a question for the aeronautical engineers among us

On Nov 2, 7:12 pm, Tina wrote:
OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as much
as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean

wrote in message
oups.com...
On Nov 2, 7:12 pm, Tina wrote:
OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as much
as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean


Now for fuel burn increases, factor in the induced drag created by the
deflected elevator to create the required down force.
Glider pilots attempt to maintain a CG at 75-85% MAC (some will go farther
aft), to keep the deflection to create the required down force to a minimum
to create the least amount of drag at cruising speed.

BT

"BT" wrote in :

wrote in message
oups.com...
On Nov 2, 7:12 pm, Tina wrote:
OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as
much as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean


Now for fuel burn increases, factor in the induced drag created by the
deflected elevator to create the required down force.
Glider pilots attempt to maintain a CG at 75-85% MAC (some will go
farther aft), to keep the deflection to create the required down force
to a minimum to create the least amount of drag at cruising speed.


huh?
75 to 85% MAC? I began to build one years ago and it's CG was in the
"usual position" about 25% to 30%, depending on how much you had for
lunch
I can't see any of the competition gliders being able for a CG in that
position either. not with the stab area that they have. A model glider
with a massive stab, yes, but not any full size as far as I can see.
Anything with a CG that far aft on the main plane is leaning more
towards being a tandem wing aircraft than a conventional one.
Can you provide an example of one of these?


Bertie


Bertie

Bertie... perhaps I miss spoke..
correct.. not 85% MAC... but 85% towards the AFT CG limit..
BT

"Bertie the Bunyip" wrote in message
.. .
"BT" wrote in :

wrote in message
oups.com...
On Nov 2, 7:12 pm, Tina wrote:
OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as
much as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean


Now for fuel burn increases, factor in the induced drag created by the
deflected elevator to create the required down force.
Glider pilots attempt to maintain a CG at 75-85% MAC (some will go
farther aft), to keep the deflection to create the required down force
to a minimum to create the least amount of drag at cruising speed.


huh?
75 to 85% MAC? I began to build one years ago and it's CG was in the
"usual position" about 25% to 30%, depending on how much you had for
lunch
I can't see any of the competition gliders being able for a CG in that
position either. not with the stab area that they have. A model glider
with a massive stab, yes, but not any full size as far as I can see.
Anything with a CG that far aft on the main plane is leaning more
towards being a tandem wing aircraft than a conventional one.
Can you provide an example of one of these?


Bertie


Bertie

"BT" wrote in :

Bertie... perhaps I miss spoke..
correct.. not 85% MAC... but 85% towards the AFT CG limit..

Ah, 85% of the range, you mean?

OK.

In airliners we actually go for the aft limit, if possible. There's a
double benefit from this. One, the drag from the stab is lowered and the
effective weight of the aricraft is also lowered. But perhaps more
importantly, the lower wing load resulting from the aft CG allows a wider
buffet margin and therfore a higher allowable cruising level.
One of the reasons that manufaturers want FBW systems is to permit flying
with what would be an extreme aft CG for a standard airplane. This allows
almost silly cruising levels at very, very low buffet margins to be had.
The reason of course is the higher you go, the more fuel and $$$$ you save.



Bertie

I agree with your statement, but I don't know where the center of lift
is on GA airplanes, hence the question. Maybe it would have been
better to ask a center of lift question. The loading moment is
calculated from an arbitrary datum, it would be neat if there was an
easily identifed point that corrosponded to the center of lift.

On Nov 2, 5:52 pm, wrote:
On Nov 2, 7:12 pm, Tina wrote:

OK, so the center of gravity (except for some airplanes with really
smart computers) is going to be forward of the center of lift for
stall recovery reasons -- that makes sense. The question I have is,
for a typical GA flying machine -- take a complex single for example
-- what really is the download (I'm thinking of it as induced weight
with fuel burn consequences) supplied by the vertical stabilizer? It
has a reasonable moment arm hanging way back there, but is it as much
as say 100 pounds if the CG is near the forward limit?

This isn't an aeronautical problem, its a basic mechanics problem.
The moment of the CG vs. the center of lift must equal the moment of
the tail plane.

If the airplane gross weight is 2500 lbs, and the CG is 1 foot in
front of the center of lift, that is a moment of 2500 foot-lbs. If
the tail plane is 10 feet behind the center of lift, the force on the
stabilizer is 250 lbs.

Dean