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  • November 17, 2018, 06:57:24 AM

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Author Topic: case pressure to the unaflow tube  (Read 738 times)

Offline bob whitney

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case pressure to the unaflow tube
« on: June 06, 2018, 06:42:37 PM »
the old brain is working overtime today. what affect dose  case pressure have on the unaflo tube ,I know it needs to stay under the fuel ,but what about the above or below the feed tube .will it affect the up right ,inverted settings if not centered
rad racer


Offline Tim Wescott

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Re: case pressure to the unaflow tube
« Reply #1 on: June 06, 2018, 07:41:42 PM »
Uh, what are you thinking?  AFAIK case pressure isn't really suitable (or necessary) for stunt, and uniflow is really only needed for stunt.

Based on the strength of the 'puff' that I get when I take a head off and turn an engine, case pressure is around 1 or 2 psi (someone please correct me, if you have real numbers!!!)  The head variations that uniflow is correcting for are about 1/10th that.  So if you are running case pressure, I'm pretty sure that running uniflow is moot.

If you meant to say "muffler pressure" instead of "case pressure", then none of the above applies.
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Offline bob whitney

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Re: case pressure to the unaflow tube
« Reply #2 on: June 06, 2018, 10:29:58 PM »
Oh young Grasshopper  LL~ . Stunt got along for years with the standard tank while the racing boys were burning up engines at the end of a run if they didn't have a shut off, Rat Race and Good Year have been using case pressure for years to make use of the bigger venture;s

before pressurized  fuel bladders almost all combat ships used case pressure ,Rad Racer

this is for a project that I am working on
rad racer

Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #3 on: June 08, 2018, 06:29:29 AM »
Not to mention, Bob, that the uniflow effect works with case pressure just like it does when the uniflow vent is exposed to ambient pressure. The uniflow controls the tank transition.

The benefit of a pressure fuel system  with a "standard" vent is that, while the amount of the transition pressure change is still the same, it is a smaller part of the total pressure the engine "sees", hence less overall effect.

The uniflow still operates, so even if if you add the constant head pressure, the transition is (essentially) zero.

Remember the  Rat tanks with the pressure vent half way out in the tank, maybe up under the quick fill? Remember the "conventional wisdom" that you needed a 7 or 8 ounce tank so that you didn't get a lean run at the end? Remember that after a 35 (earlier 47) lap segment that the tank was still half-full? Can you say uniflow?

It's all in the physics of the situation (as opposed to "stunt physics")  ;D
Bill Lee
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Offline Brett Buck

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Re: case pressure to the unaflow tube
« Reply #4 on: June 08, 2018, 10:55:33 AM »
Oh young Grasshopper  LL~ . Stunt got along for years with the standard tank while the racing boys were burning up engines at the end of a run if they didn't have a shut off, Rat Race and Good Year have been using case pressure for years to make use of the bigger venture;s

   You were burning up engines at the end of the run because you were running case pressure and bigger venturis - and put the tank vent in the wrong place. Case pressure reduced the tendency to go lean at the end of the tank, but making the venturi larger put you right back in the same (or worse) boat. as soon as your head pressure started to go down.

   Bill has a point about "stunt physics" in general and the tendency towards magical thinking,  but this has been understood with simple physics for a very long time.

     Brett

Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #5 on: June 08, 2018, 12:16:39 PM »
   You were burning up engines at the end of the run because you were running case pressure and bigger venturis - and put the tank vent in the wrong place. Case pressure reduced the tendency to go lean at the end of the tank, but making the venturi larger put you right back in the same (or worse) boat. as soon as your head pressure started to go down.

   Bill has a point about "stunt physics" in general and the tendency towards magical thinking,  but this has been understood with simple physics for a very long time.

     Brett

Right on, Brett!

Back in those days, most were using the round Don's Rat tank. The quick fill was on the top towards the front.  Most who didn't understand, ran the pressure vent into the tank and stuck it in the middle of the top up under the quick fill. Take off with a full tank and that pressure line position made the first 1/2 or so of the tank run a uniflow. But when the vent uncovered, the tank then became a "normally vented" tank and transitioned to lean. Their answer was to make the tank bigger and bigger and bigger until they could get a full pit segment out of that front half of the tank. There were many running 7 and 8 ounce tanks, filled the entire interior of the Harters Rat pan from the engine back.

Nobody could understand how my little 4-oz tanks could give me the perfect runs I always had!  :)
Bill Lee
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Offline Tim Wescott

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Re: case pressure to the unaflow tube
« Reply #6 on: June 08, 2018, 12:37:44 PM »
... and uniflow is really only needed for stunt ...

So I'm gathering that my statement above was naive and wrong?  Should I ever take up racing, I may want a uniflow tank?
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Offline Dennis Toth

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Re: case pressure to the unaflow tube
« Reply #7 on: June 08, 2018, 12:55:42 PM »
So Bill, what was the design of your 4oz tank? Did you use case pressure? Let's not lose the technology.


Best,    DennisT

Offline Motorman

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Re: case pressure to the unaflow tube
« Reply #8 on: June 08, 2018, 02:54:20 PM »
wow I don't have time to rebut all the totally wrong statements on this thread.
There will be a sunny day and we will fly our airplanes.

Offline RandySmith

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Re: case pressure to the unaflow tube
« Reply #9 on: June 08, 2018, 03:00:07 PM »
So I'm gathering that my statement above was naive and wrong?  Should I ever take up racing, I may want a uniflow tank?


LOL  ....  YES  you would want a uniflow tank, I ran these  when I flew  15-  19  racing events
Sometimes these thread get a chuckle out of people :-)

Randy

Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #10 on: June 08, 2018, 03:33:39 PM »
So I'm gathering that my statement above was naive and wrong?  Should I ever take up racing, I may want a uniflow tank?

Yes, absolutely!
Bill Lee
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Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #11 on: June 08, 2018, 03:36:27 PM »
So Bill, what was the design of your 4oz tank? Did you use case pressure? Let's not lose the technology.


Best,    DennisT

It was a simple coffin-shaped tank with the pickup in the outer most corner at the bottom and the uniflow/pressure inlet about 1/2" above. The tank was about 3-3/4 oz when I was using the K&B 40S but had to be extended to about 4-1/4 oz when I switched to the K&B 6.5.
Bill Lee
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Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #12 on: June 08, 2018, 03:39:08 PM »
wow I don't have time to rebut all the totally wrong statements on this thread.

Give it a shot Motormo .....Motorman, I for one would like you to point out "all the totally wrong statements". Will be an interesting discussion!

Bill Lee
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Offline Dennis Keeton

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Re: case pressure to the unaflow tube
« Reply #13 on: June 08, 2018, 04:55:04 PM »
Yea moto... come on tell us who is wrong, who is wronger, and who is the wrongest!!!

Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #14 on: June 13, 2018, 06:59:43 AM »
Yea moto... come on tell us who is wrong, who is wronger, and who is the wrongest!!!
It's been pretty quiet, hasn't it, Dennis!  ;D
Bill Lee
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Online Ken Culbertson

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Re: case pressure to the unaflow tube
« Reply #15 on: June 13, 2018, 07:45:21 AM »
This is a bit off topic but I have been running simple muffler pressure without uniflow in my clunk tanks recently and I am getting much better runs than I did with uniflow, especially my OS46LA.  I can set it at a very fast 4 and it holds there for the entire pattern with no 4-2 or any need for it (11oz wing loading).  I am not complaining, I love it but I don't quite understand why it does this.  Am I misreading a fat 2 for a fast 4 perhaps?

Ken
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Offline BillLee

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Re: case pressure to the unaflow tube
« Reply #16 on: June 13, 2018, 12:50:47 PM »
This is a bit off topic but I have been running simple muffler pressure without uniflow in my clunk tanks recently and I am getting much better runs than I did with uniflow, especially my OS46LA.  I can set it at a very fast 4 and it holds there for the entire pattern with no 4-2 or any need for it (11oz wing loading).  I am not complaining, I love it but I don't quite understand why it does this.  Am I misreading a fat 2 for a fast 4 perhaps?

Ken
I can't give you a definitive answer, Ken, but my guess is that when you are running rich enough to fly on a four-cycle, the transition that is in EVERY tank is covered up by the excess fuel being delivered.

In other words, the engine has enough power on the extra rich front or the tank so that when the mixture leans out due to the transition, the engine is still happy on the resultant too-rich mixture. The uniflow would simply make it stay the same and you are experiencing the difference between a poor too-rich setting that persists with one that gets a bit better as the tank empties.

Also keep in mind that the effect of a uniflow is best seen under steady-state conditions, like a speed or a racing model, and the wildly varying G-forces experienced in a stunt model makes it all different.

Just a guess.
Bill Lee
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Offline Dennis Keeton

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Re: case pressure to the unaflow tube
« Reply #17 on: June 13, 2018, 03:20:25 PM »
It's been pretty quiet, hasn't it, Dennis!  ;D

I guess moto... is too busy with "other &@$#I!!!" to enlighten us!!!


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