Hi guys,

I make my own tanks, uniflow type, and I don't see a difference between exhaust pressure or natural pressure. I've tried both set ups on the same engine and tank and don't see any difference between the two different set ups. What should I be looking for? Each set up acts the same, good engine runs, quits the same. What am I doing wrong? Shouldn't there be a difference? I don't see any use for the exhaust pressure system, maybe with high tect engines? My brother's saito 62 loves running without using the pressure tap. Its turning 8800 rpm's and uses four ounces of 10/22 gma fuel. Run time is 7:31 minutes, not bad for four ounces. Its fun doing four second laps on 63' lines. The plane is a good old Strega ARF and I love the way she flys. Would appreciate any input, Gary

Gary,

Uniflow hard, metal tanks were a great help in getting - basically - suction feed engines to do a few things they didn't with the old traditional over and under vented tanks:

=== They could be adjusted to a nearly identical run from start-up to the last bubbles left in the tank,

=== They could be adjusted so that upright and inside maneuvers ran identically the same as outside and downward maneuvers.

The engines that seemed to benefit most were older designs, where power was a bit marginal. It was there, but fuel feed irregularities (height, maneuver forces, upright and inverted conditions) moved the apparent run off ideal. The over and under venting system allowed vent air into the tank whether the model was upright or inevrted, with minimun spilling. However, "fuel head" - the "height of the fuel's surface" inside the tank varied by its full width from start to finish of a flight.

The effect is like putting an engine on a test stand and starting it with the tank several inches above the spraybar, then gradually moving the tank several inches below the spraybar as the fuel empties out. Yes, it will run richer at the beginning of that, and will get leaner as you lower the tank. (The loads on the fuel in flight include its weight, always straight down, and combinations of centrifugal force from flying at the line length radius, and from maneuvering g loads.)

In theory, a uniflow tank has a pretty constant "fuel head" of about the distance between the fuel pickup end and the vent end inside the tank that lets air enter the sheel to replace fuel drawn out by the engine. Measure this distance in regard to the equivalent load made up of simple centrifugal force, maneuvering g and weight. In level flight, the resulting loads "aim" out about 3 g and down about 1 g -something like 22° below horizontal...

Maneuvers blow that all to heck, but they go both ways in about the same number and severity...

BTW, I've never had great results using muffler pressure to a uniflow vent, particularly with Fox 35s. Muffler pressure to the overflow vent seemed more consistent and stable...

Ralph,
Have you tried crankcase pressure or bladder with a stunt engine, or combat/racing only?  I know from another recent SH post that Bob Palmer used case pressure on at least one plane.  Also, I have a plane that would benefit from being able to use an oddball tank placement, and a bladder would be good if didn't cause oversensitive needle settings or other problems.

Kim Mortimore

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Hi Kim,
Pressure, whether crankcase or bladder (I preferred pacifiers), is practically a "given" in combat. In stunt, I have used only muffler pressure in the last 25 years or so. I did try crankcase pressure on a G21/.35 on a Midwest AD-4 Skyraider back in the '70s, and the engine performed about the same as without pressure, so I went back to suction.

Ralph