Tuned Exhaust Systems
We have tried several pipes in the past, first let me say I am not a huge fan of tuned pipes on a helicopter, or for that matter, a plane. I for one can’t live without a modern governor and pipes with governors sometimes get along like two 8-year-old children. At worst they can be constantly fighting each other, disagreeing on things etc, but with a little discipline they can be forced to get along in most cases.
If not setup correctly a governor and pipe will both be trying to hold a constant rpm but if they do not fundamentally agree on the same speed you could be in for a reduction in performance. The engine and rotor rpm might be hard to control and not very smooth sounding with a mismatched configuration especially if running on the edge of the tuned pipe design resonance (where the pipe tuning starts to come into play).
Personally I don’t like tuned pipes because I prefer widely different rpms for alternate flight modes and many pipes don’t effectively allow this flexibility. On the other hand they do give you more power which many of us are always looking for, but as stated often at a narrow rpm band. Is the trade off of smooth, quiet, wide band operation and fuel efficiency worth the extra 5-15% power you gain from a tuned pipe? Well if it is then there is no question here, you need a pipe! Sometimes a long full wave tuned pipe mounted aft to its own header pipe will affect the helicopter fore/aft C of G so it might require some ballast or a selective battery placement.
A brief description on how the tuned pipe operates is in order since some people may not understand how they basically function. The tuned pipe takes sound or pressure wave energy from the combustion process and cleverly uses it to increase the volume of the unburned fuel/air charge in the combustion chamber. A larger fuel/air quantity in the combustion chamber to burn translates into more power at the crankshaft. It should be understood that pressure waves move independently to the gas flow through the exhaust system and also move at a much higher velocity.
The tuned exhaust system is made up a header pipe, a diffuser cone, a mid section and baffle cone. The point of the diffuser is to create a low pressure near the exhaust port to pull gases into the header pipe. The low-pressure reverse wave is created/directed towards the exhaust by the diffuser or megaphone process. (This reverse negative wave is in addition and separate to a positive pressure wave heading to the end of the exhaust system.)
A man named Walter Kaaden, who was at that time working for the East German company MZ, initially discovered this phenomenon to the best of my knowledge in the 1950s. Mr. Kaaden understood that there was power in the sound waves coming from the exhaust system, and so initiated a whole new system of two-stroke theory and tuning.
The diffuser action pulls the last part of combustion gas first and additionally fresh fuel/air mixture behind it out of the engine exhaust port and into the header pipe. As diffusion occurs correctly the transfer ports are still opened allowing the fuel/air mixture in that area to be efficiently scavenged or pulled towards the header pipe. There is a very small overlap of open transfer and when the rotary intake valve which is machined into the hollow crankshaft just starts to open.
This helps to encourage or prolong mixture flow through the engine and raise pumping efficiency. Shortly after this point the transfer ports will be blocked as the piston moves up in the cylinder, even though the exhaust port is still partially open. Now comes the clever part.
The positive pressure wave which has continued on has been reflected back off the rear cone shaped baffle, arrives and forces the unburned fuel/air charge previously pulled into the header pipe by the diffusion action back into the cylinder before the exhaust port is closed by the piston. A mild supercharging effect takes place adding to engine power. Remember the transfers are closed at this point so the unburned mixture charge cannot be moved further back down into the transfers where it originated.
The speed of the pressure wave is based or dictated by the speed of sound and the speed of sound varies with temperature and gas density, so the system needs to be somewhat stable in these parameters. The internal pressure in the exhaust system determines how well and how hot the engine and pipe will run. The tail pipe (stinger opening) is used to calibrate back pressure for these reasons. Engine port timing certainly plays a role in how good the overall tuned exhaust performance is, but that is best left to the design experts.
We can see relatively speaking that the speed of sound is a constant and rpm or combustion waves are not. In order for the system to be in acceptable resonance with the transfer and exhaust port timing we will have to run at the designed or designated rpm range to reap full potential. The rpm bandwidth of the tuned pipe is based upon engineering design, but suffice it to say that a system that is correctly designed for a narrow rpm band operation may develop better peak power than one intended for a wide bandwidth.
Some tuned exhausts may be shorter and less powerful, sometimes loosely referred to as half wave units. These attempt to use secondary pressure (reflection/harmonics) to do the job because of the shorter more compact exhaust length. Simply put a tuned pipe system cannot be in resonance at all rpms. A rule of thumb is the higher the rpm the shorter the pipe length. True generic full wave pipes are often fine tuned by adjusting the length of the header pipe.
A good system can supply more mixture into the cylinder than the piston displaces thus raising volumetric efficiency, the result is similar but weaker in nature to what turbo/super charging does to four cycles. While design tricks, efficiency and operation are rather complex, this is basically it in a nut shell.
I recently purchased an 8.5 gear ratio Curtis Youngblood Muscle Pipe? II, first impressions were good. While a pure muffler or a true tuned pipe it is not, it does however seem to be a fair compromise between the two. This compromise gave me the extra 5-10% power gain I was looking for over my existing exhaust but more importantly it did so without the engine hanging on the pipe when I brought the throttle to idle during an auto or landing. I could run 1400 rpm in normal mode and 1800 in idle up as I desired.
My biggest complaint is the noise; we were not too impressed with the audible level but thought that we’d give it a chance regardless (it’s only noise and the club where we usually fly is not sensitive to that issue). My second complaint is that of tuning, you either have it tuned correctly or not. Unless the mixture is spot on (both the high and low end) the engine will perform poorly given the units full potential. Lastly concerning the fuel economy, the Muscle Pipe II cut my flight times by about a third but power is not up by 30% (.5hp). For a month I’ve been flying the MP-II and am often trying to find the sweet spot on the needle, although once found it is fine for the day.
All in all I am pleased with the MP? II’s good performance, just the minor hassle of fine-tuning and noise are its weaknesses. We could get the model to do full cyclic/collective tic-tocs right off the deck with confidence because knowing the engine was not going to give out due to MP? II’s very reliable operation and power instilling confidence. A newer more quiet version is available or will be available shortly should you be considering this unit.
We have a brand new Revolution tuned muffler on the shelf we thought we might as well compare to the MP2. We bolted it on and as soon as we started the machine up we noticed right away how much nicer sounding it was. During the first hover I had to adjust my throttle curve a little bit to acquire the non-governed head speed I was looking for. It was swiftly sorted then taken for a quick flight, right away completing straight up climb outs, low flips, tail stand launches etc. I didn’t at this point notice a lot of difference compared to the MP-II (about a 10% or so power loss).
Some full cyclic/collective tic-tocs were completed at which time I could tell the engine was working hard hanging on. Minor flying mistakes made required correction with cyclic pitch possibly causing it to bog a tad. Some power was lost, but with a gain in smooth and quiet running characteristics. So, setup habits were quickly adjusted slightly to effectively get around that one by bumping the rotor rpm up by 50 (425 engine rpm increase). We are going to fly the Revolution muffler with a 25-30% nitro content fuel and see if a rise in power up to the MP2- 15% nitro spec can be had at the previously lower rpm, hopefully with reasonably good fuel economy since we have very decent flight duration now.
We had on hand another exhaust, a Century muffler smaller in length and volume than the other two. We found the sound level and throttling quite acceptable but the engine power was down. We feel that it is chocking the engine by a small amount. This muffler is intended for the older 46 engines and has its heritage in the past. This is typical of many welded/machined economy mufflers supplied with many ready to fly packages. Suffice it to say these welded and machined mufflers are more durable and better performing than the cheapest cast aluminum multi-piece types usually reserved for airplanes. They seem quite acceptable for sport flying.
Some of the problems people have had with the multi piece cast mufflers have to do with the parts loosening off. What can happen is as the tailpiece rotated by normal vibrations hot exhaust is directed to the landing gear rear strut or plastic side frames causing the plastic to melt. The cure here is to silicone the parts together after a good cleaning with the included O-rings and use a second jam nut over the elasto-stop lock nut.
A good back up is to install a correct silicone exhaust diverter, because besides keeping the glow goop mess away from the airframe it also ensures that if the muffler tailpiece ever rotates, hot exhaust gas will not be directed towards plastic components. It is best to secure the diverter with silicone to the muffler and also to use a ty-rap.
At the present time I prefer the lighter weight Revolution or the slightly shorter 50 sized 3033B Century can type tuned muffler (they look similar and fly about the same to me). The MP2 is however a good behaving “pipe-ler” compared to some true full wave units I’ve flown, but I feel it unwise to contend with the noise or tuning issues at events and/or demos we fly at. I suspect some of the spectators prefer a more quiet exhaust note too. This is the main reason I switched to the Revolution muffler. As expressed to me by another skilled pilot after flying the Revolution and MP-II, “the MP2 does add more power, but it’s coarse power”
Generally, if you want every bit of power you can extract from a 50 engine there will be compromises, but even so it can be money well spent. If you’re interested in a smooth, quieter running engine with slightly less power and are concerned with fuel economy then go for a quietly tuned muffler. I like both the MP2 and the Revolution tuned muffler, but from my perspective the sport flyer gains more from the tuned muffler then from the MP-II “exhaust system”. We’ve tested these exhaust systems above successfully on both the TT50 and the more fuel hungry OS50 engine in case you were wondering.
A newer engine governor with very good performance. The standard in engine governing loaded with bells and whistles
As mentioned governors are everywhere with the GV-1and the Throttle Jockey Pro (Tjpro.jpg) are running neck and neck. Both are very good units but each has its own virtues. The TJ Pro requires a super servo to maximize its performance, which brings the cost up to that of the GV-1, however SS support does give it a definite performance edge. There is one feature I liked about the GV-1 when using the MP-II.
It allowed the low governed throttle position to be quickly adjusted up using the “governor low” (GV1.jpg) limit setting thus keeping the exhaust from falling out of a best tuned range, thus guaranteeing a perfectly constant engine speed and faster response. RPM is displayed on the GV-1 screen while the TJ needs a bit of tweaking to hit its exact mark.
I hope this article has been interesting and by understanding some of the options available helps you better find the exhaust system suited for your personal situation, because after all you are the one who counts.
