Fuel tank shapes?

[Dennis Toth]

Guys,
I've looking at several fuel tank shapes and observed how the engine run seem to react during maneuvers. Assuming a good stunt engine set to run in a 4-2-4. What it looks like is that tanks that are deep, say 1 1/2" with fixed pickup tubes and uniflow seem to cause a slight lean burst at the bottom of round or square maneuvers. This also seems to happen with normal 1" tanks but to a lessor degree. It seems the faster the ship is flying the more pronounced this burst is.  Two shapes that seems to overcome this is either a round tank with a floppy pickup and uniflow or a rectangular tank with the same type of pickup and uniflow setup that follow the fuel. The thing that seems to cause the burst is the angle of the normal outside wedge. As a ship comes through a tight loop or corner were the "G" force pushing the fuel down allows it to move away from the pickup tube and allowing a few bubbles of air to be sucked into the fuel line, causing the burst.

Anyone notice this and how did you eliminate the burst?

Best,           DennisT

[Brett Buck]

Guys,
I've looking at several fuel tank shapes and observed how the engine run seem to react during maneuvers. Assuming a good stunt engine set to run in a 4-2-4. What it looks like is that tanks that are deep, say 1 1/2" with fixed pickup tubes and uniflow seem to cause a slight lean burst at the bottom of round or square maneuvers. This also seems to happen with normal 1" tanks but to a lessor degree. It seems the faster the ship is flying the more pronounced this burst is.  Two shapes that seems to overcome this is either a round tank with a floppy pickup and uniflow or a rectangular tank with the same type of pickup and uniflow setup that follow the fuel. The thing that seems to cause the burst is the angle of the normal outside wedge. As a ship comes through a tight loop or corner were the "G" force pushing the fuel down allows it to move away from the pickup tube and allowing a few bubbles of air to be sucked into the fuel line, causing the burst.

Anyone notice this and how did you eliminate the burst?

   Make the "apex angle" of the wedge more acute.

    Brett

[Lauri Malila]

I use 90 degr. Wedge angle and there is no problems. Also a baffle inside the tank helps. Good place for baffle is about an inch in front of rear wall. I drill 12x1/8" holes to baffle, 6 of them close to tank floor and ceiling so that fuel can flow freely.
The baffle also makes tank stronger. I've had good tanks without baffle, but they tend to fatigue crack, usually from rear bottom corner. L

[Dennis Toth]

I have noticed this "burst" with several brands of engine and with hard tanks from the classic Veco to the teardrop Perfect style. Deep tanks are more pronounced and the deeper 4 cycle you run the more noticeable it is. I've tried baffles and they help but when the wind comes up and starts to push the ship through the rounds you can feel it.

Way back in the 50's they had a design call a "clank" tank. This used a swivel pickup tube inside the tin tank. It would work but the swivel was on the inside wall and would start leaking air with made getting a consistant setting difficult. A variation could be to use silicon tubing and a pickup tip to follow the fuel. The question is how long will the silicon hold up and if it does have a problem you make a new tank.

I have not tried a "chicken hopper" arrangement mostly because they need more space than my ships have in the tank area. Is there a way to build an internal hopper?

Best,      DennisT

[Dick Carville]

Dennis

I used metal uniflow tanks for 20 years-some commercial; most  I made myself. I found the same "burst" you talked about. I used to de-compress the heck out of my engines to make the burst go away.

One plane I had would just not stop with the "hard break"  I looked at the tanks Al Rab used on his Critical Mass and saw that he used a metal clunk tank and no inflow.

I made a metal clunk tank with the pressure line going about one third back in the tank and turning INBOARD -much like, if not exactly like Al's. No uniflow at the pickup point

I loved it!  I put all my engines back to standard compression

I changed all my tanks to metal clunks about 3 years ago and have not had any problems


Used on ST60, ST46, DS 40

Dick

[Brett Buck]

I have noticed this "burst" with several brands of engine and with hard tanks from the classic Veco to the teardrop Perfect style.

   Mine do nothing like that if the wedge angle is sufficient.

  I would also not assume that it's a tank issue. David had a problem with very-repeatable breaks into a 2-stroke at a particular point in the pattern. Take the same tank and engine, and put it into a different (much more rigid) airplane, it doesn't happen.

   Brett

[Dennis Toth]

I did some searches and seems that Al Rabe used the silicon tubing uniflow clank tank in the BBFB Bearcat and Critical Mass ships.

Best,           DennisT

[Walter Hicks]

Not sure if Al uses clunk tanks for non- four stroke engines , This one is not a clunk build per Snaggletooth pattern.
  This is one I build per his specs last night  .004 Stainless Steel.

1.2 oz weight .