I have been following the theme about muffler pressure for stunt. My query is what about crankcase pressure for some of the more potent 1/2A engines? I have been experimenting with an MP Jet engine which goes like a bomb when you find the correct diameter of crankcase nipple. It all seems a bit hit and miss to me. So is there any rule of thumb for this type of pressure feed?
I took me a little while to figure out why there is an average positive pressure in the crankcase, but I still feel a one way valve would be an advantage. I know this isn't normal stunt application, but those people that have it mastered, do some very impressive stunt schedules!

Andrew.

Thanks Jim,
 had a suspicion that a one way valve would be a good bet. Are there any valves that you could recommend. The ones I have are a bit on the large side for 1/2A. I wonder if the hit and miss results I got are due to the fact that I omitted the one way valve. I tried the scientific approach and started with a very small aperture and just kept on increasing the diameter. The results were very inconsistent and I couldn't really tell what was happening!

Thanks Andrew.

P.S. The AM25 is doing sterling work and is now powering grandson's first "big" (!) stunt job.

Back in 1978, I wrote in my CL Racing column in Model Aviation:

A good question to ask here is, "How much does
this pressure change affect the total pressure in at
pressure system?" In order to find an answer to this,
you have to know the pressure that is created in the
tank by using crankcase pressure. I set up
a test stand to measure this and found that the tank
pressure from an untimed crankcase tap is about 1.4
psi, or about 1.6 psi if a ball-check is used in the
pressure line.

The entire column can be seen at http://www.nclra.org/TechTopics/UniflowPhysics.pdf

I believe these are what Paul and Howard used on the B-17 "909", very possibly modified.  Steve 


http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXG850&P=7

To: Rustler -

Timed shaft pressure is quite a bit higher than 'averaged' case pressure, as it is only pumping when the piston is on the way down. The shaft intake port hole closes before the piston starts to rise significantly. A check-valve also prevents cyclical pressure drop in the case from affecting fuel delivery.

Years back, I made a few, flawed, attempts to measure the pressure from muffler, averaged-case and shaft-timed pressure operation. Scatter of observations with cheap Pep Boys type pressure guages negated anything conclusive.

The trend was there, however:-

Lowest: muffler pressure

middle-ground: averaged case pressure

highest: timed (shaft port) pressure.

Otherwise, only inflated bladder pressure tanks  - as in glow combat - exceed any of the above as a fuel delivery pressure source.

Sometime way back in the 70's I did an experiment on an ST G20/15 to find out what untimed pressure was and from memory it was 1.5psi. The pressure guage was a simple water manometer where 1psi = 27.75". Although I've never checked it, I believe muffler pressure is around .1psi and a simple way to test it is to lower a length of fuel tube into a glass of water until it stops bubbling. Timed crankcase pressure is around 6psi so you'd need quite a tall manometer to measure that (or use mercury instead of water).