Mechanics of Elevator Trim. In Detail.

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

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 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

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!

Basically you want to set up your speeds so the main gear touches before
the canard stalls in a fully flying condition about 85/90 kts. This
gives a wide margin before the canard stalls and reduces the sensitivity
to Xwinds. Easier than a full-stall landing; all control surfaces are
fully functional plne is highly maneuverable all the way to the ground.

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.

Thx. lol

On Tue, 10 Jun 2008 18:14:41 GMT, 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 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.

Apparently only to those who don't know how to fly one.

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