MSv

Hi,
I practice CL flying since a couple of years but only 10 times/years so I am still a old beginner.

I have two related questions:

1/ I flew until this year only smaller planes (with MVVS .15 CL engines). Recently I equipped a plane with OS .20 R/C engine (inverted position, R/C carb set full open, probably “uniflow” tank. The engine run nicely except lost the power (and even stops) in inverted loops, why?

2/ I constructed this year a profile Banshee from Sig with a Fox .35 (that I bought older) (equipped 11/6) and a new, V edge 3 ounces metal tank (from Brodak?) with two vents (up and down). After filling up the tank with standard 5% nitro, I plug the down oriented vent. When I run the engine had tendency to stops in acceleration and in simple loops. When I start engine, the fuel is pushed out of the tank!

Thank you for your explications
Michal

R8893

My Feedback: (20)

MSv,
I don't have any guesses on the OS 20 powered plane but have a couple suggestions for the Banshee. I think the 11-6 prop is too big for a Fox 35. Suggest you try a 10-6. The fuel coming out of the top vent could be caused by suction/siphoning as the propeller slip stream blows past the top of the vent. I suggest you take a piece of fuel tubing and cut the end at a 45 degree angle and install it on the vent with the angled side pointing forward. I would try the same thing on the bottom vent rather than plugging it. The plugged vent will cause a change in fuel pressure when you go inverted.
Good luck,
Chuck

MSv

Thank you Chuck,

I was wrong with the propeller dimension: in fact it is a Top flight 10/6. I will install the tubing as you suggest.
You advise to not plugging the bottom vent. OK. But for “Uniflow” tanks it should be plugged, not is it?

Thanks
Michal

gcb

Michal,
You did not say what plane the OS is in. As a guess, your fuel tank height may be set up for the CL version when actually the spraybar in the RC carb is at a different height. This will be most noticeable in a difference between how the engine runs during inside and outside loops.
For instance when you adjust the needle for inside loops, the engine will run much richer or leaner in the outside loops.
Many (most?) PA flyers shim the tank and make it height-adjustable (add or remove shims).

On the Fox, you did not say what fuel you are using. For an old Fox .35 I would recommend 25%-29% all castor lube. All castor because it is an old iron/steel engine, and the high percentage because a Fox .35 is a maximum engine in a minimum case and requires lots of lubrication. If you use synthetic lube in an old iron/steel engine you may find that you will lose compression because of the cleaning affect of the synthetic. All castor will eventually restore compression by creating a castor varnish.
If the OS is an ABC engine most any fuel will work. If it is an old iron/steel, I would suggest all castor lube for it also, although it does not need as much lube (20%-25%).

Good luck.

George

Sport_Pilot

Is he talking about the Fox burp? I have one too that burps if you do a figure 8 or loop to inverted. I recall a differant tank could stop that, but I don't remember what tank that would be.

Jim Thomerson

I think your fuel tank on the 20 airplane is either too high or too low. If it is too high, your engine should go lean in outside loops, if too low then it will go too rich. Either is not good. With a uniflow tank, it is the position of the uniflow outlet in the tank that is important. (It is probably at the outside wedge near the rear of the tank. If you don't have room to move the whole tank up or down, maybe you can rotate it enough to move the uniflow outlet where you need it.

With a nonuniflow tank on your Fox you will get a run which varies from rich at the beginning to leaner near the end. One thing you might try is to put a piece of tubing between the top and bottom vents. Make a very small hole in the center of the tubing to let air in. Air only needs a tiny hole.

Jim

MSv

Your are 100% right George: the tank was to high! Consequently, the engine leaned in inside loops!!!

For my Fox .35, I observed that diffuser hole was not positioned exactly toward the inside of the venturi. Changing position ameliorated the engine run. However after each loop (inteted only iside loops for the moment) the engine stops one second. It is normal?

Should the vent of the uniflow tanks be always plugged?

Thank you for your advices.
Michal

MSv

Correction to my previous post: Your are 100% right, both, George AND Jim.
The tank was indeed positioned to high.
Thanks
Michal

Jim Thomerson

If you don't seal the overflow vent on a uniflow tank, you have a open vented tank similar to your Fox tank. If you are running a muffler on the Fox, attach a piece of tubing between the two vents and put a T fitting in the middle. Attach a tube from the pressure tap on the muffler to the T and your Fox will run much better. What you are describing is the famous Fox "burp" which may well be due to the bypass area being too large. Using muffler pressure has made the burp go away for me. A common remedy is to put a piece of wood, parallel to the axis of the cylinder, in the bypass to cut the bypass area about in half.

Jim