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A New, Easy, Bulletproof, Fuel Return-line Retrofit for RV?s.

Hartstoc

Well Known Member
[Otis, I edited the text format to eliminate the huge font in your original version; S.Buchanan]

Necessity really is the mother of invention. I've been pondering the challenge of retro- fitting fuel return lines to RVs and other aircraft for over two years. Those for my RV-7A need to be installed without removal of wings or tanks, and no access to the tank interior is possible without violating the difficult-to-access and heavily pro-sealed mid-cord sending unit flange . I’ll be posting a separate thread titled “Reasons you might need fuel return lines” soon.

I thought through several possible designs and I even prototyped a couple, but none were particularly satisfying. Then, about six months ago, Inspiration led to a design so elegant, easy, and reliable that It deserved protection, so I engaged an attorney and obtained US Provisional Patent #62/900,809. This new fitting can be used to add a fluid or gas line anywhere on any thin-walled tank with limited wall curvature, even where zero access to the tank interior is possible.

Right now I’m putting out feelers for a*production and marketing partner with CNC milling capabilities in order to offer this to the aircraft market (and the auto racing after-market?). Ironically, developing this idea has set back work on my own RV mods by about three months, but I hope that, in the end, others will benefit.

At this moment, working prototypes are at the anodizer(two for installation in my 7A and two as demo units), and I’ll soon be working on an installation video that I plan to share on VAF. Below are some preliminary photos. Note that these make use of some unfinished parts, so you will see some roughness in ring grooves and surface finish. A damaging fire at one of my properties has brought a pause to normal life, but I wanted to get the word out on this hopefully “coming soon” retrofit option. Before long I will update with more info, imagery of fully finished parts, and an installation video.

Features:
1- It is quick and easy. It can be installed in an RV in about one hour per side working within the space available between the wing and fuselage. The circular stiffener pressed into the forward root rib near the leading edge provides an ideal installation location.
2- Installation requires ZERO access to the tank interior, except through a small opening made entirely by working from outside the tank. The product kit will include an installation template that will be secured in place with aluminum tape to enable precise, easy cutting of this opening using a tiny carbide burr on a Dremel tool, and will include hole-locators for the two 3/32” retaining cap-screws. I’ve developed an installation strategy that keeps the tank interior absolutely clean during installation that will also be covered in the video.
3- The assembly includes rigid inner and outer flanges that sandwich the tank wall very firmly, preventing any significant strain upon it once installed.
4- It does not require the use of Pro Seal or any other type of sealants, relying instead upon Viton O-rings embedded in each face of the outer flange. It can be placed in service immediately.
5- It is permanent, yet can be easily removed for inspection and servicing in less than one minute if desired. (Plus one safety wire if desired but this is not required as it is intrinsically self-locking).
6- It promises extreme reliability and airworthiness.
7- *It is cosmetically beautiful, with no connectors other than the AN flair fitting visible from outside the tank(more a factor for high-wings with visible tank ends than for RVs).
8- Many, including me, might actually install it in new builds as well as retrofits and for feed lines as well as return lines, especially in aircraft with visible tank ends. Using one for feed-lines would provide for the very easy servicing of a finger screen on the pickup line inside the tank. Photos follow here:

This shows all of the parts, including a tank cutout mock-up. The working name is the “0tis elipt0flange” tm, patent pending #62/900,809:
2v2EjsgnoxBELK5.jpg

This shows the tank-contact faces of both flanges. Note small bores in the outer flange face where the cap screw heads reside. These screws secure the inner flange permanently to the tank inner wall and impart anti-rotation force to both outer and inner flanges during tightening or removal of the hollow bolt. After torquing, the screws serve to prevent any possible rotation of the entire assembly:
2v2EjsW9zxBELK5.jpg

The rigid inner flange is fully threaded and acts as very sturdy nut for the hollow bolt. Here it is shown stacked upon the outer flange. Note that the outer dimension of the inner flange exceeds that of the large elliptical O-ring groove that is cut into the outer flange. This prevents any distortion of the tank wall when torqued:
2v2Ejsgq2xBELK5.jpg

This shows the Inner flange being inserted through the trapezoidal hole in the tank mockup and placed in approximate position using one of the cap screws as a temporary handle. A 4’ piece of dental floss threaded through the central hole with both ends hanging outside tank would allow retrieval if dropped inside the tank by accident:
2v2EjsgdvxBELK5.jpg

A paint can opener is being used here to hold the inner flange against the inner tank wall as the temporary screw is removed then re-inserted in its permanent hole through the tank skin:
2v2Ejsg59xBELK5.jpg

The inner flange fully installed- there is never again a need to remove it:
https://media.fotki.com/2v2EjsgrMxBELK5.jpg
An O-ring groove is cut into the 6AN bulkhead fitting that serves as a hollow bolt, and some upper threads are also removed(the patent application provides for a custom-fabricated hollow bolt as well):
2v2Ejsg8oxBELK5.jpg

A few threads are cut into the outer flange bore to prevent the O-ring from extruding along shaft when torqued, but the flange spins freely on the fitting once screwed all the way on. This free-rotation becomes important during final assembly.
2v2EjsWovxBELK5.jpg

Both O-rings shown installed on the outer flange and the hollow bolt:
2v2EjsWvuxBELK5.jpg

The outer flange shown installed to finger-tight O-ring contact, after taking care that the cap screw heads have entered into their bores note the gaps that will be closed upon torquing the assembly:
2v2EjsWE3xBELK5.jpg

Finally, metal-to-metal contact is achieved as both O-rings are compressed during torquing of the bulkhead fitting onto the tank skin mockup. The unit is now fully operational and ready for installation of the return
2v2EjsGgCxBELK5.jpg
 
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I?ll be your first customer Otis. So, I guess the first two obvious questions...when will they be available and how much will they cost?
 
I?ll be your first customer Otis. So, I guess the first two obvious questions...when will they be available and how much will they cost?

Ha! Thanks Mark- This is a work in progress, so I?m afraid the answers on both counts are TBD! I put this out now hoping it might lead to connections with good and qualified RV/EAA friendly collaborators. I?m too busy a guy to have a lot of hands-on involvement in production or marketing, so we'll see what happens. Fabricating the prototypes was a real challenge on my old Bridgeport mill, but a good CNC shop should be able to turn these out as a pretty affordable product, at least by airplane-parts standards. Stay tuned- Otis
 
Thanks for the update Otis. Hey, if you happen to need an additional bata tester....you know where to find me. :)
 
Ha! Thanks Mark- This is a work in progress, so I?m afraid the answers on both counts are TBD! I put this out now hoping it might lead to connections with good and qualified RV/EAA friendly collaborators. I?m too busy a guy to have a lot of hands-on involvement in production or marketing, so we'll see what happens. Fabricating the prototypes was a real challenge on my old Bridgeport mill, but a good CNC shop should be able to turn these out as a pretty affordable product, at least by airplane-parts standards. Stay tuned- Otis


Otis,

Sent you a PM.
 
Consider other to make and market

This does look like an excellent solution to return line retrofit, or as noted service of the finger stainer.

If I were you I would consider partnering (i.e. license out your design for a small "royalty") to Aircraft Extras, Anti-Splat, JD Air Parts or other reputable 3rd party.
 
Stat-o-seal

Good job Otis! Awesome for blind access.

FWIW, I used a Stat-o-seal, $5; tho I had access to fish the inside nut thru the tank neck, and get a rachet wrench with extension on to the nut to hold it. Depending on how your edges are formed, a hole near enough to the tank edge could be used as a flat for said inside nut..... Doesnt take much to squeeze the seal.

https://www.speedwaymotors.com/Goodridge-SS902-07-Stat-O-Seal-Sealing-Washer-4-AN,34188.html

Dowty seals are a similar tech, tho typically used in a threaded hole application, similar to how an ORB fitting works.
 
This looks like a really good solution, but i'm wondering how you make the trapezoidal hole and the two holes on either side of it without dropping aluminum shavings into the fuel tank...
 
We'll have to wait for the video.

But I don't personally think aluminum dust from a carbide burr would be anything my fuel filter element (or sump drain) couldn't safely handle in the expected amounts.
 
What am I missing....

Why won't the AN thru-bolt back out?

Well, two reasons. First, the O-rings are substantially compressed, so act much like a spring washer to greatly increase breakout force required to loosen it. Second, the fuel line secured via the flair fitting would lock the assembly. If any concern remains, it would be very easy to drill the hex head and a meaty section of the outer flange for safety wiring it, so ultimately this will not present a problem.

Thanks for the question!- Otis
 
This does look like an excellent solution to return line retrofit, or as noted service of the finger stainer.

If I were you I would consider partnering (i.e. license out your design for a small "royalty") to Aircraft Extras, Anti-Splat, JD Air Parts or other reputable 3rd party.

Thanks for the kind words. My hope is to do exactly as you suggest, and I appreciate the list of candidates.- Otis
 
Good job Otis! Awesome for blind access.

FWIW, I used a Stat-o-seal, $5; tho I had access to fish the inside nut thru the tank neck, and get a rachet wrench with extension on to the nut to hold it. Depending on how your edges are formed, a hole near enough to the tank edge could be used as a flat for said inside nut..... Doesnt take much to squeeze the seal.

https://www.speedwaymotors.com/Goodridge-SS902-07-Stat-O-Seal-Sealing-Washer-4-AN,34188.html

Dowty seals are a similar tech, tho typically used in a threaded hole application, similar to how an ORB fitting works.

Thanks- I understand that Ross Farnham?s EFI customers have had good luck with the Stat-O-seal, bit access to the tank interior is required for this, and I?m also a bit concerned about its small area transmitting damaging vibratory stresses to the tank wall in some situations, possibly leading to a tank rupture. -Otis
 
This looks like a really good solution, but i'm wondering how you make the trapezoidal hole and the two holes on either side of it without dropping aluminum shavings into the fuel tank...

I?ll be following up on this in due course. I do have a good method that I will be sharing. Right now my time is very limited due to fire damage to one of my buildings. Really quite easy to do with a little care, though.- Otis
 

My earliest designs were along these lines, with a chopped ring inserted to reinforce the backside of the tank skin. In the end, the installation is much more difficult and involves a lot of messy pro-seal, cure time, etc. I like that my unit can easily be disassembled at any time, and that it traps the tank wall so securely. Certainly no flange should ever be attached to the external tank tall without appropriate reinforcement from the inside.- Otis
 
Well...

"...Certainly no flange should ever be attached to the external tank tall without appropriate reinforcement from the inside..."

Well, I guess we will see...

There are a bunch of ER tanks out there flying and I have yet to hear of anyone complaining about lack of "an appropriate reinforcement from the inside".

I do like your idea though...I would have been all in...four years ago. ;>}
 
But I don't personally think aluminum dust from a carbide burr would be anything my fuel filter element (or sump drain) couldn't safely handle in the expected amounts.

Grit in a sump drain is about the only problem i've ever had with my fuel system. I'd do anything to avoid having to swap out a sump drain from a nearly full tank just because it won't seal due to grit caught in the O-ring. Grit can build up at any fitting in the system, and eventually choke out the flow regardless of filters or gascolators.
 
Thanks- I understand that Ross Farnham?s EFI customers have had good luck with the Stat-O-seal, bit access to the tank interior is required for this, and I?m also a bit concerned about its small area transmitting damaging vibratory stresses to the tank wall in some situations, possibly leading to a tank rupture. -Otis

Many tens of thousands of flight hours collectively on hundreds of installations. No reported issues like this to date.
 
"...Certainly no flange should ever be attached to the external tank tall without appropriate reinforcement from the inside..."

Well, I guess we will see...

There are a bunch of ER tanks out there flying and I have yet to hear of anyone complaining about lack of "an appropriate reinforcement from the inside".

I do like your idea though...I would have been all in...four years ago. ;>}

Touché’- That was a statement of personal opinion-acknowledged. I like forces to be as widely distributed as possible when attaching a rigid body to a thin wall. Even the pro-seal contact may have a little give, allowing vibratory movement to be concentrated on the rivets protruding put the back. I’m not predicting a failure, though.- Otis
 
Many tens of thousands of flight hours collectively on hundreds of installations. No reported issues like this to date.

It Is hard to argue with a record like that. I would certainly understand anyone who has easily removable sending units to provide access going with that dirt cheap option.- Otis
 
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