Question on using Blender

This is literally the hundredth time Blender ruined my project by creating unknown lines automatically. This time it completely destroyed the back window of my Piper PA44 Seminole. Any idea why this is happening? Thanks in advance!

elaborate a little !

what tool did you use ?

happy bl

If there is a problem in the mesh editing and it happens with you a lot, it probably means that you do something wrong. One common mistake I used to do, for example, was to select vertices in solid mode without toggling X-ray. This lead to me only selecting vertices that are in front of the view while leaving those at the back. When I then extrude or edit them and return to look from different angles I realize that I messed up and left some vertices unedited mistakenly.
You probably do something similar. Maybe you press F key (which connects two vertices to form an edge or with more vertices it forms a face) not knowing its function?
As RickyBlender said, elaborating more on the problem will help.

Thanks for the reply! I always use X-Ray when selecting vertices. I have not used F key except on the engines, but that is done almost a week ago and shouldn’t affect the windows. I have used Mirror Modifier and Boolean Modifier for windows. Not sure if that’s the issue.
The interesting thing is that everything is fine before I delete the extra lines on fuselage. After the lines on fuselage, which Blender added itself, the window broke.
Hope that’ll be enough information. Thanks!

Sorry for not being specific enough. I found some extra lines on the fuselage which I don’t know when they are added. I deleted those lines and wanted to check the work, and it was only then when the window is messed up. I think it might be the removal of the extra lines which caused the broken window issue, but I’m not sure why there are extra lines at the very beginning. Could it be Mirror issue? (I used Mirror on fuselage)

It could be the mirror, it also could be something else.
I would suggest that after every couple steps you check your model (from different angles). Once you notice something wrong about the model go over what you did and find the mistake.

best way would be to upload the file with problem
then we can test it !

happy bl

Blender doesn’t destroy anything really. You can just retrieve the last auto-save and it will put you back where you were or very close to it.

temp screen shot

I always select the first one in the list, rather than the one that’s offered.

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It sounds like it’s caused by your boolean modifier. You didn’t explain what your were trying to do so I can only guess that you applied a boolean and it created those extra edges.

Sometimes the boolean modifier has a problem with complex or odd shaped meshes and will share verts between faces when it recalculates the mesh, so that when you delete one of those faces it destroys other faces as well because it removes those shared verts. It causes some extra work but it usually isn’t as bad as it might appear.

Sometimes adding a few extra cuts in the right places will make the boolean work better. Trial and error.

Thanks for the tip. Unfortunately Blender only recorded the model after the window’s broken, but I managed to get the more-odd-looking version of the windows and fuselage before they are fixed… Perhaps inproperly.

Thanks for the tip. I’ll do that later on!

Be wary of using the merge points tool. If you merge by distance and nothing is selected It gets applied to everything. I’ve done that a few times. Also I Dissolved some edges only to find I had accidentally selected edges out of view and removed them also. I had to get into the habit of Hittiing the period/Del key on the numpad to frame all selected.

Thanks for the replies! Original problem solved. I simply modeled another fuselage. Yet, another problem occured. After finishing the modeling of the new fuselage and all other stuff, the fuselage now appears to be hollow. I used Boolean again to delete a part of the fuselage for a gear, but I can now see through the fuselage. Any idea?

For a clearer view, I made a cut view of the fuselage right now. It has two walls on the side and a single sheet of mesh in the middle, and that appears to be it. Hope this can help explaining the issue.

I am not sure what you mean by the issue, maybe display it on a separate simpler model without other models around it?

However, if I guess correctly you either want one of two things.
Number one, you want to give thickness to the model. In such case just look up solidify modifier.
Number two, which is more likely, is that you wanted to make the model to remain solid after the boolean operation. So basically you want to block out this open part.
For this you have to go into edit mode and fill the faces either manually or by using grid fill tool (look it up), but for the future I don’t think you should use boolean operations for such simple task. It would have been better to edit the model in different ways to achieve the same result.
Boolean operations in 3D tools like Maya, Blender and Max are not optimal such as other 3D tools oriented more towards CAD modeling, so it might take some time to be able to understand how they work correctly.