Conference vmszoo::rc

>    	What on earth is a Flex Tank?

    Sullivan has a relatively new line of flexible fuel tanks. I think
    it's just thinner plastic that will allow you to mal-form it into tight
    spots. The tank in the SS20 will stick through the first bulkhead,
    and rather than shave away the bulkhead to make it fit, I'm hoping
    the flexible tank will just squeeze through. Before, I did shave away
    the bulkhead and it was a weak spot that broke in the crash (probably
    would have broke anyway, I hit pretty hard).

    Has anybody used those tanks? Comments?

    re: DD shock mounts -
    I'm intending on gluing a piece of thin ply on the nose of the fuel
    tank to prevent the screws from stabbing the tank. It's real tight
    inside the nose - no removable hatch - and I don't want to gum up
    the screws because I might want to remove them some day.

    Charlie Watt had a very good suggestion of running the screws in
    from the back of the shock mounts, and then using nuts on the front
    to mount the engine mount. I'd do that, except that the bottom two
    holes in the engine mount are too tight for nuts to clear. If I could,
    I'd do it that way, and add a washer under the screw head as a safety
    stop in case the shock mount fails or pulls out.
    
    +---||---+
    |___||___|
       |  |
       |  |
+------|  |-------+
|      |  |       |
|______|  |_______|
   *   |  |   *
    \ \|  |/ /         Dave "Spin-it-in" Hughes
---------------------

    The tank is designed to be mounted horizontally. The clunk can fall
    to any side of the tank, keeping it in the fuel during banking and
    even inverted flight.

    The clunk cannot fall to the front of the tank. If you mount it
    vertically, it might fly ok, but it will not fly inverted - you'd
    run out of fuel in a few seconds.

    There are also fuel flow issues having to do with the distance the
    fuel has to be siphoned, etc. I'm sure many of the "how to get started"
    books will have some good info on mounting fuel tanks.

    Dave

    The fuel tank should be mounted horizontal and with it's centerline
    just about even with the needle valve on the carb.  The reason for
    this is that as the fuel level decreases, the engine has to pull
    or push (with muffler pressure) the fuel up hill to the carb.  This
    causes the mixture to lean out (less fuel for given volume of air).
    You want to minimize this effect because it means your engine can't
    be adjusted to give uniform performance during the whole flight.
    If the Tank is too high, it will lean out when you pull the nose
    up because the tank will then be lower than the engine in the nose
    up position.  Any mounting position is a compromise which can only
    be overcome with a pump mounted at the carb.  Follow the instructions
    that come with the tank on setup.  Your engine may have come with
    info on tank mount position.  If not, take a look at some completed
    planes and see where the ones that run well have their tank mounted.
    
    Charlie
    

   
    The following is about the routing of the line from tank to carb
    and is a direct quote from one of Duke's Mixture ads/articles. Now
    Duke Fox is a savvy old timer, but this doesn't set right with me.
    Any comments or experiences?
    
       "Take-off hesitation can be eliminated and acceleration out of
        a turn can be improved by simply re-routing your fuel line.
        .... This applies to standard tank without pump set-ups.  What
        you do is route your motor's fuel line around the _right_ side
        of the motor, over the front bearing housing in front of the
        carburetor, and back, then loop it up to the fuel nipple.  The
        reason that it works is that during acceleration the fuel wants
        to move back, creating a pressure reduction or total cavitation
        at the jet in the usual set-ups.  By routing the fuel line
        around the front of the carburetor, the fuel to the left of
        the foremost point is pushed toward the carburetor, not the
        tank.  It is the small amount of fuel in this section of fuel
        line that keeps feeding the motor for the two or three seconds
        of severe acceleration."

    I'd title this entry "Onboard storage of spare fuel line", but that
    wouldn't help future retrieval.

    Al, 
    
    Sounds to me like ol' Duke is referring to racing...I doubt most of us
    ever see this problem as we're not slamming through sharp, maximum-G
    turns like a racer does.  Acceleration is gained through the fact that
    this fuel-line routing prevents the momentary starving the engine can
    suffer during such turns, thus maintaining top rpm's rather than
    sagging for a moment through the turn, then picking back up when the
    G-load diminishes.    

      |
      | |      00	 Adios,      Al
    |_|_|      ( >o
      |    Z__(O_\_	(The Desert Rat)

    As quoted, he referred explicitly to take-off hesitation, but I had
    thought that hesitation then was apt to be a carb setting issue, not a
    forces-on-the-fuel-in-the-line issue. 
    
    To avoid poisoning replies, I didn't say why I thought the idea
    suspect.  Well, I still won't, except to say that  _if_  it works 
    it might depend upon having the line diameter reduced during these
    moments. 

    I left out one sentence;   where I used "....", read:
    
    	"No Bull - it really works."
    
    There was no mention of racing.
    
    Alton

Yes All's, it is true, I have done similar myself. Another way to overcome the 
problem is to use small diameter tubing as it takes less time to get the fuel 
back to the carb.

When using a peripheral jet venturi the positioning of the fuel inlet is also 
an advantage. If the inlet is on the top the only fuel sucked out is in the 
tube, if it is on the bottom it also sucks the fuel from the banjo ring and the 
engine is more lightly to quit.

Christmas greetings to you all in notes land.

John

    i read that thingy by duke fox sometime back and didnt know whether
    to believe it or not (either).
    
    stated yet again, the problem is that as a plane accelerates sharply,
    the fuel in the line is pushed back from engine to tank, momentarily
    starving engine of fuel.
    
    dukes fix is basically to extend fuel line even further beyond and loop
    back towards engine, the theory being that the fuel in the line beyond
    the loop gets pushed back towards the engine, not the tank.
    
    but.  to get an inch of line going back towards the motor, you had to
    extend the line going forwards from the tank by the same amount.  so
    why isnt it a wash?  i don't know, apparently duke has some emperical
    evidence that this works.  i don't have that problem much in my
    relatively low performance planes anyway.
    
    kevin

        Re:                        <<< Note 940.8 by ROCK::KLADD >>>

                It will  make  no difference whatsoever.  The pressure at
        the tip of  the fuel line is the same no matter how you route the
        fuel line. Placebos work great!
        
        Anker

    Re .9
    
    
    Anker, think again about the routing!  In the normal routing
    centrifigal force from accelleration or some fast manuvers will tend
    to cause the fuel to flow back into the tank, thus reducing the
    pressure at the needle inlet. With the fuel line looped around the
    engine, the same thing will happen except that the fuel thats in the
    line at the front of the engine will tend to flow into the needle
    valve, not away from it.
    
    
    						Dave

    Yes, there will be some tendency to flow towards the needle valve,
    but this will be counter-acted to some extent  by the siphon force 
    of the other side being pushed back into the tank.   Since the mass
    is greater on the tank side, I would imagine that it would out weigh
    any positive direction flow....
    
    What we really need is a person versed in fluid dynamics and/or
    physics...  Anyone want to post this problem in the Physics notesfile?
    
    cheers,
    jeff
    

        Re:             <<< Note 940.11 by WRASSE::FRIEDRICHS "Go Bruins!!" >>>

                Jeff is  right,  as  I am!  Accelleration and gravity are
        indistinguishable.  I  am  shure you all remember the the physics
        demo that shows that  the  water  level is identical in connected
        pipes no matter what shape  and  length  they  have.  Same effect
        here!
        
        Anker

    
    It might be a mistake to get into this discussion, but I'm good 
    at mistakes so here goes...
    
    I interpret Kevin's note to mean that the guy extends his fuel
    line forward past the carb, then loops it back to the carb.
    Anker is correct that the routing of the tube does not affect the
    pressure at the end of the tube, but _only_in_a_closed_system_.
    
    But imagine the following possibility...
    
    	.----------------------------------
        | .-------------------------------- to carb
        |*|  ------> force of acceleration
        | `--------------------------------
        `---------------------------------- to fuel tank
    
    	* = air bubble
    
    There is force generated by acceleration on the fuel in the carb
    side of the loop, and on the tank side of the loop.  These forces
    work against each side of the air bubble.  If the air bubble expands
    as a result of this force (air will certainly expand easier than
    liquid fuel), then you have a net gain in the amount of fuel flow
    to the carb during acceleration.
    
    So, it's just an idea but maybe our fuel systems don't always behave
    like ideal closed systems... with tiny air bubbles floating through
    the fuel, maybe there _can_ be a net gain to looping the fuel line
    this way.
    
    Bill

    Re. .13
    
    The other possibility I see is that with the fuel line set up as
    in Bill's picture, the two forces would cancel each other out. Rather
    than that being a negative, what you would end up with is CONSTANT
    preasure at the carb, rather than a loss of preasure due to
    excelleration.
    
    
    Steve 

Like Bill, I didn't want to get into this discussion. Like Bill,
I'm doing it against my better judgement. But as long as we're
pooling our collective ignorance, I've got plenty to add to the
discussion!

Regarding Bill's reply (yes, Bill, it was a mistake to reply), you
don't need an air bubble. The liquid fuel is not compressible, but the
fuel line is very compressible. Furthermore, if you actually create a
partial vacuum, the fuel may vaporize, creating a gas bubble (of fuel,
not air). Most of the discussion here is assuming the fuel line is
rigid. I can imagine several possible explanations for why Duke's
suggestion might work, but I can also think of ways to refute those
explanations. We got too many engineers here trying to redesign the
world. I think this is one of those cases where: If you have a problem
with fuel starvation on acceleration, try this approach. If it works,
great, if it doesn't, try something else. If you've got nothing better
to do because it's frigid outside and you're winter project is finished,
then go ahead and ponder some of these mysteries of the universe!

Dave

    RE: .14 & .15.

    Bill and Dave,

    Thanks for putting in exactly what I was thinking but was too lazy
    to type.  Especially the part about the freezing temperatures
    outside, etc...  :-)  :-)

                       _____
                      |     \
                      |      \                          Silent POWER!
      _        ___________    _________   |            Happy Landings!
     | \      |           |  |         |  |
     |--------|-  SANYO  + ]-|  ASTRO  |--|              - Dan Miner
     |_/      |___________|  |_________|  |
                      |       /           |     " The Earth needs more OZONE,
                      |      /                       not Caster Oil!! "    
                      |_____/

    Just to reactivate an old note...
    
    Did anyone else notice the dual clunk fuel tank in the 1/92 Model
    Aviation? You put a clunk line on the muffler pressure line so the
    pressure bubbles through the fuel and gets trapped and pressurizes the
    tank. You need a third line as a vent that is capped off when not
    filling the tank. I was thinking of giving this a try since I seem to
    be leaning out as my tank empties.

    I saw it, looked at it for a moment and decided I needed to go back
    to it again..
    
    If you are delivering X psi from the muffler pressure line, your are 
    going to increase the pressue in the tank by Y.  I don't understand
    why the value of Y would increase just because X is delivered below
    the fuel line..
    
    still confused..
    
    jeff
    

In a normal tank setup, the muffler pressure line is always above the fuel
level. That way, pressure is built up in the open space above the fuel and
when the motor stops, the tank depressurizes through the same line. If you 
put a clunk on the end of the pressure line so that it's always at the bottom
of the tank, the air from the muffler bubbles up through the fuel and is
"trapped" at the top of the tank. It's can't escape because the pressure
line is below the fuel line. Under water so to speak. The advantage is that
you maintain a constant pressure regardless of engine speed.

The thing that I would question in this setup, is the constant "stiring up"
of the fuel. We take great care (sometimes) to wrap tanks in foam to isolate
them from vibration yet in this setup, your doing just the thing your trying
to avoid by blowing air through the fuel and "blowing bubbles" in your tank.


Steve

If the pressure line is immersed in the fuel, and the engine stops running,
it seems to me that the higher pressure in the tank would force the fuel
through the pressure line back into the muffler. Doesn't sound like a good
solution to me.

Dave

    or into the carb. They suggest a check valve in the system to avoid
    going back into the muffler while fueling (you need a capped vent for
    fueling) Depending on the pressure into the tank, it's likely to be
    empty when you're done with the flight whether you run it all out or
    not...
    
    I figured it was worth a shot...

    I once got a one way valve from an auto parts house and installed it in
    the pressure line from muffler to tank.  It built up so much pressure
    that the carb would flood the motor after running about thirty seconds
    and the motor would die.  I tried it in a couple of two stroke motors
    and one small four stroke with no luck.  Good tank positioning seems to
    be the only cure that works for me in trying to get a consistent run
    from a motor.
    
    Will

    I just wanted to report back on the 2 clunk pressure system. I couldn't
    go flying yesterday but I decided to try the tank in the driveway. I
    needed to adjust the idle on the engine anyway. I installed the system
    in my Panic which has a ST .75 2 stroke on it. I carefully tied it to
    the garage door with rope and fueled it up. I had replaced the needle
    valve assembly so I started about 1.5 turns out so I wouldn't flood the
    engine initially. After a few attempts I got it running and it seemed
    quite happy with the pressure system. The needle wasn't excessively
    sensitive and I started working on the idle mixture. I finally got it
    to crank open without sputtering and decided to run the rest of the
    tank through it. I didn't see any of the expected foaming until about
    the last ounce of fuel and then it stopped when I throttled back to
    about 3/4ths throttle. If the weather cooperates, I'll flight test it
    sunday.

Well, I finally flight tested it yesterday. Charlie Watt helped readjust the
carb for the 80+ degree temps and I got in several uneventful flights. There 
was a tendancy to load up on idle which might be due to the check valve 
keeping the tank pressurized but I didn't fiddle with it too much. I may 
remove the check valve for a couple of flights next time. The system is flyable
but who knows if it's better or not.