is it necessary to use a header tank if I want to fly 3d?
with an x-cell gasser?

No need for a header tank unless you need to see your fuel level for some reason.

The Walbro carb has a built in pump, if you suck an air bubble, the pump just plays catch-up and no problem.

You may want to run your vent line a full wrap around your tank to keep it from leaking in odd attitudes.

I use a one way check valve on the vent tube so their is no chance of fuel leaking out.

I tried that once and still had leaking as there is negative pressure in the tank as the machine uses fuel. There just didn't seem to be enough pressure to keep it closed in inverted backwards flight when the end of the vent was below my fuel level. Then it leaked anyway. And I didn't like all the extra plumbing to provide for venting when not in use. I like the KISS method, lees to go wrong.

I actually closed my vent tap down to a pin hole and wrapped the line past the front of the tank and then back to the rear of the frames. This way no matter the attitude, there is always air in the line and no leaking. There IS a small amount that drips out when I return to upright though. I figure this is as good as it gets.

I suppose if you could find a spring type check valve, that would probably work OK.

I'm using a high volume, one way that is made for gas, no problem yet.

Thanks for the info I think I will just wrap the vent line
Thanks