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Mechanics of Elevator Trim. In Detail.



 
 
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  #81  
Old June 11th 08, 11:28 AM posted to rec.aviation.student,rec.aviation.piloting
Tina
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Posts: 500
Default Mechanics of Elevator Trim. In Detail.

On Jun 11, 12:45 am, Ron wrote:
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


Yes, it seems to me (again, ignorant of the reality) that the airplane
has to be flown onto the runway, rather than stalled onto it. When we
land the airplane is done flying, period, but flying it on means it's
fast enough to take off again.

The higher angle of attack causing drag in cruise trade-off is a bit
of a surprise since what is gained is aerodynamic positive lift from
those little wings in front of the airplane, instead of the negative
lift from those wings most of us have on the back end that are
increasing the aerodynamic load.

Fun discussion, thanks.
  #82  
Old June 11th 08, 11:15 PM posted to rec.aviation.student,rec.aviation.piloting,rec.aviation.homebuilt
Gezellig
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Posts: 463
Default Canards PAin In The Ass To Land? (was: Mechanics of Elevator Trim. In Detail.)

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.
  #83  
Old June 11th 08, 11:31 PM posted to rec.aviation.student,rec.aviation.piloting,rec.aviation.homebuilt
Gezellig
external usenet poster
 
Posts: 463
Default Mechanics of Elevator Trim. In Detail.

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
  #84  
Old June 12th 08, 04:05 AM posted to rec.aviation.student,rec.aviation.piloting
Ron
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Posts: 15
Default Mechanics of Elevator Trim. In Detail.

On Wed, 11 Jun 2008 03:28:05 -0700 (PDT), Tina
wrote:

On Jun 11, 12:45 am, Ron wrote:
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


Yes, it seems to me (again, ignorant of the reality) that the airplane
has to be flown onto the runway, rather than stalled onto it. When we
land the airplane is done flying, period, but flying it on means it's
fast enough to take off again.

The higher angle of attack causing drag in cruise trade-off is a bit
of a surprise since what is gained is aerodynamic positive lift from
those little wings in front of the airplane, instead of the negative
lift from those wings most of us have on the back end that are
increasing the aerodynamic load.


True, but remember all lift, whether up or down is drag. The
balancing "down lift" from the elevator is much less than the load
bearing "up lift" of the canard.

Fun discussion, thanks.


Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.

Ron Kelley
  #85  
Old June 12th 08, 01:08 PM posted to rec.aviation.student,rec.aviation.piloting
Tina
external usenet poster
 
Posts: 500
Default Mechanics of Elevator Trim. In Detail.



Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.

Ron Kelley


Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.
  #86  
Old June 12th 08, 05:43 PM posted to rec.aviation.student,rec.aviation.piloting
Ken S. Tucker
external usenet poster
 
Posts: 442
Default Mechanics of Elevator Trim. In Detail.

On Jun 12, 5:08 am, Tina wrote:
Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.
Ron Kelley


Hmm, how the XB-70 or this,
http://en.wikipedia.org/wiki/Boeing_Sonic_Cruiser

I think the aircraft business is extremely conservative.
KISS applies, also canards are difficult for the average
pilot to understand, (Hey man, you got that thar tail on
the wrong end :-).

Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.


Canards are NOT simple, I've designed quite a few,
and studied others, especially Rutan's.
I find they can be optimised for a given air speed
and are much better than the conventional lay-out.
The main problem is designing the stall.
Ken
  #87  
Old June 12th 08, 08:57 PM posted to rec.aviation.student,rec.aviation.piloting
Bertie the Bunyip[_25_]
external usenet poster
 
Posts: 3,735
Default Mechanics of Elevator Trim. In Detail.

Tina wrote in news:54595019-a2b0-413e-aea8-
:



Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.

Ron Kelley


Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.


Yeah, that's one of the main reasons they're going for FBW. An aft cg also
improves buffet margins and allows a higher cruise altitude because of
that. Some airplanes pump fuel aft after takeoff to bring the CG well aft.
Even ones that aren't FBW. you have to have the autopilot engaged to
utilise this feature though.


Bertie
  #88  
Old June 12th 08, 08:58 PM posted to rec.aviation.student,rec.aviation.piloting
Bertie the Bunyip[_25_]
external usenet poster
 
Posts: 3,735
Default Mechanics of Elevator Trim. In Detail.

"Ken S. Tucker" wrote in news:108f98cd-aed5-
:

On Jun 12, 5:08 am, Tina wrote:
Yes, the time my son spent with canard aircraft brought out all

sorts
of interesting information about canards and the history of trying

to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there

is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.
Ron Kelley


Hmm, how the XB-70 or this,
http://en.wikipedia.org/wiki/Boeing_Sonic_Cruiser

I think the aircraft business is extremely conservative.
KISS applies, also canards are difficult for the average
pilot to understand, (Hey man, you got that thar tail on
the wrong end :-).


Dat's ok kenny , yu kin fix it wif duk tape.

Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.


Canards are NOT simple, I've designed quite a few,
and studied others, especially Rutan's.
I find they can be optimised for a given air speed
and are much better than the conventional lay-out.
The main problem is designing the stall.



Boggle.

Bertie



  #89  
Old June 13th 08, 05:53 AM posted to rec.aviation.student,rec.aviation.piloting
Ron
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Posts: 15
Default Mechanics of Elevator Trim. In Detail.

On Thu, 12 Jun 2008 09:43:07 -0700 (PDT), "Ken S. Tucker"
wrote:

On Jun 12, 5:08 am, Tina wrote:
Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.
Ron Kelley


Hmm, how the XB-70 or this,
http://en.wikipedia.org/wiki/Boeing_Sonic_Cruiser


True, the XB-70 was a qualified success of a large canard type
aircraft. Most of it's problems were due to system failures and
trying to fly at Mach 3.0. The only control issues I know of were
related to overly sensitive pitch response to control inputs. Ref:
http://www.labiker.org/xb70.html

As far as I know the Boeing Sonic Cruiser hasn't gone beyond the
artists concept stage. I guess it's no accident that all the current
crop of passenger jets look alike. That could be partially due to the
reluctance of any large airframe manufacturer to take a gamble on
trying to certify any new "radical" design. Who knows.

Personally, I am intrigued by the three surface aircraft like the
Piaggio. They seem to have done pretty good with their design. One
wonders if given enough time, money and talent, there is some room for
improvement there.


I think the aircraft business is extremely conservative.
KISS applies, also canards are difficult for the average
pilot to understand, (Hey man, you got that thar tail on
the wrong end :-).

Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.


Canards are NOT simple, I've designed quite a few,
and studied others, especially Rutan's.
I find they can be optimised for a given air speed
and are much better than the conventional lay-out.
The main problem is designing the stall.
Ken


Anyone who can design a successful canard aircraft has my respect. I
didn't learn a whole lot about the design aspects from my son (he was
in flight test, not design), but what we did learn was everything
interacted with everything else. The job was interesting, but didn't
last long.

Ron Kelley
  #90  
Old June 13th 08, 07:02 PM posted to rec.aviation.student,rec.aviation.piloting
Ken S. Tucker
external usenet poster
 
Posts: 442
Default Mechanics of Elevator Trim. In Detail.

On Jun 12, 9:53 pm, Ron wrote:
On Thu, 12 Jun 2008 09:43:07 -0700 (PDT), "Ken S. Tucker"

wrote:
On Jun 12, 5:08 am, Tina wrote:
Yes, the time my son spent with canard aircraft brought out all sorts
of interesting information about canards and the history of trying to
scale up Rutan's original concept Beech Starship. The smaller true
canards like the Long-eze are pretty good aircraft. However there is
a reason why we don't see large (six plus passenger) true canards.
It's the relationship between CG, fuel load, payload and range.
Apparently in the scale up process there is a point where it is no
longer practical.
Ron Kelley


Hmm, how the XB-70 or this,
http://en.wikipedia.org/wiki/Boeing_Sonic_Cruiser


True, the XB-70 was a qualified success of a large canard type
aircraft. Most of it's problems were due to system failures and
trying to fly at Mach 3.0. The only control issues I know of were
related to overly sensitive pitch response to control inputs. Ref:http://www.labiker.org/xb70.html


Thanks for that link.

As far as I know the Boeing Sonic Cruiser hasn't gone beyond the
artists concept stage. I guess it's no accident that all the current
crop of passenger jets look alike. That could be partially due to the
reluctance of any large airframe manufacturer to take a gamble on
trying to certify any new "radical" design. Who knows.

Personally, I am intrigued by the three surface aircraft like the
Piaggio. They seem to have done pretty good with their design. One
wonders if given enough time, money and talent, there is some room for
improvement there.


Yes! That Piaggio is one real impressive piece of
aerodynamics, and it sounds very pilot user-friendly.
Very remarkable how they utilized the canard.

I think the aircraft business is extremely conservative.
KISS applies, also canards are difficult for the average
pilot to understand, (Hey man, you got that thar tail on
the wrong end :-).


Would it not also be true that passive stability is not as important
in larger modern airplanes? I would guess (again, I admit being
ignorant of the realities) that adding a 10% aerodynamically induced
increased load on bigger aircraft would be avoided for efficiency
reasons? It might be better to have enough fly by wire and computer
induced stability instead. I don't know enough about this stuff to
even find the back of an envelope, let alone do a calculation there.


Canards are NOT simple, I've designed quite a few,
and studied others, especially Rutan's.
I find they can be optimised for a given air speed
and are much better than the conventional lay-out.
The main problem is designing the stall.
Ken


Anyone who can design a successful canard aircraft has my respect. I
didn't learn a whole lot about the design aspects from my son (he was
in flight test, not design), but what we did learn was everything
interacted with everything else. The job was interesting, but didn't
last long.


Your son sounds like a cool dude.
The major PITA is designing aircraft to be efficient
at cruise, but safe all the way to stalling, and
recoverable. The difficulty is the movement of the
Center of Lift forward on the main wing as stall
begins.

Ron Kelley


Regards
Ken
 




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