Thorp Air Command - T18.net

Help!

I purchased A T-18 built in the late 60's N252F with 0-360 and had fuel flow problems since the get-go. this a.c. has 1800 tt. with 500 smoh. I could climb through about 4000-5000 ft and the fuel pressure will start dropping then fuel flow drops off followed by eng. loosing pwr.
After new fuel pump - rerouting fuel lines, ellison throttle body overhaul, engine overhaul for other problems, fuel pressure falls off going though 4000-5000 ft.
Any help will be appreciated

Robert Phillips
0A7

Robert: From your post it sounds like you are developing full power for takeoff and you regain power for landing and you only experience the fuel problem at altitude (4-5000ft). Also from your msg it sounds like you have a fuel pressure gauge to show the drop in fuel pressure and a fuel flow meter ? Is that correct ? Has a fuel flow test been done on the ground ? Problem occurs with a full tank of fuel, 1/2 tank or ?

Going thru your list of things replaced, I would start looking at the things you did not replace/checked or cleaned.

1. Finger screen in the tank.
2. Tank vent and vent line. Same line that was installed in the 60's ? An old rubber line.
3. An old fuel line that was not replaced, possibly from the tank to the FSOV or to the firewall that is failing internally.

RB

I have Airflow Performance fuel injection. It has a very fine finger filter in the fuel inlet fitting of the throttle body . Mine blocked after 35hrs. From the building dust etc.That caused a partial power loss.. After cleaning the engine purrs nicely now. Some carbies have a filter at the fuel inlet to the bowl too.Under that big brass fitting.

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James Ball

Ellisons use a low pressure fuel pump like carburetors. You should have pretty low fuel pressure to begin with--3-5#. You may want to talk with the Ellison tech guys--I have found them helpful in the past. Also there is a screen at the fuel inlet to the Ellison that should be checked/cleaned.

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Dave M
Sacramento
ex-T-18 owner

When I first bought 7077J it was from a man who spent (literally) 30 years building. He just drug it from house to house (he was a house builder) and I think he should have stuck to building houses. Anyways I would get same thing, but but mostly when increasing speed and or rpm. He obvioulsy was fighting the problem and installed an inline pump. To make a long and scary story short, I traced it down to the ANGLE at which he had the breather (vent) tube bent to. The angle was just right that when I hit cruising angle of attack it was making a vacuum to the tank. Like holding your finger on a straw, and would run out of gas in flight. Even turning on the pump would not solve the starvation completely. But it really was as simple as changing the angle on the vent line. Plan to remove the pump during this rebuild, caused some anxious moments on a trip to Copperstate once when my son inadvertently turned the switch on. and I didnt monitor the situation as we arrived at the air port and had to wait in a holding pattern I saw the gauge bouncing off the bottom and I didnt know how long it had been there. It was obviously running rich and drank more than its allotted share. I was able to convince traffic control that I NEEDED to land next with out declaring an emergency. Sorry I went long but thats why the pump is coming off!
Skeet

Same problem here, not as bad.
N883FF has an O290, so the fuel demand is lower, before take off it shows good pressure from engine driven fuel pump, and even more with the boost pump on, but at cruise altitudes and speed, it reads zero.
Un-nerving, but keeps running strong, and I can richen the mixture, so fuel is available.
The fuel vent line is slightly bent forward. I wonder if I should bend it forward to a nice straight facing elbow?
Marilyn and I flew today, with good pressure in the pattern while she worked on the landings, then my turn, we flew to 1S9 in Eastern WA. Over an hour with zero or fluctuating near zero fuel pressure.
It returned to normal in the approach and taxi to tie down.....?
Maybe it's the cruise speeds above, say 140mph? After climbing to cruise alt at 110 to 120?

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Scott Emery
EAA Chapter 326
T-18 N18TE

Are we talking the "external" vent line (ram) into the air stream or the tube bend off the top of the tank ? Here is the "stream line" vent I installed on the botom of the A/C. The tube on top of the tank is to plans.

RB


RB

Yes, and that part is a beauty.
I swear I've got the slowest & ornryest t-18. It flies crooked. It's slow. It hobby horses down the runway if you don't flair just perfect.
It's one saving grace is that it tracks straight on landing, and can be horsed back into obedience even at 35 mph, the worst speed.
But that O-290 just purrs.
I've had a real fun time improving it's manners. It's a challenge that I'm enjoying.
This fuel pressure issue is important enough to move to the front of the list.
I'm using a 1 1/4" automotive fuel pressure gauge, liquid filled. It's in the lower part of the upper panel, which might be 2 ft above the carb inlet fitting were the pressure tap is mounted. 2ft head of gas is less than 2 psi gage.
I want to see the fuel pressure above 1 psi in flight on the mechanical pump. I get near 6 psi on the ground with the boost pump on.
Right now I am willing to try improving the vent. I see gas vapor comming out on the ground, on a hot day. I can blow into it without restriction, until back pressure builds up. I don't think it is restricted.

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Scott Emery
EAA Chapter 326
T-18 N18TE

Ship,

I had a question about the location of the vent on the gas tank years ago. The answer I received at that time was "build it to plans and it will work". I've always wondered why JT had the vent so far to the rear of the tank. Always seemed, to me, that with a full tank, the gas could fill the vent line creating a vacuum. (especially if the vent tube is NOT flush and extends into the tank) Could this become worse on steep climbout? What if the vent line filled with fuel and managed to run down the VL in front of the gas tank. What if the vent line is pointing up when parked and fills with gas. Is there enough air stream presure to lift the fuel back up the vent line into the tank? Or will the tank have a vacuum from the blocked vent line. The easy fix is never over fill the tank (if you know about this possibility). I've thought about adding a second VL to the front or what if the vent was moved to the center/top of the gas tank?? Maybe there is no problem or maybe you have not had the right combination of variables.

I know there are hundreds of Thorp's that have never had a problem. But, this is not the first time I've heard of a Thorp climbing out and the engine sputtering or quiting after fillup. I recall two in the last few years with no explanation. Maybe you always turn left or never over fill or you level out and the vent clears. I don't know if there is a potitential problem, but I would like a little more discussion than "build it to plans".

Bob MO

My setup is "built to plans" and I have over 2500 hours of trouble-free operation. I see as high as 16 gph fuel flow on climbout (sea level, 180 hp). The vent is terminated on the bottom of the plane in a 3/8" tube cut at 45 degrees with the openning facing the wind (forward). It extends maybe 1/4 to 1/2" below the floor skin. The tube exits the aircraft through the floor about 1-2" aft of the front edge of the foor and offset to the right the width of the front tunnel to which it is clamped. I have no scoop on the bottom of the cowl to interfer with airflow, however. It is important to vent the tank at the rear because at level flight attitude, that is the highest point on the tank.

One thing to check on older aircraft is the hose connecting the top of the tank to the vent. If the builder didn't use fuel line grade hose, it could partially collapse or get soft and restrict the flow of air. I had plastic for a while and it hardened and cracked.

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Bob Highley
N711SH
SN 835
KLAL

Bob stole my thunder. I was going to suggest that you check the hose from the tank to the bottom vent. No telling what type of hose was used. It may be old and soft (like me) or it may have a kink in it. I used top of the line Goodyear fuel hose.

RB

Your right, Bob. But, I have never seen how to run the vent line in the plans. I thought the line was all aluminum. I may be making a mountain out of a molehill. As you can see, the vent has always bothered me.

I did not build my gas tanks, but I am going to look inside at the vent tube to make sure they are flush welded and not protruding. I know one tank has 1/4" tube on the vent, not 3/8. How about the tank in the picture. Is the filler cap to far forward? Can the tank be over filled blocking the vent? I don't know. I even searched the Mustang list to see how they ran their vent. One person posted "is there any reason why he couldn't run the vent aft of the tank". No one came back with a negative reason. I thought it would kill him.

I know I have read to many "they just filled up with gas and the engine quit on takeoff". (The thorp that hit the house and Scott in FL had this senario) I plan to have redundent vents. Then it will take two mud dobbers to put me down. Or, how about a small vent hole in the gas cap (or a check flap in the cap). If you run sixteen gallons with no vent hole, maybe twelve with. How much gas flows when you have two gallons left? How much does a boost pump restrict gas flow? A Lot to consider.


Well, I've spouted enough.

Bob MO

I too was concerned about a rubber hose for a vent. I used aluminum from the tank down the firewall side just above the rudder pedals, used a rubber bib hose to connect it to the vent tube going out the floorboard and vented forward. No chance of the hose collasping. This keeps positive pressure on the tank. My cap is not vented and the vent outlet is towards the rear of the tank. We have discussed this before, I have four fuel pumps on my Thorp. Two for fuel transfer from the wings to the header tank and the other two, mechanical and electric to supply the motor. twelve hundred hours and never a fuel flow problem. Never installed a fuel pressure gauge, never thought I needed one, cause the fan out from will tell me when I have fuel pressure.

Both the fuel cap/bung and the fuel sender need to be positioned between the top skin ribs (can't remember the plan #). Since you still have to installed the tank, spend an extra hour and make an access hole on the top skin for the fuel sender. Looks like you are using the same fuel sender I did. I calibrated the sender out of the tank (per the instructions) and then installed it in the tank. It was still off a few gallons when I filled the tank. W/O the access hole I would not have been able to "re-calibrate" the sender. When you have the sender calibrated don't forget to put a "dab" of silicone on the adjustment screws to keep them in place. The fuel guage is accurate to less than a gallon when I compared it to fill ups and the newly installed FC-10 fuel computer.

RB

Bob,

Before mounting the fuel tank to the craddles on the firewall, I measured the distance from the top of the tank where the vent fitting is welded to the tank down to the floor mounted AN 837-6D sticking out the bottom facing forward. I cut, flared and installed a 6061 T6 line with no rubber hoses connecting the tank directly to the fittings. The fittings torqued to 100 in lbs which is the recommended torque values for those nuts. The tank you show whick was tack welded for fit check has an AN 822-6D elbow welded into the tank with about 1/4" of the threads inside the tank. On each tank ends (left and right) near the aft end of tank you'll also notice two welded AN 822-6D elbows that I used for connecting the lines from each of the wings for fuel transfer. No rubber hoses used in those connections except where the fuel tank connects to the shut off valve and to the QDs at the wing joints connections.

Awhile back I filled the fuselage tank all the way to the top which actually took 29.5 gallons and no fuel ran out of the vent untill I fell in a hole similar to the one Gary Green fell into except not as deep Put two 1/2" long cracks at the bottom edge of the wheel pant and nothing else, but my pride hurt. Once the wheel was out of the hole, the fuel quit running out the vent.

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In CowTown USA,
Don D-Day
NX18DD