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Aerobatic Fuel tank

NorthernRV4

Well Known Member
Good day fellow RV'ers

This winter I've been building the starboard wing and I'm at the point of putting the finishing touches on the tank. Firstly, this tank is getting a flop tube and the standard Van's resistive fuel senders. I'm looking for some tips & advice specifically on mounting the sender in the second rib bay and the anti-hang up guide strap. Van's plans are pretty simplistic and don't show at all any dimensions for mounting the sender.

1. For the sender, do you mount it in a cover plate similar to the end rib to allow for greater access and ease the inserting of the sender/float arm? Or is a simple hole for the sender sufficient?

2. At what vertical position would the sender be mounted in the rear baffle? I would think roughly the same position as mounting it in the end rib would put it.

3. Do you bend the rod to the same dimensions as if it was mounted in the rib? I'd rather not have the tank all sealed up only to find the fuel gauge is way off. I expect some inaccuracy but I'd like it to be as close as possible.

and lastly, in regards to the strap. Did you just fabricate a guide that looks like the diagram provided in the plans? Any specifics here to consider?

Thanks in advance, keep calm and build on!

Starboard fuel tank by James Soutar, on Flickr

Starboard fuel tank by James Soutar, on Flickr
 
many questions here, and a long time since I built the tanks... last century...
You might be able to access my build, pictures on https://drive.google.com/drive/folders/1-LIBd7KXJZhC-BVw-9BjnALbl5jqrp3x
Pictures #128 onwards detail the tank building (don't use any sloshing compound, not necessary. It is still holding good after all these years, but only thanks to the scuffing I did). Installed the inverted fuel on both sides.

- the hole for the sender was big enough for the float, just a hole did it
- sender was mounted mid-way up, guarantees a linear motion of the float arm
- I remember hooking up the gage to the sender and did a dry calibration, don't remember about the rod though
- yes, the guide was fabricated, main consideration is for the flop tube not to tangle itself in the flap doors (which I guess you will install?)
- don't forget the o-ring on the brass head

Hope that helps, enjoy the build
 
Thanks Dan, your pictures are very helpful. Interestingly I still put the access plate in the end rib in case I ever decide to ditch the flop tube I can go back to a standard pickup. Everyone has their own unique style to their build I guess.

Yes, I still need to fabricate the trap door but I noted from your pictures that you also put a trap door on the middle fuel transfer hole. I hadn't considered that, I'll have to have another look at my plans but it makes sense.

NO slosh! Ever, nope! way too many bad stories with that stuff.

What do you mean about the o-ring on the brass end? My prefab'd flop tube from Vans has one installed already. I assume that's simply to act as a cushion.

How many years has your a/c been flying? Have you ever changed out the Flop tube hose? One thing I realized recently was that if I ever run MOGAS I'm wondering about the compatibility with dreaded ethanol. I'll have to ask around about that one.
 
I made something similar to what was in the plans. I don't think it really matters as long as the anti-hang guide keeps the flop tube from contacting the rib.

 
hadn?t notice the presence of the o-ring, so u all good Jim.

The -4 was in storage for a number of years and has only been flying for like 2 years, AVGAS only, no feedback on the hose.
 
Ever thought of doing 2 normal fuel pickups with the 3rd being a flop tube?

That way you can have the maximum amount of usable fuel for normal flight.
 
Ever thought of doing 2 normal fuel pickups with the 3rd being a flop tube?

That way you can have the maximum amount of usable fuel for normal flight.

For a gravity fed system, fine. For a system utilizing a pump, not so much. A "vent" (e.g. uncovered fuel pickup) would, vapor lock the fuel system/starve the engine. If this weren't the correct physics, you would ditch the flop tube and just have high and low fuel pick-ups tee'd together. You could get creative with valving applications or a third pump but it's not worth it. IMO, the OP should accept whichever compromise he prefers; range versus inverted flight. Fly safe
 
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I was never considering using the flop tube and the standard pickup at the same time. Just have the option of converting if I find I don't like the flop tube.

Thanks gents, I don't know how much time I'll spend inverted. I just want the option, it's more about future proofing to build one tank this way.
 
I'm curious about the statement reference usable fuel in the aerobic tank in an RV4. My tanks are built to plans and all 16 gallons are usable.

Joel
 
For a gravity fed system, fine. For a system utilizing a pump, not so much. A "vent" (e.g. uncovered fuel pickup) would, vapor lock the fuel system/starve the engine.

Only if the flop tube and normal pickup are T'd together

With a L Aux R fuel valve you wouldn't have that problem
 
I'm curious about the statement reference usable fuel in the aerobic tank in an RV4. My tanks are built to plans and all 16 gallons are usable.

Joel

Complete brain fart on my behalf. I had senders on my mind from the OPs original questions. Was taking a break from this "work at home" ****. Sorry. My bad.

BTW. Where in VanAire, CO?
 
Ok. Understand.
Both fuel QUANTITY indicators will show zero when tanks are zero, but aerobatic tank will not show more than 13 gallons, when full, on my build.
VanAire is 15 miles north of DIA.
 
spent way more time than I expected fiddling with the trap door but I'm pleased with how it turned out. I'm left wondering about the hole in the center of the rib. Should I cover it over or put a door on it too? I don't quite understand why the plans call out for this hole if the hole in the bottom rear corner of the ribs allows for fuel transfer.

Starboard fuel tank rib by James Soutar, on Flickr

Starboard fuel tank rib by James Soutar, on Flickr
 
I needa...

Good day fellow RV'ers

This winter I've been building the starboard wing and I'm at the point of putting the finishing touches on the tank. Firstly, this tank is getting a flop tube and the standard Van's resistive fuel senders. I'm looking for some tips & advice specifically on mounting the sender in the second rib bay and the anti-hang up guide strap. Van's plans are pretty simplistic and don't show at all any dimensions for mounting the sender.

1. For the sender, do you mount it in a cover plate similar to the end rib to allow for greater access and ease the inserting of the sender/float arm? Or is a simple hole for the sender sufficient?

2. At what vertical position would the sender be mounted in the rear baffle? I would think roughly the same position as mounting it in the end rib would put it.

3. Do you bend the rod to the same dimensions as if it was mounted in the rib? I'd rather not have the tank all sealed up only to find the fuel gauge is way off. I expect some inaccuracy but I'd like it to be as close as possible.

and lastly, in regards to the strap. Did you just fabricate a guide that looks like the diagram provided in the plans? Any specifics here to consider?

Thanks in advance, keep calm and build on!

Starboard fuel tank by James Soutar, on Flickr

Starboard fuel tank by James Soutar, on Flickr

Hey Jim,
I built my Four BE-FORE the internet, VAF, heavy weldments, long gear and yes, flop tubes. Over the past 2000 hours of ownership by two(major league ADD) former Fighter pilots, the need for sustained inverted never materialized.
Positive G Acro, 1V1's and lots of yanking a banking never (OK maybe once :)) exceeded the fuel cavitation abilities of a Carb and mechanical pump.
So, don't burn too many brain cells on inverted fuel/oil or associated systems if you don't have to...

Your bigger concern over time will be leaks, trust me.
:)
V/R
Smokey
 
What do you mean about the o-ring on the brass end? My prefab'd flop tube from Vans has one installed already. I assume that's simply to act as a cushion.

I believe there is an SB/letter/some sort of recommendation to use some proseal to glue that O-ring in place, otherwise it has been known to go astray.
 
Thanks Smokey, I'm not sure how much I'll need the inverted option. Just nice to have for the future potential and hey, there is a certain cache' to be able to say "because I was inverted" ;-) Seriously I wanted the experience of building one tank this way as much as anything.

Last night I bent up the float arm as per the Van's drawing and located it in the rear baffle. I put it in the inboard rib so I could check the position of the low fuel condition and found the float was 1/4" off the bottom tank skin so I used this as the baseline for the vertical position. For my setup this proved to be 4" from the bottom of the rear baffle lower flange. I located it as close to the 2nd rib as possible while keeping the float clear of the rib so it would be as accurate at the low fuel position as possible. I fabricated a reinforcing ring to be mounted in the rear baffle. While the plans don't show it, I wanted to keep that area as stiff as possible for mounting the sender to prevent leaks.

For the trap door(s) in rib #2 I think I"ll rivet a cover plate over the center fuel transfer hole. I think the lower aft hole is plenty to transfer fuel and in this case, I can't see the benefit of the complexity of a second trap door.

RH fuel sender by James Soutar, on Flickr
 
Reality check.. you absolutely will install the (complicated, heavy, and expensive) inverted fuel and oil systems on the engine, right?

My experience is the same as Smokey's - I installed this flop tube thinking I would use it some day, but after over 1000 hours that need has never materialized. I ended up taking it out and putting in a standard aluminum pickup tube.
 
Yeah, I know I may never spend much time inverted, or I might. The point is, I won't have to worry about the tank later on in the build. I do also think, some future buyer might appreciate the option too.

In the end, it wasn't that much harder to build the tank this way. Just a few unknowns I had to solve for as I went. Overall, I'm happy with the results.
 
While looking for info on the subject I think I found an answer which is good enough for me. Thanks Chad.

"Tonight I was able to work on the trap doors.. It's little things like this that take a long time and you never realize. These guys as I said earlier keep the fuel in the inner most fuel bay in the wing where the fuel pick ups are, so that when you are turning (especially on final) the fuel doesn't run to the outside of the low wing and... ut oh..."
 
Access plate mount

I’m starting my fuel tanks. For the flop tube install, is there any problem with mounting the fuel sender onto the supplied access plate and installing that plate onto the rear baffle in the second bay? No reason to cut an access hole on the inboard rib, correct? Thanks!
 
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For the trap door(s) in rib #2 I think I"ll rivet a cover plate over the center fuel transfer hole. I think the lower aft hole is plenty to transfer fuel and in this case, I can't see the benefit of the complexity of a second trap door.
Perhaps little value in a second trap door, but there is still value in having that hole. The trap door prevents fuel from running out rapidly when G-forces push fuel to the outside. The trap doors don't seal perfectly so you will get fuel flowing back out, just less rapidly.

However, if the door ever sticks, that's your only way for fuel to get into the inboard bay. The hole in the rib gives you a "backup" that will compensate for this. The flow outboard will be restricted by the size of the hole, but it will retain any fuel below the hole, regardless of whether you're right-side-up or upside-down.
 
Knife edge flying, I remember that was the reason I built the 2 trap doors in. Well, I was young at the time...

As for inverted flying, matter of taste, I quite like it, looking at the Planet upside down :)
On the-6 without any inverted system and a carb, the engine keeps running for like 4-5 seconds before quitting. CSP takes over, and the airspeed drops real fast. What ain’t fast is cleaning the spilled oil off the belly and tail... still dripping 2 weeks later :p
 
Being that the fuel transfer hole in the center of the rib is the same size as the lower one with the trap door I feel that any G's that would cause fuel to spill through would do so in either hole. I decided to cover over the middle hole with a plate. Even if the trap door were to become "stuck" in a closed position, it doesn't seal well, so fuel would still leak through to the inboard bay. I felt the trap door was really there to restrict the sloshing of fuel out of that bay and having the second hole open would negate the purpose of the door.

As for building it, yes it's fiddly and I spent way too much time on making the hinge pretty but I feel good knowing it's so nice :)

Aerobatic fuel tank by James Soutar, on Flickr
 
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