I have a 2 1/2 year old son, and the Barnstormer (or the Goofy Roller Coaster, as we call it) is now his favorite ride - but it did take a little bit of effort to get him on it.
First, I would show her a YouTube video and make sure it's something she's interested in riding. We had to watch it several times so he got excited and knew what to expect.
Our biggest issue was making sure he knew and understood that he couldn't sit on my lap like other rides. We had to drill this in, but also explain that mommy or daddy would be sitting right next to him. He kept insisting he was going to sit on my lap (not allowed), so finally I sat down next to him and wrapped my arm around him to show him I could still hold on to him on the ride - but his tush had to be on the seat.
As others mentioned, it is a little rough, so I would recommend wrapping an arm around her for the whole ride. Also, make sure she understands that she needs to bump her head on the height measurement. A lot of kids shrink to not hit it, but they will actually use a piece of paper to try to slide under to make sure a kid is tall enough, if it is close.
And if you think she will ride it, get a FastPass+... less time in line to dwell on it.
My husband was actually upset when we got off the first time and said our son looked terrified the whole time. Then we asked him how he liked it, and the reply was "That was fun! Let's ride it again!"
I think there was fear of the unknown the first time, but he's been on it two more times since and now yells "Woo hoo!" throughout the whole ride.