Pressurized OS 25fp gloom

Well, last week, I was able to get the engine running just fine on the test stand, but this week, I could not get a consistent run in the plane. I may have to shelve this particular line of research, and put in a conventionally plumbed engine. I need this plane fully tuned in for VSC, and can't spend more time fiddling with the engine just for fun.

But I'm not giving up completely!

Larry, Why not contact Derek Moran in Santa Barbara. He runs the same engine with good success. I actually flew his plane and the motor performed fine.

>Larry, Why not contact Derek Moran in Santa Barbara. He runs
>the same engine with good success. I actually flew his plane
>and the motor performed fine.

Larry said he got a good run with conventional methods - he just want's to run pressure, which may or may not require the violation of basic physical and engineering principles!

Derek's airplane, in addition to running well, is also beautifully done with excellent craftsmanship.

Thanks, Brett.
I've ruminated on Larry's "challenge" since his tread began. I ran crankcase pressure on combat and sport planes when I was a kid, but there isn't much I can contribute that hasn't already been said. I never had any problems, but I eventually switched to pacifiers because I found them easier to run.
One small suggestion may be helpful. I always routed my pressure line to the inside, top, front corner of the tank. This helped prevent flooding while starting the engine. If you, Larry, are running pressure through a uniflow vent, it's possible the pressure bubbles are interfering with fuel flow to the pickup. Perhaps you could try relocating the pressure line to the front of the tank.
The plane that Milton and Brett are referring to uses a Tettra Bubbless tank, vented to atmosphere. These same tanks are used by pylon racers with crankcase pressure.
Derek Moran

>I've ruminated on Larry's "challenge" since his tread began.
> I ran crankcase pressure on combat and sport planes when I
>was a kid, but there isn't much I can contribute that hasn't
>already been said. I never had any problems, but I
>eventually switched to pacifiers because I found them easier
>to run.

Howard and I have speculated, with no direct information, that the 25FP crankcase pressure goes down with RPM (as Howard directly measured on the 15FP). If it does, the rest of the system setup doesn't really matter very much, and it will be unstable regardless of compression, fuel, tank, etc.

I'm sure he knows how make it work normally - he's just experimenting. He can always put it back, assuming it doesn't fry in the attempt.

Alright then, perhaps something like a Cline regulator http://www.billsroom.com/pcfs/fuelsysdesc.htm would help.
Derek

That looks like it would be just the ticket, but it costs more than the engine did! It looks like it should cost more like $6, not $60!

One approach I haven't tried, is a pressure relief valve for the pressure line before it gets to the tank. It would be very easy to make on my lathe, and could even just be a modified fuel filter (substitue a ball and spring for the screen) Then, if done right you can set the pressure in the tank to be below the lowest pressure the engine will generate, and it will remain absolutely constant, similar to a bladder in the main part of the run. (as I recall, bladders go rich at the very end, then die lean)

my pressure line does go to the front of the tank, and I am using a Fourmost check valve in the line to prevent back flooding of the engine.

I have switched the engine back to the restricted venturi and muffler pressure that has worked so well in the past. I forgot to pull out the head shims I put in, and am debating whether to leave them in or pull them out before trying the plane again. I can always do that operation at the field if it is needed.

Larry,
While reading about your woes with crankcase pressure, I remembered something we used to do when crankcase pressure was used for combat. We built a "dome" on top of the tank in the forward inboard corner for the pressure fitting. On mine I used the metal cap off pint cans of thinner soldered to the top of the tank with a piece of 1/8th brass tube soldered into it for the pressure line. The hole into the tank under the dome was small, i think about 1/32. If we were careful and did not fill the dome with fuel during the fueling process, we got pretty good runs. Pacifiers seemed easier to me, so I converted early, but there were always some guys who stuck with metal tanks. This simple mod made for much more consistant runs.