Tank problem?

Hi Guys!
I am using a homemade metal uniflow tank on my new PA60 powered Geo XL. The problem I am having is that the lap time increases by about .3 or .4 seconds from the start of the flight to the middle of the pattern. This situation is unacceptable. Has anyone else had this problem using a uniflow tank? Any suggestions?

It's a common problem on uniflow tanks. One way to fix it is to reduce the diameter of the hole in the 1/8" copper tubing that you are using as a vent. stick a piece of fuel tubing on your 1/8" copper tubing vent and stick a piece of 3/32" copper (or brass or aluminum) tubing into the fuel tubing. You have now effectively reduced the i.d.
of the vent tubing.

Test fly it and see if it doesn't solve your problem.

>Hi Guys!
>I am using a homemade metal uniflow tank on my new PA60
>powered Geo XL. The problem I am having is that the lap time
>increases by about .3 or .4 seconds from the start of the
>flight to the middle of the pattern. This situation is
>unacceptable. Has anyone else had this problem using a
>uniflow tank? Any suggestions?

One other thing you might try is to use muffler pressure. That seems to even out the run throughout the entire flight.

>Hi Guys!
>I am using a homemade metal uniflow tank on my new PA60
>powered Geo XL. The problem I am having is that the lap time
>increases by about .3 or .4 seconds from the start of the
>flight to the middle of the pattern. This situation is
>unacceptable. Has anyone else had this problem using a
>uniflow tank? Any suggestions?

I doubt that it has anything to do with the tank.

First thing to try is to break and remake the header/engine connection - face off both sides, and of course use silicone sealant to seal the joint. Then check the pipe for leaks as usual, under extremely hot water and pressure. If that doesn't fix it, shorten the pipe by 1/8" or so. If that doesn't change it signicantly try another pipe. I had one pipe (the one that I ran at the 95 NATs) that did funny stuff like that all the time. Never did figure out what was it was, but another pipe of identical design and setting worked perfectly.

I have to agree with Brett, if your tank is shimed to the correct height I believe it will work fine. It doesn't take much of a leak anywhere to run things amuck in a big hurry. Fuel lines,header joint, coupler,pipe,venturi.

Be absolutely certain that the tank vent is not leaking also. If the vent leaks, the run will change because the uniflow will be non-functional.

>Be absolutely certain that the tank vent* is not leaking
>also. If the vent leaks, the run will change because the
>uniflow will be non-functional.
>
>Curt

*the overflow vent. Actually any air leaks anywhere in the tank will defeat the uniflow.

>Be absolutely certain that the tank vent is not leaking
>also. If the vent leaks, the run will change because the
>uniflow will be non-functional.
>

True, but that would make it go lean as the flight goes along, not rich.

Yes--Absolutely--Misread lap time/lap speed.

Never Mind!

Hi,

I have exactly the same problem using the tank in the picture. Engine PA51 and pipe ran OK in one plane (different tank). When I moved it to another plane (engine+pipe) I got the same problem as described above; 5.1 s laptime initially, somewhere between inverted loop and inverted square loop (during the manouver) the engine was going richer to a laptime of about 5.5 s. It stayed at that laptime throughout the flight. This happened every flight.
I checked the tank for leakages, replaced all fuel lines and filter. Same thing.
When I replaced the tank with a tank having the uniflow tube at a slightly different position the problem was gone. It now runs at about 5.35 s the complete flight.

I removed the end of the tank that did not work as you can see in the picture.

Staffan Ekstrom
Sweden

Staffan,
Thank you for posting that very clear picture. My tank is plummed exactly like the one in the picture. How did you change the uniflow vent on the sucessful tank?

Jim,
I havn't made any changes to the tank yet. I replaced it with a similar tank. The two main diffrences is that the uniflow tube enters further back in the tank and that it has a tube right thru to hold it in place in the tank comparment.

I see no reason why the tank should cause these kind of problems. It might be a combination of things.

My plane was built in 2002 for a PA61 for the WC:s in Germany. I checked my log-book and have a note that I had the same problems at that time. I had an idea that the tank was moving forward at the beginning of the flight and somewhere during a manouver returning to its original position. I put a L-shaped piece in front of the tank to hold it in place. The problem was still there, but for some reason (which I don't remember) it was not as big as in beginning. A prop change might have lessen the problem. The plane was however set aside after the WC:s. I have just started using it again (PA51 equipped), due to an accident with my first choice of plane (picture).

/Staffan Ekstrom

Brett,
I took apart the engine header joint and resealed it as you suggested. I am wondering exactly how you check a pipe for leaks? You fill it up with hot water? How do you apply pressure?

>Brett,
>I took apart the engine header joint and resealed it as you
>suggested. I am wondering exactly how you check a pipe for
>leaks? You fill it up with hot water? How do you apply
>pressure?

First, make end plugs. I use a "stinger" with a dowel rod tie-wrapped to the small end, and a piece of header tubing with an old cap from a bottle of Zap tie-wrapped into the other end. Then plug both ends, and apply pressure with a fuel syringe attached to the pressure tap.

To do the test, fill the bathtub or some other receptacle big enough to submerge the pipe with HOT water, I'm not talking bathwater hot, I am talking full hot - let the hot water run by itself for a minute or two, and then fill the tub enough to compeltely cover the pipe. Hold the pipe under water for a minute or so (push it down with something or you will scald you hand) and then apply pressure and look for bubbles. If you find any leaks, mark them with a sharpie.

To fix it, scuff up the leaking area with a wire brush, and cover with J-B Weld. You can set it off in minutes with a heat gun. Then retest, and repeat until any leaks are gone.

BTW, you are not running it too slowly. That's about my launch RPM in the Midwest, using a Eather 11.5-3.7 (cut from a 12-3.75) UCT (red) 3-blade. My pipe length is about 17 3/8-17 1/2, and I use a #5 spigot venturi and 15% nitro.

I assume you have checked the portion of the pipe that resides under the coupler for collapsing tendencies. I also assume you sealed the spraybar to the case as well, although, usually, leaking there only makes it surge on the ground.

I am starting to think I am running the engine too rich. I made a new tank with a 1/16 ID uniflow line. I changed to a different glow plug. I remade the header connection. This made no difference. What RPM are piped PA60 users taking off at? I am taking off at 9,800 to 10,000 RPM.