Last week we managed an 8 hour flight with 3 ds's - 4,3 & 15 months and it was actually less stressful than you would have imagined. My biggest tips are:
Try and keep dc awake for as long as possible before the flight and then get a drink ready as you taxi. 9 times out of 10 mine have looked a bit shocked at the noise of the take off but then settled down pretty quickly for a nap with the drone and vibration of the plane. If you're lucky, cos you'll be holding her, she'll sleep for half of it!
Take a small bag for her with a couple of books and some favourite toys and then loads of snacks - I'm sure I fed mine constantly for most of the flight! They especially like stuff they don't get very often like sweets and lollies, although your dd is still quite young.
Think about all the parts of your journey from home to the plane and talk it through with your DH/dp. That way it takes all the stress out of the journey as you already have specific jobs to do and there's no last minute frustrations. You will also have thought about whether to go for a sling or buggy, how many flight bags to take, where to put any liquids or gels so they can be grabbed quickly for security. For example, my DH and I usually arrange it so I sort the kids and he does the bags, this includes getting to check in, through security and onto the plane. We usually always take a buggy all the way to the gate as well which is really useful. Sounds a bit anal and I was never such a planner in my singleton days but it honestly helps massively to do the virtual journey beforehand!
Stay relaxed and that rubs off on the dc.
Try not to worry about other passengers. At the end of the day there is little you can do if dc decides to have a blowout so they'll have to put up with it like you. If they have kids themselves they'll know! Also, while it seems loud to you, the engine noise dulls it a lot by the time it reaches other rows. Sometimes getting up and taking them to the toilet helps cos you can shut yourself in for a minute and they get distracted by the new environment. I've let my ds play with the running water for a minute before, just as a distraction.
Hopefully you'll find that she is so interested in the new experience that she'll be better than a car journey!
Sorry it's a bit long but hopefully it might help a bit. Enjoy your sunshine!