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Author Topic: Bladder pressure regulation  (Read 1464 times)

Offline Mike Urban

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Bladder pressure regulation
« on: May 01, 2022, 06:50:31 AM »
Hi folks,
I was a pretty active combat flyer in the 80’s & 90’s, but work and family took me away from it until recently. Now retired, I’m trying to climb back into it, starting with speed limit. It’s going pretty well. I have several OS 25 FP’s which run really well, until about the last half ounce or so in the bladder, when the pressure increase causes a rich run for the last few laps.

This is not a new problem. We had to deal with it in fast combat, but with the 36’s it only seemed to last a couple laps, plus by that point I had usually killed or been killed. With these speed limit engines it seems to last about 10 laps, and those can be important laps when flying for cuts only.

I think Norm McFadden designed and used a pressure regulator that fit inside the back cover cavity of a Mk III which solved the problem. Does anyone have details of that design?  Are there other solutions?  I understand one solution is to carry enough fuel to run 5 minutes. Unfortunately, the bladder tube on some of my planes is a bit too short for that.

Suggestions?

Thanks,
Mike

Offline Paul Smith

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Re: Bladder pressure regulation
« Reply #1 on: May 01, 2022, 11:06:50 AM »
I don't know enough about the McFadden unit to make one, but they sure worked when I saw them in action.

His Nelson were at full power in every attitude of flight: upside done, upside right, inside & outside loops, etc.  Good from the first flip to the last drop of fuel.
In addition, he didn't need to pinch the fuel line while starting.  Just prime & flip. Maybe not even prime.

The late great Hardy Broderson had that on his F1C's also.  He could just connect the battery, engage the starter and be running at full speed.

It uses a ball check valve actuated by crankcase pressure to admit fuel in sync with the RPM.

The last few drops isn't the main point.  The key element is absolute full power in every phase of flight.

Paul Smith

Offline Bob Mears

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Re: Bladder pressure regulation
« Reply #2 on: May 09, 2022, 06:03:32 PM »
Pete Jaden and I used to put a restrictor in the fuel line. We would build several with different size holes and end up running the one that would let us set the motor a little rich, but could open the needle way out and not flame out, if that makes any sense. So basically a restrictor as small as we could get it and not go lean. It made a great difference at the end of the bladder.
Home of the control line combat museum.

Offline phil c

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Re: Bladder pressure regulation
« Reply #3 on: May 11, 2022, 04:09:23 PM »
Mike- the simplest fix is to make the bladder long enough to get a good run before the engine goes rich.

But Norm's fuel regulator did work very well.  They were built just like an industrial regulator- a piston pushed by a spring with adjustable pressure on it.  The movement of the piston would pull/push a valve in the wall of the intake to open or close the fuel input valve to keep the pressure constant.  A screw in the end of the regulator piston compartment changes the pressure on the inlet valve to keep it constant.

I had one for while and it  did work.  It was rather fussy if fuel got a spec of dirt past the regulator valve.  Everything had to be small and fairly delicate.

A couple of suppliers back in the 70's did make inline adjustments to keep the flow rate steady.  I tried one, but for combat it was more bother than it was worth.

Refrigerator capillary tubing is still available from Grainger industrial supply.  10ft. is around$25 and they have ID's from .026 to 0.1in.  They might have some short pieces to try.

But the best solution is a needle valve that can adjust the range you need.  The OS 1a for the old OS 10 works pretty well as long as you put a piece of tubing around the threads to keep from leaking fuel.  There are some specialty model suppliers that that supply NVA's for small engines that work too.

An alternative also is an F2D needle valve assembly. They all use a very fine thread which works from Half A to a Fast 36.
« Last Edit: June 26, 2022, 11:31:26 AM by phil c »
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