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Author Topic: Pinto / Big Mig 061 fuel tank  (Read 1883 times)

Offline Dave Moritz

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Pinto / Big Mig 061 fuel tank
« on: July 10, 2023, 03:43:11 PM »
I need to break out the soldering iron and build a tank for this bird. Any suggestions on what works best? Any to avoid? My choices:
 
1. Standard stunt w/ three pipes
2. Standard stunt w/ muffler pressure
3. Uniflow w/ prop wash pressure
4. Uniflow w/ muffler pressure

My first guess is that it doesn’t make much difference. Straighten me out here if I be wrong.

Dave Mo…
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It’s a very strange world we live in, Master Jack.” (4 Jacks and a Jill)

Offline 944_Jim

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Re: Pinto / Big Mig 061 fuel tank
« Reply #1 on: July 10, 2023, 03:51:51 PM »
I only offer the following as another unlisted variation...I had a twin .074 Big Mig powered DH Mosquito (Black Hawk Models profile fuselage, slab wings). I built single feed tube tanks with the single feed tubes ending in the front of the tanks, and the pickups at the rear. It was a pain to fill the tanks with the plane nose down so the tank vented through the spray bars as I filled both tanks. Then the supply lines needed to be pinched off to keep from gravity feeding non-running engines. The vents were plugged to the muffler nipples.
Except for the refueling inconvenience, the engines/tanks worked well enough. I would do it again. If you are careful with plumbing, you could build the vent tube to be in the airflow ala Uniflow, yet also where you could sneak muffler pressure on the same vent. Then you could compare naturally-vented vs muffler pressure.
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Offline Dave Hull

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Re: Pinto / Big Mig 061 fuel tank
« Reply #2 on: July 10, 2023, 05:12:43 PM »
Curly Moe,

If you have the Hobby Fasteners Pinto plans, they show a tank designed for the plane. It is an "Aldrich style" pinched-back tank, but with only two lines. It is the max size that will fit between F-1 and F-2 and should hold about 1.4 flowing ozzes.

As shown, it is standard vented, not uniflow. It could be used with or without pressure. As a two line setup (only two tubes soldered in) you fill it like Porsche Jim suggested: pick up the plane by the tail, turn the prop to close the crank port, let the nose hang down then push fuel up thru the vent line with your syringe until you see fuel dripping from the vespucci. Set the plane back down on the ground and remove your syringe hose from the vent. You are now fueled and ready to get flippy. As drawn, the vent line goes to the upper forward inside corner, so it is no more likely to siphon when on the ground than any other tank.

A flying buddy of mine has one with this setup and it works. I think his has a Medallion running suction. I made him another stock tank for a second Pinto kit he's got...so I think he likes it.

If you are planning on building a taller tank because you think your engine will need it, the hydraulic geometry is no longer so favorable, and I'd be thinking seriously about a uniflow tank running pressure. The Medallions have decent draw and shouldn't need it. I have a standard vent wedge tank on a Baby Clown feeding a Medallion and the run is nice and even. I don't know if the Big Mig is going to be as robust or if a tall/offset tank will cause it to get finicky on you.

The Divot

---edited to spell an important guy's name right...and while I was at it, I added a picture....
« Last Edit: July 11, 2023, 01:53:29 PM by Dave Hull »
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Offline 944_Jim

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Re: Pinto / Big Mig 061 fuel tank
« Reply #3 on: July 10, 2023, 08:31:38 PM »
Mr. Dave,
Consider me subscribed to this thread...I have a Pinto in the box for some day. The other Mr. Dave's comments make me want to pull the instructions and review the tank notes.

Good luck with the build!
« Last Edit: July 12, 2023, 09:02:28 PM by 944_Jim »
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Offline Motorman

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Re: Pinto / Big Mig 061 fuel tank
« Reply #4 on: July 10, 2023, 09:12:40 PM »
Definitely uniflow without pressure. I forget the size but you want to make it pretty big. (20%)High nitro burns fast.

MM
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Offline Dave Moritz

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Re: Pinto / Big Mig 061 fuel tank
« Reply #5 on: July 12, 2023, 02:17:00 PM »
Good gents all:

I'm glad the Divot provides some backdoor experience with the tank shown on the Pinto plans. At this point, I'm leaning towards that approach as the simplicity has some appeal and I don't have any good reason for doing something else. I can probably manage the fancy ground-based fueling maneuvers without crashing!

Thanks for the discussion.

Dave moe
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Offline Dave Moritz

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Re: Pinto / Big Mig 061 fuel tank
« Reply #6 on: July 13, 2023, 07:53:55 AM »
Jim

Building the Pinto has been both a challenge and fun. My experience:

1.  The directions that came with my kit were poor, so I had to think through the building sequence very carefully.
2. I used threaded control rods with clevis ends. Bending music wire with enough accuracy would be impossible for me.
3. Made my own control horns using flattened brass tube, music wire and solder.
4. A “go-slow” approach to building this works best.

Dave Mo…
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It’s a very strange world we live in, Master Jack.” (4 Jacks and a Jill)

Offline Dave Hull

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Re: Pinto / Big Mig 061 fuel tank
« Reply #7 on: July 13, 2023, 04:21:11 PM »
Moe,

You got instructions with your Pinto kit???  Deeeee-Luxe!

I use threaded clevises (clevi?) all the time with 1/2As. Haven't had one fail. I prefer the Sullivan gold ones. I also put a jam nut on to snug up against the shank of the clevis to avoid working the threads and things loosening up.

If you study a bend diagram for forming metal (applies to wire, too), and get your head wrapped around the concept of "setback," you can bend pushrods quite accurately--most of the time--and eliminate a source of potential failure. I like to use a solid flap pushrod for that reason, but still use something adjustable for the elevator pushrod for trimming.

What kind of solder did you use on your custom horns?

I guess I kind of have to differ with you on the "go slow" approach to building. I have to go as fast as I can. That cuts my build time down to only 3-4 years per model. I need to get it flying before either the balsa petrifies or the covering gets brittle. It's a given that the fuel tank will be rusty before I can get it to the field....

The Divot
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Offline Dave Moritz

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Re: Pinto / Big Mig 061 fuel tank
« Reply #8 on: July 15, 2023, 12:41:24 PM »
El Maximo Divot

Good stuff you’re offering here, some of which can still go into the pending Pinto. As for the solder, I used lead-free plumbing solder containing silver. It melts at a higher temperature than the stuff for electronics.

You might be a slow builder, but at least you didn’t use neutron decay as a time scale.

Dave Mo…
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It’s a very strange world we live in, Master Jack.” (4 Jacks and a Jill)

Offline Dave Hull

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Re: Pinto / Big Mig 061 fuel tank
« Reply #9 on: July 16, 2023, 12:45:46 AM »
Moe,

I did some testing on homemade 1/2A wire joiner-type horns. Different solders, different techniques. Data sheets for different solders.  If you are interested, lemme know.  I had Oatie Safe-Flo Silver Bearing Wire Solder (Sn-Cu-tin-Ag) in my table, but never found a data sheet with shear strength values. Perhaps not surprising for a plumbing solder used in joints with a ton of surface area in the lap joint and all they want beyond that is something that won't leak....

Just in general, the strength of "silver bearing" solder is roughly twice as high as electronic solders, which are very weak. (Harris Stay-Brite #8 with 5.5%Ag vs. Kester "44".) There is a slight advantage if you are doing sequential assembly, where you can do one area with one type, and then using a lower liquidus solder after that and with care, not cause the first area(s) to come undone. If you want the strength of the solder to go up, use "silver brazing" material. That is going to take a torch, not an iron, but is much stronger and not any harder to do.

A critical factor is the joint clearance. You want maybe .0015" gap for the solder to fill. As the gap gets larger, the joint strength goes down.

The easiest way to building in some strength margin using soft solders is to add surface area to the joint. Formost brass washers help and they have precision diameter fits.

The horn in the picture failed at a sustained static load of 11-12 lb. I don't recall the arm length.

I get all confused about particle physics. All I know is that I decay at the speed of light. With or without neutrinos....

The Divot
« Last Edit: July 16, 2023, 01:20:22 AM by Dave Hull »
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Offline Dave Moritz

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Re: Pinto / Big Mig 061 fuel tank
« Reply #10 on: August 20, 2023, 09:41:01 PM »
Maestro Divot

In again reviewing your advice above, it dawned on me that there is a nugget buried in the fine verbiage. That would be your suggestion concerning the silver brazing material with a torch. As long as the torch would be propane, I’d be good to go. Will look into it.

A thousand and one thanks!

Dave Moe…
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It’s a very strange world we live in, Master Jack.” (4 Jacks and a Jill)

Offline Dave Hull

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Re: Pinto / Big Mig 061 fuel tank
« Reply #11 on: August 21, 2023, 01:52:18 AM »
Curly Moe,

Here's the last one I messed with. It was supplied in an older ARC "kit." The brazing was incomplete, with only about half of the diameter of the joiner attached to the arm on both the flap and elevator horns. And, the arm was twisted, suggesting that the hole was oversize which would also reduce the strength of the joint. Seemed easier to fix it now rather than after the plane crashed....

I scuffed up the area to get it clean using red Scotchbrite, then fired up a little BernzOmatic propane torch. Fluxed the area thoroughly with Stay-Clean flux and added a bit of true silver braze material. I had some pretty thick stuff, so it was a little harder to make a neat job but it can be done. The goal was no gaps and smooth fillets.

I might solder a 1/2A horn with soft solder, but no way on anything bigger. Gotta go with the brazing alloys....

Good luck with your project!

The Divot
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