Hey Guys, quick question.

Flew my Kougar today on a os 46ax and tuned pipe. First flight for this combo after some checking/cutting and fiddling on the ground. Anyways at one point I throttled back from WOT and it took a LONG time to come off WOT. Fearing my RX Pack was low <slow servo> I landed the model.

I could not recreate the problem when I taxied back to the pits. Throttle response was good, and it came right down then closing the throttle.

A flyer made the comment when a 2S is on a tuned pipe sometimes that happens. Ok... I thought to myself, how in the world does the engine maintain WOT if I mechanically close the barrel? It's got to pull air <and a lot of it at WOT I presume> from somewhere so how in the world does the addition of a tuned pipe affect that?

Ideas??

Tommy D

A tuned pipe is actually injecting unburned fuel back into the engine!


Hers a page with a good graphic of how it works. http://tech.flygsw.org/tuned_pipe.htm

The green is unburned fuel and you can see how the exhaust pulses send it back into the head.

You are starting to get the system tuned. What you experience is what is known as the pipe over running the throttle. This is normal. Now for most sport application one would want an undersized carb to try to minimize this effect. But with full wave pipes this will be a feature ( I know I sound like MicroSoft salesman. "thats not a bug it is a feature") you will have to learn to deal with. I sometimes will come off pipe (come down to idle) then bring the power back to what I want below the resonant point of the pipe.

If you did come down to idle and she still was over running the carb it is an indication that your pipe is still to long. Also opening up the stinger diameter can help minimize this.

While on the pipe did she pull good?

All the best,
Konrad

Thanks guys for the education.

The model was no doubt faster on it's last flight <with the tuned pipe vs without it>. On the ground it spun a 10x7 @ 14,500 RPM after trimming the length down some. Best it did before was 13,400 <same prop/nitro ect> However with some elevator linkage issues it was akin to a tiger chasing it's tail at some points. So when the throttle was slow to respond <close> and the wacky elevator I landed. I had some suspicion that the flight pack might have been low. So much for that thought.

Have to finish the elevator linkage mod, and put some more time on the model.

Tommy

<P.S. I like the bit about it's a "feature">

Put the model back into the air for another flight today. No doubt I have some tuning/diagnosis to do. While at WOT I noticed an occasional <forgive the 4 stroke automotive lingo> missfire. Had a smoke stream behind the model at times, but with the gray sky it was hard to see clearly what was coming out of the pipe. Running the motor up static in the pits for sure showed a rich condition at WOT.

I'm wondering just how rich off peak RPMs I should be setting this motor? 500 RPM????

Was having too much fun with a electric Pylon Plane to put the Kougar back up.

Tommy D

Hi!
Remember that when using a tuned pipe you raise the compression inside the combustion chamber which mean you better lower the amount of nitro in the fuel if you use a "untouched" sport engine like an ASP,OS Magnum etc.
Most engines don't like more than 15% nitro and using a tuned pipe makes the engine "belive" the nitro content is higher...No good!
Use max 5-10% nitro!

Tommy,


Take heed to Jan's words...

Most who use a certain fuel with a normal muffler, do not understand why it may not be suitable for a different, tuned-exhaust setup.
Also, the procedure for adjusting the length of the system (cutting the header), is mis-described in some web sites.

The ability to immediately transition back to full on-the-pipe RPM, should take prescedence to getting the very last RPM from the engine.
Too short a setup, will potentially get you institutionalized, trying to get the engine to run properly...[sm=bananahead.gif]

Thank's for the input guys.

You really don't believe that 15% Nitro fuel will work on the OS 46ax with the addition of a tuned pipe? I'm to understand that it is possible the surge/misfire I'm encountering could actually be detonation caused by too much compression or Nitro?

If no amount of enriching or switching to a colder plug will help, would the addition of a 2nd head gasket work?

Initially I was curious as to why the throttle seemed not to retract. Now the issue is the surge/misfire.

Thanks

Tommy D

Take a look at the glow plug, see if the element is starting to come out of the glow plug bore. If so this is an indication that the engine is detonating. A "misfire" is not usually an indication of detonation, well not unless it is preceded with over heating.

Now most sport engines do not have a great deal of blow down (time in degrees that the exhaust port is open yet the intake port is closed) to allow the tuned pipe to supercharge the engine. So unless your engine has a high compression head to begin with or you are using rather high nitro, the pipe's supercharging of the cylinder is not much of an issue with detonation. This is assuming the engine is propped for the same rpm of higher as it was with a muffler. Loading the engine down into a lower rpm with the pipe set up to resonate at lower rpm then detonation can become a concern even with "Sport Timed" engines. As you are running in the 14k to 15K rpm band you look to still be in the normal rpm band for the engine. I seem to recall that the OS46AX is rated at 16K rpm

Most European engines have rather high compression ratios as they are designed to run on no or low nitro fuels. So adding a pipe even with sport timing can cause detonation issues. But your OS is a sport engine designed to run on moderate nitro 15% to 25% max fuel I'd look to adjust other tuning factors before shimming the head.

I like to use as cold a glow plug as practical. This forces you to lean the fuel mixture to get the best on pipe high rpm. This often results in a lean midrange. A lean midrange is often needed as the higher fuel flow needed at full power from a piped engine will flood the midrange, or off pipe performance.

I'm glad to see you are trying to use a high performance set up ( I think all pipes are beyond the scope of what is called a sport engine). But I have to make clear that a pipe does add a lot of dynamic variables that make tuning them rather tedious. Again the piped engine is not for the beginner or for those that aren't willing to put in the time to understand what is happening.

I'm glad you made mention of pulling the plug for a look. If I was track tuning a drag car thats exactly where I would at right now <before I made a jet change> or ignition change for that matter.

I do enjoy learning as much as I can about an engine. Specifically understanding why it reacts as it does. Also I understand trying to help someone tune a power system via the net is daunting at best. Heck I could have a tank vibrating/foaming in the air for all we know!

What you say has a lot of truth to it, tuning that is Konrad. I expect a trade off as there is no free lunch in HP for sure. Move the powerband one way, or another and something may end up suffering.

Funny, one "old-timer" at the field asked me what size motor was on the Kougar, to which I replied "46". To that he asked why a pipe on the motor? My reply was I'm trying to go faster. His retort was I should have just put a 55ax with a Ultra-Thrust muffler on it and been done with it. My reply was "thats too easy, and expensive!... besides now I get to learn about how to tune a pipe". He did agree with me the pipe does look kinda cool on the Kougar as well.

=)

Konrad,


The maximum recommended nitro for the OS.46AX is only 20%.
I am quite sure this is for the engine with the 'standard', P-Box muffler.

Even simply mounting a large prop on any engine, will lower its tolerance to nitro.
A tuned-pipe will do this for sure.

Any change in the engine's setup, that causes it to be be more 'volumetrically efficient' (as in cramming more fresh mixture into the combustion chamber); will cause it to act 'like a dog'; as if too much nitro is used. Shimming the head up is one of the easiest modification to perform; only a bit more difficult than changing a glow-plug.

May I ask a question?

I'm going to assume that the higher the RPM the more 'volumetrically efficient' the motor will be, with a tuned pipe? So that said could a engine run "ok" on the ground at say "X" RPM <in my case 14,5K> and "Y" Nitro <again in my case 15%>. However once in the air, and possibly unloading over the 14.5K it now "acts like a dog" as it's being .... how should I say it to make sense.. over boosted/supercharged at it's newer/higher unloaded RPM range?

Tommy D

I have no issue with any of this. My detailed knowledge of OS in limited. From bitter experience, I'd like to keep it that way. So 20% nitro it is with the large props OS calls out, 11"?..

Tommy,
Volumetrically efficiency has nothing to do with rpm. It has to do with how much fresh (dense) charge there is in the cylinder. The denser the charge the (higher volumetrically efficiency) the greater the effective compression ratio. With our two stroke engine the term "effective" is very important. As the cylinder porting and resonant gas flow have a drastic impact on the actual (dynamic) compression ratio. Detonation is a speed thing, that is the speed at which the flame front is travailing across the piston. If the pressure is high the flame will travel too fast. The pressure is controlled by both the speed of the burn and the speed at which the piston moves. The speed of the burn is a function of the fuel and effective compression ratio, the speed of the piston is controlled by the prop load. So as the engine unloads the piston moves faster lessening the tendency to detonate. Now if the engine is now coming into the point that the pipe resonates (supercharges more effective) the effective compression ratio goes up increasing the tendency to detonate.

I hope you can see that there are few hard rules. You will need to experiment with the variables you have. Given some time you will get a set up that will be the awe of the sport pilots at your field. I will say that the stock timed OS 46AX is not an engine I would have too much concern about having too high a compression ratio on 15% nitro in the 15K to 16K rpm band. I'm not say that it is not an issue just that it would be way down the list on my trouble shooting tree.

To test how the engine is responding in the air you might want to run a smaller prop on the ground like a 10x5/6. You are trying to mimic the in flight condition. If the engine now responds like it did the air you can change your tuning variables to correct for any issues you have. Put back your flying prop and see if the engine responds like you want.

I do think the OS is a mild engine and should not give you too much problem as far as compression ratio and 15% nitro. Pipe resonants, prop load, carb setting and glow plug would be what I'd adjust. I have found that historically the OS is a mild (read dog) when it come to a high performance set up. This would be an advantage for the newcomer when it comes to the application of a pipe.

Later you can go with real high performance engines with over 180° exhaust timing and late crankshaft timing, and of course true ABC construction.

A fast Kougar? It really isn't a fast plane with that thick wing. I think Sig marketed it as an advanced aileron trainer. Now to go fast have you thought about dropping down to say an 9X8 prop set up to run in the 18K rpm band. Now that will scare or bring on a smile from the old guys at the field.

Yes the Kougar has a wing that resembles a park bench in it's cross section for sure! That said it's one of my favorite flyers as is anything designed by Claude McCullough. Somehow I get more personal satisfaction from making one run in the 120 MPH range then throwing up my electric Surprise 9.

Yes it's a waste of energy <like discharging one of my lipo's to storage voltage on Sundays> but I truly enjoy it.

In all honesty I think as draggy as the Kougar is, and with my RPM being what it is, Ill stick with a 10" diameter prop. Me thinks you would have to spin a 9x? awfully fast to get this brick moving.

Thanks for your help. It sounds like I am/was confusing volumetric efficiency with what I assume is the compression of the motor. I assumed at higher RPM the pipe injects more fuel/air then at a lower RPM, changing the compression and efficiency of the motor.

Tommy D

If the plane was a fair ways out when you cut the throttle it will take some time for the sound to get back to you. This would explain why you couldn't recreate the problem on the ground.

Tommy,


Too short a pipe leads to the engine getting 'on the pipe', only as it unloads...
It then becomes too lean, with all the consequences.

A tuned exhaust is nice for its extra power; but one needs to set it up exactly right and it is very easy to get it wrong...[]


Konrad,


I only corrected your quote of the nitro percentage, from looking in the Tower web site's recommendations.

I have to admit that was more effort than I'd have taken for an OS .
Your point about the pipe is correct regardless of the engine.

Tommy,
As to speed you will find that the 9x8 will give you much more potential than the 10x7 even at the same rpm, but you will get even more RPM. The Diameter address the drag and acceleration issue. Pitch deals with speed. And yes rpm is important to the overall performance of the prop. The ability of a prop to absorb power is related the Diameter to the 4th power and RPM to the 3rd power, Pitch is a linear. Since pitch speed is RPM times pitch it is a benefit to loose some Diameter to gain RPM if speed is the goal.

He also designed the Kavalier for about the same thing and I know some guys who pylon race them. It was sad to read about his passing away.

When did Claude McCullough pass away? As to racing I have even pylon raced Kadet trainers. Now that was a challenge keeping the Kadet where you wanted it. Basic trainers are not known for going where you point them! [X(]

1-30-2008

3 years ago, I now seem to recall his passing.

Hi!
Using as much nitro as 15% (which is totally unnecessary,5% is all it takes) and a tuned pipe demands a good tank! That means getting a Tettra "Bubbleless tank", or if you don't want to invest that amount of money...an ordinary tank set up with two clunks (Called Uni-flow set-up)
It's also important to understand that the more nitro you use (or the higher the compression is) the colder glow plug you have to use .
My experience is that Rossi 6 or 7 plug...or even the coldest there is an 8 might work .The Rossi 8 was more or less the plug to use when I flew F3D pylon 12-15 years ago. (we used 80/20 fuel, no nitro).
You will notice that the colder the plug you try to use the more senitive the engine will become.
Adding a head shim (0.1mm) will give the same results as using lower percentages of nitro!

If we ever get any warm weather in the next month or so, I might for giggles try the smaller diameter prop. I do think while no doubt it will be spinning faster then the 10"er it will not have the ability to fly the model faster, in top speed. Do hope you prove me wrong however.


Would you care to educate me on the correct way to set one up? If this is a far too common question, by all means a link would be more then adequate as well.

Thanks!

Tommy D

Tommy,


Setting up the pipe requires that you use a tachometer.
It is best done with the engine mounted in a test-stand!
You cannot trust your ears, to determine, if the frequency of the sound produced by an engine (hence the RPM), has increased or decreased.
Always write the RPM and also the conditions for every reading (how much was cut from the header), on a sheet of paper, or in a spreadsheet on your PC.

You will also need several tie-wraps (or reusable pipe-clamps), a metal tubing-cutter and something to de-burr the inside of the aluminium tubing, every time you cut it.


Select the prop; size model and make, with which you want to fly your model.
Take note that if you want to use a different prop, you will need to go through everything, all over again.

Mount this prop (for a .46 engine on a sport/pattern model, you would typically want one in the 10x6 - 11x7 range) on the engine.
You first must take the reading peaked (backed a bit rich), with the standard muffler installed.
Write it down!

Install the header, full length; and the tuned-pipe. The rear-end of the header must be pretty snug with the front-end of the pipe (leave just ~1 mm/.04" between them within the coupler).
Batten everything down with tie-wraps (or pipe-clamps). You will not need to undo the end that's on the tuned-pipe's front; only the one on the header.


Now, 'play with' the high-speed needle and take an RPM reading (as always, peaked and backed up a bit); and write it down. Repeat this after every cut that you make from the header.
Use the tubing-cutter to cut short lengths from the header. Don't forget to de-burr the inside of the rear-end of the header.
Make the cuts 1/4"(6 mm) at a time, taking RPM readings again and again; as long as RPM is increased noticeably. When the rise becomes marginal, make the last cuts only 1/8" (3 mm) at a time.

And now, this is very critical! Every time; after measuring max-RPM, close the throttle to idle; open it to full and make sure the engine reverts to the same, 'on the pipe' RPM!

With the setup at its original, full-length, it will probably not be a problem, but as you continue to cut and test; you will eventually reach a point where you would need to momentarily lean the mixture, or to pinch the fuel-line, for the engine to come back to maximum RPM.

Once this happens, the setup has become too short to fly with! Insert the last piece of aluminium tubing, that you cut from the header into the middle of the silicone coupler and reconnect the whole installation.

Check RPM and 'after idle' recovery to full', to make sure it is as it was.