PA 65 and pipe questions

When the pipe too short, what is the effect?

Can a short pipe cause runaways, or the engine to speed up a lot until a runaway occurs?

I think you were saying that when it cools off and the motor is getting more air volume beause it is more dense. This is why we use more fuel when it is cold out. This could also be the reason we need to lengthen the pipe. It is getting alot more air in there in that same distance and it is acting like it is too short. Is this correct?

Also how much to pull the pipe to get that smooth transition back?

DMoon

Smaller venturi may also be appropriate for colder weather.

From South Florida to Muncie Indiana and ranging 48 degrees to over 100, this is what we do to achieve consistent runs. Vary the percentage of Nitro in your fuel according to weather conditions. In cool conditions run 5%, moderate temperatures use 7.5%, and in the thin & very hot summer air go to 10%. Seldom do we change pipe length, that would be a real pain since on one flying day it can start out the morning at 48 degrees and be in the 70’s to 80’s by the time of a Contest’s first round. Then it can rise to the high 90’s after the typical judges’ lunch break. Imagine trying to chase that between rounds with pipe adjustments? By just starting the day with 5% when the air is cold and has lots of bite, and then possibly switching to 7.5% when the conditions change and more nitro is required, you can achieve steady and predictable performance of your run through the day/season. Purchase both 5% and 10% fuel, and then mix some 50/50 to make the 7.5%. For times when a small amount of additional power is desired, switch from the green air filter to the black one. When that is not enough, put back the green air filter and go up in Nitro.

Prop pitch is the next part of the equation. As a general rule, go down in pitch when the air has more bite, like when it is cool, and up in pitch as it gets warmer or “thinner”. Whatever the pitch is that you settle in at, you should always carry one prop pitched just a little more and one a little less in your box at the field.

A pipe that is too short can do many bad things. First, you can blow raw fuel out the venturi. Secondly, that same hot exhaust charge that pushed the cool fresh charge out the venturi is now super heating the motor. I have been in Randy’s room at the Nationals when he showed me the scorched evidence of running a motor with a pipe that was too short. He then showed me the inside of a motor that ran correctly for years and looked brand new. As for the running characteristics of a motor running with a pipe too short, it will charge and hit too hard. This is also a symptom of too much Nitro, so consult Randy's Pipe Tuning chart if in doubt. Another symptom is that lap times will be quicker after the Clover than before the Wingover. The reverse is also true.

Shorter pipes are for Higher RPM’s and longer pipes for lower RPM’s. If your pipe is too short for the RPM’s you are running, when it hits a two cycle and jumps in RPM it will develop too much power as it begins to “enjoy running faster” on its short pipe. When that happens take the pipe out a little, or lengthen it. Longer Slower, Shorter Faster.

Hope this helps,
Curt

I’m not an expert; I just pretend to be one on the Internet.

RPM Length
At launch Plug to first baffle

8,438 19.19”
9,000 18.61”
9,562 18.01”
10,125 17.44”
10,680 17.04”
11,250 16.61”
11,812 16.02”
12,375 15.43”
13,500 14.24”

Slower Longer...Faster Shorter

I am presently running a PA 61 turning a 12x4.0 3-blade Bolly (cut and re-pitched from a 13x4.5). My ground RPM’s at launch is 10,400 and my pipe length is 17 3/16” (17.18”). This week, I am running 7.5% Nitro.

RPM / Length
At launch / Plug to first baffle

8,438...........19.19”
9,000...........18.61”
9,562...........18.01”
10,125..........17.44”
10,680..........17.04”
11,250..........16.61”
11,812..........16.02”
12,375..........15.43”
13,500..........14.24”

Slower Longer...Faster Shorter

I am presently running a PA 61 turning a 12x4.0 3-blade Bolly (cut and re-pitched from a 13x4.5). My ground RPM’s at launch is 10,400 and my pipe length is 17 3/16” (17.18”). This week, I am running 7.5% Nitro.

You said you saw the inside of a motor that had been run correct for two years and it looked new. Mine is two years old and has many runs on it.(maybe 500+) The cylinder looks great you can still see hone marks in the liner. The back of the piston when you look in the exhaust port is clean. However the top of piston is balck with minor buildup of carbon on top. Did this motor you saw have any carbon on the top of the piston? Mine builds up, every once in a while I might very carfully clean off any carbon buildup that is on the top of the piston only. I usually just leave it alone because it runs darn good. Also I build up carbon on my plugs and they grow a little wart and then fail. I use the same fuel as everyone else around here. I have no idea what this could be.

Any black on the op of that piston?

DMoon

Typically, no. Curt has posted the chart Randy sends with the engine. If you're much off the posted lengths (more than about 3/8-1/2") and you're using all the standard parts, you're either really way ahead of the rest of us, or you're messing up. Even Ted, David, and I, running a lot different setup, run about the same pipe lengths per RPM as everyone else.

The physics are simple enough. The time it takes the wave to go from the exhaust port to the reflecting surface, and back to the exhaust, needs to pretty close to the time it takes the exhaust port to crack open, go around bottom dead center, and then close again. The variables are the exhaust timing, the pipe length, and the speed of sound in the exhaust gasses. The last is about 1400 fps or so, and is essentially independent of the conditions. If you make a mistake, I suggest making the mistake on the long side instead of the short side.

Changes in conditions are handled almost like before. I like nitro changes because it's easy. Less nitro in low density altitudes, more in high density altitudes. At home, I run 10% SIG in the summer. In Muncie, I use 15% with the same everything else, or in extreme conditions, more pitch.

Changing the pitch is also a traditional way to adjust for conditions. While you used to use maybe 5" in the cold, 5.8 in medium, and 6.3 in hot on the ST46 or 60, now use 3.5 in the cold, 3.75 in medium, and 3.9 in the hot. The reasoning is that the air density effects the advance ratio, so you need more pitch to make up for more "slip". This is just exactly like it used to be in 4-2 rbeak days, for the same reason.

Pipe length tweaks are good for slightly moving the break point up or down, or slightly tweaking the lap times up or down. Using by current favorite prop (11.5-3.7 Brian Eather "red" 3-blade), my airplane wants to fly about 5.35-5.4 second laps. If I push it in 1/4" and left the needle alone, it would probably want to fly about 5.25. If I then backed it off with the needle to get it back to 5.4, I would then be running richer, and get more boost/brake. I am running a ground launch rpm of 10400-10500 with a 17.375 pipe length, which is on the long side of the tuning chart. I like it that way, because I want a bare minimum boost/brake.

Note that I use the spelling "brake", not "break". It's not necessary to have the motor break into a 2-stroke to get mnore power. It has plenty of power variation even in a solid 4 or a solid two, whether you can hear an audible change or not. There's nothing wrong with running it in a solid 4, a solid 2, or somewhere in between, as long as it gives you the right power variation, don't worry about the noise it makes. I personally try to run in a constant 4, because I find the boost from 4-2 not to my liking even with a lot of head shims, but that's just a personal preference.

The moral of the story - just because you can adjust it, doesn't mean you should. A real problem with those who are inexperienced with piped systems is that they tend to see any issue with the engine as a pipe length problem. The length is a no-brainer, it everything else that is a question mark, and most of these question marks are the same as any other engine system.

>When the pipe too short, what
>is the effect?

A little too short, and it charges too hard in the maneuvers. A lot too short, it causes runaways, or just flies too fast. This is because it wants to run at the tuned speed shown in the chart.

If you need a 10400 rpm to fly the airplane, and you set the pipe to 16.5", you'll probably end up going like a bat out of hell when the engine decides it want to run at 11,500. Solution: lengthen the pipe, or use a lot less pitch.

>Can a short pipe cause runaways,
>or the engine to speed
>up a lot until a
>runaway occurs?

see above.

Bear in mind I have a 61, not a 65, so it may end up differently. I had .016 head clearance as delivered, and I am currently at .024. This is with the Hemi, not the derby head. I had it up as high as about .036, which smoothed out the break some, but also just didn't feel very strong.

The timing should be the same, so the pipe length/rpm should be the same as well.

My current PA61 is set up as follows:

Stock crank and sleeve timing
#5 Spigot venturi with a 1/16 SIG eyelet for the spigot, and only about .020 protrusion
Pipe length = 17 3/8, standard 61 pipe with exhaust straight out the back
largest Du-Bro stinger.

Hemi with .024 head clearance
Thunderbolt 4-Cycle plug
Brian Eather 11.5-3.7 "red" 3-blade, helical, cut from 12-3.5
7.4 oz of standard SIG 10% for 6:30

I'm very happy with the way it runs out here, in the typical 1000-2500 foot density altitudes and low humidity. The guts are completely stock, the pipe is completely stock, and the pipe length is pretty much consistent with the chart (although I use a spreadsheet). The spigot venturi helps with the inside/outside difference.

I like it slightly less well in the Midwest, with everything the same but SIG 15%. This is probably because I haven't had time to refine anything, just throw together something that works in time for qualifying. The only changes I have made was the nitro, and occasionally props. I liked the 11.75-3.9 Eather in the super-miserable weather at the NATS early in the week, but I switched back to the other prop about 30 seconds before my first qualifying flight, since it had cooled off considerably.

The point at which a given engine breaks from a four to a two cycle run is pretty much determined by the heat of compression (controlled by adjusting the height of the head with shims) and the "octane" of the fuel (nitro percentage). The bad result of a shorter than ideal pipe (for otherwise copacetic conditions) is a hard break from four to cycle operation.

With engines timed as are most schnerle ported engines this is a big change in RPM and, as a result, thrust. The effect on our stunter is a "charge" or rapid increase in speed. Not ordinarily what we desire.

When we shorten the pipe length (up to the point that we force already burned exhaust back into the cylinder) we increase the amount of air/fuel mixture in the combustion chamber for the next compression/power cycle. This increases the "heat of compression) and results in an earlier ignition of the fuel/air charge. Ignition takes place sooner as a result and the engine breaks into a two cycle...sort of like advancing the timing on a spark ignition.

If we have shortened the pipe (advanced the timing) to the point that we have caused the engine to charge under load we can make a further adjustment...increasing the combustion chamber volume at top dead center...to delay the ignition. We do this by adding head shims.

This fact can be utilized by the flyer to allow him to increase the torque of the engine (same combustion pressure in a larger volume due to the raised head) while precluding it from breaking into a high speed two cycle.

Thus, an additional opportunity for trimming the power package is available. If you need a bit more "punch" from your system try shortening the pipe and adding enough head shim to eliminate the break.

On Brett's use of a number 5 venturi with a short spigot, this would give the equivalent choke area of a number 7 venturi without the spigot. If the spigot went to the center of the bore, then it would be more like a number 9 venturi without the spigot. The larger venturi will give more power, but less draw (and, also, use more fuel.) We have never used as much fuel as Brett uses, so the difference has to be one of set up.

The DuBro "stinger" will rob power, but is a "necessary evil" with that type of pipe. We tried one with the 35 FP on one of Art's tube mufflers and it was totally unsatisfactory for us in that application. But, as mentioned, it is a "necessary evil" when using a pipe with a stinger coming straight out the back, and it will rob power. So, everything with these applications becomes a compromise, and if anyone is copying specifically what someone else is doing, they must make sure they are copying everything or it won't be the same.

One thing that was not mentioned in this discussion is the pipe, itself. Matt had been using the 61 pipe on the 61 engine that has the angled exhaust and doesn't need the extra stinger. Although we have not tested one against the other, it should provide more power than the straight pipe with the added stinger,while, also, offering a weight savings.

Matt's original set-up was to use this pipe at a 17 1/4 inch setting, a number 9 venturi, and a 13 inch 3-blade prop. He used 5-10% nitro depending on the weather here in the mid-west with a .024 head clearance, and this combination seemed to be working good.

After about 800 or so flights, however, he noticed there was a significant carbon build up in the pipe that had resulted in a loss of power. Switching to a new pipe restored the power. He talked to Billy at the Nats about this and Billy said this was "normal". We were wondering if this was aggrivated by a too-short pipe, but, apparently, others have experienced the same situation. (It is something to check on if you get a lot of flights on the same power train, as this "sneaks up" on you and you don't notice it right away.)

Now, final item, at the Nats we picked up a new 65 pipe and Matt installed this on his engine. This is a bigger diameter pipe and allows more power when used on the 61 engine. He pulled this pipe out to a longer length and still went up to about .032 head clearance to reduce the tendency to charge. It might be pointed out that we like a little more "charge" than Brett, but were still running a higher head clearance due to the difference in pipe.

With that combination, Matt experienced more power and migrated to a 13.5 inch three blade prop. He had gone from a number 9 to a number 8 venturi and was using 5-10% nitro depending on weather. When the temperatures went back to a high in the mid 80s at the FAI trials (and maybe even 55 in the wee hours of the morning during practice) Matt went back to a number 9 venturi and 5% nitro with this set up, which is what he used for both practice and all official flights.

But, yes, you should be able to set up your pipe length to your liking and simply adjust nitro (or maybe venturi size or air filter if you are using one) to compensate for differences in temperature or altitude density.

Leonard Neumann

Brett says,

"Bear in mind I have a 61, not a 65, so it may end up differently. I had .016 head clearance as delivered, and I am currently at .024. This is with the Hemi, not the derby head. I had it up as high as about .036, which smoothed out the break some, but also just didn't feel very strong.

The timing should be the same, so the pipe length/rpm should be the same as well."

Ted says,

"When we shorten the pipe length (up to the point that we force already burned exhaust back into the cylinder) we increase the amount of air/fuel mixture in the combustion chamber for the next compression/power cycle. This increases the "heat of compression) and results in an earlier ignition of the fuel/air charge. Ignition takes place sooner as a result and the engine breaks into a two cycle...sort of like advancing the timing on a spark ignition.

If we have shortened the pipe (advanced the timing) to the point that we have caused the engine to charge under load we can make a further adjustment...increasing the combustion chamber volume at top dead center...to delay the ignition. We do this by adding head shims."

Ted says shorting the pipe will change the timing. Bring the timimg back to a flyable situation by adding head shims. Brett says that the timing should not change by adding/losing head shims.

Did I read this wrong?

I have talked with Randy before about Breaks and stuff. He told me that if you squeeze the head down you will get a tough hard 4 stroke. If you raise the head you will get a "break". I like to run it the way Brett has described his. Almost all four stroke all the time. It is a smoother cleaner easier to fly run. I have found that getting this type engine run had alot more to do with the prop then the shims or pipe length.(AFTER I GOT THE INITIAL SETUP) Once the length was set and it ran well(solid 4 stroke everywhere) Then you add nitro as it gets hotter(raise compression to keep the four stroke alive) As fall came start baking off in nitro becasue then it would it would start breaking do to the over compression. When it breaks with this head setup it charges hard. I run the eather 4 blade 11 7/8 X 4.25 at station 8 and 5.25 at station 12. I run a bone stock 1999 pa 61. The pipe has the angled stinger other than that it is the stock 61 pipe. I used .017 shims this year in the dead heat. It really runs best when it has .026 in it.(Nats setup) The venturi is the same as when he sent it to me, 11.(green filter)

After reading all of this it sounds like the timing is always changing. The more the temp goes up the less density there is so you have to add nitro to keep the compression up. Then the break or no break will occur at the same time it did a month ago when it was cool. Or lose head shims. Either way it will work. As for Pipe length. Once you find the rpm you like it should never be moved unless you start all over and try a new rpm range. If you go all the way down to FAI fuel(no nitro) and it is still running off on you then you might want to start adding shims. Try baking off on the needle a little too. Not just 50 rpms try 300 what the heck. You could be over compressed.

Leonard you talked about dubro angle deflectors robbing power. They can be very helpful in calming down a hard charging motor. I had a 46 that I could not get shims for so I put on long deflector. It took to much power away so I started trimming it flight after flight and it finaly came in perfect. It was a cool little trick that was really easy.

Confused and trying to figure it out...

Doug Moon

>Confused and trying to figure it
>out...

Ted was talking about ignition timing. I was talking about exhaust timing. You advance the ignition timing through a variety of means. The exhaust timing is fixed by the way the sleeve is cut.

The pipe length is adjustable to vary the run slightly over a narrow range. What I was concerned about in this particular thread was that pipe newbies tend to wildly vary the pipe length over large ranges. For instance, this very weekend, I had someone fairly knowledgable come up asking about pipe lengths, and found that he was changing it in 1/2" increments. Doing this, you can jump entirely from one side of right to the other, and never, ever find the right range, or even worse (with baffled pipes*), jump from too short from one tuning peak to too short on another tuning peak.

I strongly suggest that everyone that doesn't have a lot of experience with this at least get it running reasonably well at the pipe length shown on the chart. If it won't run right at all, then there's something else wrong, NOT THE PIPE LENGTH. Then, once you get a consistent system, then start adjusting it over very small ranges. This way, you get the other bugs worked out before adding complications of tuning.

Brett

*The multiple tuning peaks is a detail that not a lot of people have picked up on. The pipes with baffles can actually be tuned almost equally well at several different lengths. You send one acoustic wave down the pipe, and you get 3-4 reflections back at different times, one peak related to each baffle position, and the tail cone. This explains while Paul Walker runs his 40vf pipe at 18 1/2" and I run it at the same rpm at 16 3/4.

Sounds like it would be a good idea for someone to produce a pipe run trimming chart! I'm still a rookie at pipes too so possibly Randy, Ted, Brett and Paul can tell us what is a basic setup and then the finer points to tuning on an Aero pipe engine should be. That would be way cool for us rookies to be able to say OK this is basic and these other things can fine tune the run the way I want it to be!

Brett,

I understand now what you were meaning. Thanks for the clarificaion.

Jim the pipe chart listed on this thread is excellent. This chart will give you excellent starting points as far as where the motor will run good at what rpm. Then start sticking props on to get the right speed.

I find that the more I run these motors the better I become at figuring out how to get good consistant runs. I may not always understand exactly what is going on but I can get a good run.

As always thanks for all the info.

DMoon

Is that deck clearance or head shims? Mine was delivered with .024" HEAD SHIMS. I was told the deck clearance is substantially more that the HEAD SHIMS. Bob runs as little as .008" HEAD SHIMS.

By the way, I am running .030" HEAD SHIMS. My break is very soft.
My Venturi is a #9.

> Brian Eather
>11.5-3.7 "red" 3-blade, helical, cut
>from 12-3.5
> 7.4 oz
>of standard SIG 10% for
>6:30

My prop is 12.5 X 4.5 Bolly 3-blade. Pitch is 4.5. Ground launch RPM is 10,000 to 9,800.

>I liked the
>11.75-3.9 Eather in the super-miserable
>weather at the NATS early
>in the week, but I
>switched back to the other
>prop about 30 seconds before
>my first qualifying flight, since
>it had cooled off considerably.

That is interesting that you consider Muncie to be “miserable” weather for power. The Dallas guys look for the huge BOOST in power when they go to Indiana. Dallas in the summer requires much power and pitch.

Long story short, it appears I am running too much pitch and not enough RPM, especially for cool air. The 4.5 pitch will probably serve me well in August, but right now I should probably be closer to 3.75” to achieve 10,000 to 10,500 ground RPM. Everyone here has dropped RPM 500-1000 in the last month or two (with the same summer props), hence the pipes have all come out in length. The other approach would be to change props and leave the pipe and engine alone. Nitro alone will only work for a select few engines.

If I get the ground RPM up, my pipe length should be fine.

This thing still acts weird… I am going to try my new Saitos.

Ted said:
Thus, an additional opportunity for trimming the power package is available. If you need a bit more "punch" from your system try shortening the pipe and adding enough head shim to eliminate the break.

I say:
How short?
And I think that is EXACTLY what Randy says NOT TO DO.
This would essentially be the same as running a butt load of nitro and raising the head to compensate for the ZOOM. I have seen that done.

I run .024 HEAD CLEARANCE. Mine was delivered with .016 HEAD CLEARANCE.

I know the difference between the two, and I have a depth micrometer. The clearance is the fundamental measurement, not the number of shims.

>
>By the way, I am running
>.030" HEAD SHIMS. My
>break is very soft.
>My Venturi is a #9.
>
>> Brian Eather
>>11.5-3.7 "red" 3-blade, helical, cut
>>from 12-3.5
>> 7.4 oz
>>of standard SIG 10% for
>>6:30
>
>My prop is 12.5 X 4.5
>Bolly 3-blade. Pitch is
>4.5. Ground launch RPM
>is 10,000 to 9,800.
>
>>I liked the
>>11.75-3.9 Eather in the super-miserable
>>weather at the NATS early
>>in the week, but I
>>switched back to the other
>>prop about 30 seconds before
>>my first qualifying flight, since
>>it had cooled off considerably.
>
>That is interesting that you consider
>Muncie to be “miserable” weather
>for power. The Dallas
>guys look for the huge
>BOOST in power when they
>go to Indiana. Dallas
>in the summer requires much
>power and pitch.

Some days that's true, and others it's not. Monday at the 2001 NATS was mid-90's and 80-90 percent humidity. This is not a lot different from Dallas in the summer. The 1999 Team Trials was much hotter than the same day in Dallas. Thursday at the 2001 NATS it was much cooler than Dallas. By "miserable day at Muncie" I was referring to Monday, which had a quite comparable wet density altitude to Dallas the same day.

I ran 3.9 instead of 3.7" of pitch, and used 15% nitro instead of 10% like at sea level, and the same pipe length. It ran pretty well. The Bolly will require much more measured pitch to get the same effects as the Eather undercambered prop. This is because the measured pitch is not really what determines the characteristics. It's the effective pitch, and the airfoil of the Eather prop makes the measurements and the effective pitch much further apart than the Bolly airfoil. This doesn't make one any better or worse than the other, it's just different.

>Long story short, it appears I
>am running too much pitch
>and not enough RPM, especially
>for cool air. The
>4.5 pitch will probably serve
>me well in August, but
>right now I should probably
>be closer to 3.75” to
>achieve 10,000 to 10,500 ground
>RPM. Everyone here has
>dropped RPM 500-1000 in the
>last month or two (with
>the same summer props), hence
>the pipes have all come
>out in length.
>The other approach would be
>to change props and leave
>the pipe and engine alone.
> Nitro alone will only
>work for a select few
>engines.

I don't know why you say this, it may be true, but I havn't seen a lot of evidence of it. It's worked ever time I tried it.

>If I get the ground RPM
>up, my pipe length should
>be fine.
>
>This thing still acts weird…
>I am going to try
>my new Saitos.

What is it doing wrong?

>Ted said:
>Thus, an additional opportunity for trimming
>the power package is available.
>If you need a bit
>more "punch" from your system
>try shortening the pipe and
>adding enough head shim to
>eliminate the break.
>
>I say:
>How short?
>And I think that is EXACTLY
>what Randy says NOT TO
>DO.

I think Randy is trying to keep everybody from using the stunt flyer's maxim, e.i. "if a little's good, a lot's better". If you get a bit more power from pushing the pipe in 1/4", why not get a lot more power by pushing it in 3/4"? It doesn't work that way. A proper setting means it's running on the down side of the power curve from the local tuning peak. This makes the torque climb if you load the engine, prevent further rev decrease. If the load is reduced, the torque drops, meaning the engine doesn't want to speed up.

Pushing it in too far can put you on the wrong side of the local tuning peak, making it want to go faster once it's unloaded, and slower under load. This is an unstable situation, and can cause "runaways" and "crapping out under load" just like the old schneurle wars motors.

Put it to the length on the chart, fix all the other problems, and it will work. Then, and only then, should you try small changes to tweak it.

>This would essentially be the same
>as running a butt load
>of nitro and raising the
>head to compensate for the
>ZOOM. I have seen
>that done.

Not the same at all. Pipe length effects are frequency-dependent, and nitro and compression aren't (at least to first order).

I think this was the answer to my original question.

Weird stuff is generally having unusable engine runs. Running away and going screaming lean after a few laps (and it is not the tank).

Very good stuff this thread.