Coordinated turns and the little ball

"Jose" wrote in message
om...
If the 38 is rolled fast enough at any g above 1g, (with the airplane
loaded) you can couple the roll axis with another inertia axis, usually
pitch in the Talon. Its quite a complex issue, and involves both the
inertial axis and the aerodynamic axis of the aircraft.

Ok, I see it's interesting, but I'm still not sure what it means. Is
"pitch" referenced to the earth or to the (rolling) aircraft axis?

Is it something like "If you are rolling fast, and then stop the roll, the
aircraft will pitch towards the pilot's feet."?

Its a complex model. Basically, you can visualize inertia coupling by
splitting the airplane into 2 basic elements of mass; one element
representing the mass in front of the cg and the other behind it.
There are 2 separate axis systems in play for an airplane in maneuvering
flight. The first axis is the is the aerodynamic axis system acting though
the cg in the relative wind direction, and the other is the inertia axis
acting through the cg lined up with the two mass elements I've described.
If you are maneuvering at say 1g or even unloaded where the 2 axis systems
were in alignment, there would be no coupling in a roll.
But if the inertia axis is inclined to the aerodynamic axis for some
reason....say you had a slight pitch input in play as hard aileron was being
applied (the airplane loaded above 1g,)
now you have a condition where the 2 axis systems are not aligned. As roll
input progresses under this condition, a pitch moment can be produced, and
its the coupling of the two axis systems that constitutes an inertia
coupling.
Dudley Henriques

Could make for an interesting discussion on the physics forum :-)
I'm not all that up on the physics of spinning tops these days as I'm
getting older and more feeble minded by the day :-), but off the top (no pun
intended) of my head, I'm guessing that as the speed slows on a spinning
top, a mismatch similar to the mismatch between the two axis systems in the
T38 would probably cause a gravitational torque change in the top,
accounting for a precess determined by the torque.
I think you could easily get into angular momentum here :-))
Anyway....I see the main difference between the two examples as the
existence of the aerodynamic axis produced by an extremely predominant
relative wind in the T38 as being missing in the spinning top.
Dudley

"Morgans" wrote in message
...

"Jose" wrote in message
om...
If the 38 is rolled fast enough at any g above 1g, (with the airplane
loaded) you can couple the roll axis with another inertia axis, usually
pitch in the Talon. Its quite a complex issue, and involves both the
inertial axis and the aerodynamic axis of the aircraft.

Ok, I see it's interesting, but I'm still not sure what it means. Is
"pitch" referenced to the earth or to the (rolling) aircraft axis?

I'm betting it is a motion not unlike a child's spinning top, as it slows
down too much, and starts to wobble on it's axis, right before it really
wobbles and falls down. That is when you depart from controlled flight!
g

Kinda' right, Dud?
--
Jim in NC

There are 2 separate axis systems in play for an airplane in maneuvering
flight. The first axis is the is the aerodynamic axis system acting though
the cg in the relative wind direction, and the other is the inertia axis
acting through the cg lined up with [front and back] mass elements [...]
...if the inertia axis is inclined to the aerodynamic axis for some
reason [...] the 2 axis systems are not aligned. [...] a pitch moment
can be produced, and its the coupling [...] that constitutes an inertia
coupling.

Thanks. That makes it much clearer to me.

If I'm banked, and I "pitch up", does that mean the nose rises up with
respect to the horizon, or with respect to the pilot's feet?

Jose
--
"Never trust anything that can think for itself, if you can't see where
it keeps its brain." (chapter 10 of book 3 - Harry Potter).
for Email, make the obvious change in the address.

"Jose" wrote in message
...
There are 2 separate axis systems in play for an airplane in maneuvering
flight. The first axis is the is the aerodynamic axis system acting
though the cg in the relative wind direction, and the other is the
inertia axis acting through the cg lined up with [front and back] mass
elements [...]
...if the inertia axis is inclined to the aerodynamic axis for some
reason [...] the 2 axis systems are not aligned. [...] a pitch moment
can be produced, and its the coupling [...] that constitutes an inertia
coupling.

Thanks. That makes it much clearer to me.

If I'm banked, and I "pitch up", does that mean the nose rises up with
respect to the horizon, or with respect to the pilot's feet?

Pilot's feet. The pitch axis is considered controlled by the elevator or
stabilator as the case may be. Any pressure both positive or negative to the
elevator/stab regardless of the aircraft's position in relation to the
horizon is considered a pitch input.
If you rolled the airplane upside down and applied back pressure, the Split
S would be a pitch input. Rolling into a turn and applying back pressure is
also a pitch input.
Dudley Henriques

Jose schrieb:

Rudder applied while rolling a T38 at certain lateral deflections
above 1 g can actually couple the airplane and then be followed
immediately by a departure from controlled flight.

What does "couple" mean in this context?

I guess he means precession. If you roll fast enogh, your aircraft acts
as a gyroscope. Now apply a force perpendicular to the roll axis, and
the result will be a precession motion which can be pretty impressive.

You've probably seen (live on an airshow or canned in a video)
gyroscopic maneuvres flown with propeller driven planes. There, the
propellor is the gyroscope. When your aircraft lacks a propellor, you
can still fly gyroscopic maneuvres. The trick is to roll fast enough and
transform the entire aircraft into a gyroscope. You don't need a high
performance jet for this, it works pretty well with an aerobatic glider
with a sufficient roll rate, too (Fox, Swift).

Very funny stuff, yet completely irrelevant to the average spam can
driver. And I doubt MSFS's aerodynamic model canhandle it.

Stefan

I guess he means precession. If you roll fast enogh, your aircraft
acts as a gyroscope. Now apply a force perpendicular to the roll axis,
and the result will be a precession motion which can be pretty
impressive.



ok, I knew it must have soemthing to do with a gyro....I couldnt reason it
out...but this makes sence. I hope your right...or else ill just have to
go back to being confused again.

"new_CFI" wrote in message
...


I guess he means precession. If you roll fast enogh, your aircraft
acts as a gyroscope. Now apply a force perpendicular to the roll axis,
and the result will be a precession motion which can be pretty
impressive.



ok, I knew it must have soemthing to do with a gyro....I couldnt reason it
out...but this makes sence. I hope your right...or else ill just have to
go back to being confused again.

That's a shame it makes sense, as its not correct, and understanding all
things related to flying are worth knowing as a flight instructor. There's
really no reason for any instructor to be "confused" when a little research
will enhance understanding.
If you are actually interested in inertia coupling, perhaps a little bit of
research might bring you up to speed on it. If not, please accept my sincere
apology for having mentioned it.
Dudley Henriques

Dudley Henriques wrote:
"new_CFI" wrote in message
...


I guess he means precession. If you roll fast enogh, your aircraft
acts as a gyroscope. Now apply a force perpendicular to the roll axis,
and the result will be a precession motion which can be pretty
impressive.



ok, I knew it must have soemthing to do with a gyro....I couldnt reason it
out...but this makes sence. I hope your right...or else ill just have to
go back to being confused again.

That's a shame it makes sense, as its not correct, and understanding all
things related to flying are worth knowing as a flight instructor. There's
really no reason for any instructor to be "confused" when a little research
will enhance understanding.
If you are actually interested in inertia coupling, perhaps a little bit of
research might bring you up to speed on it. If not, please accept my sincere
apology for having mentioned it.
Dudley Henriques

Googling "inertia coupling" I found this, which cleared things up a bit
(at least for me):

A few of the experimental aircraft encountered a new type of behavior
known as inertia coupling, a behavior that was not fully appreciated
until the F-100 and F-102 also encountered it. Inertia coupling
resulted from the tendency of the new generation of high-speed aircraft
to concentrate most of the weight in a long thin fuselage, a departure
from the distribution of subsonic fighters. The X-3 configuration is an
excellent illustration. Even though its high-speed performance was
disappointing, the X-3's unanticipated susceptibility to loss of
control from inertia coupling contributed to understanding the problem.
With much less weight in the wing and tail, the dynamic motion in a
maneuver could cause the inertia of the fuselage to overpower the
aerodynamic stabilizing forces of the wing and tail. In the worst cases
the pilot lost control and the resulting abnormal air loads caused
airframe structural failure. The early F-100A models are remembered as
a classic example of susceptibility to inertia coupling, although the
initial F-102A models also encountered the problem.

--Walt
Bozeman, Montana

"Walt" wrote in message
ups.com...

Dudley Henriques wrote:
"new_CFI" wrote in message
...


I guess he means precession. If you roll fast enogh, your aircraft
acts as a gyroscope. Now apply a force perpendicular to the roll axis,
and the result will be a precession motion which can be pretty
impressive.



ok, I knew it must have soemthing to do with a gyro....I couldnt reason
it
out...but this makes sence. I hope your right...or else ill just have
to
go back to being confused again.

That's a shame it makes sense, as its not correct, and understanding all
things related to flying are worth knowing as a flight instructor.
There's
really no reason for any instructor to be "confused" when a little
research
will enhance understanding.
If you are actually interested in inertia coupling, perhaps a little bit
of
research might bring you up to speed on it. If not, please accept my
sincere
apology for having mentioned it.
Dudley Henriques

Googling "inertia coupling" I found this, which cleared things up a bit
(at least for me):

A few of the experimental aircraft encountered a new type of behavior
known as inertia coupling, a behavior that was not fully appreciated
until the F-100 and F-102 also encountered it. Inertia coupling
resulted from the tendency of the new generation of high-speed aircraft
to concentrate most of the weight in a long thin fuselage, a departure
from the distribution of subsonic fighters. The X-3 configuration is an
excellent illustration. Even though its high-speed performance was
disappointing, the X-3's unanticipated susceptibility to loss of
control from inertia coupling contributed to understanding the problem.
With much less weight in the wing and tail, the dynamic motion in a
maneuver could cause the inertia of the fuselage to overpower the
aerodynamic stabilizing forces of the wing and tail. In the worst cases
the pilot lost control and the resulting abnormal air loads caused
airframe structural failure. The early F-100A models are remembered as
a classic example of susceptibility to inertia coupling, although the
initial F-102A models also encountered the problem.

--Walt
Bozeman, Montana

Sounds like a winner to me. Thank you for taking the time and interest.
Dudley Henriques

With much less weight in the wing and tail, the dynamic motion in a
maneuver could cause the inertia of the fuselage to overpower the
aerodynamic stabilizing forces of the wing and tail. In the worst cases
the pilot lost control and the resulting abnormal air loads caused
airframe structural failure.

Sounds like a winner to me. Thank you for taking the time and interest.
Dudley Henriques

I'm not quite sure what that means, as far as what motion the fuselage actually
takes.

Does the nose veer off of the line of flight, or does something else happen?
--
Jim in NC