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ITEM NO. 1
On September 22, 2001 Bill Kennedy asked if adjusting the standpipe vent
pressure so the measured level stayed constant with airspeed would be an
acceptable method of calibrating a level measuring system which used separate
vents for the fuel tank and standpipe.
All I can say is maybe. Keep in mind three things.
Number 1 - The differential pressure required to produce an error in the
system is extremely small. If the difference in tank level between full and
empty is 9 inches then the differential pressure required to introduce a 10%
error (i.e., 0.9 inches of fuel) is only 0.024 PSI. This is equivalent to one
vent seeing 200 mph and the other seeing 201.7 mph (assuming 100% recovery
and sea level conditions). This is a very minor difference resulting in a
very big error.
Number 2 - There are a lot of variables involved. Perhaps the biggest is that
the vent on the wing tip is going to be in the free stream while the vent on
the header tank is most likely in the propwash. Since we are talking about
the efficiency of recovery of dynamic pressure, everything from density
altitude, gross weight, angle-of-attack, etc., may also come into play. I
just don't know how you could eliminate all these variables or determine if
close was close enough.
Number 3 - You are trying to calibrate the vent on the header tank to the
vents on two different wings. In others words you are going to have to adjust
at least two of them to get all three to agree and I doubt if that's possible.
You are going to spend a lot of flight time and effort calibrating a system
which shouldn't require any calibration (at least not this kind) in the first
place. Assuming you finally get it to work, then the question is will it be
accurate throughout the flight envelope? Finally, will you be willing to
trust it once you get it adjusted. For a system as important as fuel level
gauging this is not the way to go IMHO. If you do proceed with building your
plane with this system, I would strongly recommend that you have a Plan "B"
in mind.
ITEM NO. 2
On September 22, 2001 Jerry Grimmonpre asked if a system wherein both the
standpipe and the tank were vented to a common NACA duct (presumably located
on the fuselage) would work.
IMHO this is a system that has some possibilities but it also has some
potential problems. Venting both the standpipe and the tank to the same
location should equalize any pressure differences and the levels in each
should be the same. If it were my plane, I think I would carry it one step
further and tie the two vents together thus insuring that the pressures are
the same and then use only one common connection to provide ram air
pressurization. In either case, the ram air pressurization connection(s) must
be located at the low point in the system and provide the secondary function
of a system drain.
The problem here is drainage. You have to assume that fuel or condensation
will get into one or the other vent line at some time or other. Therefore,
you must provide positive drainage using the pressure vent(s) at the NACA
duct as the drain. Although not stated, I am assuming the NACA duct is
located on the bottom of the fuselage. To make the system work, each of the
vent lines must slope continuously downward from their source to the NACA
duct with absolutely no low points. This should be fairly straight forward
for the vent on the standpipe but will be quite a challenge on the wing tank
vent.
I am confused by your description of the vent line to the wing which you say
will be routed "to the fuel bay at the wing root only" and then go on to
say "To prevent siphoning fuel both of these lines would be installed so the
highest point would be higher than the highest fuel level, or a check valve.
It sounds like you intend to connect the vent line to the tank at the BL51
rib (which is not the high point of the tank) and then route it down to the
fuselage and then up and over a siphon break and then down to the NACA duct.
If I understand your system correctly then I don't see how it is going to
work since it's not venting the vapor space in the tank (at least not when the
tank is mostly full) and it will form a standing leg of fuel in the vertical
runs of the siphon break which completely defeats the purpose of connecting
the two vents to the same pressure source.
Instead, why don't you start the vent near the top of the tip rib and then
route it down the inside of the upper wing skin, out the BL51 rib, through
the stub wing and then inside the fuselage to the NACA duct. This will allow
the system to positively drain if you can route it continuously downward. By
having the end at the top of the tip rib, fuel is prevented from entering the
vent line except while flying uncoordinated or when parked on a side hill. If
you want to prevent even this from happening then you can add some type of
ball float valve to the end of the vent but at some point you will begin to
violate the KISS principle. I cannot recommend the use of a check valve as it
will prevent the vent line from properly relieving pressure within the tank
such as might occur during the daily heat up cycle.
I can't emphasize enough the importance of keeping the vent lines drained if
you expect to have accurate fuel level readings. If you use 1/4 inch OD tube,
then all you have to do is kink it upward less than 3/16 inch (the ID of the
tube) to form a pocket which will trap moisture or fuel. Ultimately, this
will form a leg of fluid which, if it has a vertical component of say 1 inch,
will cause a 1 inch error in the level measurement (11% error).
Although completely off-topic, I am wondering about the use of the NACA duct
as the source of ram air pressure. I see many IV/IVP builders are putting
them in their winglets for what I assume is the source of ram air pressure
for the fuel tank vent. Does this really make sense? The NACA duct is
intended to maximize pressure recovery in a free flowing stream. If the tank
vent is the only thing connected to the NACA duct then the stream is
basically dead ended. Intuitively, I would think that a dead ended NACA duct
would act just like any other depression in the side of the plane with a
pocket of very turbulent air in the duct area and the pressure at the vent
line very close to the free stream static pressure rather than the ram air
pressure. Is this a case of form overriding function or does someone have
data to show that the NACA duct is an effective way to convert ram air
pressure without a free flow of air?
ITEM NO. 3
On September 23, 2001 Rob Wolf suggested drilling a hole in the standpipe to
vent it.
I must respectfully disagree with this suggestion. This is tantamount to
venting the fuel system inside the cockpit! Ignoring the obvious health and
fire safety concerns, I don't think it will work anyway. Such a vent will
insure that the pressure in the standpipe is equal to the cabin pressure
which, for un-pressurized aircraft, is something which approximates static
pressure. As I stated in my previous post, the pressure above the fuel in the
tank could be as high as 26 inches of fuel above static assuming 100%
recovery by the fuel tank vent, 200 mph and sea level conditions. Ignoring
friction losses this would produce a fuel geyser in the cockpit of similar
height! Of course pressure recovery is probably a lot less than 100% so the
effect would be diminished but you get the point.
I freely admit that I don't know squat about rockets but I don't think your
analogy is appropriate. If you are talking about cryogenic propellants then
the pressure in the vapor space is determined by the fluid inside the tank
which is at saturated (boiling) conditions. The pressure is determined by the
temperature of the propellant which is at equilibrium with its vapor and
contained within a pressure vessel. The pressure will go up and down as the
temperature changes and is independent of ambient pressure assuming that the
vent is closed.
In an airplane, since the fuel temperature is significantly below its boiling
temperature at atmospheric pressure, the space above the fuel is a mixture of
air and fuel vapor. Each component contributes its partial pressure to form
the total pressure of the mixture. The total pressure is determined by the
conditions at the vent. If the pressure in the tank rises above the pressure
at the vent then is equalized by an outflow of air/fuel vapor mixture. If the
pressure in the tank falls, then air flows in to equalize the pressure.
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