what are the pros and cons of each for stunt?

Terry

the cons are you still have to do the same maneuvers over and over again

Terry,
For the 4-2-4 type run muffler pressure to the uniflow line is said by some to reduce the "up wind / down wind" rich/lean in higher winds. open uniflow usually provides a constant setting in low to moderate winds and with some engines like it better depending on the nitro % and mostly the particular tank. Crankcase pressure (CCP) was at one time popular for combat. For stunt with todays 2-2-2 run style using CCP to the uniflow it could work well and should all but eliminate the up wind/down wind variation for this type setup. The downside from the CCP is you need to have a way of pinching off the pressure line for starting.

Best,    DennisT

Thanks very much, Dennis, for the informative overview.
At the moment, just one question re: clamping the CCP line - for starting or for filling?

Terry

Both...A simple wire configuration operated by a plunger to the outside of the fuselage can work well.  Also a check valve in the pressure line simplifies the system but either way the pressure can become extreme and tends to increase needle sensitivity.

A number of folks, including me, tried this back in the 60's and 70's and gave up on it as an unnecessary complication that worked but was less reliable, run to run, A few, like me even tried using accumulators (tiny tank between the main tank and the needle valve) which also worked but not well enough to be worth the extra effort and reliability factor.  One thing that generally holds true in any engineering situation is "more parts" less reliability".

Most modern stunt engines that are specifically designed for stunt really do not need pressure, especially with a tuned pipe.  The pressure change from up wind to down wind in a stiff breeze is very real but can be overcome by simply running a line (fuel tubing) back into the fuselage out of the airstream.  Muffler pressure works to do just that as does pipe pressure and doesn't provide enough pressure to become a problem with needle sensitivity.

Typically I like muffler pressure with uniflo on side exhaust engines due to the simplicity of installation and the pressure thing discussed earlier.

On rear exhaust piped engines I prefer uniflo with no pressure and simply put the uniflo line out of the airstream and back into the engine tank area for high wind conditions.  Unless the wind is extreme my PA's and RO Jetts do not seem to be drastically affected by wind pressure difference when running on a pipe.  The pipe tends to regulate the peaks and valleys and make the engine less sensitive to pressure changes at the tank.

I'm sure there are other opinions but those are mine and definitely work in general!

Randy Cuberly

Muffler pressure is somewhere around .1 or .2psi, depending on mainly outlet area. Untimed crankcase pressure (where the pressure tap just goes directly into the crankcase at some convenient point) is around 1.5psi which is why setting the needle starts to get a bit tricky. Timed crankcase pressure, which is controlled by the inlet port, is probably around 6psi.

Thanks, Brian - just another misconception on my part, but now I know.  
And you're fight, of course, Chris - there's always some pressure from somewhere.

Terry

Muffler pressure certainly does fix the rich/lean crap as the model goes up/downwind. But so does just putting a hose from the uniflow vent into a null area, such as the backplate cavity. I've done both. I didn't like pressure off the tuned pipe, but do like it off a tube muffler. Currently have one of the piped models setup with the "null" system. Not at all interested in ever using crankcase pressure again, timed or untimed.  Steve

Thanks for the explanation, Dennis.
I was curious because I have an unmuffled ignition engine with a crankcase port (of sorts) that I thought, not knowing about CCP, might be useful but after my education here I don't plan on trying it.

And Steve - thanks for the null area vent tip - neat and simple.

Terry

Just to throw the cat amongst the pigeons here, has anyone tried a mechanical pumped Heli or Pattern style fuel system?

It would seem to be the best if not overly complicated and heavy.

   People have used pumps. Not much improvement on TP engines, and reportedly "too smooth" for 4-2 break engines. Although I would have probably preferred a pressure pump rather than the Perry displacement type.

    Several people have also tried bladders with regulators, most famously Paul Walkers B-17. That's also were it was found that the pressure sometimes goes down with RPM in some ranges of operation, which is fatal to any crankcase-pressure referenced system.

      Brett

In fact, Chris, it's for the tank mount screw, open to the intake rotor housing, therefore to the crankcase.
I have it plugged for external tank now.
Not knowing any better about CCP, I thought maybe it might sub for unavailable muffler pressure.
It's moot now, since I don't want to fiddle with CCP anyway.

Terry

K&B 40 , a 1980 ish ringed baffle piston , light front end sucker , I ran in my M B 3  flying brick , as the G-51 sounded a bit odd on a 10 x 4 three blade

The Idea was to ' lock in ' the device , and keep it moving . Was flying in ' some wind ' - usually around 15 - 20 Knots at that time of year , up at
Whangaporoa - a Penisular they ran the Americas Cup a way out from . So was usually 15 - 25 . Theyed cancel it if it was over 30 . Though the
planes were usually the Folkerts , Straight Wing Spit , or the wee Phantom .

The M B 3 did the unbelievable . stayed on track . A outside loop , or inside from inverted I think youd be on full down ( or up ) and it'd hurtle round
with the 5 ft ( or was that 4 ) clearance of the deck . The Unbelievable bit was the Squares , as hitting the controls for the corners it hit the corners
fine & steady & consistant .Maybe it just felt like full control in the rounds - it pulled like a s.o.b. .

G -51 it'd get dragged about a bit - so the Idea was to go to the 10x4 3 Bl Tornado . This meant the K&B as it bolted in .

The power would come on and off , uphill & down - audibally . The PERRY PUMP steadied it up a bit ( tried it with and without ) .
But still essentially the same power characteristics . Just less of a switch - if you call it that . And naff all diferance in needle setting .

Was a home made ( carved ) nylon intake . I melted some lead into one of the backplates at some stage - months later I left it on a pot belly stove
to melt the lead out. Unfortunately it melted the backplate too . Sort of putt me off the suckers as I wondered if the case was that low a melting point too .

ANYWAY - the perry pump works o.k. , but you might need a larger intake to get the sensitivity for the 4/2 bit , Id think .

The rough Air bumping it around was the inspiration for the experiment . It tends to ' jump ' the fuel flow aroud a bit , and the power .
where its struggling uphill against a strong gust , you want to keep the fuel up to it . It seems to do that - so worthwhile there .