S

Steve Brown

Well Known Member
I've been considering getting an extended fuel bladder for the baggage area:
http://turtlepac.com/sportdetails.htm

The only thing holding me back is deciding how to tie it into the fuel system.

I'm not great at wrenching so I'm thinking of minimally invasive approaches.

One variant is to use one of the tank vent lines to feed fuel into the tank. The idea is to gravity feed into the vent line, then let the airplanes fuel pump do the work, pulling aux fuel back through the line to fill the vacuum created by spent fuel.

So:
Aux bladder higher than the wing tank, but lower than the top of the vent line.

To do this I would install a T in the vent line with a valve. Opening the valve while running off of that tank starts the process. When axillary fuel is empty (bladder collapsed) the vent line would start puling air again.

Any pitfalls with this method?
Any simpler or better ideas?
 
Safeair1......

......sells extended range tanks that fit in the large holes in the nose ribs, just in front of the spar with access from a door you install in the wingtip. They hold ten gallons and feed directly into the outboard end of the mains.
Visit www.safeair1.com.

Regards,
 
They don't but....

Safeair does not make one for the 9, but I am wondering if the diameter of the lightening holes in the 7 are the same as the 9. The er tank should work for the 9 then....You would just have the difference of wing length between the 7 and 9 and have to run a longer transfer line to the 9's existing tank. Dont' think the extra wing length on the 9 would make too much difference, but I don't really know. Once the wing is flying it does it really matter? I am considering this, so I have not put the bottoms on my wings yet to allow easier access.
 
I'd bet....

....that if you called the manufacturer, that they would. However, the extra wingspan of the -9 might make them too heavy when they're full.

I'm with Jim in using the -7 tanks and having a little longer transfer fuel line to the mains.

One caveat, though....with the weight outboard, spin avoidance would have to be a must, IMO. The little Grummans are placarded against spins for this reason. The fuel is carried in their round tubing spars and the fear of not being able to recover from spins as all the weight would tend to aggravate recovery.

Regards,
 
The reason that HW don't yet have tanks for the -9 is not because of the tube diameter, but because of the tip shape. Because of the different airfoil on the -9, the tip is shaped quite differently. This is where the "fill" box and filler is installed.
 
The moment is big...

Not having built a 9, but having studied it for years, I'd suspect that the lower g (load) rating of the 9 wing combined with the longer arm of outboard wing tanks, eats more into the 9's wing safety margin compared to any other RV wing. Some people have done it and are happy with it. Its not for me. It would be interesting to compare V-speeds on a V-N chart with the two different wings.

I'm focusing on extending my range by some combination of: smaller engine, electronic ignition, prop optimized for cruise, and tight plenum.
 
Steve Brown said:
...One variant is to use one of the tank vent lines to feed fuel into the tank. The idea is to gravity feed into the vent line, then let the airplanes fuel pump do the work, pulling aux fuel back through the line to fill the vacuum created by spent fuel.

Any simpler or better ideas?
Click to expand...
It is just as easy to tie into the fuel line as the vent line and probably safer. IMHO

ergie63 said:
Not having built a 9, but having studied it for years, I'd suspect that the lower g (load) rating of the 9 wing combined with the longer arm of outboard wing tanks, eats more into the 9's wing safety margin compared to any other RV wing...
Click to expand...
I don’t believe this is true, someone correct me if I’m wrong. The weight is carried by the wings and not the fuselage, thus it does NOT compromise the G loading for the same reason fuel is not counted in the acro weights, if carried in the wings. However, the tanks and extra fuel will count towards your GW and will move the CG forward, when full.
 
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N941WR said:
It is just as easy to tie into the fuel line as the vent line and probably safer. IMHO

I don’t believe this is true, someone correct me if I’m wrong. The weight is carried by the wings and not the fuselage, thus it does compromise the G loading for the same reason fuel is not counted in the acro weights, if carried in the wings. However, the tanks and extra fuel will count towards your GW and will move the CG forward, when full.
Click to expand...
By this same observation, extra fuel carried in the fuselage would be more of a factor.
Of course as long as you stay within GW & CG limits, you should be OK.
 
Its true that the wings create all of the lift needed for flight and that g-loading is normally thought of as the amount of work the wings have to do in order to heft the fuselage around. The difference between what the flight surfaces want to do (turn) and what the fuselage wants to do (straight line) is revealed by how much the wings bend. This is a fine visual model for flight and wingtip tanks would not be much of a factor. But I don't think that this is the correct visual model for the occurence of disturbances at Vne, Vc or even Va.

There is always a tradeoff somewhere. Putting mass on the wingtip will affect the wing's natural frequency. It likely reduces the frequency and increases the amplitude caused by a given disturbance. The wing ends up bending less often, but more severely each time. If true, then V-speeds must be reduced in order to maintain a given margin of safety.
 
Minimally invasive

I'm definitely planning to put the fuel in the baggage compartment. The only question is how I'm hooking up to the fuel system.

The trip that I need the range for is PAO-to-APA. We've already staged clothes and gear at that end so we only need to carry a few pounds with us.
 
Fuel system design is very exacting.

I would never approve a fuel system that uses vent lines to provide for fuel transfer. Fuel vent lines need to remain open for the purpose of venting as designed.
 
I have not put much thought into this, however. How about a fuel line connected permanently to the spare port on the Vans fuel selector valve and run the line back to the baggage area where it can be connected to the removable?? aux bladder tank as needed. Normal operation would be to climb to cruise height on main tanks, then select and drain aux tank first so you could always go back to mains if there is some problem with the feed??

Fin
9A Flying
 
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Steve,

I also wanted to have the option to put aux fuel in the baggage area on my 9a. We used the extra port on the stock Vans fuel selector valve (the one you plug with a machine plug) and routed an aluminum line from it back to the baggage area along and under the center elevator tube-adeled to the side wall. It terminates in a bulkhead fitting beneath the baggage floor on the left side and is mounted in one of the floor stiffeners. I have an access panel in the baggage floor and can remove the cap and hook it up with minimal fuss--you do need to also allow for a vent to the outside.

On my fuel selector valve I simply turned it so that left tank selection is at 9 o'clock, right at 3, off at 12, and aux at 6.

BTW, I have not fitted an aux tank as yet, as the range on my 9 is greater than I expected--even with the 0360 (I normally run 66% power/8.5 gph). I do however, like having the option of additional fuel.

Hope this helps.

Cheers,

db
 
Anyone know what the story is as regards the need for a vent with these bladder type tanks. Theoretically the pump could suck the fuel out and it would just collapse on itself until it was empty?? Presumably some type of vent/valve would be needed to allow for air pressure changes during climb if the bladder was only partially filled??

Fin
 
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For all the time you will spend trying to kludge something, you will be better off time and cost-wise by making one stop ...such as at MLF.
 
Finley Atherton said:
Anyone know what the story is as regards the need for a vent with these bladder type tanks. Theoretically the pump could suck the fuel out and it would just collapse on itself until it was empty?? Presumably some type of vent/valve would be needed to allow for air pressure changes during climb if the bladder was only partially filled??

Fin
Click to expand...
No vent is needed for a bladder as they collapse as you drain fuel, as you stated. When climbing with a partially filled badder, they don't have air in them, or enough air, to worry about venting or expanding.

Ron Lee said:
For all the time you will spend trying to kludge something, you will be better off time and cost-wise by making one stop ...such as at MLF.
Click to expand...
Best advice yet!
 
Extra port very interesting

Since I didn't build the airplane, I didn't know there was one. I need to look into that. Sounds like a possible minimally invasive solution. I'm guessing the plugged hole is the off position, so I would still need a valve in the aux line in case of emergency.

Mel,
In the scheme I suggested, the vent would definitely remain open all the time. While the aux tank was emptying, gravity would fill the lower part of the vent line with fuel where it would be sucked into the tank, but the original vent path would remain intact. If the vent demand ever exceeded the fuel flow from the aux tank, air would be pulled directly though as in usual operation.
Let me know if this alleviates your concern or not.
If I was sure this was a good idea I wouldn't have made my original post.
 
Survival consideration

You are likely to go across terrain that is inhospitable, rugged, far from civilization and with no cellphone coverage.

I would prefer survival gear to extra fuel in the baggage area.
 
Not sure how this will work

db1yg said:
Steve,

I also wanted to have the option to put aux fuel in the baggage area on my 9a. We used the extra port on the stock Vans fuel selector valve (the one you plug with a machine plug) and routed an aluminum line from it back to the baggage area along and under the center elevator tube-adeled to the side wall. It terminates in a bulkhead fitting beneath the baggage floor on the left side and is mounted in one of the floor stiffeners. I have an access panel in the baggage floor and can remove the cap and hook it up with minimal fuss--you do need to also allow for a vent to the outside.

On my fuel selector valve I simply turned it so that left tank selection is at 9 o'clock, right at 3, off at 12, and aux at 6.

BTW, I have not fitted an aux tank as yet, as the range on my 9 is greater than I expected--even with the 0360 (I normally run 66% power/8.5 gph). I do however, like having the option of additional fuel.

Hope this helps.

Cheers,

db
Click to expand...

I use the 4th port to turn the valve into an on/off valve..I'm not sure how the 4th port is connected to the tanks ports...Like if it is pointed to the left tank and you pump from your aux tank...Does it fill the left tank?

It may, i just don't know for sure.

Frank
 
Why?

asav8tor said:
From a safety angle only:

Don't put fuel in the fuse.

Don't tie into the vent line.
Click to expand...

No one seems to like my "fuel in vent line" solution. Seems to be touching a nerve or something. I feel a bit rude asking for advice, and then asking "why" when I get it, but I'm trying to learn more about fuel system issues before I mess with mine in any way whatsoever.

My perception is that of all the engine out situations I have read about, the majority seem fuel system related. Thus seeking nuggets of information from the VAF collective, plus the desire to modify the existing system in a way that upsets the airplane's original design considerations as little as possible.

I got the idea because my vent line puts fuel on the tarmac every time I fill up on a hot day. Once its done with that, there is still fuel in the vent line well above the tank. If I take off the filler cap, that fuel above the tank in the vent line pours out the filler hole. So, if I don't take of the cap, that fuel gets pulled back into the tank as it is drained.

My conclusion was that fuel in the vent line is a normal situation, plus that the filler cap is able to handle the pressure created by the vertical without leaking or popping off.

The thought line evolution started with putting a reservoir in the upside of the vent line vertical so that I lost no fuel due to expansion. Basically just a high volume vent line that stored the expanded fuel until there was room in the tank.

Then I thought, why not enlarge the reservoir even farther and put a filler cap on it? From there is was obvious that gravity permitted putting it in the baggage compartment.

Anyway, on the vent line "why" question. Having raised teenagers I'm well aware that the inability to articulate the reason why something should not be done doesn't mean that doing it is a good idea. I just want to understand as much as possible.

FYI: The builder messaged me back channel indicating that he put removable plugs at the outer side of the tanks to allow for aux fuel connection. That gives me a third option of feeding from the baggage compartment via gravity or pump to get the aux fuel into the system. The wing weight due to the lines and fuel in them would be minimal and not centered near the wing tips, which is something I'm concerned about (see below). Also, this does not interrupt the fuel flow path of the original design in any way, which is my other big concern.

*****Fuel in fuselage .
Fuel in the fuselage is an ongoing debate. One certified aircraft has it in the seats., others have it in the fuselage. Personally I prefer it in the wings and if there was a well proven system for the 9 that put it there I would probably use it. The 9 has unknown spin recovery qualities and I don't want to exacerbate that but putting weight in the wingtips. My gross weight & CG can handle fuel in the baggage compartment. I plan to burn that fuel first during climb out on long trips. The risk will be higher during that time, but the time will be relatively short. How high that risk is and how it compares to the risks associated with an additional landing and takeoff for refuel could be the subject of a long debate.
 
From a safety angle only:

Don't put fuel in the fuse.
Click to expand...

Then I guess you would not be a candidate for an RV-12! (But, that said, I agree with you, though it is being done successfully.)
 
Frank,

The aux line does not go to, nor refill either, wing tank. When I install an aux tank in the baggage compartment I will simply uncap the aux feed line and attach the pickup line from the aux tank to this dedicated line. Selecting aux (not left or right) at the fuel valve draws directly from the aux tank--it does not involve flowing through either wing tank.

Hope this helps!

Cheers,

db
 
Why

Steve,

Let me try to explain why the vents are only for venting.

In the RV's the fuel vent line is designed to pressurize the fuel tank. This is done by the vent exiting the fuselage and pointing forward under the fuselage, just behind the firewall.

As fuel drains from a wing tank, it has to be replaced by air. This is true for our tanks because they are rigid. Should the vent become blocked for some reason, one of two things, or both could happen. The fuel will stop flowing to the engine and it will stop running is one option. The other is the tank will start to collapses due to the negative pressure. Bladders don't have to be vented because they are designed to collapse as fuel is used.

Let's run this scenario.
Aux tank to the right tank's vent line via a simple "T" fitting with a shutoff valve for the aux tank.

You fly along on the right tank, draining it of enough fuel to accept the volume stored in your aux tank.

While the engine is running off the left tank you open the shutoff valve on the aux tank which is tied into the vent on the right tank.

If you have no fuel pump on your aux tank, nothing will happen because the positive pressure on the vent line will pressurize both the aux and right tank keeping the fuel right where it is.

Should you put an electric transfer pump on our aux tank and turn it on, there is now no place for the air in the wing tank to go as the fuel level rises. Chances are you would start pumping fuel out the vent and into the air.

If fuel where to get to the tank there is still no way for it to displace the air, thus you may expand the tank until something starts to leak.

I hope that helps.

If you feel the need to increase the fuel capacity of your -9, "T" into the fuel line and not the vent. That or add another fitting on the tank as mentioned earlier.
 
Ferry flights

Jon Johanson has successfully ferried several RVs from the US to Australia. That's a long long haul across water (particularly the leg to Hawaii). He does it with a collapsible bladder tank directly connected to the normally plugged third port in a standard Vans fuel selector. Very simple and obviously works very well.

I plan on doing the same but will use an Andair FS20X5 selector which has LEFT, RIGHT, and a third AUX port (which faces forward and will normally be plugged in my case when not being used for ferry purposes).

There's a lot to be said for fuel systems that have a proven track record. It's not an area to get too extrapolative and "clever". Sometimes untested systems eventually fail in modes that defy prior prediction. For instance builders have had a lot of quirky problems with tip tank set-ups in RVs. It all seems simple...but it's not!!!
 
Thanks All

N941WR said:
Steve,
.............................................
In the RV's the fuel vent line is designed to pressurize the fuel tank. This is done by the vent exiting the fuselage and pointing forward under the fuselage, just behind the firewall.
.........................................
Click to expand...

The vent feed idea would be gravity only. When opening the valve, fuel would run into the lower portion of the vent line due to gravity. The vacuum created by that tank being emptied would pull this fuel into the tank, the same way it pulls from from there on a hot day now.

Running sitting still on the ground, I'm sure that would work.

The thing you pointed out that I hadn't considered is the pressurization of the tank via the vent line. Minimally it has to be at outside pressure to allow the tank to empty, but as you pointed out the hole is forward facing, which probably creates some positive pressure.

I'm guessing that at cruise this could easily be enough pressure to overcome the gravity pressure, preventing the aux tank from emptying into the vent at all, or at best allowing it to gurgle in unreliably.

So, this thread has been quite helpful for me. The vent feed is DOA, but I've got two new options which are:
-Pump feeding the aux into the tank via convenient hole provided by the builder
-Gravity feed into the "extra" port of the fuel valve and let the engine pump do the work.

The main problems I see with the "extra" port:
-Losing the "off" position unless I add another in line valve - fixable
-Using all the fuel in the aux tank will involve a moment of silence.
The POH for my first Mooney (M20E) recommended alternating tanks periodically (pretty standard), and ultimately running one completely dry to be sure you used all the fuel in that tank (unusual). Obviously that's the only way to have access to all your fuel, but I haven't read another POH that recommended it specifically. Except for the fact that my heart would probably stop for as long as the engine did, I guess there isn't anything wrong with that technique if you're up high. I haven't actually tried it though. Interested to know if others routinely do this.

I think the next step for me is to get out the screw gun and start looking around to see which method looks cleanest to do.
 
Aux tanks

I got very spoiled having a normal 100 gallon capacity wing tanks in my Mooney. It was a great feeling never to have to worry about having enough fuel for most domestic trips. Even with a planned trip of 5 or 6 hours, it there was ever a question of weather, it was never a problem to put in an extra few hours of fuel. You know what they say "The only time you worry about having too much fuel aboard, is when you're on fire"!!.

I did 12 Atlantic crossing in the Mooney, and all but 2 trips carried a ferry tank. I had carried, 100, 150, 160, and 210 gallons, depending on my destination and souls aboard.

I'm not recommending this method of plumbing a aux tank, but it was simple, effective, and easy to install and remove. If I were to build an aux tank for my 9A, this is how I would do it.

Since the tank would be in the baggage area, it will be higher than the wing, so it can gravity feed.
On the Mooney, right about where a rear passengers ankle would be on the right side, there's a panel that covers up plumbing that goes to the right wing tank. I made a small access door there so I could reach that area.
There are 2 lines in that area.
One is the fuel feed going from the fuel pickup, to the cabin fuel selector valve. The other line is the tank vent line. It was a simple matter to put a "T" fitting into these two lines.
The ferry tank had a outlet fitting at the bottom with a On/Off valve. A hose was connected from this valve to the fuel line "T". There was a hose fitting installed into the top of the filler neck on the ferry tank. This was connected to the vent line "T"
I would start flying on the right tank. When the tank had burned X amount of fuel, I would switch over to the left tank, and open the valve on the ferry tank, and carefully monitor the fuel gauge. When the tank appeared almost full, I would close the ferry tank valve, and switch back to the right tank. I'd do this until I'd burned off more fuel than I knew was left in the ferry tank. Then I'd switch back over to the left tank, and completely empty the ferry tank, knowing that there would not be enough fuel left to overflow the tank and go out the vent.
At times, when I knew there was enough fuel in the ferry tank as to not introduce air into the fuel line, I would run the engine and replenish the right tank at the same time.
This system just requires that you be diligent with the fuel gauge when the ferry tank is replenishing.
When you want to remove the aux tank, simply undo the lines, and cap the "T's".
Might sound complicated, but it's very simple, and it did give me something else to do on those long flights.
Installing the above mentioned "T's" would be very easy in the RV.
Just how I would do it.
Happy plumbing
Jack
RV9A
6 hours left on phase 1 !!!
 
Steve Brown said:
. The vent feed is DOA, but I've got two new options which are:
-Pump feeding the aux into the tank via convenient hole provided by the builder
-Gravity feed into the "extra" port of the fuel valve and let the engine pump do the work.

The main problems I see with the "extra" port:
-Losing the "off" position unless I add another in line valve - fixable
-Using all the fuel in the aux tank will involve a moment of silence.
Click to expand...

A couple of points on the above comments.

1. If you connect from the ferry tank directly to the third Vans port on the fuel selector it is not a "gravity feed" system per se. The fuel will be pumped uphill from the ferry tank (as it is from the wing tanks) by the mechanical fuel pump. No difference. Whether the actual fuel level in the ferry tank is above or below the mechanical pump at any particular time is irrelevent to safe operation.

2. You don't lose the "off" position on the Vans selector. The "off" position on the Vans selector is a fourth position.

3. You don't have to run the ferry tank dry and have the moment of silence. You can manage your ferry tank the same as your wing tanks...there's no need to run any of them dry. The only time I'd run any of them dry is if the situation was becoming serious. I've installed a fuel flow transducer for optimized fuel management.
 
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We can kill off two threads:
"Extra Fuel for ......"
"Why pilots won't fly with ...."

with a single photo. Lots of extra fuel, designed to withstand 6 G, and no one will ask to fly with you:

normal_new%2520cockpit.jpg
 
A reason not to run tanks dry

Engine failure Bold Face

Push-70 mph min
Boost pump-on
Fuel-select opposite tank
Alt air-pull
Mags-check


What good is the above engine failure procedure if you have no fuel in the opposite tank?

Keeping fuel in a known feeding tank gives you options you don't have if you run it dry. Kind of nice to have options.
 
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