Mechanics of Elevator Trim. In Detail.

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?

The more specific your answer about the mechanics on airfoil outside
the plane, the better.

And remember, no cheating!

Also, for experienced pilots that know the answer, please do not ruin
it by telling everyone before any answers have been given, though if
you are not certain of the answer and would like to speculate, that's
fine.

-Le Chaud Lapin-

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

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

On Jun 10, 9:29 am, 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.

CG range for most typical lightplane airfoils is 25 to 33%
of the chord, while the centre of lift is around the 40% mark. The
load on the stab/elevator isn't all that big, but it's enough that
we'll teach you in groundschool that the aircraft's stall speed is
lower when loaded to the aft limit than when it's loaded to the
forward limit, and that the cruise speed is a little better at the aft
limit.

Dan

Le Chaud Lapin wrote in news:426e092d-74b7-4668-
:

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?

The more specific your answer about the mechanics on airfoil outside
the plane, the better.

And remember, no cheating!

Also, for experienced pilots that know the answer, please do not ruin
it by telling everyone before any answers have been given, though if
you are not certain of the answer and would like to speculate, that's
fine.

-Le Chaud Lapin-

Good grief. Have you checked your room for solvents?



Bertie

"Le Chaud Lapin" wrote in message
...
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.

Probably because there are several different ways to make it work -

I assume you are most familiar with the use of a servo tab. Can you name at
least two other ways that it is commonly done in light aircraft?


But it is good to understand how the aircraft systems work (and what can go
wrong) - so continue on with the discussion...

--
Geoff
The Sea Hawk at Wow Way d0t Com
remove spaces and make the obvious substitutions to reply by mail
When immigration is outlawed, only outlaws will immigrate.

On Jun 8, 11:05*am, "Capt. Geoffrey Thorpe" The Sea Hawk At Wow Way
D0t C0m wrote:
"Le Chaud Lapin" wrote in ...

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.

Probably because there are several different ways to make it work -

I assume you are most familiar with the use of a servo tab. Can you name at
least two other ways that it is commonly done in light aircraft?

Nope.

But it is good to understand how the aircraft systems work (and what can go
wrong) - so continue on with the discussion...

Also, for the record, my assumption of what was going on with elevator
was invalid. The Jeppesen Private Pilot mentions the servo tab, and
technically, they do say what it does, but very briefly, so if student
is not paying attention, s/he might miss it.

-Le Chaud Lapin-