|
Interesting question Eric. I don¢t know
the answer, and need to leave soon, but here are some thoughts.
At 18,000 ft half the atmosphere is below
you, the absolute pressure is about 7.5 psi. Neglecting ram air pressure the
pressure inside the tank is 7.5 psi.
Now imagine we lower the wing instantly
to sea level. The pressure outside the wing is about 15 psi and the tank
differential is 7.5 psi, (1,080 lb/sq ft). The wing will be crushed.
My 360 wing holds 23 gal, 5,313 cu in. Assuming
the fuel tank is empty and does not crush, we have to add 2,637 cu in of sea
level air to equalize the tank.
Using Grayhawk¢s ID of 0.156, the area of
the tube is 0.019 sq in. Assuming an average velocity of 20 ft / sec the flow
rate will be 4.6 cu in / sec and time to equalize will be 9.6 min.
Comments.
I don¢t know what differential gives 20
ft/sec, it depends on tube length, fittings, bends, burs, dirt, bugs etc. Maybe
Grayhawk's calculator can give a ballpark number.
Air at altitude is less dense so more
cubic inches are required.
LIV tanks are larger, so proportionally
more air and time required.
The critical cases would be emergency
descent with empty tank, and extreme climb rate possible with turbine engines and minimum fuel.
Bill Hannahan
--- On Tue, 11/25/08, Sky2high@aol.com <Sky2high@aol.com> wrote:
From: Sky2high@aol.com <Sky2high@aol.com>
Subject: [LML] Re: 360 Fuel Vent
To: lml@lancaironline.net
Date: Tuesday, November 25, 2008, 6:00 AM
Eric,
Assuming all kinds of things and using pipe flow calculator for a gas known
as air:
If your engine is using 15 gal/hr = 2 cu ft/hr = .0335 cfm
Suppose the inside diameter of 1/4" tubing is .156", the pressure of the
air at the vent is .7 psi (about 200 kts) and the temp is 31F, then the air is
traveling in the tube at 3 Kts.
"So What?" You might ask.
OK then, take a gallon jug, fit a 3/8 inch tube (engine pump fuel line
size?) at one end and a quarter inch tube at the other end (vent
line). Use a good sealing putty. Fill the jug with water.
Let the water drain out the 3/8 tube and time it. If it drains in 3
minutes or less (1 gal/4 min = 15 gals/hr., 1 gal/3 min = 20 gph), the vent is
large enough. That's just using gravity - without assistance
from the engine pump or the vent pressure and 100LL may be less
viscous.
Grayhawk
PS Let us in on the results.
In a message dated 11/24/2008 10:01:34 P.M. Central Standard Time,
cassutt@windstream.net writes:
I am not
using the header tank in my 360 and have installed 1/4" od tubing as the vent
for each wing tank. My old SB manual says nothing about fuel vent size even
with the header and what published updates there are for the extended bay
don't have a size listed either. Is this big enough dia. for pulling
fuel straight from the wings or should it be larger? What have others
done.
Eric Demaray
360 SB
--
For archives and unsub
http://mail.lancaironline.net:81/lists/lml/List.html
|