Why do some airplanes have their winglets at different angles?

While travelling recently a friend of mine noticed that the tips of the wings on some planes were at different angles on the left and the right hand sides of the plane. Is this a thing and what is the reason for this?

Some googling and searching on this site did not reveal an answer.

• It's possible that when looking out the windows of the plane from his seat, the winglets appeared to be at different angles due to her different viewing angle from one side of the plane. The closer he was to one window the farther she was from the other and that parallax could have made a difference. Sep 24, 2018 at 12:44
• @FreeMan - He was looking at other planes, from straight ahead or behind, not the plane we were in. The angles were obviously different on the different wings on least two planes he saw. Sep 24, 2018 at 18:29

Winglets are symmetric, meaning they are the same shape, size and angle on both sides of the airplane. Different shapes and/or angles would mean the wings would have different aerodynamic properties, i.e. they'd fly differently from one another, which is generally a bad thing. It's probably been done for testing purposes, but not in production aircraft.

It's possible that your friend noticed that different makes and models of airplane have different winglets. Some look like this:

Some look like this:

There's other shapes too. Viewed from different angles it's possible they may look like different shapes, but they're going to be the same even if it doesn't seem that way.

• He was looking at the planes from straight ahead or behind. The angles of the winglets were obviously different on the different wings on least two planes he saw. Sep 25, 2018 at 6:12
• Perhaps that's how it looked to him @Lizzan, but it's not the case, winglets are symmetric.
– GdD
Sep 25, 2018 at 8:19
• Sure, that makes the most sense, which is why he was so surprised to see it. I'll keep this question open for a while longer in case someone else knows of an exception. Sep 25, 2018 at 8:52
• I'm accepting this now as I've been on a trip and watched some winglets on my own. My first instinct on seeing a plane with them was that they were indeed different, but as we travelled behind the plane the opposite winglet seemed to be pointing almost inwards to the first I saw. It seems to be a powerful optical illusion unless you are watching from perfectly straight ahead or behind. Nov 13, 2018 at 7:34