Newbie seeking advice

Eric Greenwell wrote in message 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.

If it is confusing it is only because a previous poster made specific
reference to elevator forces when perhaps they meant the net force
acting on the horizontal stabilizer.

Parts is parts but they all have names.

Andy

"Andreas Maurer" wrote in message
...
On Wed, 16 Jun 2004 09:14:13 GMT, "Gldcomp"
wrote:


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.

Correction : The tail is generating LESS negative lift.

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.

Exactly. And this is because the tail generates less negative lift.

1: A parachute is not very expensive (relative to the other things you
may buy as you get into soaring)

2: Most ships have seats designed to take a parachute (I assume this
is a result of the fact that chutes are mandatory in contests) so you
usually either need to wear a chute or use some cushions. I have found
that in most gliders a good chute is more comfortable than cushions.

3: These points being taken into account, why would you not wear a
chute? Maybe you will never have the opportunity to use it (I
certainly hope I never do!). Maybe you wouldn't be able to get out if
you nedded to. However maybe you WILL need to bail out and maybe you
WILL be successful in doing so. No matter what happens wearing a chute
gives you another option and has no real drawbacks.

4: Just think how stupid you would feel if you DID need a chute and
didin't have one.

5: If you and your ship end up down in the wilderness and have to wait
a day or longer to get rescued all that nylon could be a useful thing
to have on hand.

If you can try out a number of different types of chutes in your
glider to find out which suits you and your ship best, do it. My club
has nearly a dozen chutes and after trying them out I bought a
National backpack because it fits perfectly into the seat recess of my
ASW15b. It's as comfortable as lying in bed! On the other hand, when
flying my club's Grobs, I used a bigger Security chute because they
raise me up a bit so I can see out better. In the backseat of our L13
and L23 I use these huge ex-military chutes we have because they raise
me up and push me forward so my head isn't as buried in the wing root
as it would be with a backpack chute.

As I said: get a textbook, and don't confuse powered aircraft with gliders.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
m...

"Andreas Maurer" wrote in message
...
On Wed, 16 Jun 2004 09:14:13 GMT, "Gldcomp"
wrote:


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.

Correction : The tail is generating LESS negative lift.

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.

Exactly. And this is because the tail generates less negative lift.

Bert,

You should do that yourself.
You all seem to be confusing how the forces interact.

The tailplanes can produce positive lift in extreme aft CG positions,
nothing to do with low speeds.


"Bert Willing" wrote in
message ...
As I said: get a textbook, and don't confuse powered aircraft with
gliders.

--
Bert Willing

ASW20 "TW"


"Gldcomp" a écrit dans le message de
m...

"Andreas Maurer" wrote in message
...
On Wed, 16 Jun 2004 09:14:13 GMT, "Gldcomp"
wrote:


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.

Correction : The tail is generating LESS negative lift.

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.

Exactly. And this is because the tail generates less negative lift.

On Thu, 17 Jun 2004 05:12:08 GMT, "Gldcomp"
wrote:


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.

Correction : The tail is generating LESS negative lift.

Nope.
As Bert has been pointing out repeatedly - for a typical powered plane
setup this might be correct, but not for a glider.

BTW: The tail of a powered plane is also designed not to deliver any
lift at the typical cruise condition of this plane (speed, wing
loading, CG, density altitude).



Just take a look at the AoA of the tail:

AoA_tail = (AoA_wing) - (longitudinal dihedral)

with longitudinal dihedral typically being 1.5 degrees.


Grab one of the available airfoil simulators (e.g.
http://www.mh-aerotools.de/airfoils/javafoil.htm), use a symmetrical
tail airfoil with a, say, 5 degrees upwards elevator deflection, and
simulate it at a typical AoA of about 5 degrees (simulating a wing
AoA of 6.5 degrees for a typical thermalling situation).

Voila - you'll see that that the lift vector of the tail points in the
same direction as the wing's one, despite the upwards-deflected
elevator.





Bye
Andreas