Engine Exhaust Systems, II
If we want to fly our aircraft for years to come without disturbing our neighbors with the noise coming from our engines we need to take measures to silence the exhaust. In some countries its required by law and they classify the aircraft in noise categories which are coupled to the landing fees. The less noise your aircraft emits the less you pay.
As we, or rather you, have built our own aircraft, why not do the same with the exhaust?
You will need to learn how to weld mild steel of even stainless steel pipes and will need to work with ball joints. But it gives you the chance to form that exhaust exactly how you want it. And possible repairs later on are easy as you built it yourself.
Before you go on, make sure that you have read the previous page which outlines some rules laid down by the FAA regarding safety and exhaust systems.
Building yourself gives you the chance to form it to the internal dimensions of your cowling and in such a way that inspections can be easily done during annuals and daily preflights.
Most exhaust systems are made from stainless or plain mild steel pipes. Each has their own advantages. Mild steel corrodes quickly and is not as light as stainless steel (its thicker) piping but are more economical and easy to weld.
Stainless steel piping weighs much less, it is very strong, can be bent easily, is non magnetic and corrosion resistant. Disadvantages are thats its difficult to weld and drill holes in. Basically it is the perfect material for an aircraft exhaust. Inconel is sometimes used for exhaust piping but this material more difficult to obtain. It is a nickel-chromium-iron alloy where stainless steel does not have the nickel.
Drilling into stainless steel can be tricky if the correct technique is not used. Make sure the drill is grind at a slightly less angle than normal, set a punch mark and use slow drill speeds. The trick is to keep the drill cutting. If does not then the steel will get very hard and you will have to soften it again with heat.
Rebuilding a factory build exhaust is much easier than starting from scratch. It gives the opportunity to adjust the tailpipe bending it parallel to the airflow as a jet exhaust. Take care that the muffler does not heat up other components under the cowling should it be repositioned.
Building a new exhaust from parts is the best option when a standard stock exhaust is not available for your aircraft. Some aircraft have their engine installed in such position that a stock exhaust will not fit, but one can be made with stock parts to fit it to the aircraft in question.
Flanges & Bends
Flanges are where it all starts, these are bolted to the engine at the exhaust studs with special nuts (brass or steel, do not use self locking nuts these contain a plastic and this will melt). The rest of the exhaust is welded to the flanges and there is higher possibility (compared to pipes) that cracks may develop in this area.
Make sure that you obtain the right flanges for your engine, size does matter here. Rotax types are easy enough, these a somewhat smaller than the average Lycoming or Continental flange.
You will probably need some bends in the right angles, 45°, 90° or even u-bends. Bending pipe steel yourself can be done with the right equipment and some experience.
Slip & Ball joints
You will need these as long pipes will tend to crack due to movement and vibration after some time in use. With the correct joint the exhaust system is able to move when the engine is running and can expand when it heats up. This will raise the reliability of the system and maintenance will be a no brainer.
You will need at least two heat muffs: one to heat the cabin and the other to supply hot air for the carburetor. Make sure to keep the air flowing in the muff when hot air is not needed as it will melt the muff due to the absence of air flow.
A good way to transfer heat from the pipe to the air space in the muff is by using coiled springs inside the muff. Tack these to the pipe and at both ends in the muff. To get the cold outside air routed to the muff you will need some heat resistant flexible hose, Aeroduct will do fine for this purpose.
After the exhaust is completely welded it pays to have it sandblasted to remove slack and any rust and then have it painted with a high temperature paint (>1200°F) to prevent corrosion.
You will need to make sure that you create an exhaust system that route its pipes in such a way that preflight inspection does not become impossible without removing parts, this also is valid for the annual and 100 hour inspection by you.