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Pitts s1 header tank vent line size

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Active Member
Supporting Member
Feb 23, 2016
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Hi all,

I'm building a Pitts S1ss and would love to get your comments on what size vent line is appropriate for the header tank - main tank interconnect. I did some tests to give an idea of relative performances between two tube sizes, 1/4" or 3/8".

I assume that the aim is to support a good combination of header tank (upright flight) fill speed (fast as possible) and (inverted flight) drain speed (slow as possible), but I don't know which is more important in real life use. Below is a pick of the intended install and the header internals. The header tank vent fitting is 1/4"npt which can support the 3/8" or 1/4" pipes..





The best test would have been to completely plumb in the tanks to the frame and do in situ tests but - nah... This is I guess more about relative differences rather than absolute numbers.

Header Fill Tests
Based on:
- Half main tank filled with water and blue dye.
- empty header tank with inline tap between tanks and 20mm connection tube about foot long.
- tanks positioned to correct relative heights.
- Filler cap fitted
- 1/8" npt main tank Tee fitted with one end blocked (to roughly simulate actual restriction to atmosphere)

1/4" tubing
Header Fill test (upright)
1/4" vent pipe 1m long.
Time to fill 10 Litres= 56sec

3/8" tubing

Header Fill test (upright)
3/8" vent pipe 1m long.
Time to fill 10 Litres = 45sec

Header Drain Tests

Based on:
- Full header tank with water and blue dye.
- tank positioned in inverted position slightly raised front end.
- 20mm Filler neck disconnected from main tank and exposed to atmosphere

1/4" tubing
Directed vertically down 1m.
Time to drain 1 Litre = 40sec

3/8" tubing

Directed vertically down 1m.
Time to drain 1 Litre = 14sec

Supplemental test with 3/8" tubing

end of vent tubing fitted with 1/8" fitting to simulate connection to 1/8" main tank vent system.
Time to drain 1 Litre = 26sec

What would you pick? 1/4" or 3/8" ?

Any guidance greatly appreciated.

Last edited:

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