On Fri, 15 Aug 2003 13:25:09 -0700, Jim wrote:

I suppose another way to say this is that the changes in indicated
airspeed may be due to angle of attack changes that are not due
to changes in the aircraft's attitude, but rather due to changes to
the direction of the airflow (which are felt as changes in lift and
sink.

That's pretty much what I was trying to say.

The change is AOA is instantaneous, but inertia effects will delay the
change in attitude and (probably) this delay is responsible for quite
a lot of the indicated airspeed increase on entering the thermal
because it makes the required correction bigger than an instantaneous
correction would require.

I'm sorry that I can't easily diagram the velocity vector using only
ASCII text! This was why I suggested you draw the still air vectors
for forward speed (l-r horizontal), sink in still air (downward) and
the resultant path (sloped down completing the triangle). There's a
simplifying assumption that the wing's AOA is given by the angle of
the resultant path. That's not strictly true, but doesn't affect the
argument. Now draw the thermal velocity vector (upward, starting from
the bottom of the sinking speed vector) and draw a new resultant
slope. This will have a lesser slope than the still air situation and
shows that the instantaneous AOA has been reduced, which reduces the
wing's lift. This is an unstable situation which must be corrected and
the normal reaction of a stable aircraft is to pitch down and
accelerate to restore the lost lift.

The attitude change in a free flight model is often quite obvious. Its
pitching inertia is minimal by design: large efforts are made to
concentrate its mass at the CG by shortening the nose as far as
possible and reducing the weight of the tail group and boom. I've
often seen them pitch down quite sharply on entering a thermal but not
noticed a parallel speed increase.

I dunno. This is absolutely wonderful stuff, but it leaves me
really wanting a wind tunnel so I could test these things.

This is actually quite difficult to show in a wind tunnel because it
is a dynamic effect. Wind tunnels, OTOH generally show static effects.
The best tools I know for showing dynamic effects are visualisation
tools, vector diagrams and carefully watching free flight model
planes.


Martin has interesting points, but I'm not understanding
them just yet (it may be I don't understand the
terminology quite yet...)



During a flight yesterday I realised that you can feel the pitch-up as
you enter sink when dolphinning: as well as the sudden soggy feeling
there is a distinct sensation that the rear of the glider is sinking
fastest. I still can't say I saw a pitch up, just that the tail feels
like its sinking faster. The resulting speed loss is almost certainly
masked by pushing forward accelerate and the resulting acceleration is
certainly slower than you can get by pushing over before leaving a
thermal.

--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :