Bending loads on the spar will be less if tanks are wing mounted, because
the lift from the wings acts directly on the mass of the fuel.

It has to do with the concept of load path -- when a laod is applied to the
airplane at any point other than the CG, the load must get to the CG through
the structure of the airplane.

That's why the Questair Venture has its nosegear attached to the engine
instead of the airplane, because it is always advantageous to feed loads
directly into a major mass.

Regards,

Gordon.

PS: What design are we talking about?


"Ernest Christley" wrote in message
. com...
Lou wrote:
Ok, a question or 2 about the wings. I'm building a wooden aircraft
that could go either fixed tri-gear or retract. I choose fixed seeing
how this is my first plane to build and I wanted to keep it simple. The
fuel was supposed to go behind the firewall but I choose to build tanks
in each wing where the wheels where to go.
What will this do to the loading of the aircraft? Does this effect
the useful load and how? Does it raise my useful load or lower it or
niether? Since the wings create lift, and the fuel was in the fuselage
and now in the wing should it screw up the center of gravity? Anybody,
Anybody, Anybody????


You have to worry about 3 axis. With now knowledge of the airplane, it is
possible that the fuels position may not have not changed fore or aft. If
that it true then your CG is safe. If not, then you will have to be very
careful in your weight and balance calculations. The nice thing about
wing tanks is that the fuel tends to sit on the center of lift. When this
is the case, the draining fuel doesn't cause your trim to creep.

Next is the vertical axis. If you move a lot of weight from up high to
down load, but leave the center of thrust untouched, then you've created a
situation where adding thrust will cause the plane to tend to nose over.
You're trying to move a filing cabinet by pushing the top. So, you're on
short final, just before the flair, all trimmed out and smooth. A deer
jumps on the runway. You firewall the throttle. Do you have enough
elevator to keep from eating a mouthful of dirt.

The other issue is lateral loading. Move all that weight to the wings and
you've modified your roll response. Just like an ice skater that changes
her spin by extending her arms or retracting her arms (moving weight in
and out.) When you try to start a roll, you have to have enough
differential lift in the ailerons to get everything moving...and THEN get
everything stopped. Moving the tanks could make the plane difficult to
control.


The upshot is that the wing ROOT doesn't have to carry the bending moment
of the fuel load. If the wing root was the limiting factor in your load
allowance, then this move very well could increase your plane's usefull
load. Airplanes have to be light, though. It's doubtful that the
designer cut the wing root to the bare minimum and then beefed up
everthing else, like landing gear, tail surface area, elevator surface
area, etc. I wouldn't bet MY life on an increased load allowance from
moving the fuel tank.


--
This is by far the hardest lesson about freedom. It goes against
instinct, and morality, to just sit back and watch people make
mistakes. We want to help them, which means control them and their
decisions, but in doing so we actually hurt them (and ourselves)."