Tank height: Uniflow v. Standard vent...

I know, I know, I'm sure this has been covered 10 jillion times, but are they both as sensitive about tank height, or is one less sensitive than the other?

They should be about the same as concerns tank height. Fuel draw with whatever engine you are using should have more to do with how sensitive it is than the type of tank.

A large part of my experience with both venting arrangements is on profile planes, on which it has seemed to me that uniflow tanks did in fact make the tank height variations more noticeable. Atmospherically vented tanks worked quite well lined up on the negine CL. There most likely was some small difference in speed, but most fliers just didn't use stop watches nearly as often before Uniflow venting became so popular.

The variations seem mostly related to individual engine types, such that the Fox Mark IV Combat Specials (and Mark VI's as well), when operated on GRW Chicken Hopper tanks, needed 1/4" to 5/16" of offset upward. I haven't found any other engine that wanted as much offset as that.

Upright and inverted mounted engines typically want between 1/16" and 1/8" of upward offset (and the truth of the matter is probably that either tank type actually needs the same offset), and using a set of engine motor mount "pads" is a good way to get that offset with an inverted mount (just make allowance for realigning the nose block and cowl with the altered thrust line).

.

Kiwi

Tom,

Open vented tanks tend to lean out as fuel is used up. Uniflow vented tanks don't. Because uniflow tanks run more consistently, any relative-height 'error' shows more noticeably.

I have a special test stand to determine "relative tank height" for equal running upright and inverted. Tank and engine mount to a baseplate that I can rotate -more or less about the crankshaft axis- while the engine runs. The tank is in an adjustable fixture so "relative height" can be changed between runs. The tank fixture can be set up to simulate upright, inverted, and head-in or head-out profile installations.(Think about the positions of cylinder head and tank vertex in each of those.)

Rotating the baseplate simulates directions of g loads on the fuel in flight. Flight loads combine gravity's 1 g and centrifugal loads of 3-plus g. We can't recreate dynamic load QUANTITIES in static conditions, but we can use the ANGLES at which they work. After starting the engine, I rotate the baseplate to a neutral mid-point, with tank vertex straight down, and set an easy RPM.

Rotating the baseplate 45º in the 'cockpit up' direction from neutral crudely simulates upright flight loads. Rotating it past 'neutral' to a 45º 'cockpit down' angle simulates inverted loadings. Engine RPM changes in response to relative tank height differences, or gravity head fuel feed, between the two 45º's.

If the tank is too high on one side, the engine richens. It will also lean a bit when the baseplate is rotated to the other 45º. So, check RPM at each 45º. If there's a difference, pinch off fuel to stop the engine. Adjust the tank fixture to correct tank height. Test-run the adjustment. This usually takes a few test runs to get right. When there is no detectable RPM change between the 45º's, the tank height is centered for gravity feed effects. And, as flight loads are the same, only larger, than static loads, it is centered for them, too.

Those 45º tilt angles are not critical. They are large enough to have effect, and easy to eyeball. Improves confidence in getting the same test angles both ways from 'neutral'.

It works. I rarely make tank height adjustable in a model... only if I'll use different engines, with differing tank height needs. Examples of a type respond consistently... usually... Why do engines differ about tank height? That's complicated. I doubt there is any simple, useful explanation. My bench setup lets me build-in a tank, and know it will feed right.