A aviation & planes forum. AviationBanter

If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below.

Go Back   Home » AviationBanter forum » rec.aviation newsgroups » Soaring
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Newbie seeking advice



 
 
Thread Tools Display Modes
  #21  
Old June 15th 04, 08:43 AM
Bert Willing
external usenet poster
 
Posts: n/a
Default

You're right - the elevator produces lift (same direction as the wings) at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing" wrote in
message ...
That doesn't make sense to me. At high speeds, the elevator produces

lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as the wings
(most of the time anyway).




  #22  
Old June 15th 04, 03:03 PM
Gldcomp
external usenet poster
 
Posts: n/a
Default

Bert,

It has little to do with airspeed. The position of the CG will determine the
force on the elevator.


"Bert Willing" wrote in
message ...
You're right - the elevator produces lift (same direction as the wings) at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing" wrote in
message ...
That doesn't make sense to me. At high speeds, the elevator produces

lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as the

wings
(most of the time anyway).






  #23  
Old June 15th 04, 04:07 PM
Bert Willing
external usenet poster
 
Posts: n/a
Default

Not quite correct. At high angles of attack, the elevator produces lift and
at of angle of attack, it produces negative lift. The crossover (i.e. zero
lift, minimum drag) is a design criterium and is usually placed at the max
L/D angle of attack. But then, this will of course be influenced by a large
variation of the CG.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

It has little to do with airspeed. The position of the CG will determine

the
force on the elevator.


"Bert Willing" wrote in
message ...
You're right - the elevator produces lift (same direction as the wings)

at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing" wrote

in
message ...
That doesn't make sense to me. At high speeds, the elevator produces

lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as the

wings
(most of the time anyway).








  #24  
Old June 15th 04, 05:56 PM
Andy Durbin
external usenet poster
 
Posts: n/a
Default

"Bert Willing" wrote in message ...
You're right - the elevator produces lift (same direction as the wings) at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing" wrote in
message ...
That doesn't make sense to me. At high speeds, the elevator produces

lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as the wings
(most of the time anyway).



Ok, so let me see if I've got this straight now. I cruising along at
60 kts in trim and elevator close to neutral. I want to go 140kts so
I push the stick forward, the elevator goes down, which pushes the
tail up, which pushes the nose goes down, I go faster. And all this is
because the elevator is producing more lift in the downward direction?

For a fixed stab with moving elevator don't we have to consider the
forces on both components separately to predict the failure mode?


Andy
  #25  
Old June 15th 04, 10:22 PM
John Giddy
external usenet poster
 
Posts: n/a
Default

Andy Durbin wrote:
"Bert Willing" wrote
in message ...
You're right - the elevator produces lift (same direction as the
wings) at low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing"
wrote in message ...
That doesn't make sense to me. At high speeds, the elevator
produces

lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as
the wings (most of the time anyway).



Ok, so let me see if I've got this straight now. I cruising along

at
60 kts in trim and elevator close to neutral. I want to go 140kts

so
I push the stick forward, the elevator goes down, which pushes the
tail up, which pushes the nose goes down, I go faster. And all this

is
because the elevator is producing more lift in the downward

direction?

For a fixed stab with moving elevator don't we have to consider the
forces on both components separately to predict the failure mode?


I think we are confusing transient and steady forces here. The
overspeed problem occurs with the controls almost in neutral, but the
plane in a dive, where the speed builds up steadily. In this case the
tailplane will be generating an increasing downward force in relation
to the longitudinal axis of the aircraft. This downward force is to
counteract the forward rotation force generated by the wing. At a high
enough speed these forces will increase beyond the capacity of the
structure to support them.

The transient case is when a large control excursion is input at high
speed, and in this case the force on the tailplane could be in either
direction, depending on the direction of control input. However
downward total force is likely to be more severe in a pull-up than an
upward force in a push-over, since the contribution of the elevator
adds to the existing downward force in the first case and subtracts
from it in the push-over case.

That's my 2c worth...
Cheers, John G.


  #26  
Old June 16th 04, 06:23 AM
Eric Greenwell
external usenet poster
 
Posts: n/a
Default

Andy Durbin wrote:

The elevator does produce lift, but in the opposite direction as the wings
(most of the time anyway).




Ok, so let me see if I've got this straight now. I cruising along at
60 kts in trim and elevator close to neutral. I want to go 140kts so
I push the stick forward, the elevator goes down, which pushes the
tail up, which pushes the nose goes down, I go faster. And all this is
because the elevator is producing more lift in the downward direction?


It is confusing! Here's what happens, simplified:

*The horizontal stabilizer (with the flap we call the "elevator") is
pushing down (at least at "higher" speeds - maybe not at 60 knots -
dependes on the glider)
*You push the stick forward
*the elevator flap goes down
*this _reduces_ the downward force of the horizontal stabilizer, but
doesn't elimanate it
*this allows the tail to rise

There is more to it than that, of course.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

  #27  
Old June 16th 04, 10:14 AM
Gldcomp
external usenet poster
 
Posts: n/a
Default

Bert,

I'm sorry, you are not correct or not expressing yourself correctly.
I'm not expressing an oppinion here, I'm telling you how it is.

The CG is by design located ahead of the Neutral Point of the glider
(otherwise the glider would be uncontrollable), so, it has the overall
tendency to push the nose of the glider down, since the Neutral Point is the
place where all the Lift is acting.
To counteract this force, the tail planes are always pushing the tail DOWN,
thus keeping the forces balanced.
We vary the amount of down force produced by the tail planes by moving the
elevator with the stick.

When we move the CG aft, we bring it closer to the Neutral Point, which
reduced the required down force produced by the elevator.
In extremeley aft CG situations, the tailplanes MAY IN FACT produce an
overall UP force on the tail, but this is the exception, rather than the
rule.

The more forward the CG position, however, the more DOWN force is necessary
on the tail.
This is the very reason pilots try to place the CG aft in competition
gliders : so that the elevator doesn't have to produce quite so much DOWN
force on the tail. The result is improved climb because of this.



"Bert Willing" wrote in
message ...
Not quite correct. At high angles of attack, the elevator produces lift

and
at of angle of attack, it produces negative lift. The crossover (i.e. zero
lift, minimum drag) is a design criterium and is usually placed at the max
L/D angle of attack. But then, this will of course be influenced by a

large
variation of the CG.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

It has little to do with airspeed. The position of the CG will determine

the
force on the elevator.


"Bert Willing" wrote in
message ...
You're right - the elevator produces lift (same direction as the

wings)
at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing" wrote

in
message ...
That doesn't make sense to me. At high speeds, the elevator

produces
lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as the

wings
(most of the time anyway).










  #28  
Old June 16th 04, 12:56 PM
Andreas Maurer
external usenet poster
 
Posts: n/a
Default

On Wed, 16 Jun 2004 09:14:13 GMT, "Gldcomp"
wrote:

Bert,

I'm sorry, you are not correct or not expressing yourself correctly.
I'm not expressing an oppinion here, I'm telling you how it is.


Sorry to intrude, but Bert is correct.

The induced drag of a low aspect-ratio horizontal stab is
considerable, therefore the designer tries to minimize it at the speed
of max L/D - since L/D is still the main number to characterize the
performanc eof a glider, this is the number that needs to be
maximized.
The only case where induced drag is 0 is when the tail does not create
any Cl at all.

Situation at low speeds:
Don't forget that the center of pressure (CP) moves forward with
rising AoA, creating a nose-up momentum - and this needs to be
encountered by the tail (wich is therefore creating lift at speeds
below the speed of max. L/D). And vice versa.


The more forward the CG position, however, the more DOWN force is necessary
on the tail.
This is the very reason pilots try to place the CG aft in competition
gliders : so that the elevator doesn't have to produce quite so much DOWN
force on the tail. The result is improved climb because of this.


The aerodynamical benefit of an aft CG is the fact that the tail
airfoil with upwards deflected elevator has got an extremely bad L/D
due to its negative camber. Less upwards elevator deflection (due to
aft CG) drastically improves the L/D of the tail.

Bye
Andreas
  #29  
Old June 16th 04, 12:57 PM
Bert Willing
external usenet poster
 
Posts: n/a
Default

If you had made your statements for powered aircrafts, you would be
completely right. However, the design criterons for sailplanes are slightly
different because the airfoils used are different (well, today's airfoils
anyways).
The forces produced by the tailplane are fixed by the pitching coefficient
of the airfoil in the first place. At high angles of attack, the airfoil
pitches up which has to be compensated by a lift vector on the tailplane. As
you need to have the CG in front of the neutral point for stability reasons,
it reduces the required lift on the tailplane as the CG moves forward.

To use your words, this is not an opinion. A good textbook wood be "Concept
and design of Sailplanes" by Fred Thomas (the wording of the title may vary
as I just know the German title).

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

I'm sorry, you are not correct or not expressing yourself correctly.
I'm not expressing an oppinion here, I'm telling you how it is.

The CG is by design located ahead of the Neutral Point of the glider
(otherwise the glider would be uncontrollable), so, it has the overall
tendency to push the nose of the glider down, since the Neutral Point is

the
place where all the Lift is acting.
To counteract this force, the tail planes are always pushing the tail

DOWN,
thus keeping the forces balanced.
We vary the amount of down force produced by the tail planes by moving the
elevator with the stick.

When we move the CG aft, we bring it closer to the Neutral Point, which
reduced the required down force produced by the elevator.
In extremeley aft CG situations, the tailplanes MAY IN FACT produce an
overall UP force on the tail, but this is the exception, rather than the
rule.

The more forward the CG position, however, the more DOWN force is

necessary
on the tail.
This is the very reason pilots try to place the CG aft in competition
gliders : so that the elevator doesn't have to produce quite so much DOWN
force on the tail. The result is improved climb because of this.



"Bert Willing" wrote in
message ...
Not quite correct. At high angles of attack, the elevator produces lift

and
at of angle of attack, it produces negative lift. The crossover (i.e.

zero
lift, minimum drag) is a design criterium and is usually placed at the

max
L/D angle of attack. But then, this will of course be influenced by a

large
variation of the CG.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

It has little to do with airspeed. The position of the CG will

determine
the
force on the elevator.


"Bert Willing" wrote

in
message ...
You're right - the elevator produces lift (same direction as the

wings)
at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing"

wrote
in
message ...
That doesn't make sense to me. At high speeds, the elevator

produces
lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as

the
wings
(most of the time anyway).












  #30  
Old June 16th 04, 01:00 PM
Bert Willing
external usenet poster
 
Posts: n/a
Default

Had a look at Amazon: The Fundamentals of Sailplane Design, Fred Thomas &
Judah Milgram

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

I'm sorry, you are not correct or not expressing yourself correctly.
I'm not expressing an oppinion here, I'm telling you how it is.

The CG is by design located ahead of the Neutral Point of the glider
(otherwise the glider would be uncontrollable), so, it has the overall
tendency to push the nose of the glider down, since the Neutral Point is

the
place where all the Lift is acting.
To counteract this force, the tail planes are always pushing the tail

DOWN,
thus keeping the forces balanced.
We vary the amount of down force produced by the tail planes by moving the
elevator with the stick.

When we move the CG aft, we bring it closer to the Neutral Point, which
reduced the required down force produced by the elevator.
In extremeley aft CG situations, the tailplanes MAY IN FACT produce an
overall UP force on the tail, but this is the exception, rather than the
rule.

The more forward the CG position, however, the more DOWN force is

necessary
on the tail.
This is the very reason pilots try to place the CG aft in competition
gliders : so that the elevator doesn't have to produce quite so much DOWN
force on the tail. The result is improved climb because of this.



"Bert Willing" wrote in
message ...
Not quite correct. At high angles of attack, the elevator produces lift

and
at of angle of attack, it produces negative lift. The crossover (i.e.

zero
lift, minimum drag) is a design criterium and is usually placed at the

max
L/D angle of attack. But then, this will of course be influenced by a

large
variation of the CG.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
. com...
Bert,

It has little to do with airspeed. The position of the CG will

determine
the
force on the elevator.


"Bert Willing" wrote

in
message ...
You're right - the elevator produces lift (same direction as the

wings)
at
low speeds, not at high speeds. Got mixed up.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
om...
"Bert Willing"

wrote
in
message ...
That doesn't make sense to me. At high speeds, the elevator

produces
lift
so
in case of structural failure, the bits would go upwards.

--
Bert Willing

ASW20 "TW"

Bert,

The elevator does produce lift, but in the opposite direction as

the
wings
(most of the time anyway).












 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Seeking advice on pilot training approach... Rob General Aviation 8 December 15th 04 01:58 AM
Midland: Seeking Advice [email protected] Piloting 8 September 29th 04 01:44 PM
Seeking Route Advice - OH -> SD -> MT -> BC Darrell Clay Piloting 0 January 28th 04 02:20 AM
Newbie seeking glider purchase advice Ted Wagner Soaring 19 January 2nd 04 08:00 PM
Newbie seeking glider purchase advice -- II Ted Wagner Soaring 11 December 26th 03 06:04 PM


All times are GMT +1. The time now is 05:48 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2024, Jelsoft Enterprises Ltd.
Copyright ©2004-2024 AviationBanter.
The comments are property of their posters.