Thorp Air Command - T18.net

Here is my installation of an Andair valve using their shaft extension. This makes it very easy to shut off the fuel in a normal sitting position. Previously I had a valve with no extension on the handle and if I would have needed to shut off the fuel in flight I would have then had to do an unusal attitude recovery.


The fitting on top of the valve is a male pipe thread to a pipe nipple to the tank, the bottom fitting is an 90 angle -6 AN flare to a -6 hose, I have more than 2 90 elbows in my gravity system. I did do a flow test with the nose at a climb angle and my minimum fuel level is 5 gallons for 150% full throttle fuel flow.

The purple disk you see on the shaft near the valve is an adapter that allows you to reposition the handle, if I had not used it the ON position would have been 90 degrees from where it is now. There is also a universal coupling on the valve to allow for misalignment of the knob mount.

This was after I had just installed new carpet and the instrument panel has not been re-installed.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

Well done Bruce ! The possibilities are endless . Now if you find a way to turn political bull squirt into fuel that would make me very happy !

RB
Rectus Bustus

My valve hangs from the tank also... it's an inexpensive teflon-seated brass ball valve. Very light & smooth operating, even after 26 years. I have a home-made extension. My belief is that the problems related to directly mounting the valve to the tank outlet that have been reported over the years stem from two issues. Number one is the use of the heavy conventional brass plug valves that were (are) used on many aircraft. They can get very stiff to operate, creating a lot of stress on the tank flange when opening or closing, and the weight contributes to the extra stress, especially if the fuel line isn't routed nice & short to the firewall penetration where is is supported. Light & simple is usually always good on an airplane. BTW, brass is used on fuel systems for many airplanes - the aforementioned brass plug valves are an example. Strength or brittleness is really not too much of a consideration, IMHO. Brass is stronger than the aluminum used to fabricate most valves & fittings, in tensile strength... the main issue is extra weight. As an aside, I believe that my little brass valve, which by the way is used by many experimental builders, is probably actually lighter than that nice Andair valve.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

While looking through some of my photos, I found these of John Evens' installation.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

It is too cold and windy to work outside long enough to accomplish anything useful so I am back to this project that wasn't suppose to be a project. I have seen a lot of information about what is "required" in a fuel system (shutoff valves, routing lines downhill, flow requirements...) but not a word on sumps until I stumbled across an article in a magazine concerning experimental aircraft. So I did a little digging in the regs:

Sec. 23.971
Fuel tank sump.
[(a) Each fuel tank must have a drainable sump with an effective capacity, in the normal
ground and flight attitudes, of 0.25 percent of the tank capacity, or 1/16 gallon, whichever
is greater.
(b) Each fuel tank must allow drainage of any hazardous quantity of water from any part
of the tank to its sump with the airplane in the normal ground attitude.
(c) Each reciprocating engine fuel system must have a sediment bowl or chamber that is
accessible for drainage; has a capacity of 1 ounce for every 20 gallons of fuel tank
capacity; and each fuel tank outlet is located so that, in the normal flight attitude, water
will drain from all parts of the tank except the sump to the sediment bowl or chamber.
(d) Each sump, sediment bowl, and sediment chamber drain required by paragraphs (a),
(b), and (c) of this section must comply with the drain provisions of Sec. 23.999(b)(1)
and (2).]

Is anyone using a fuel system in a Thorp or any other homebuilt that complies with this part of the regs? By my math, a sump for a 29 gallon tank would need to hold 9.28 ounces (29 gallons * 128 ounces * .0025 = "required" sump size in ounces). Strainers only comply with item (c). I know when I drained my tank through the carburetor connection there wasn't anything left in the tank and, according to this reg, there should have been more than enough to make a mess.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Jeff,

If I am reading it correctly, I would hazard a guess that very few actually meet all of these requirements. For instance, how do you reasonably accommodate the "required" sump in wing tanks whose outer surfaces are the wing skins? It would be difficult to get the minimum "1/16 gallon" capacity with many of the designs that are prevalent. The "sediment bowl or chamber" in paragraph (c) - a gascolator - is also missing from quite a few homebuilts that I have seen. I've frequently seen a simple non-drainable fuel filter in lieu of that... not saying that that is the best idea, but it is done. One way to get the "sump" in a T-18 tank would probably be with a short stand-pipe, along with another drain point/valve on the bottom of the tank to make it "drainable". It would reduce usable fuel a little.

The way the sump thing is done on the Kitfox I'm building is with a small drainable sump on the bottom of a header tank that is fed by the two wing tanks. Each wing tank has a very shallow "sump" with a drain valve, but I doubt that they meet the volume requirements of this regulation... I'm not sure.

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John Evens
Arvada, Colorado
T-18 N71JE (sold)
Kitfox 7 SS N27JE

Whoa !! All this fuzzy math is taking me back to the 70's ! I installed the Usher Gascolator #4 from Spruce on the firewall and all is well !

RB

Integral wing tanks would be easy if there is any dihedral at all...just look to what the aircraft manufacturers have done. The fuel pickup is located a little off the bottom of the skin. All water and debris collects at the inboard end below the outlet. A lot of fuel tanks and bladders are built that way too. No actual "sump", just an area that cannot drain into the fuel line with it's own drain.

The J-3 I get to fly from time to time has the fuel line outlet at the front of the tank and a Curtis drain at the back. When on the ground, water and debris collect at the drain.

I have been kicking around the idea of moving my strainer to a position below the tank and having an uphill run all the way to the carburetor. Even some production aircraft have the strainer in the cockpit. There isn't much space there so it may require short hoses with barbs and clamps to make the connections but there are production aircraft examples that use hose clamps in the main fuel system too.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Rich,
Please explain "fuzzy math". The definition I have been using for 50 years could be compared to a rough estimate.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

You young guys haven't been around long enough to know the important stuff . Fuzzy Math goes right along with NU-CU-LAR ! Fuzzy Math is more of a political definition meaning we're not sure what it will cost , but it will be more than you think !

RB

I think the thorp system can be very simple. Out the tank through the shut off valve then down hill to the gascalator (I have a Lunkenheimer ex FU4 which holds 1/2 pint) then up hill to carb. Done! Every last drop of fuel can be used and any water collects in the gascalator and is easily drained during preflight. If you have a mechanical pump and electric then out of the gascalator into electric pump then mechanical then carb.
Tony
ZK-VMS
T18C

Thanks Rich...must be a Navy thing.

I agree Tony but I tend to overthink everything when problem solving.

One of the things I have been struggling with since buying a homebuilt is way words like "required" and "must" are thrown around when it really isn't. Having been military trained, the transition to working on Cessna and Piper aircraft was pretty easy because "required" and "must" still meant exactly that....not just that it is a good idea. When I read something like a "gravity feed fuel system must flow 150% of the max the engine can burn" I look it up and wonder why the rest of the "requirements" in that section are ignored and what makes that one safety "requirement" so much more important than the rest of them. The magazine article I read is a good example. It claimed a sump is required but we are getting along just fine without.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

I finally finished the fuel lines. I removed the valve and the fuel line that was cork-screwed uphill to the right. Replaced the small finger screen with the larger one that actually fit the fitting on the tank and connected it to a J-3 fuel shutoff valve I mounted to the firewall. To keep everything flowing down hill I had to build a bracket to lower the the strainer about 2 inches. Shutoff valve is controlled with a push-pull cable on the panel. I initially intended to use hose and hose clamps but the bends were too tight and caused kinks so I used aluminum tubing for everything between the tank and the strainer. The tubing bends had to be closer to the flared ends than my bender will allow so I filled the tubes with sand and bent by hand to get the final bends needed to make the nuts thread on by hand. There are 90° bends but no 90° fittings. Not necessarily the prettiest of setups but very simple and functional.

Someone was kind enough to PM me a warning about the thermos style fuel cap but I didn't give it much thought until I went to reinstall the tank. When I picked up the tank I grabbed it by the PA-23 cap and it came off in my hand. I double checked it was "latched", tightened it, reinstalled it and the same thing happened again... pulled it out easily with the tab screwed down as tight as I could get it latched. Replacing the cap with a new one would have likely fixed the problem since the rubber was hard on it and nearly impossible to compress any meaningful amount but I chose to change to a different style. I couldn't find any drop in options I liked that would fit until I started searching for fuel tank components geared toward people building tanks but before I actually placed an order I realized I had the fix out in the scrap pile. I cut the filler neck flange off of an old fuel tank for a '51 CJ-3A, "adjusted" it (cut, bent and welded) to fit the airplane, drilled a hole to secure it to the tank filler neck with a screw and sealed it with some B-2. I stole the cap from the Jeep itself since it's currently not running anyway.

An after thought... if people are cracking brass fittings, I believe they should invest in a torque wrench. Not everything needs to be as tight as humanly possible. This is possibly one reason why the tanks were cracking too. How often does a shutoff valve get actuated? I drained and removed the tank without shutting mine off. I also did the flow checks without using the valve. Barring any other issues I can't think of any reason to operate the valve other than once a year to check the fuel screens at the condition inspection.

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Thorp T18
O-320-B3B (160 HP)
68x74 Sterba Propeller

"The joke in aviation is, 'If you want to make a million, you'd better start with £10m.' " -Bruce Dickinson

Jeff,
I shut off the fuel after every flight, just in case. Also, I know that the valve is operational and not sticking in case I really do need to shut it off in flight, hope that day never comes.

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Bruce Finney
N18JF T-18C #262
Yelm, WA USA

After I had a float stick (needle seat) and drip fuel all over the deck , I shut off the fuel with the FSOV at the end of the day or if the AC is going to be stored indoors . It also provides me with an Ops check of the valve .

RB