Fuel Delivery Problem with Long Narrow Tank
 

Hi,

I'm learning to fly with a Duraplane trainer that has the gutterpipe fuselage. After several too short flights of 5 minutes with the 5 oz. tank (OS LA40), I found a 12 oz. tank at the LHS that would actually just fit inside the fuse. It's roughly 2 x 2 x 7" lg. I rebuilt the plane to accept the long tank, assembled it per instructions, but worried about the clunk position being far aft.

Subsequent flights have proven my fears to be true. After a long training flight (believe me I need the airtime!) my base leg causes the motor to abruptly quit every time. If I moved the clunk to the middle of the tank, would this be the best I can do without going to a bladder?

Has anyone made their own bladder? I don't suspect I can buy one for this application, can I? I see the racers use latex bladders in 5-6 oz. tanks, but I don't want to go back to a small tank. Would you use latex or mold a convolute out of silicone?

I'm wondering why this isn't a more common problem with all the different tanks and fuselages out there. Any recommendations welcome!

Dan

RE: Fuel Delivery Problem with Long Narrow Tank
 

Are you using muffler pressure on he new tank? I would expect things to be OK the way you described it. It may need slight adjustment of both fuel valves. Will it run wide open holding the nose vertically up, or at least at 45 degree up?

RE: Fuel Delivery Problem with Long Narrow Tank
 

Yes I am using the muffler pressure per the tank's instructions. It runs fine both 45° up as well as 45° down with a full tank, at idle and at full throttle. New plug didn't help. Idle mixture properly adjusted.

No wind yet - The eerie calm before the storm. I'm 15 miles from Galveston Island on the Bay and we're gonna get wet! Hoping for no tree limbs on the roof!

Dan

RE: Fuel Delivery Problem with Long Narrow Tank
 

I wonder if the engine can supply enough pressure to fill the tank...it's ok with a full tank..but as the tank empties you just aren't getting enough head pressure from the engine..(too much of a void)..can you try an 8 oz tank packed in foam?

RE: Fuel Delivery Problem with Long Narrow Tank
 

a 2 x 2 x 7 tank seems to me to be far too long a tank for the 2 x 2 x-sectional area, and is probably only used in large models which at speed and aerobatic has centrifugal force keeping the fuel in reach of the clunk at the rear of the tank. If you watch these birds landing more often than not the approach is slightly nose up at the slower speeds and this also helps with fuel feeding from the rear of tank.

At the end of a training flight- usually nose down attitude for landing - as the nose goes down then fuel will run down the near empty tank away from the clunk which is not meant to bend back on itself, and certainly not that length,and therefore the engine will be starved of fuel and stops.

exhaust pressure will be maintained on the tank regardless of size once pressure is built up as exhaust is normally more than adequate to keep the pressure up as it replaces the fuel draw off which is minimal compared to exhaust and pressure produced and is relatively constant for each engine irrespective of tank size.

shortening the fuel pick up to get the clunk 1/2 way back up the tank simply hastens the time when the clunk wont see fuel in the tank even if there is fuel - and this will vary according to the attitude of the plane.

If there was plenty of room I would tilt the tank back 20/30 degrees to try and keep the fuel at the rear of the tank and next to the clunk which should just about cover most flying attitudes for normal training flying including landing.

Failing that, I would get a smaller tank as suggested above or time your flights to have more fuel left.

I think I get more time out of my LA 40 , but then I throttle back after take off and fly at a speed more compatable with both the planes natural speed and my own innate flying skills.;)
John

RE: Fuel Delivery Problem with Long Narrow Tank
 

Dan, I'm not sure I would call that wimp a storm. I've seen worse summer-time thunderbumpers here in Texas. What doesn't surprise me is how the media handled it; Typical blow it out of proportion type thing. Hope you get the fuel prob figured out man.

RE: Fuel Delivery Problem with Long Narrow Tank
 

Can you try feeding the 12 oz. tank into a small (1-2 oz.) header tank?
That way the engine is always drawing from a full tank.

RE: Fuel Delivery Problem with Long Narrow Tank
 

How would a header tank be situated? Just an in-line second tank next to the needle valve?

Dan

RE: Fuel Delivery Problem with Long Narrow Tank
 

That's pretty much it. The small tank feeds the needle and it's vent connects to the pick-up line from the larger tank.
The muffler pressure still connects to the larger tank's vent line. That way the header tank is always full until the larger tank is empty.

RE: Fuel Delivery Problem with Long Narrow Tank
 

Dan,
why dont you rubberband the tank on on the outside of the gutterpipe fuz in various levels and attitudes to see what position helps to run fuel right down without premature stopping ??
John

RE: Fuel Delivery Problem with Long Narrow Tank
 

Look I have a similar set up in the belly of a full scale 727-200....no pumps just bleed air supplying 10 psi of head pressure to what amounts to 2 tanks one in each bag pit.....pressure remains constant.....as the fuel is transfered into the nbr 2 tank the volume of space increases..(transfer from 1 tank at a time only) pressure has to decrease and the fuel transfer slows to a crawl....
In case your wondering the volume increaseing is off set in flight by the outside air pressure dropping..you transfer as fast as you burn
now in this case with a 40 LA...the 12 oz tank seems a bit large...8 oz seems to be more what I remember for a 40 size....so look at this at full throttle.. lots of pressure..as you throttle down the supplied pressure drops, head pressure decreases and volume is increaseing(fuel burn)...we as RC'ers never get the altitude for an outside pressure drop....if the engine can't produce the volume needed to fill the void transfer of fuel will stop just like a fuel line coming off....
A header tank will only increase the volume....
remember this is only a possibility...but I'd try and plumb in an 8 oz tank and run that on the ground and see what happens
as always Good Flyin