Aerodynamic Pressure Loads in FEA of Lifting Surfaces - Query

Question

Let me give a brief summary of the process involved in the FEA of lifting surfaces with the wing as an example: 1. The wing is modelled as a hollow surface body in Ansys, meaning only the exterior aerodynamic surface is specified. 2. Internal structure made of composites is modelled next. 3. Aerodynamics loads and Boundary conditions (fixed edge at the root of the wing) are specified. 4. The solution is obtained in terms of displacements, stresses and failure criteria etc.

As part of Step-3, aerodynamic loads are generated in the form of Cp values (Coefficient of Pressure) at discrete points on the exterior surfaces of the wing. This data is generated from XFLR5. Cp values are converted into pressure values and applied on the exterior surfaces of the wing. To do the same, the following formula is used:

Pressure values obtained from the LHS of the above expression are applied over the exterior surfaces of the wing. FEA process is completed and the solution is obtained.

FEA predicts that the wing cross-section is unable to maintain its airfoil shape and the wing cross-section essentially gets "crushed"'.

Observe the crushing of the wing cross section in this figure where the top surface and bottom surface come close to each other and airfoil geometry is completely lost

However, it finally struck me that there has to be an internal air pressure that will support the cross section from the inside as what happens with hollow structures on earth, eg. a hollow can be placed on the ground doesn't get crushed into itself since the external and internal pressures cancel each other out.

Hence, we should not be using absolute pressure but rather gauge pressure (difference in external and internal pressure) should be used. Otherwise, it essentially means that there is a vacuum on the inside and air pressure on the outside.

Once this change was made, we no longer observe the "crushing" effect.

Finally, we still want to be sure if the internal air pressure would be equal to the static pressure at that altitude as has been assumed in the usage of gauge pressure. Our assumption is that the pressure inside the wing body would be equal to the static air pressure at that altitude from thermodynamic relations even if the wing is completely sealed. IS THIS ASSUMPTION CORRECT? ANY SUGGESTIONS WOULD BE HELPFUL?

Answer 1

There is fuel inside the wing, and there is definitely a connection with the outside atmosphere for venting while fuel is flowing into the engines. The fuel tanks are sealed from the outside airflow. In a document about the B737NG from Smartcockpit.com (now via Scribd) it says:

The purpose of the fuel vent system is to prevent damage to the wings due to excessive buildup of positive or negative pressure inside the fuel tanks and to provide ram air pressure within the tanks. The tanks are vented into surge tanks which vent through a single opening at each wing tip.

A recent development, long recommended by the NTSB but deemed too expensive to mandate by the FAA, from this site:

Boeing has developed a Nitrogen Generating System (NGS) which decreases the flammability exposure of the center wing tank to a level equivalent to or less than the main wing tanks. The NGS is an onboard inert gas system that uses an air separation module (ASM) to separate oxygen and nitrogen from the air. After the two components of the air are separated, the nitrogen enriched air (NEA) is supplied to the center wing tank and the oxygen enriched air (OEA) is vented overboard. NEA is produced in sufficient quantities, during most conditions, to decrease the oxygen content to a level where the air volume ... will not support combustion. The FAA Technical Center has determined that an oxygen level of 12% is sufficient to prevent ignition,

Answer 2

I wouldn’t work out the static pressure at the inside as equal to the static pressure of the undisturbed air away from the plane because the low pressure is forced at the inside of wing due to outside airflow vacuuming the inside air. What you need is to build an internal structure, add resistance to the skin and redo the calculations. Let the inside static pressure be as low as it should be as a result of airflow, as if the inside communicated directly with the outside at trailing edge and a differential vacuum ( pressure lower than the outside static air pressure) was created internally. I would do that based on the fact that control surfaces ( flaps, ailerons) allow free air flow between inside and outside.