At very high speeds where angle of attack approaches zero, lift induced drag must be zero, your graph says that. My graph at zero angle of attacks shows a positibe coefficient of drag, this is parsite drag coefficient.
which is exactly what I said I was talking about.
Free body diagram of the two cases of weight.
Energy in arrow forward
Energy out Drag arrow to the back.
Energy in the system potential inside the box.
Energy out arrow pointing down (gravity)
Energy in pointing up (lift)
Max airspeed equals max drag which is the same for both as the Vmo doesn't change with the weight. As per the graph I showed if you look at the total drag.
As soon as you go away from Max Drag you are creating more variables that you have to account for and also will affect the result. Which is why I took the situation of Vmo because it zero's out or decreases a multiple of variables to insignificants. Induced drag tends towards insignificant so your variation in L/D ratios can be ignored the AoA of the wing can be ignored. By taking any other speed other than maximum you are into calculus to work out what all the different variables will actually mean and how they relate to each other.
Your 737 data proves the point, the only variable that is constant is the speed. Drag is different for each weight, the energy removed is different for each weight. You have 4 variables which can't prove a relationship between two of the variables.
Where as my solution the drag is the same for all weights, the energy removed is the same and the speed is the same. We only have two variables the weight and the rate of decent. Which you can prove the relationship.
It is a very simple problem which is given to second year Mech Eng students to teach them how use freebody diagrams for pratical problem solving and manipulations of variables to form simplified systems from complex ones and to also learn how to choose boundary conditions to find a relationship.
Yes you could go into into L/D ratios and all that but they would fail because they wouldn't have proved the relationship.
Now with your 737 data get a set of weights where the drags the same and the energy removal rate is the same and the speed is the same. Then we will be talking about the same thing. I will save you some time it won't happen but if you try Vmo it will be quite close.