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Author Topic: fuel tank location?  (Read 1692 times)

Offline Charles Hofacker

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fuel tank location?
« on: July 12, 2015, 08:29:41 AM »
I have read in several threads that it is desirable to locate the c/l of the (traditional metal uniflow) tank slightly (approx 1/8") above the center line of the venturi/needle valve on profile models with side mounted engines then adjust to equalize to upright and inverted lap times and inside/outside engine run. Does this approximately 1/8" above the needle valve also apply to inverted and up right mounted engines? Does it also apply to "clunk" tanks? Do y'all allow for the greater height of the clunk tank - approx 1.5" vs. 1" - during construction if you are planning to use a clunk tank? The c/l of the needle valve is about .5" below (inverted engine) the surface of the engine mount. If the typical clunk tank is mounted on the rear ward extension of the engine mount the tank c/l will be about .25" below the needle valve. Is this a non-issue? For educational and experimentation reasons I'd like to be able to switch back and forth between tank types.
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Offline Tim Wescott

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Re: fuel tank location?
« Reply #1 on: July 12, 2015, 09:20:19 AM »
Yes, tank location matters with clunk tanks.  Usually if you're using a clunk tank at all it's with muffler pressure, which makes the thing less sensitive to fuel draw issues -- but moving the tank up and down to fine-tune operation is still necessary.

I don't have much personal experience with upright & inverted engines, so this is mostly regurgitating things I've read: on a traditional inverted engine installation with a clunk tank you need to relieve the engine bearers for the tank so that it'll go up a bit, and then adjust from there.  I'm not sure what the ideal starting point is.
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Offline Steve Helmick

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Re: fuel tank location?
« Reply #2 on: July 12, 2015, 01:16:52 PM »
With the typical clunk tank (i.e., not Hayes), you can fiddle with the position of the internal end of the uniflow tube to adjust the engine run for insides/outsides rpm. This feature discourages one from fiddling with the tank, once set.

Also, the installation depends on tank cross sectional shape, round, square, or rectangular.

There are a few advantages and disadvantages to a typical R/C clunk tank. Advantages are that they're cheap and easy enough to carry a spare, if you're inclined, and they will allow you to do short flights, to check needle setting or to check wings level, etc. The main disadvantage (I'm using one) is that you can't do a "cutoff loop" to position your engine shutdown for best landings on a windy day.

I recently started running pipe pressure on mine, to avoid rich running into the wind and lean running downwind. Seems to have fixed that, although the alternative is to just hook a hose to the uniflow vent that ends somewhere inside the cowl, inside the backplate cavity, etc., where there is a constant pressure. I'll probably try that next.  H^^ Steve
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Offline Tim Wescott

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Re: fuel tank location?
« Reply #3 on: July 12, 2015, 01:32:59 PM »
Quote from: Steve Helmick on July 12, 2015, 01:16:52 PM
With the typical clunk tank (i.e., not Hayes), you can fiddle with the position of the internal end of the uniflow tube to adjust the engine run for insides/outsides rpm. This feature discourages one from fiddling with the tank, once set.

To clarify Steve's statement: you can take a bottle & plug style RC clunk tank and make it into a uniflow by arranging the tubes right.  As a bonus, you can then take advantage of the fact that a uniflow tank doesn't "care" where the tank top & bottom are -- uniflow tank behavior depends on the height of the uniflow outlet inside the tank.  With a bottle & plug style tank you can rotate the uniflow tube to move the outlet up or down and tune the tank.

Unfortunately, not only can you move the uniflow, but airplane vibration can, too.

As an added note: I've been building lots of metal tanks, mostly to go into unique spots.  Most aren't very good runners until I give up on a fixed pickup and shove a clunk in there.  Metal clunk tanks work really well, and there's no reason you can't make one uniflow.  Just sayin'...
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