Tank questions for the experts

[Gary Dowler]

I'll post these pics of my tank setup.
Just did some static tests to compare to flying. Seems my static run time is almost double flight time at similar throttle settings. This surprises me.
Tank capacity is 7oz. The only full tank flight to date lasted about 7 minutes , static run today was 12:26 with the same 11/7 TopFlight prop.  Avg rpm at 3/4 throttle (where it flies best) was 9000-9200, max rpm was 10100. (10/20 fuel, 50/50 syn/ castor).  Switching to a master airscrew  11/4 nylon prop brought run time to 10:30, and max rpm to 13,500.



This is before adding end cap on the tank. Shown sitting on the outboard side of the tank. Note uniflow vent terminating near fuel pickup line.


Basic installation of engine and tank.


Completed tank. Outboard side atvtop

So the basic question is, why is there such a difference in flight time vs static run time?  True to uniflow use the overflow vent is capped on every flight so I can't have a fuel siphoning issue due to the airstream, I don't think.
No changes were made to needle valve setting.

[TDM]

Tank should be symmetrical pic up line in the midle
Then you adjust to position up down to even motor run for normal and inverted flight
One more thig. The nose has to be rigid for best runs. I do not see cross braces between the hardwood motor mounts and that is weak

[Gary Dowler]

Motor is on a nylon mount securely bolted to 1/4" ply firewall. This was early in construction and it's much stronger than what this pic would indicate

[bill bischoff]

Where does your uniflow vent go outside the tank? Does it stick straight up from the tank as shown in the pictures? If so, try making a fitting to relocate the open end of the uniflow vent to the inboard side of the fuselage, and above the tank. This will eliminate any possible fuel siphoning. The test setup can be as simple as a piece of brass tubing rubberbanded to the fuselage, connected to the tank uniflow vent with fuel line. The photo shows the same concept on a profile model.

[Brett Buck]

Tank should be symmetrical pic up line in the midle
Then you adjust to position up down to even motor run for normal and inverted flight

   The tank does not need to be symmetrical. What determines the "height" of the tank is the uniflow vent, not the shape. I have made many tanks that were that asymmetrical or more and they worked fine.

      Brett

[Gary Dowler]

   The tank does not need to be symmetrical. What determines the "height" of the tank is the uniflow vent, not the shape. I have made many tanks that were that asymmetrical or more and they worked fine.

      Brett

It does indeed work fine. The pick up line is pretty level with the carb. The idea above about rigging up the uniflow vent to the inboard side and facing forward will be tried.   I also can try capping both and using muffler pressure as the muffler fitting is 2" away from the uniflow vent line.
The question is still why does fuel usage in flight so greatly exceed fuel usage on the ground if there is only one open line to the tank when in flight.

Pic of the uniflow vent/fill line on the completed plane.

[EddyR]

Gary      Let me get away from the take problems and address another problem you need to address in the next model. I understand why the metal bellcrank as it is a throttle control model. Using stranded leadouts with out some bearing in the bellcrank can cause the leadouts to fail very quick. All so it was found out years ago that soldering stranded leadouts also leads to early failure. Many use solid leadouts on metal bellcranks to solve this problem.
NOTE  If they are solids forget what I said.
Ed