TAF (total aerodynamic force) question.

[George Chityat]

Please see attached photo.
I could not wrap my head around this one yet. Maybe someone here can explain it and it will click for me.
Look on the right in figure B.
The red line in the middle goes thru the center of pressure.the green Lift line comes out of the center of pressure too. The dashed blue line which represents drag and together with lift forms a rectangle for TAF.
The question is where does the dashed blue drag line come from? It seems like it just comes off an arbitrary point along solid blue relative wind line. How do you determine where that line goes?

 

[George Chityat]

Well if you look at diagram A and Diagram B. Look at how much further back the drag line is. I could bring it much further forward, there by bring TAF further forward.

 

[Velocity173]

Well if you look at diagram A and Diagram B. Look at how much further back the drag line is. I could bring it much further forward, there by bring TAF further forward.

The drag line is further aft on A compared to B because the drag is greater in that blade region (driven). It's greater because the velocity of the blade is greater there compared to B (equilibrium). Parasite drag.

 

[George Chityat]

Correct, but how does one determine how far the drag line goes back or forward? It doesn't seem like it is based on any measurement.

 

[Capt. Geoffrey Thorpe]

Correct, but how does one determine how far the drag line goes back or forward? It doesn't seem like it is based on any measurement.

it could be from wind tunnel data, computational fluid dynamics, or pulled out of a warm dark place for the sake of illustration.

What you measure in a wind tunnel are the total aero forces in the lift (perpendicular to the tunnel axis) and drag (parallel to the tunnel axis) directions and the moment about wherever your model is attached.

 

[Velocity173]

Correct, but how does one determine how far the drag line goes back or forward? It doesn't seem like it is based on any measurement.

That's just one diagram of many on autorotative regions. It just depends on the author / artist on how far any of those vector quantities go. Most depict the same distance / ratios but it's just there to give you an idea of the basic magnitude of each force.