It increases drag, reduce structural strength, reduces the top speed and the climb rate while improving the best turn rate by 1%.
What else does this upgrade do? It seem to make the plane worst.
Before the “Plane cover modification” upgrade
After the upgrade

Changes to the stats
I’ll make an educated guess and say it actually decreases drag. It’s just written in a weird way.
Probably due to the math and code calculating physical simulation.
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Yeah, im sure its supposed to be a - instead of a + there.
If it was the case of typos then there should only be a few errors. The drag coefficient increases, and as expected the top seed is reduced.
All seem to be counterintuitive to what an upgrade should be.
As I wrote before, it may be caused by calculations the game makes.
For example (it doesn’t make sense but ilustrates what I mean)
speed = acceleration / drag coeficient
This way higher drag coeficient means more speed.
the cover upgrade is broken, they put the wrong modifiers (+/-)
i made a bug report some time ago, feel free to click ‘i have the same issue’
it’s better to simply not upgrade planes for now
https://community.gaijin.net/issues/p/enlisted/i/ZUz1PTeMEGdJ
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IIRC this is an issue with all the modifications - they are expressed in the wrong manner.
Haven’t got my game here at the moment but I’ve always presumed that it was just a cosmetic error, but basically yeah - drag co-efficients should be LOWER for LESS drag - the classic drag equation is

cd is the drag co-efficient, and so the force exerted by drag is lower when it is lower.
Also I think perhaps the lift co-efficient is similarly wrong??

CL is the lift co-efficient so you want it to be high for more lift