I saw the hint but felt it should be hammered home. Not too many years ago our fuel was pretty much just Castor oil and the smoke trail thing was true. It's a hold over from bygone days. Bygone, hope that's a word!! Another thing that has changed and some of the peanut gallery hasn't grasped is the anti foaming agents put in the fuel, it really works well and we no longer have to pack our fuel tanks in 6 inches of foam to keep the fuel from foaming up with vibration. The best video I have seen showing this was one from Germany. They were showing how fuel moved around during flight. A clear fuel tank was mounted on the outside of the plane with a wing mounted camera, the plane was flown through a complete series of stunts, it was a split screen so you could see what the plane was doing. Not only was it an eye opener as to how the fuel moved around, it showed how fuel no longer foams up. The tank was hard mounted. I had the thread in my favorites but it is no longer active. I showed it to people just so they could see the fuel moving to different locations through out the flight. Wish the site was still working!!