Nightpilot

Quick question:::: Why would a Norvel .061 AME running on a pressure bladder tank, turning a 5x4 master airscrew at 23100 on the ground. Go RICH midway thru the flight???? Yes the needle valve is tight, no air leaks that I can tell in the fuel system, oh and its a brand new engine..

Anyone care to field this one?
Cause I am outta Ideas.

NP

William Robison

My Feedback: (3)

NP:

This has always been a bother with bladder tanks.

For a practical demonstration, remember blowing a balloon up? It takes a whale of a blow to get it started, but after you have the first bit expanded it becomes a lot easier to finish. And it works in reverse too. When you release the balloon and let it fly, almost always there will be a surge of accelleration just as it finishes deflating. This pressure increase isn't very obvious with the balloon until it is almost fully deflated, but it starts when the first part comes back to its normal size, and increases in a linear manner until it's empty.

So for a fuel bladder the best material will remain equally distended over its entire length, and it will all come back to normal size at the same time. Selecting a bladder material for this characteristic can be a pain. Surgical tubing seems the best, but it will still vary from one batch to the next, even from one end of the same tube to the other.

So toss the one that has too great a variation, cut another, and try it. When you get a good one make another, build up a supply of good ones and take care of them.

Bill.

BMatthews

Just to add.

The pressure will be even as long as the bladder looks like a weiner. Then as it uses the fuel the weiner will shorten and finally you only have a ball shape left. It's around the time where the last of the weiner shape is gone and the ball is left that it'll stiffen up like Bill says and run rich. The cure is to use more fuel so you get a long enough run from the weiner portion. If that's too much fuel then use a smaller diameter line for the bladder. But it's important that you keep the weiner shape as long as you can for an even run.

combatpigg

My Feedback: (3)

I make mine out of 3/16" tan surgical tubing, or out of neon surgical tubing, and make them 2-3" long. I fill them up with 1 0z of fuel, and the rich portion of the run is only the final 10 seconds or so. SULLIVAN small fuel line is good stuff to plumb with from the bladder to the needle.

Nightpilot

Guys, thanks alot....makes total sense. You could time the flights to almost the second it was going to go rich, which was right at about 2 min. and 15 seconds.

Man so simple of a fix!!!!!!!!!!!
Thanks, you all just made my Sat. a little more fun!!!!

NP