Drag cubes - how to calculate aerodynamic stress?

[Rahjital]

Hello, recently I've been trying to calculate aerodynamic stress on vessels in atmosphere for the purpose of damage when the stresses get too great. To that purpose, I've been trying to calculate pressure on each part, for which I need the aerodynamic force and the surface area of the part.

Getting the aerodynamic forces wasn't hard, but I'm struggling to get the area. Part.aerodynamicArea is very small for parts in the middle of a vessel (like the fuselage of an aircraft). I've tried using Part.DragCubes.GetCubeAreaDir(Part.DragVector), but it suffers the same problem. I've been trying the same thing for the lift force as well, and I think it may suffer from the same problem, although it wasn't as apparent when testing the mod.

Is there anything I can use to calculate the aerodynamic pressure properly?

[sarbian]

You used the proper values & call. Why are you surprised that parts with surfaces parallel to the air flow reports small surfaces ? 

[Rahjital]

The problem I'm having is that for inline parts, area gets smaller much more quickly than drag, meaning they get disproportionally high pressure compared to other parts.

Look at this: a default Osprey modified with the ramjet engine, in level flight:

Spoiler

(F is force, S is area, DragP/LiftP pressures are simply force divided by area)

The inline Mk1 fuel tank gets far higher drag than other parts, including the Tail Connector acting as the nosecone.

 

Here's how it looks like when pitching down as much as possible:

Spoiler

(F is force, S is area, DragP/LiftP pressures are simply force divided by area)

 

Pressure is higher for every part... except for the fuel tank, where it dropped because the area got so much bigger.

 

There's no way this can be right. What is it that I'm doing wrong?

Edited October 24, 2017 by Rahjital

[sarbian]

Hum, have a look at MJ code there (in other line about drag in that file) and see if th result is more in line with expectation. 

[Rahjital]

The line concerning areaDrag, correct? How would I go about using it? The result do seem more reasonable, but I'm unsure what the number actually means and how to actually use it. Do I use it instead of area, or replace the pressure calculation with it entirely?

[Rahjital]

Ended up 'solving' this issue by calculating drag force (and from there, drag pressure) independently for each side of the drag cube, and then using the highest of these values. This gives a fairly decent result, but I'm afraid that it will be a fairly hefty computing load for larger vessels. Any help to resolve this properly would still be very much appreciated!