Day tank

[jared payne 40]

What is the purpose of the day tank ? Why not just tee the tanks together, and use vented fill caps? You would still have gravity feed.

[1avidflyer 1983]

One thing is it acts as a large gascolator  You can also put a low fuel warning system in it to warn you that things are about to get exciting.  Wish I had had one of those a few years ago. JImChuk

[Yamma-Fox 403]

The header tank?

Here's my guesses:

Low point in the system with a vent return which is most positive way to keep air out of the system.

With just a T fitting, In a steep turn or slip you might have fuel flowing really fast through the T from one wing to another.... and the fuel flow down to the engine might be reduced or even reversed (venturi effect).   Kind of like those fertilizer/weed killer bottles work where the garden hose flow draws up the solution.

Also it acts as a bit of a pressure reservoir with some volume to keep the engine feed constant.

Now someone smarter than me can give you the REAL reason!  (Inverted flight)

Edited 14 Dec 2017 by Yamma-Fox

[jared payne 40]

All right I will leave it in.