Integration using Cumtrapz backwards instead of forwards
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Ramses Young
on 19 Nov 2021
Commented: Ramses Young
on 20 Nov 2021
Hey all, I am trying to create a program that will take distributed point loads from lift generated by a wing and create moment and shear diagrams using numerical integration through cumtrapz
The way I am doing this is by integrating the points and locations which gets me a shear force, but because cumtrapz integrates numerically forward, it technically creates the right graph but backwards from a conventional beam analysis, which creates cuts. Is it possible to make Cumtrapz work backwards? I have attached images of what I want vs what I get for an example
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/806999/image.png)
Desired Shear diagram
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/807004/image.png)
Obtained Shear Diagram
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/807009/image.png)
The obvious solution is just to subtract the maximum value from the graph but this does not encapsulate the essence of what's going on, and also creates a further problem in the bending moment diagram.
Here's the code
L_dist =[10 9 8 7 6 5 4 3 2 1 0];
locations=[0 1 2 3 4 5 6 7 8 9 10];
shear.z = cumtrapz(locations,(L_dist));
figure('Name','shear - z','NumberTitle','off');
plot(locations, shear.z);
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Accepted Answer
Sean de Wolski
on 19 Nov 2021
Easy solution is to just flip everything
L_dist =[10 9 8 7 6 5 4 3 2 1 0];
locations=[0 1 2 3 4 5 6 7 8 9 10];
shear.z = flip(cumtrapz(flip(locations),flip(L_dist)));
figure('Name','shear - z','NumberTitle','off');
plot(locations, shear.z);
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