For the majority of R/C model fliers Tuned Pipes are the things of myth and legend, only used by experienced competition flyers and therefore often regarded by some as a "Black Art".

With a reputation for being tricky to "set up" and coupled with a distinct lack of first hand information telling us "how to do it", it is not surprising that this myth remains.

In an effort to alleviate some of your concerns I have put together at the request of Just Engines, information I have gathered over my years of dealing with Tuned Pipes for the benefit of you, their customers. This document assumes that the modeller is taking a responsible attitude towards his flying and only refers to modern silenced or "Quiet" Tuned Pipe systems. Whilst on the subject of noise, Just Engines tell me that one of the most common requests they get is for an engine "that's really powerful and really quiet" - and here is also where a tuned pipe can be of real benefit.

I would like to take this opportunity to thank my friend, the late (great) Arthur Burley (of Art Burley Pipes) for sharing his wealth of knowledge with me, for the many late night 'arguments' on Tuned Pipe theories and for the opportunity to test his prototypes.

Index

1. Introduction
2. Mini Pipes
3. Fixed Length Pipes
4. Calculating Pipe Length
5. Exhaust Port Timing
6. Setting up - with formulas
7. Setting up - without formulas
8. Summary

1) Introduction: Put quite simply, Tuned Pipes work as engine Superchargers.
Tuned Pipes manipulate the pressure wave made by the exhaust of the model engine to help to scavenge the exhaust gases from the engine. They draw the fresh fuel/air mixture more effectively through the inlet ports of the engine and into the combustion chamber before ignition and then ensure a minimal loss of the fresh charge out of the exhaust port. Due to the presence of a now "larger than normal" charge in the combustion chamber, the engine produces more power. How this extra power is then used is up to the user, but could be higher rpm or the ability to turn bigger props etc. Further benefits are typically better throttle response, cleaner running engines, better fuel efficiency (less wasted through the exhaust port), cooler running and of course quieter running. Most modern tuned pipes are amongst the quietest exhaust systems around and it is a testament to the attitude of UK aeromodellers towards the protection of flying sites that their use is becoming more widespread.

Various types of Tuned Pipes exist but are generally either Twin Cone tuned pipes, Flat Disk tuned pipes, Fixed Length tuned pipes or Mini Pipes. The principles of operation are the same for all, but variations in set up, power band, tolerance, performance and adaptability are all contributory to the "black art" perception.

Flat Disk Pipe Twin Cone Pipe Fixed Length Pipe

Mini Pipe

I will attempt to cover all aspects of these but first let's establish a better understanding of what is happening inside the pipe.

As the exhaust gases leave the exhaust port area and enter the first (diverging) cone they expand and give off heat. The effect creates a negative pressure (or pulse) wave travelling down the pipe giving a scavenging effect which actually sucks gases out of the combustion chamber along with all the other matter associated with combustion.

The combination of normal engine operation and the scavenging 'suction' of the tuned pipe gives the exhaust gasses an extra boost out of the engine and this, in turn, helps suck the new fuel / air mixture into the engine more efficiently through the inlet ports of the engine which will now be open. The pulse wave travels down the pipe until it reaches the start of the second (converging) cone, flat disk, or other reflecting surface of the pipe internals. At this point part of the pressure wave is reflected back up the pipe towards the engine and, if correctly set up ( or tuned ) this reflected wave arrives at the exhaust port just before it closes and prevents the loss of any of the new fuel / air charge (some of it is actually pushed back in through the exhaust port ) to give the supercharging effect. Because of this "Super-charge" we are now burning, more power is produced.

Whatever make or type of pipe you select it should match the size of the engine i.e. .40 size, .60 size etc. You should also have some indication of the application in which you are to use the pipe. With this in mind you can set about determining which pipe is best for you.

2) Mini Pipes.

Mini pipes tend to be very much fit and forget systems which are very tolerant to variations in prop size, fuel, plug type etc. The very wide power band associated with these pipes and their relatively small dimensions make them a good introduction for modellers looking for a modest power increase. This is typically manifested as a 10% increase in RPM over the standard exhaust system. Mini Pipes are mostly used on sport type models as a direct replacement for the standard muffler system. They do have benefits over standard exhaust systems but not as much as the "Full Specification" Tuned Pipes will allow.

3) Fixed Length Pipes.

These pipes are of the same type as those used by most Ducted Fan flyers, Pylon Racers or Control Line Speed flyers. They are design to fit directly onto the engine manifold stub (usually rear exhaust) and are aimed at either a specific application or a specific RPM. The adjustable factor in the fixed length pipe system is of course the load on the engine. Most fixed length tuned pipes are of the Twin Cone variety (although not all of them). These have a very narrow power band ( RPM is more important and they are less tolerant to slight variations in revs) so some experimentation with props will be required to correctly match the system with the fixed pipe length but an initial calculation can be made to put you in the ballpark. Ducted fan Tuned Pipes are mostly aimed at a specific Engine / Fan / Pipe combination and will usually work straight away.

Calculating the pipe length therefore is more applicable to the "adjustable" Tuned Pipes systems and before we discuss these further we need some additional information. It is however possible to correctly set up tuned pipes without doing any calculations although the process is more time consuming.

4) Calculating pipe length.

On variable length Tuned Pipes it is vital that they are of the correct "tuned" length. If the length is wrong for the RPM of the engine, destructive interference takes place and this hinders power. With too short a pipe the returning pressure wave gets back to the exhaust port too soon and actually forces exhaust gasses back into the engine causing rough running and overheating. With too long a pipe, the returning pressure wave doesn't get back to the exhaust port before it closes and the supercharging effect is lost.
The information which we need to calculate the length of the pipe is the following :

1). The Exhaust Port Timing of the engine (ExT).
2). The expected maximum operating RPM.
3). A Formula Constant.

5) The Exhaust Port Timing of the engine (Ext). This is the angle measured in degrees through which the crankshaft of the engine rotates whilst the exhaust port is open. This begins as the piston crown passes the top of the exhaust port on its way down after firing, and ends as the port closes again with the piston on its way back up. The vast majority of engine manufacturers sadly do not include this in the instructions for their engine, (so please don't ring Just Engines and jam up their phone lines - as they haven't the information either!!), but it can be easily measured using a 360 degree protractor mounted on the shaft. Typical timings for sport motors are around 150 degrees, with motors specifically for tuned pipe use are 170 degree and upwards. These figures are indicative but not absolute. You must measure you own engine ExT to enable a satisfactory calculation.

The expected maximum operating RPM. This is very tricky to decide in advance, but it is the one thing you can check to adjust the pipe length later. The RPM is chiefly determined by the size and type of prop you are to use against the ability for the engine to turn that prop. This will be determined by the type of model you are flying and the way in which you want it to fly. The aim is to set up the tuned pipe so that maximum power is achieved at maximum RPM for that prop. If this is also set at the maximum BHP/RPM setting quoted by the engine manufacturer, better still. The maximum RPM will vary according to pipe length and you will need to redo the length calculation a number of times to ensure you achieve the correct setting. You must also understand that the RPM achieved on the ground will be less than the RPM achieved in flight. This has resulted in a school of thought suggesting that the pipe should be set "short" on the ground so the length will be correct for the in flight RPM.

Therefore……There is no substitute for in flight testing!! It goes without saying that an essential piece of equipment for quickly setting up a variable length tuned pipe is a rev counter !!

6) Setting up with Formulas - A Formula Constant.
All the formulas circulating for calculating tuned pipe length contain a different constant and this leads to confusion. The constant is derived from the type of Tuned Pipe, the eventual length units (i.e. inches, centimetres, millimetres etc.), the point of measurement at each end of the length dimension, and the speed of the exhaust pulse wave (over 1100mph!). I have used many different formulas over the years but now tend to settle with the following two. I have achieved great success with these as they result in a conservative (long) pipe length from which to make adjustments. A good, safe, starting point if you will. Each formula is for a different type of Tuned Pipe. One for Twin Cone pipes and one for Flat disk pipes. I will explain the difference shortly.

NB You can tell which pipe you have by looking down it with a torch from the engine end.
Please pay close attention to the points of measurement.

Note that the length in Inches is measured from the Face of the piston to the back of the second cone as per Fig 1.

Note that the length in Inches is now taken from the Face of the piston to the surface of the reflecting flat disk as per Fig2

The differences between a Twin Cone pipe and a Flat disk pipe are as follows : A Flat Disk pipe has a pulse wave reflector like a big flat washer inside the pipe. A Twin Cone pipe has a second "converging" cone set at a steeper angle than the first cone to reflect the pulse wave back to the exhaust port. A Twin cone pipe generally produces more power than a flat disk pipe owing to the longer and smoother reflection of the shock wave, but has a smaller power band and is less tolerant to changes in the application (Plugs, Fuel, Props, RPM etc.) than a Flat Disk pipe. Flat disks pipes have a much wider power band but still require some tuning. This means that they can tolerate slight changes in prop size etc. and still work adequately. Twin coned pipes are, as you can imagine, more tricky to set up. It can be quite a long trial and error process to get the pipe, engine and prop matched to each other, whereas Flat Disk pipes after initial setting up, tend to be more 'fit and forget', although clearly, if you change anything in your setup: (Plug, Fuel, Prop), then an amount of 're-tuning' will be necessary.

7) Setting up without calculations, final setting up and in-flight testing.
Essentially the calculation process is an ongoing thing. A typical scenario for setting up your pipe would be as follows; Decide on the prop you want to use and guesstimate the RPM at which you expect it to work. Obviously in some applications you can draw on the experience of other Aeromodellers with similar set ups. This is especially true of Ducted Fans. Occasionally the more enlightened engine manufacturers, or those with engines specifically designed for Tuned Pipes, will provide some of the initial information for you. In any case do a preliminary calculation and set the pipe at this length. Err on the side of caution if you really have no idea what the RPM is expected to be so that you don't start with the pipe too short. Run the engine and check the RPM at maximum. This will now give you a more realistic figure to use in the formula. Adjust the pipe length and run the engine again. You should notice a change in RPM, usually higher, although if the RPM is lower the pipe is probably too short. However, you should now fly the model unless you are still a mile off with the calculation. If so adjust the pipe to suit. As you approach the point where your calculation with the actual RPM measured is near to the actual length of the pipe you really should be thinking of flight testing. The most noticeable way that the benefits of piped motors manifest themselves in the air is greatly improved vertical performance. Essentially if you can climb the model vertically without the motor running roughly (assuming the needle setting is correct) for a substantial distance, you are not far short of the mark. Remembering what I said earlier about increased airborne RPM the pipe should really be initially set a little two long. If your engine runs unevenly, constantly "coming on and off" the pipe, it should be shortened. If however the throttling is adversely affected to the point where there is a laboured pick up coupled with a sudden jump "onto the pipe" or rapid RPM, then the pipe is too short, and should be backed off a small increment. Herein lies a problem if you have been a little too keen with the old Hacksaw ! Again, you should fly the model to be sure. My JETT .50 powered Windsock Delta runs rough on the floor and won't throttle properly, the pipe is too short for the ground based RPM. The engine however being fitted with a coarse pitch prop unwinds dramatically in the air and never misses a beat in flight where it matters ! It is often adequate to leave the pipe on the long side if it runs OK in the air. This is more applicable to the Flat Disk pipes which have the wider power band and are much more tolerant to modifications in the system. Basically, if you fit a tuned pipe based on your initial calculation, and when the model is flown you are entirely happy with its performance, leave it alone and fly ! It will be possible to tune it further at this stage but you may only notice small additional benefits. The exceptions to this are the higher RPM applications.

If you are missing some of the variables for the formula it is still possible to tune your pipe without using a calculation. This process can take longer (unless you are lucky first time) and will result in modifications being made to the pipe and manifold between flights. If this is the case, mount the pipe and run the engine to measure the RPM. Shorten the pipe by a small increment 5 - 10 mm or so at a time initially, then run the engine again. You should notice a modest increase in RPM. Shorten the pipe again and keep repeating the sequence until almost no RPM increase is observed. If you reach a point where the RPM drops, the pipe is too short. Everything said above about in flight testing is applicable here.

8) Summary
It is true that Tuned pipes can be used to increase RPM, but chiefly they are used to increase power. This can be used to turn a larger, more efficient propeller, or one with a courser pitch, thereby, typically reducing noise. In this instance you would wish to keep the revs low instead just as in F3A Aerobatics. Typically these are propped to rev at 10,000RPM or less with pipe lengths of around 24 inches or more, but the tuned pipe means that prop pitch can be around 10 to 14 inches for a 12 to 14 inch diameter. A fast flying sport model may however be propped to turn at 15,000+ RPM with Pylon Racers and Ducted Fans doing over 20,000 RPM. In all of these cases the engine has to be capable of running at these revs whilst producing maximum Horsepower and you should take care to select your engine appropriately. Just Engines can help here, with their 25years of experience.

Good luck and happy flying..........Andy Ellison