The Engineer Mindset

Aerodynamic Modeling

Systems Modeling

Austin email on first/last body points.

OK!

Let's start with SUBSONIC dynamics!

We <> want to close the first and last sections to a single point!

This gives us a body with FRONTAL area, SIDE area, and TOP area!

X-Plane finds all those out by simply looking at the resulting shape from the front, side, and top!

Once we have all of these areas, we multiply them be the coefficients of DRAG (for FRONT) and LIFT (for side and top) and the dynamic pressure to get the body forces!

So WHAT ARE these coefficients of lift and drag?

WELLLLL, the coefficient of DRAG, multiplied by the FRONTAL area and dynamic pressure to get drag, is the number you enter in plane-maker, PLUS the drag added when the body is NOT going directly into the flow, but is instead crooked (some angle of attack and/or sideslip)

The coefficients of LIFT (used for the both the side and lift forces of the body) are based on the angle of attack and sideslip of the body, respectively, and the also the side and top areas of the body, respectively.

Note that if you are making a jet engine nacelle, a coefficient of drag of ZERO might be the best thing to enter in plane-maker, since your frontal area will be HUGE, and the air will run right THROUGH IT in reality.

So it's like this for the forces for each body:

drag = frontal area times coefficient of drag * dynamic pressure lift = top area times coefficient of lift vertical * dynamic pressure side = side area times coefficient of lift lateral * dynamic pressure

Where:

coefficient of drag = what you enter for the body in plane-maker interpolated to the sideforce of a cylinder getting dragged through the air sideways when the body is 90 degrees out of airflow alignment coefficient of lift vertical = 0 when the body is aligned with the flow, with additions as you go to positive angle of attack, according to the lift of a cylinder at some angle of attack coefficient of lift lateral = 0 when the body is aligned with the flow, with additions as you go to positive angle of sideslip, according to the lift of a cylinder at some angle of sideslip

You COULD LEAVE the body open, but it won't make any difference to the area calculations done above for subsonic flow.

Now, for SUPERSONIC flow, we look at the ANGLES of the PLANES of each little triangle on each body, and run compression shocks or expansion fans on each of those triangles!!!

Yah, this is getting a little bit close to CFD! Compression shocks and expansion fans, per triangle, based on the flow-angle change from the previous triangle!

The forces from these shocks and fans are applied normal to the plane of each triangle, so high pressure on the front of the body pushes you back, and low pressure on the back of the body pulls you back! Ugh, with supersonic flow, you just can't win!!!

NOW, if you left a body OPEN, then these supersonic calculations would sure by confused and messed up, so, um, don't!

NOW, there is one OVER-RIDING EXCEPTION:

IF this is a JET ENGINE NACELLE (as indicated by the body being attached to a jet engine in the body attachment menu right in the body-design pages in plane-maker!) then for SUPERSONIC flight, the flight model IGNORES THE BODY TOTALLY!

That's because ppl build these bodys as jet engine nacelles with open ends (would confuse supersonic dynamics) or huge vertical faces for the compressors (would be a BRICK WALL for supersonic dynamics) so we just IGNORE the nacelles for supersonic dynamics to avoid these confusions.

Whew!

SOOOOO, there you go.

austin