What everyone says is true, PolySci's can and DO succeed in flight school every week.
But I will say that if your background is Engineering because that's what interested you, then you are heading down a great path. If so you'll probably be more interested in some of the stuff here others find mundane. While you certainly don't need to be able to design the plane, concepts may stick easier and your understanding will deepen as you attach these concepts to those you picked up with your degree. The continuation of studying physical sciences from your engineering background will only help.
But honestly, flight school is a refreshing step back to focus on the bigger picture. In school as an engineer, you're introduced to a widget in a class. You don't even know what that widget is or does before you go about trying to mathematically define or parameterize the thing. Next thing you know, your a wiz with equations that you can't explain what the widget actually does to your parents in laymen's terms for the life of you.
In Flight School, especially ground school, you are presented with a widget again. This time you're told what it is and what it does. You will, naturally as an engineer, attempt to delve to deeply into the widget to understand WHY it does what it does. For this you will be scolded. Instead, you spend most of your time beating the WHAT into your head and glazing over many of the why and hows. There will be lots of times when things'll come easy, and lots when you'll trip over routine concepts that make sense to everyone else. You will get very frustrated at these times. But if you work hard enough it will all (most of it) make sense in the end.
And in the end, you will understand that widget just a little bit better than most. Because you didn't spend your ENTIRE four (or in my case five glorious) years learning JUST equations for control volumes, wave dynamics, finite element, and fourier analysis. No, you built an understanding of physical concepts and problem solving that this stuff relates right back to. (Of course you may have ALSO learned some of those concepts in the 8th and 9th grade...)
And did I forget to mention... then you get to go and strap in to your widget and fly the thing. Back in school, the best I hoped for was to model my widget using spreadsheets in Excel.
Of course, as others have mentioned, once you strap in all bets are off and you're on your own.
Plus engineering degrees look pretty good on a resume for say... NASA...