Thorp Air Command - T18.net

After reading through the forums and scanning the news letters I believe I have ram air issue with my fuel tank. There is a quarter inch vent line routed from the tank to the scupper. The scupper has a quarter inch drain line connected to a forward facing elbow on the belly. Between the fuel door, latch and gaps in the scupper seal, I don't believe there can be any pressure building inside the tank. I am trying to decide if connecting the vent directly to the scupper drain line would give me enough head pressure in the tank. I would have come up with an alternate scupper drain but the tank would not have to come out for that. If the ram air vent requires 3/8 inch then I will pull the tank. It is a gravity fed system.

My aircraft used to have a V6 running a fuel pump so this probably never surfaced for the builder.

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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

Scuppers suck and vents blow. Can't believe someone thought you could do both with one line. Scupper can't be a tight seal so you are losing any ram advantage of a forward facing vent. A few years back, I had to remove my tank FOM and found the original flexible tubes connecting the tank to the outside airflow were cracked and full of holes. You may find the same when you look back there. My vent consists of a #6 tube with a 45 degree cut on the end. It sticks down about 1/2". Scupper is the same; it just faces aft. No problems in 2800 hrs.

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Bob Highley
N711SH
SN 835
KLAL

Mine is a 1/4" line coming out of the bottom of the cowl facing forward. I'm gravity fed as well. I don't know anything about a scupper. I only have only 1 vent line coming from my tank that is routed as stated above. Haven't experienced any issues so far with it.

If the tank is built to the plans then the scupper drain goes thru the tank and out the front about mid way up the fwd face of the tank then a tube runs down to a aft facing 1/4" tube. There should be a fwd facing tube and this runs up to the top of the tank at the highest point tail down and pressurises the tank (provided your tank cap is sealing properly) in forward flight.
This is how my Thorp is configured.
Tony
ZK-VMS

Oops! The original filler point did not have a scupper as it was welded flush with the tank top and protruded through the upper fuse skin. I think most chose to recess the filler into the tank a couple of inches so a common tank cap could be used and a flush door on the skin. But this means if you spill fuel into the scupper you need a means of draining. Mine has a 1/4" pipe welded into the lowest corner of the scupper then the pipe runs internal to front face of the tank where it protrudes and turns 90 degrees then down (via flex tubing) to the aft facing vent. The tank dwg is #514 which shows the vent position on tank but no detail on the vent outside. Probably in a newsletter some where.
Tony
ZK-VMS

It sounds like quarter inch will work so, as an experiment, I may just connect the tank vent directly to the scupper drain and see if I notice an improvement. I think it can be done without removing the tank and I only need the drain if I spill. I don't understand the thinking on the existing routing unless one of the lines from the T was originally plumbed as a fuel pump bypass.

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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

Interesting. Mine has nothing like that unless this is something internal.

What symptoms or problems does only having the single forward facing vent that faces into the wind when flying? That's all I have. I don't mean to hijack the thread but it seems like I'm missing something as well.

Binder, I don't think you would have any problems if your vent line goes to the tank unless it gets plugged somehow. I am hoping that connecting the elbow on mine to the tank vent will alleviate some of what I am seeing. My plan is to connect the line on the right end of my tank to one of the lines on top while plugging the other one. I should be able to do that without pulling the tank. I wanted to experiment a little today but the wind is 13G32 (mph) which is more gust than I want to deal with even though it is mostly down the runway.

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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

Oh ok. Whew, I was afraid I had yet another issue to fix on this plane.

Do you have any issues with full power and gravity feed? I just read somewhere that we get about 1 psi of pressure in the fuel line for every 40" the fuel level is above the carb. That seems way too low for me. Mine flows 25+ gph when tested and I was thinking around 2psi and it's nowhere near 40" above my carb.

I'm just hoping that isn't an issue with my full throttle lean bog as well. I don't think it should be as the static drain test is enough for 25gph and I'm not using near that on climb. I think my 290 might need about 12-13gph on climb and I'm getting about 7gph on my flights at 2500rpms full mixture.

I had a surge on takeoff last week. The strip was kind of rough and a bad bounce got the fuel sloshing around. That is the only time I recall anything at full power. My static drain test was closer to 22-23 gph with about 2 quarts in the tank when I redid the supply lines last winter. I checked that filling a pint didn't take more than "x" number of seconds rather than timing how long it took to fill the container so I only had to do the math one time before I started. If the container overflowed before the time expired it was good, if not, it was bad. Fuel flow was considerably faster with more fuel in the tank, I just didn't do the math.

I had 10-11 gallons in the tank when I got the surge on takeoff. I flew it again the next day (climbs and steep turns) without adding fuel with no issues until I was on short final. I topped it off Saturday and flew about 220 miles with four landings without issues but it still has at least 16 gallons in the tank. I want to get it down around 10-12 gallons again to confirm it misses at low power settings then make the changes to the plumbing and fly it again to see if it duplicates.

What I hope to fix with getting ram air to the tank is a bad stumble and occasional surge on short final. This only seems to happen when I have less than half a tank of fuel and seems to worsen as the fuel level decreases. I think it is possible at higher speeds that I am getting enough ram air through the current setup to not notice any issues. I am mostly just guessing though. Fuel system design is a little out of my norm.

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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 installed to 1/4" aluminum lines. one from the top of the tanks highest point running down the front of the tank through the floor and a 45* angle facing forward to pressurize the tank in flight. Second line is a drain attached at the highest point of the tank and a second line from a "scrubber" that is around my recessed gas cap. Run down in front of the tank through the floor and the 45* is facing aft. This acts as a vent if tank overfilled or spill gas while filling. It's work for 25 years and 1500 hours

I have heard that gravity feed fuel systems can be tricky on T-18. (Not my anecdotal story)
If the fuel tank is filled to the top, fuel can run down the ram air tube when the tail is lifted for take off, plus maybe a bounce over a bump. If the tube is full of fuel, and is a small diameter tube, the dynamic pressure of forward speed might not be able to push that fuel back up-hill to the tank. Then, the air does not enter the tank to replace the fuel being used from the tank resulting in loss of power. A 3/8" vent tube would be much safer, the surface tension of gasoline probably couldn't hold up in a 3/8" tube.
In the story, the engine did recover full power before a serious incident occurred, so this is conjecture.
I'm a belt & suspenders guy, N18TE has an electric and a mechanical fuel pump.

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Scott Emery
EAA Chapter 326
T-18 N18TE

I have an HA6 carburetor and have both electric and mechanical pumps.

Mine is 3/8" teflon fuel line then right before it exits the cowl it has a 1/4" ID aluminum tubing into the rubber line that is bent just shy of 90* (large sweep) to face it forward. I don't often keep fuel up to the top of the tank when I fill it. I'm usually a few inches below the top.

Jeff: I haven't experienced any of that with mine so I don't think I have the issue you are running into.

Yes I had the engine quitting issue with mine before I modified it. My impression was that when the tank was completely full of cold fuel, it heated up and expanded up and out the vent. The real problem lies in that my vent tube is welded such that it sticks 3/4 inch down into the fuel, as opposed to being flush. I now have an electric pump. No problems since.
This is definitely not what is happening to you, Jeff.
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