Pressure Distributions for Aerodynamic Surfaces

johnkent

I'm modeling stresses and vibrations for small airplanes and need to input the pressure loads over the wing and aerodynamic surfaces. Since my structural elements are thin, shell elements are most convenient. I start with the airfoil profile and import as a dxf file, then extrude or loft into a wing of shell elements. But manual adding hundreds of pressures over the surfaces for each angle of attack is not practical. So at Victor's suggestion, I edit the liml file for the element geometry and use a math program to read in text files, find the pressure (from aerodynamic program) for each surface face, and prepare an output file in Mecway format. Then I use cut&paste in WordPad editor to add the pressures to the liml file under loads. Now I can readily assign the detailed pressure distributions for as many elements as desired. Attached is a png graphic of my test case for a crude airfoil with 32 quad4s on top and 32 on the bottom. By clicking on each pressure load in the tree, I quickly verified that each face was assigned the correct pressure value. Initially some pressures were backwards and I found that surface faces could be a 5 or a 6 from the same extrusion, so now my math program reads in the face as well as element number and so assigns.

In my test case, I used elliptic loading for the span distribution, constant pressure on the bottom and a linear chordwise suction for the top of the wing panel. The graphic shows the positive pressures applied to the bottom of the wing. Unfortunately, Mecway now displays suctions as from the inside so I have removed part of the bottom skin to illustrate this; front and rear spars are visible as well. After applying the pressures, I change the shell elements to quad8 and can refine as desired for stress calculation.

In case other users are interested in vehicles with aerodynamic surfaces, I took the Aircraft Designs, Inc course in Monterey, CA a few years ago and now with Mecway, I can perform more detailed analyses with a lot less effort.

johnkent

Here are the two text files for pressure loads which my math program outputs in liml format for the above test case. These are edited in WordPad and must be saved in unicode format to handle superscripted units. My math program kept outer quotation marks, so I added leading and trailing x's and using global replace for (eg "x) in WordPad quickly removes them; then cut&paste into the liml file.

Now I'm exploring the stresses and vibration modes: skin balloons out from my limited number of ribs (as expected) and a lot of extra frequency modes due to stressed skin effects (not anticipated).

Challenge now is to edit more ribs and stiffeners on the inside of the wing structure, but how to get inside? I use orientation and the slider to come in from the fat end of the wing and just discovered right click to consolidate new ribs with the old ones. Most convenient would be the ability to click on surface elements and make them "invisible" to allow editing the elements inside the structure.

johnkent

Oops, just discovered the "visible" option by right-clicking component - marvelous.

mike262

Hi Im looking for more info on how to do this. If i have cfd outputs in spread sheet format that gives a say elliptical lift distribution how would i input it. or even being able to put a simple parabolic or elliptical pressure distribution would probably be good enough. Anyway im also an Aero engineer and am keen to talk to anyone else using mecway to stress aircraft stuff!
Cheers
Michael

Victor

Here's a simple example of an elliptic pressure distribution done with a formula in the pressure load.

mike262

Hi Victor,
Thanks for this ....i think this is all i really need. Can you input a distribution that vary along two axis? I.e a parabolic lift distribution along the X-axis (wing span) and that varies linearly along the y - axis (wing width or Chord)
thanks michael

Victor

Yes, you can use all 3 coordinates (x,y,z) in the formula for pressure.