Several years ago, a guy I knew described a mom nursing her baby on his flight. He's a gruff, older, eagle-eyed, stern looking former Boston police officer, to give you an idea how he might come off to people on first impressions.
He said the woman across the aisle had a baby with her, and that baby was quiet as could be, her mom just nursed her throughout the flight.
After the flight was over, he told her what a good job she was doing with her baby (by nursing). I'm glad he did, because otherwise she probably would have assumed he was glaring at her (he just comes off that way).
All that to say ... even if someone does seem to be glaring or crabby about your nursing (on a plane or anywhere else), I think the best way to handle it is with a big, friendly smile. As if they are happy to see you doing what you're doing. Because, there's a significant chance that they are. And if they aren't - most people are cowardly enough to back down and ignore a nursing pair, if the mom looks confident and comfortable.
My sister just flew cross-country with her three month old and was worrying about this. She got a plugged duct on the first leg of her trip, because she'd been trying to hurry the nursing sessions. I told her - look, A's not crying over the altitude shifts, because you're nursing. You're doing the right thing. Most people agree with you; and anyone who's uncomfortable can look elsewhere. But most people are just happy your baby isn't screaming! And told her to smile at them and assume they were happy to see her ... she said it worked like a charm on her way home.
:
