Mechanics of Elevator Trim. In Detail.

"Le Chaud Lapin" wrote in message
...
On Jun 9, 10:48 pm, "Robert M. Gary" wrote:
On Jun 9, 8:18 pm, Le Chaud Lapin wrote:

Both these explanations are true, but one of them eliminates the need
for $30 product (or $30,000 aircraft) to know what is going on.

I find that its pretty difficult to teach students to fly without the
aircraft.

That brings us full-circle to a theme that was mildly explored in
another post - what utility, if any, is there in using a simulator to
learn things that do not require actual flying.

So here the question would be whether it is possible to understand how
a trim tab works without ever having flown an aircraft.

I have flown in DA-20 and Tomahawk, but I do not think actual flight
would have been necessary to understand how trim tab works.

-Le Chaud Lapin-

Well, it's been a long time since I flew a Tomahawk and I never flew the
DA-20, and my Tomahawk manual has gone AWOL; but I don't recall the Tomahawk
having a tab--IIRC, it had a spring system. That gives a different "feel"
and a different contribution to the feel of the primary controls; but there
should have been little motivation for MS to attempt to model those
subtleties--especially since the purchaser has choices in the physical
controls (yokes, etc.) attached to his computer.

Knowing how the systems work can be intellectually interesting for the
technically oriented; essential for designers, builders and mechanics; and
can easily save your life in the event of a systems failure in a real
aircraft. Therefore, most members of this group need to know the systems on
the aircraft they actually fly; but have no need for all of the possible
combinations and permutations.

The bottom line is that you are apparently part of the primary market for
MSFS and it gives you enjoyment. OTOH, most members of this group (who use
it at all) use it as a tool for procedure training, such as practicing
intercepts and approaches, and do so for efficiency rather than
entertainment.

Peter

On Jun 10, 9:32*am, "Peter Dohm" wrote:
Well, it's been a long time since I flew a Tomahawk and I never flew the
DA-20, and my Tomahawk manual has gone AWOL; but I don't recall the Tomahawk
having a tab--IIRC, it had a spring system. *That gives a different "feel"
and a different contribution to the feel of the primary controls; but there
should have been little motivation for MS to attempt to model those
subtleties--especially since the purchaser has choices in the physical
controls (yokes, etc.) attached to his computer.

Knowing how the systems work can be intellectually interesting for the
technically oriented; essential for designers, builders and mechanics; and
can easily save your life in the event of a systems failure in a real
aircraft. *Therefore, most members of this group need to know the systems on
the aircraft they actually fly; but have no need for all of the possible
combinations and permutations.

My OP did not mention anything about combinations and permutations.
It was asked in general, and then just for C172, as an example.

The bottom line is that you are apparently part of the primary market for
MSFS and it gives you enjoyment. *OTOH, most members of this group (who use
it at all) use it as a tool for procedure training, such as practicing
intercepts and approaches, and do so for efficiency rather than
entertainment.

Yes, I do enjoy understanding things, but my primary motivation is
efficiency of savings.

It was a lot cheaper to check my understanding of how trim tab works
using a simulator than it was to drive 40km out to airport and fiddle
with a real plane.

-Le Chaud Lapin-

"BDS" wrote in message
...
Some students want the first approach, some may want the second - it's
probably 95% to 5%. If you assume they all want the more detailed
explanation for everything, you will have frustrated students who either
think that flying is too complicated for them to learn, or who quit
because
they are bored.

When I took driver's ed in high school they didn't explain the inner
workings of a carburetor, nor did they need to in order for me to be able
to
learn how to drive. Knowing how a carburetor works and having the ability
to tear one down and put it back together again does not make me a better
driver.

You want to know every detail - fine. You just need to find an instructor
who is willing and able to do that for you. As you know, not all of them
are nor do they need to be in order to be able to teach you to fly and fly
well. I suspect you dispute that fact, but history has already proven you
wrong.

In general, I agree with you--one certainly didn't need to know the inner
workings of the carburetor. However, it could be very usefull to understand
what the throttle (accelerator) return spring did! That sort of information
is in the POH for any aircraft certified under Part 23.

OTOH, the answers to the original question posted by the OP might be
included over the course of an airframe mechanic's curriculum--or might not.

Peter

One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since the
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.



. On Jun 8, 5:18 am, WingFlaps wrote:
On Jun 8, 5:08 pm, Le Chaud Lapin wrote:



Hi All,

This post is primarily directed toward student pilots like myself.

First, I am not asking because I want to know the answer (I already
know), but do a little experiment. I have maybe 7 or 8 different
sources of flight information that I rely on for ground school
(Jeppesen, FAA Handbooks, etc), and none of them said _how_ it worked
in sufficient detail, they only said what one must do to make the
plane pitch up or downard.

So for you students, please do not cheat and do what I did, which is
watch the airfoils move as you move the trim control. Also, it would
help if you did not think about the correct answer too much, which
would lead you to the correct answer, thereby defeating the purpose of
my experiment.

So, without cheating, and without asking an experienced pilot or
mechanic,...

What exactly happens when the trim is adjusted to point the nose
upward?

Well, since you frame it as a troll:
You scare the lift faries to run forward by waving a very nasty bit of
metal at them.

YAWN
Cheers

Tina wrote:
One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since the
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.

I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi

On Jun 10, 1:09 pm, Tauno Voipio wrote:
Tina wrote:
One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since the
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.

I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi

Thanks for the rule of thumb, Tauno. I have watched how busy the
flippers are on fighters when they are in the flare -- no human pilot
is working that hard for control. I knew the fighters are designed to
be aerodynamically unstable.

So the aerodynamic longitudinal stability the tail provides might
cost us 5 to 10%, The obvious question is, do canards buy back that
fraction? They would be offering positive lift, and if they stall
first would provide the same sort of longitudinal stability, wouldn't
they?


be

Tina wrote:
On Jun 10, 1:09 pm, Tauno Voipio wrote:

Tina wrote:

One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since the
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.


I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi


Thanks for the rule of thumb, Tauno. I have watched how busy the
flippers are on fighters when they are in the flare -- no human pilot
is working that hard for control. I knew the fighters are designed to
be aerodynamically unstable.

So the aerodynamic longitudinal stability the tail provides might
cost us 5 to 10%, The obvious question is, do canards buy back that
fraction? They would be offering positive lift, and if they stall
first would provide the same sort of longitudinal stability, wouldn't
they?

Yes - they do bring back some, and this is the reasoning behind
e.g. Rutan's Voyager,

The price is that the canard (front wing) has to stall first
unless you want to fall to ground in reverse when the thing
stalls. The rumours are that the canards are a PITA to land
nicely.

--

-Tauno

Thanks again. My intelligent but ignorant guess is designing canards
so that they stall first should not take a genius, but there may be
traps I don't see. The world is safe, though, since I don't design
airplane.

The landing issue you raised is pretty neat, since most of us --
especially Mooney drivers -- are careful about airspeed on final and
in the flare, and like to land with the wings almost stalled. But in
the case of a canard if that stalls first I think the airplane would
very enthusiastically want to pitch forward hard enough to bend the
nosewheel!

At least with the stabilizer still flying the nose might be able to be
put down more gently. You've provided some nice insights, thanks.

On Jun 10, 2:14 pm, Tauno Voipio
wrote:
Tina wrote:
On Jun 10, 1:09 pm, Tauno Voipio wrote:

Tina wrote:

One point about the lift fairy sitting on the tail I'd like to
understand is this -- actually a serious question. As I understand
it, nearly aways the tail is exerting a downward force, since theI
center of lift is aft of the center of gravity on general aviation
airplanes (that is true, isn't it -- that the cg is forward of the
center of lift?). If so the tail really is imposing an increased load
on the airplane, adding to its effective weight. The question I have
is, how many pounds of weight is imposed aerodynamically for an
airplane that might be loaded with its CG at the forward limit? I
don't know where the center of lift is on ga airplanes -- a third of
the way aft of the leading edge of the wing is an ok approximation,
but a few inches error on an airplane weighing what ours does at max
could make a huge change in the required force to overcome the nose
heavy moment.

A rule of thumb is that the force on the horizontal tail
is 5 to 10 per cent of the wing lift. This translates
to a loss of 10 to 20 per cent of the raw gross lift
availbale from the horizontal airfoils.

I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

This is the reason why modern military aircraft are designed
aerodynamically unstable, and the electronic gnomes of the
flight control system have to work all they can do.

The loss of gross lift is the proce to pay for simple and
safe longitudinal stability.

--

Tauno Voipio
tauno voipio (at) iki fi

Thanks for the rule of thumb, Tauno. I have watched how busy the
flippers are on fighters when they are in the flare -- no human pilot
is working that hard for control. I knew the fighters are designed to
be aerodynamically unstable.

So the aerodynamic longitudinal stability the tail provides might
cost us 5 to 10%, The obvious question is, do canards buy back that
fraction? They would be offering positive lift, and if they stall
first would provide the same sort of longitudinal stability, wouldn't
they?

Yes - they do bring back some, and this is the reasoning behind
e.g. Rutan's Voyager,

The price is that the canard (front wing) has to stall first
unless you want to fall to ground in reverse when the thing
stalls. The rumours are that the canards are a PITA to land
nicely.

--

-Tauno

On Jun 11, 4:00*am, Nomen Nescio wrote:
-----BEGIN PGP SIGNED MESSAGE-----

From: Tina

I'm obviously thinking about increased efficiency -- extra weight
added because of either fat people, full fuel, or aerodynamically
imposed, all cost horsepower (OK, watts for you purists) to move
around.

I prefer ergs/minute.


Small units to make a couch potato feel happy?

Cheers

On Tue, 10 Jun 2008 11:24:39 -0700 (PDT), Tina
wrote:

Thanks again. My intelligent but ignorant guess is designing canards
so that they stall first should not take a genius, but there may be
traps I don't see. The world is safe, though, since I don't design
airplane.

The landing issue you raised is pretty neat, since most of us --
especially Mooney drivers -- are careful about airspeed on final and
in the flare, and like to land with the wings almost stalled. But in
the case of a canard if that stalls first I think the airplane would
very enthusiastically want to pitch forward hard enough to bend the
nosewheel!

I haven't flown a canard, but my son has done a lot of flying in one
that was under development. You are right... you don't want to stall
the canard on landing. You fly it all the way to the ground. Three
problems with the canard, as my son saw it, was lack of forward
visibility on landing, drag from the canard in cruise flight (a fixed
canard has to have its AOA greater than the wing and enough surface to
generate lift) and ice shedding off the wings through the propelllor.
Piaggio solved the drag problem, partially, with a three surface
aircraft and a relatively small canard. I believe Beechcraft
attempted to solve it with a variable sweep canard, but I could be
wrong.


At least with the stabilizer still flying the nose might be able to be
put down more gently. You've provided some nice insights, thanks.

My son says canard landings are like the "Little girl with the curl in
the middle of her forehead"... when they are good, they are very very
good, but when they are bad they are horrid. :-)

Ron Kelley