Thorp Air Command - T18.net

I have standard T-18 N1051Q with a 29G header tank to a O290-D2 engine.

This summer I fabricate a ram air scoop on the bottom of the cowl. Recently on a long cross country I noticed the aircraft start to loose power when I only had 10 Gallons in the tank. I checked the gascolator and fuel vent but on the next leg the same thing happened.

When I got back I went through the fuel system and replaced all lines, gascolator, and fabricated a new fuel vent pointed into the air stream. Ground test with only 1 Gallon in the tank and the aircraft would develop full static power. I tested it out over the airport and everything seemed good. But then on Sunday with 7 Gallons in the tank same thing.


In all cases I would be in a cruise setting and experience a rapid rpm drop of around 300-400 rpms with a slight recovery. I would pull the throttle back slightly and apply carb heat to even it out. This is not a case of carb ice though.

Searching the old thorplist I saw some posts on how ram air can increase the static pressure in the carb and increase the needed head pressure. Has anyone heard this before ? If so what would be a solution? I'm leaning toward installing a second fuel vent for backup to help increase head pressure. Anyone have similar problems and if so what was your solution?




Thanks
Jeff

I would think that you could run the tank vent into the ram air scoop to negate the ram effect on the carb. This should be simple to try.


Jim Mantyla

Is it possible your vent line is collapsing from the tank to the vent ? Is your finger screen clear ? You also state that it "smooths out" when you reduce power and apply carb heat ? In effect changing the fuel/air ratio. Are you running auto fuel ?

RB

Fuel vent is aluminium and doesn't seem collapsed anywhere. I can see the finger screen and it looks clear but I did not pull it out.

Uploaded a picture with the ram air modification. If you look close you can see the fuel vent below the rear of the cowl.

Since this pic the ram air snorkel is 1 inch shorter. Also the fuel vent has been replaced and points into the air stream. I made some edits in red to picture it.



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Are you sure the problem is not the snorkel?
It looks very thin and pointy maybe it is stalling in certain attitudes or power settings maybe open it out or give it a blunter entry. Effecting a (partial?) recovery by using alternate air might support this.
I do know of a Europa in the UK which had a similar problem, the suggestion was that something about the inlet design was creating a turbulent or unstable air flow in the air inlet.
I don't think a second vent will increase your pressure.
Did you have this problem before adding the snorkel?

Dave

The snorkle size crossed my nimble mind also. [:p] The area appears to be very small. High angle of attack or slip might create an air flow problem ? Any Aero Majors out there ? [;)]

RB

Thanks for the tip on snorkel size. Actually snorkel opening is a little larger now than the picture and is well above the minimum size of 10% larger then venturi. I referred to Kent Paser's book Speed with Economy when I made it. The power loss happens in cruise attitude at cruise power settings only with fuel 7-8 Gallons or below. I'm going to pull the tank screen tonight and double check that.

I found the problem or at least the major contributing factor. There was a piece of fuzz inside the screen in the bottom of the tank.


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Before removing this fuzz it took 40 minutes to drain 5 Gallons through the gascolator. After removing the fuzz and draining the same gas it took 14 minutes. What would that be 7.5 vs 21.4 GPH ? I know draining from the gascolator does not indicated what the carb gets. This is just a comparison.


One other concern I saw is that the tank does not effectively get all the water to the lowest point. Looking in the tank I can see long bubbles of water on either side of the bottom drain. Anyone else see this in their tank?


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I check my snorkel vs a RV-9 next door. Mine is actually larger.


You can also see the new vent in this pic.
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Here is the RV-9 inlet.

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Ceason: Was the fuzz in the screen mesh or further down stream in the valve?

Mesh screens like all filters need to be looked at regularly.

Good find.

_________________
Victor J Thompson
C-GIRQ
"In Memory Of Dad"

Possible that the fuzz was "floating" in the tank with more fuel. As the level dropped, the fuzz made it's way to the finger screen, partially blocking it ? What kind of fuzz was it ? Sorry...I re-read your post and you said the fuzzwas inside the finger screen ? Was this a homemade screen ?

RB

Well spotted- that's a lot of water though does your gas cap leak?

Dave

Jeff,

I also found I had a substantial amount of water in my gas tank this past spring. And, found that either the tank does not have a low spot or the base in which the aircraft sits isn't level. Fortunately, I found the leak (access panel for the fuel sending unit)and am cautiously aware if water does manage to get in that it will not drain toward the screen while stationary (in my aircraft). Also, added an aircraft cover that covers that portion of the aircraft. Moral of the story for me.....keep the water out in the first place.[:)]

Chuck Kincer
N575CK

_________________
Chuck Kincer
Richmond, Me
575CK
KIWI

Although hard to know for sure but it kinda looked like carpet fuzz from the old interior that was in the plane when I first got it. That interior hasn't been in the plane for 4+ years now. I do look inside the tank fairly frequently and can only guess it was in the screen for some time and maybe just worked itself down.

The fuzz was inside the screen at the top of the nipple that the screen goes into. I had to use a piece of safety wire to fish it out. The screen appears home made similar to an oil screen.

I have an old friction style fuel cap. There is a door that goes over the cap. But this setup does not stop rain from puddling on the cap. The plane was parked outside at Kalispell for 5 days and it rained just about every day. That must be how the water got in there. But I plan on replacing the cap with a twist lock style over the winter when I redo the panel again.

Shag or low pile carpet ? [:p] Kind of like fuzz in the old belly button...how do it get in there ? Your little do da will remind me to check my screen. [;)]

RB