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Author Topic: Bypass Ports  (Read 2293 times)

Offline Dick Pacini

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Bypass Ports
« on: July 02, 2010, 03:07:01 PM »
I have read in several places about bypass ports being plugged to improve performance and/or eliminate the "Fox burp" or similar behavior.

What was the purpose of the bypass port in the first place, if the engines run better with them blocked off?
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Offline Jim Thomerson

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Re: Bypass Ports
« Reply #1 on: July 02, 2010, 07:59:45 PM »
I think there are two separate things here.  Many Schnierle engines have a third transfer port, called a boost port.  People sometimes block  these ports in an attempt to get a run that suits them better.  I've flown a Tower 40 with and without blocked boost port and could tell no difference. 

The other is that many think the fuel transfer bypass is too large in the Fox stunt 35, and that blocking most of it with a piece of wood is a good thing to do.

Online Brett Buck

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Re: Bypass Ports
« Reply #2 on: July 02, 2010, 09:27:12 PM »
I have read in several places about bypass ports being plugged to improve performance and/or eliminate the "Fox burp" or similar behavior.

What was the purpose of the bypass port in the first place, if the engines run better with them blocked off?

      I think you are mixing some terms up. The boost port in a schneurle engine *greatly* increases the power capability of the engine. The problem is that we *don't want* and *can't use* the extra power in many cases. People seem to want to run 40FPs in Noblers, and if you run a 40FP the way it wants to run and near it's capability, the airplane is far too fast for stunt. If you somehow manage to run the engine weakly enough to fly the airplane at a reasonable speed/power output, the total bypass area is far, far, too large, leading to poor performance like a huge difference in the run characteristics from inside turns to outside turns, unstable runs (runaways), burps and other issues. All because the total flow of the gas charge is too small to get a decent flow velocity in the bypasses and ports.

   Another, far better solution to the problem (in my opinion) is to use a smaller engine that can run nearer it's capability without generating too much power. A 40FP in a Twister is generally a big problem, because it's so powerful that you need to run it at 40% of it's capability. Get a 20FP, and it runs at 80%, the boost port and bypasses are properly sized for that amount of power, and you still have enough reserve to allow it to pick up in the maneuvers.

     All case-induction 2-stroke engines need a bypass of some sort to transfer the charge from the case to the cylinder. A Fox has a single bypass, for example. A baffle on the top of the piston deflects the incoming charge to the top of the cylinder. As this goes up the side of the cylinder, it forces out the exhaust. This is commonly referred to as "loop scavenging" but that's probably not technically accurate. A Fox is a single-bypass engine, where the bypass is nonetheless still too big for the power generated, thus the burps and funnies when you run it on a profile and the charge is deflected into the sides of the bypass from acceleration. If it hits the liner (on inside turns), no problem, it's really hot and the charge just stays a gas and works normally. If it hits the other side of the bypass (outside turns), it's cold, it condenses, no charge, burp or quit.  

   A 40FP is a 3-bypass engine, two fore and aft , and a "boost" port. It counts on the angle and arrangement of the ports, and the momentum of the incoming charge, to run the charge to the top of the cylinder and force the exhaust out. If the charge comes in too slowly, acceleration of cornering alters where the charge winds up. That can cause the ignition to happen differently on inside and outside turns. At stunt power, this charge is very slow, since there is so little total flow compared to the volume and cross-section of the bypasses. Once it speeds up, it can also prefer that, pump better, which makes it speed up more, which it still likes better, faster still, etc, until it approaches its maximum output. In stunt, this is called a "runaway", in the rest of the world this is very desirable as the power output is higher. It's just running the way it was designed, the problem is that we want it to at half power or less.

    Blocking the boost port does two things - it gives less bypass area so the same amount of charge goes through with more velocity, making the directional effects of the porting more consistent, AND, it cuts the maximum power and the RPM at which maximum power occurs to be closer to the power levels you might want out of a 40FP in a Nobler. If you put a 20 or 25 in the same airplane, it pretty well works without much problem  with all three ports since you are generating the same amount of power as the 40, but the total bypass area is far smaller and the velocities far higher.

     Brett

Offline Kim Mortimore

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Re: Bypass Ports
« Reply #3 on: July 03, 2010, 03:01:04 PM »

Brett,

How does the porting and/or timing of the 40 and 46LA compare with the FPs?  The 46 in particular seems to be enjoying a fairly successful stunt career in the Intermediate/Advanced classes.

Thanks
Kim Mortimore
Santa Clara, CA

Online Brett Buck

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Re: Bypass Ports
« Reply #4 on: July 03, 2010, 04:31:56 PM »
How does the porting and/or timing of the 40 and 46LA compare with the FPs?  The 46 in particular seems to be enjoying a fairly successful stunt career in the Intermediate/Advanced classes.


  No boost port. That was the big change from the FP to the LA. It cut the maximum power but made it more amenable for stunt.

     Brett

Offline RandySmith

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Re: Bypass Ports
« Reply #5 on: July 04, 2010, 01:28:19 AM »
      A Fox has a single bypass, for example. A baffle on the top of the piston deflects the incoming charge to the top of the cylinder. As this goes up the side of the cylinder, it forces out the exhaust. This is commonly referred to as "loop scavenging" but that's probably not technically accurate. A Fox is a single-bypass engine, where the bypass is nonetheless still too big for the power generated, thus the burps and funnies when you run it on a profile and the charge is deflected into the sides of the bypass from acceleration.   

     Brett


Hi Brett

You would be correct again ..it isn't technically correct, Many many people miss this and think the FOX is a loop charged engine, the FOX is not a "loop charged" engine, it is a single bypass , or crossflow scavanged engine.
 Dr Schnuerle  invented loop charging, his two opposed ports, looped over each other, A PA , FP, HP FSR and others are loop charged, the FOX 35 is a single bypass motor   A loopcharged (schnuerle) engine utilizes a sophisticated transfer port design and a flat top piston for dramatic horsepower increases. The later addition of a 3rd boost port adds to the HP increase, Variations on this were PDP porting where you add two small boost ports to a crossflow scavaged engine.

Regards
Randy

Offline Kim Mortimore

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Re: Bypass Ports
« Reply #6 on: July 04, 2010, 07:54:54 AM »

Is all the extra power from loop charging gained at higher RPM than we use for stunt, or does stunt benefit too?

Regards
Kim Mortimore
Santa Clara, CA

Offline Andrew Tinsley

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Re: Bypass Ports
« Reply #7 on: July 04, 2010, 12:46:08 PM »
Brett,
  You suggest using a 20 FP instead of a 40 FP in something like a Twister. Is this because of the well known habits of the 40 FP, or are you saying that this is generally a good thing? I presume not, as most expert pilots seem to do exactly the reverse, with very large PA and RoJett engines, running at a fraction of their max power. Pity one can't get 2 inch pitch props and run most RC engines in their preferred rev range. Then I suppose we would run into prop noise from near sonic tip speeds!
  Maybe I am missing something here?

Regards,

Andrew.
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Online Brett Buck

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Re: Bypass Ports
« Reply #8 on: July 04, 2010, 12:52:42 PM »
You suggest using a 20 FP instead of a 40 FP in something like a Twister. Is this because of the well known habits of the 40 FP, or are you saying that this is generally a good thing? I presume not, as most expert pilots seem to do exactly the reverse, with very large PA and RoJett engines, running at a fraction of their max power. Pity one can't get 2 inch pitch props and run most RC engines in their preferred rev range. Then I suppose we would run into prop noise from near sonic tip speeds!
  Maybe I am missing something here?

      The difference is that you have a tuned pipe to shape the output to our needs. And one might examine the size of the ports and bypasses on a RO-Jett 61 compared to an R/C pattern 60, I found it illuminating.

      Brett
« Last Edit: July 04, 2010, 02:29:27 PM by Brett Buck »

Dave Adamisin

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Re: Bypass Ports
« Reply #9 on: July 04, 2010, 01:53:18 PM »
At the heart of Brett's clear explanation is the fundamental point that engines are air pumps. The comparison of the 20/25 to the 40 is apt in that the trapped airflow of the 20 and 40 is the same (within reason) at the same power level.

Online Brett Buck

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Re: Bypass Ports
« Reply #10 on: July 04, 2010, 02:28:20 PM »
At the heart of Brett's clear explanation is the fundamental point that engines are air pumps. The comparison of the 20/25 to the 40 is apt in that the trapped airflow of the 20 and 40 is the same (within reason) at the same power level.

   Precisely.

     Brett

p.s. the same effect is why we run very close to the same choke area on engines from 20 to 75!
« Last Edit: July 04, 2010, 06:29:23 PM by Brett Buck »

Offline RandySmith

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Re: Bypass Ports
« Reply #11 on: July 04, 2010, 04:32:37 PM »
Is all the extra power from loop charging gained at higher RPM than we use for stunt, or does stunt benefit too?

Regards

Hi Kim

Most definate, loop charged motors, timed and setup right are a big increase in power and work very well in the RPM range we use in stunt

Randy

Dave Adamisin

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Re: Bypass Ports
« Reply #12 on: July 04, 2010, 08:20:00 PM »
  Precisely.

     Brett

p.s. the same effect is why we run very close to the same choke area on engines from 20 to 75!

Hmmmm, you noticed that to......

Offline Gordon Tarbell

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Re: Bypass Ports
« Reply #13 on: July 05, 2010, 09:37:35 AM »
Since I don't own a PA or Ro Jett engine ( I really want a PA65RE) would it be too much to ask how do the port sizes, shapes , and locations(timing) compare to those of the untethered use engines.
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Offline RandySmith

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Re: Bypass Ports
« Reply #14 on: July 05, 2010, 01:08:01 PM »
Since I don't own a PA or Ro Jett engine ( I really want a PA65RE) would it be too much to ask how do the port sizes, shapes , and locations(timing) compare to those of the untethered use engines.

Several PA engines are, for our uses, timed lower both intake and exhaust , the overlap is smaller and the ports are smaller with more attention paid to angles, crank timing is generally differant too

Regards
Randy

Offline Chris Wilson

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Re: Bypass Ports
« Reply #15 on: July 14, 2010, 09:10:18 PM »
My MVVS 40 that is converted to diesel use in stunt runs better with the boost port blocked.

The diesel just can't seem to make use of that much gas flow and completely blocking off the third port lowers the crankcase volume and speeds up the gas flow in the remaining ports, and seems to produce a finer fuel air mix when viewed with the head off the engine and its turning over with a starter.

Thanks.
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