
all of the rotation will be around the yaw axis of the aircraft.

In all the cases between these two extremes, part of the rotation will be
around both the longitudinal and the yaw axis of the aircraft. The bank
will mean some of it is around the pitch axis as well, which is a problem
to be considered in, among other things, turn rate gyros.

As for the approach, it remains interesting. To help my understanding,
I've been using a train. Imagine a sensor on a rail that only measures
side force. A train going straight on level ground registers zero side
force. As the rail bends though, the sensor would measure a side force
proportional to the train's acceleration. However, a straight rail
with a side pitch would register a force as well. When viewed this
way, a train rolling on a straight rail with several degrees of side
inclination could be said to be "turning." Of course, it isn't. Unlike
a curved rail, no additional power is needed to maintain speed.

The equivalent to a banked railway track would be straight slipping
flight. And in both cases, the normal force on the rail (the lift) will
have to be larger if it is to keep the train/aircraft from accelerating
downwards, and there will have to be a lateral (train/aircraft frame of
reference) force as well, assuming that there are no other forces
perpendicular to the direction of travel than the normal and lateral (to
the wings/rail). Both these forces will add friction/induced drag and
require additional power.

Cheers,
Fred