Constant speed or constant attitude?

On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed

I thought a stable aircraft has the C.G. forward of the center
of lift. If this is so, and this effect only happens if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this effect
still occur? If the C.G. is behind the center of lift (my
understanding of "unstable") does this occur?


My guess, and it sure is only a guess, is that the changes in
the indicated airspeed as a result of the glider flying into lift or
sink WOULD occur regardless of the stability or instability of
the aircraft. I'm guessing this is so because I'm also guessing
that THESE changes in the indicated airspeed are not the
result of instaneous pitch changes in the aircraft's attitude, but
rather are changes in dynamic and/or static pressure directly
created by the changes in lift and sink themselves.

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.

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


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

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

On 15 Aug 2003 12:36:05 -0800, (Mark James Boyd)
wrote:

With regards to vertical gusts...

A stable aircraft will react to this by pitching down
and increasing its airspeed

I thought a stable aircraft has the C.G. forward of the center
of lift. If this is so, and this effect only happens if
the aircraft is stable, then C.G. is important, right?

If the C.G. and center of lift coincide, does this effect
still occur? If the C.G. is behind the center of lift (my
understanding of "unstable") does this occur?



My guess, and it sure is only a guess, is that the changes in
the indicated airspeed as a result of the glider flying into lift or
sink WOULD occur regardless of the stability or instability of
the aircraft. I'm guessing this is so because I'm also guessing
that THESE changes in the indicated airspeed are not the
result of instaneous pitch changes in the aircraft's attitude, but
rather are changes in dynamic and/or static pressure directly
created by the changes in lift and sink themselves.

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.

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



I think I only further muddled this by my saying "actual airspeed" may
not be changing. This is not at all the way to look at things.
Indicated airspeed DOES change as a glider flies into lift and sink.
Period. What I wanted to describe is a situation in which the changes
in indicated airspeed are reflective of changes in the airflow over
the glider created by the changed lift and sink, not of accelerations
of the glider itself.

Phooey. This probably only made it worse. I know what I want to say,
I just can't find the right way to say it.

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 :

I didn't use a wind tunnel, just noticed this effect on
landing and takeoff in a 172. I suppose one could
trim for min sink, and then with no stick input watch
a plane or glider fly into ground effect.

From there you should see the nose initially drop. Due
to oscillation, it may come up again, but I always saw
an initial drop.

I saw the reverse when flying out of ground effect, although
it was also subtle because it was masked by the
osscillations a bit. Once out of ground effect, the nose
would pitch up, and the horn would be just chirping for
the stall. I'm guessing this is why the PTS soft field
power technique is to "remain in ground effect while accelerating
to Vx or Vy, as appropriate" after the wheels come off the ground.

As pointed out, this is a dynamic effect, and combined with
oscillation, pretty subtle.