Ram air assist

I am building an air scoop to direct ram air to my fuel tank vents. The
vents (2) exit the bottom of the fuselage at 90° to the airstream. The scoop
will fit over the top of the vents and is shaped like ¼ of a football. It is
in the propeller arc.

The inlet area of the scoop is .84 in². There will be three 3/8" OD aluminum
tubes inside the scoop through which the air will flow. Two of them are the
vents for the front and rear tanks, while the third is the drain tube for
the rear tank fill. (There is a fuel door on the top of the fuselage for the
rear tank. The tube is intended to drain any liquid such as rainwater or
fuel overflow from the area inside the door.) Any air entering the third
tube will exit around the fuel door with only the friction loss from the
tube to restrict flow.

Am I wasting my time, or will there be enough ram air pressure to actually
assist fuel flow in the event of multiple fuel pump failure or vapor lock? I
suppose I could put a check valve in that third tube so air couldn't flow.

Rich S.

In article ,
"Rich S." wrote:

I am building an air scoop to direct ram air to my fuel tank vents. The
vents (2) exit the bottom of the fuselage at 90° to the airstream. The scoop
will fit over the top of the vents and is shaped like ¼ of a football. It is
in the propeller arc.

The inlet area of the scoop is .84 in². There will be three 3/8" OD aluminum
tubes inside the scoop through which the air will flow. Two of them are the
vents for the front and rear tanks, while the third is the drain tube for
the rear tank fill. (There is a fuel door on the top of the fuselage for the
rear tank. The tube is intended to drain any liquid such as rainwater or
fuel overflow from the area inside the door.) Any air entering the third
tube will exit around the fuel door with only the friction loss from the
tube to restrict flow.

Am I wasting my time, or will there be enough ram air pressure to actually
assist fuel flow in the event of multiple fuel pump failure or vapor lock? I
suppose I could put a check valve in that third tube so air couldn't flow.

Rich S.




It depends on how fast you are going. At 200 Kt,you may get about 2" or
so of ram air pressure, which translate to about 1 PSI.

"Orval Fairbairn" wrote in message
news

It depends on how fast you are going. At 200 Kt,you may get about 2" or
so of ram air pressure, which translate to about 1 PSI.

I would probably need it most at best glide speed - you know, when there's
that eerie silence up front.

Say 80 mph or so.

Rich (knots are for boats) S.

On Mon, 10 Nov 2003 10:57:00 -0800, Rich S. wrote:

"Orval Fairbairn" wrote in
message
news

It depends on how fast you are going. At 200 Kt,you may get about 2" or
so of ram air pressure, which translate to about 1 PSI.

I would probably need it most at best glide speed - you know, when
there's that eerie silence up front.

Say 80 mph or so.

Rich (knots are for boats) S.

The ram air pressure at 80 mph would be worth about 1/4 inch of HG, or
about 1/8 psi. It doesn't seem worth the trouble to me.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

"Rich S." wrote in message ...
"Orval Fairbairn" wrote in message
news

It depends on how fast you are going. At 200 Kt,you may get about 2" or
so of ram air pressure, which translate to about 1 PSI.

I would probably need it most at best glide speed - you know, when there's
that eerie silence up front.

Say 80 mph or so.

Rich (knots are for boats) S.

Fuel tank venting can be tricky. If you have two tanks, and
there is a "both" position on the fuel selector valve, any difference
in vent pressure will cause one tank to flow faster than the other. If
the difference is large enough, it could actually prevent fuel from
flowing from the lower-pressure tank.
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.
Ram pressure won't be worth the effort, if you're counting on
pressure to make up for failed pumps. The fuel pressure, and therefore
flow, would be very inadequate and the engine would quit anyway.
The area of the scoop will have no effect on ram pressure.

Install a generic automotive electric fuel pump. Not expensive.

Dan

"Dan Thomas" wrote in message
om...

Fuel tank venting can be tricky. If you have two tanks, and
there is a "both" position on the fuel selector valve, any difference
in vent pressure will cause one tank to flow faster than the other. If
the difference is large enough, it could actually prevent fuel from
flowing from the lower-pressure tank.
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.
Ram pressure won't be worth the effort, if you're counting on
pressure to make up for failed pumps. The fuel pressure, and therefore
flow, would be very inadequate and the engine would quit anyway.
The area of the scoop will have no effect on ram pressure.

Install a generic automotive electric fuel pump. Not expensive.

Dan..........

Got a Bendix on the firewall and a mechanical on the Lycoming. The tanks are
either/or - not both. I need to move the Bendix to a cool location, but
haven't yet. That is a *bunch* of work and I haven't been up to the project.

The scoop, though is a small matter and easy to do. I think the area of the
scoop does make a difference in ram pressure due to the "escape" line that
runs up to the fuel fill enclosure. How much, I don't have a clue.

When you say, "by law" I assume you are speaking about production aircraft.

Regards,
Rich S.

On 10 Nov 2003 17:04:49 -0800, (Dan
Thomas) wrote:

"Rich S." wrote in message ...
"Orval Fairbairn" wrote in message
news

It depends on how fast you are going. At 200 Kt,you may get about 2" or
so of ram air pressure, which translate to about 1 PSI.

I would probably need it most at best glide speed - you know, when there's
that eerie silence up front.

Say 80 mph or so.

Rich (knots are for boats) S.

Fuel tank venting can be tricky. If you have two tanks, and
there is a "both" position on the fuel selector valve, any difference
in vent pressure will cause one tank to flow faster than the other. If
the difference is large enough, it could actually prevent fuel from
flowing from the lower-pressure tank.
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.

I think you will find even certified planes like Bonanzas have an
either, but not both.

As I recall the Cherokee was the same way, but didn't have aux tanks.

Ram pressure won't be worth the effort, if you're counting on

It probably wouldn't be enough to push the gas half the height of the
tank unless you were really moving.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

pressure to make up for failed pumps. The fuel pressure, and therefore
flow, would be very inadequate and the engine would quit anyway.
The area of the scoop will have no effect on ram pressure.

Install a generic automotive electric fuel pump. Not expensive.

Dan

"Rich S." wrote in message ...
\
When you say, "by law" I assume you are speaking about production aircraft.

Yes, it's a part 23 requirement that says that tanks with interconnected outlets
must have interconnected venting.

"Roger Halstead" wrote in message ...
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.

I think you will find even certified planes like Bonanzas have an
either, but not both.

So? That has nothing to do with the venting. The problem is that if you
can't gravity feed to a common point, then you are in deep dodo if you try
to pump and one tank is full of air. High wings generally have no problem
gravity feeding (and frequently have a both position, and in sometimes
only a both position). Low wings generally don't have a BOTH, although
my Navion for example drains both wing tanks to a common low point
and pumps from there. The (stock) Navion fuel valve is OFF-ON.

The Cessnas with a BOTH position have a bent line that runs between the
two tanks just behind the pilot's head. This line is actually the source of
some interesting fuel flows that cause the Cessna tanks to drain unevenly
when filled above the vent line. Likewise the Navion fuel vent line runs
between the two tanks (forward of the pilot).

Roger Halstead wrote in message . ..
On 10 Nov 2003 17:04:49 -0800, (Dan
Thomas) wrote:

Fuel tank venting can be tricky. If you have two tanks, and
there is a "both" position on the fuel selector valve, any difference
in vent pressure will cause one tank to flow faster than the other. If
the difference is large enough, it could actually prevent fuel from
flowing from the lower-pressure tank.
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.

I think you will find even certified planes like Bonanzas have an
either, but not both.

As I recall the Cherokee was the same way, but didn't have aux tanks.

Ram pressure won't be worth the effort, if you're counting on

It probably wouldn't be enough to push the gas half the height of the
tank unless you were really moving.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

pressure to make up for failed pumps. The fuel pressure, and therefore
flow, would be very inadequate and the engine would quit anyway.
The area of the scoop will have no effect on ram pressure.

Install a generic automotive electric fuel pump. Not expensive.

Dan

High-wing airplanes typically have a "both" position and
often rely on gravity flow. Low-wingers can't, as running one tank dry
could result in the pump being quite happy sucking air instead of
fuel.
We finished a Glastar (high-wing) that had only an on-off valve
and two wing tanks. The tank plumbing was teed together just upstream
of the valve. The tanks had separate vents, and one had a bit more
pressure than the other. That tank would drain first, but we didn't
fly it long enough to see if there was enough pressure to prevent the
full tank's fuel from flowing. We modified the vent system to generate
similar pressures, but it still needs to have the tank vent systems
plumbed together.
My quick calculations give me a static head pressure for
gasoline of about 0.32 psi per foot, so a 1 psi vent pressure could
lift the fuel three feet, more than the typical high-wing tank-to-carb
distance on a gravity feed system. There is a real danger here, I
think, though the difference in vent pressures would have to be pretty
serious. A plugged vent on one side would do it.
Cessna has their tanks vented from a single underwing vent
behind the left strut. This line goes into the top of the left tank,
and another line runs from the inboard top end of that tank across the
cabin roof to the top of the right tank. In addition, the fuel caps
have vents with one-way check valves to allow air in (but not fuel
out) in case of vent icing.

Dan