Why are center tanks not used?

Mxsmanic wrote
Why does it seem that wing tanks are filled in commercial airliners in
preference to center tanks? What is the advantage to filling the wing
tanks while keeping the center tank empty or partially filled?
Wouldn't heavy wing tanks increase the inertia of the aircraft around
the roll axis?

In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.

Bob Moore

Bob Moore writes:

In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.

But wouldn't putting weight inside the wings cause them to flex
_more_, since the weight is now outside the wing root instead of
within it? That's the part I don't understand.

I guess that if the weight is in the fuselage, and the wings have to
move the fuselage, there might be more stress on the wing roots; is
that what you mean? In that case, I can see where putting the weight
in the wings might reduce the stress at the point where they join the
fuselage; although it still seems that it would put the wings under
more stress when landing, and it would make the aircraft slower to
respond to roll commands (which I presume isn't desirable).

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic schrieb:

fuselage; although it still seems that it would put the wings under
more stress when landing,

You don't typically land with full tanks.

and it would make the aircraft slower to
respond to roll commands (which I presume isn't desirable).

I thought you didn't like aerobatic style roll maneuvres?

Stefan

Stefan writes:

I thought you didn't like aerobatic style roll maneuvres?

Every turn involves some degree of roll movement. I don't mind it as
long as the net acceleration vector continues to point down through my
seat and the G load doesn't climb significantly.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic wrote:
Bob Moore writes:


In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.


But wouldn't putting weight inside the wings cause them to flex
_more_, since the weight is now outside the wing root instead of
within it? That's the part I don't understand.

I guess that if the weight is in the fuselage, and the wings have to
move the fuselage, there might be more stress on the wing roots; is
that what you mean? In that case, I can see where putting the weight
in the wings might reduce the stress at the point where they join the
fuselage; although it still seems that it would put the wings under
more stress when landing, and it would make the aircraft slower to
respond to roll commands (which I presume isn't desirable).

One reason weight and balance. Another if you put a non compressible
(realtively) fluid in a confined space it gets stiffer. Think full soda
can vs empty. Try to crush a full one.

Michelle P

"Michelle P" wrote in message
ink.net...
Mxsmanic wrote:
Bob Moore writes:


In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.


But wouldn't putting weight inside the wings cause them to flex
_more_, since the weight is now outside the wing root instead of
within it? That's the part I don't understand.

I guess that if the weight is in the fuselage, and the wings have to
move the fuselage, there might be more stress on the wing roots; is
that what you mean? In that case, I can see where putting the weight
in the wings might reduce the stress at the point where they join the
fuselage; although it still seems that it would put the wings under
more stress when landing, and it would make the aircraft slower to
respond to roll commands (which I presume isn't desirable).

One reason weight and balance. Another if you put a non compressible
(realtively) fluid in a confined space it gets stiffer. Think full soda
can vs empty. Try to crush a full one.

Michelle P

Be sure to shake it vigorously first...

Jay B

On Wed, 20 Sep 2006 13:52:21 +0200, Mxsmanic
wrote:

Bob Moore writes:

In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.

But wouldn't putting weight inside the wings cause them to flex
_more_, since the weight is now outside the wing root instead of
within it? That's the part I don't understand.

Remember the wings are supporting the airplane, not the other way
around. Putting fuel out there makes enough difference in my plane
that they increased the gross weight allowance to include all the fuel
in the tip tanks, but that increase applies only if the extra weight
is fuel in those tip tanks.

I guess that if the weight is in the fuselage, and the wings have to
move the fuselage, there might be more stress on the wing roots; is
that what you mean? In that case, I can see where putting the weight
in the wings might reduce the stress at the point where they join the
fuselage; although it still seems that it would put the wings under
more stress when landing, and it would make the aircraft slower to

That is why most commercial aircraft have a maximum take off weight
that is more than the allowable landing weight.

respond to roll commands (which I presume isn't desirable).

Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

Darrell R. Schmidt
B-58 Hustler Web Site URL (below)
http://members.cox.net/dschmidt1/

"Jay Beckman" wrote in message
news:%5eQg.39$La2.35@fed1read08...

"Michelle P" wrote in message
ink.net...
Mxsmanic wrote:
Bob Moore writes:


In order to reduce the bending moment on the wing spar at the
juncture with the fuselage. Longer airframe life.


But wouldn't putting weight inside the wings cause them to flex
_more_, since the weight is now outside the wing root instead of
within it? That's the part I don't understand.

Think of it this way, Bob. In level flight the wings basically support the
aircraft total weight thereby tending to bend the wings upward at the wing
root. Fuel in the wings make them heavier which tends to bend the wings
downward from the wing root. If you put all the fuel weight in the fuselage
it adds to the first situation only, tending to bend the wings upward. Fuel
you place in the wings adds to the weight but also adds to the 2nd situation
which fights the upward bending movement because of the weight of fuel in
the wings themselves. (Some fancy aircraft like the SR-71 produce
considerable lift from the fuselage which changes the situation somewhat)

In many commercial jets the fuel is first burned from the center tank
(within the fuselage) and then the wing tank fuel is burned. Sometimes, due
to Center of Gravity (CG) situations it becomes necessary to retain some
center tank fuel for CG purposes. That fuel is considered unuseable and it
the only time that all the center tank fuel is not burned first. In the
MD-80, fuel in the wings that is in contact with the upper wing surface
contributes to ice forming on top of the wings. Because of that many users
burn the wing tanks down a little before burning the center tank fuel.

You were correct in your statement below that the stress at the wing roots
is what is the most important consideration. You are also correct to state
that fuel in the wings reduces roll authority, but you can just use more
aileron/spoiler input and it is normally not a controlling element.

I guess that if the weight is in the fuselage, and the wings have to
move the fuselage, there might be more stress on the wing roots; is
that what you mean? In that case, I can see where putting the weight
in the wings might reduce the stress at the point where they join the
fuselage; although it still seems that it would put the wings under
more stress when landing, and it would make the aircraft slower to
respond to roll commands (which I presume isn't desirable).

One reason weight and balance. Another if you put a non compressible
(realtively) fluid in a confined space it gets stiffer. Think full soda
can vs empty. Try to crush a full one.

Michelle P

Be sure to shake it vigorously first...

Jay B

Darrell S wrote
Think of it this way, Bob.

Darrell, I think that you have your attributes mixed-up.

Bob Moore

Darrell S wrote
Think of it this way, Bob.

Darrell, I think that you have your attributes mixed-up.

Bob Moore

??????????????????????????????????