Exhausting!

One of the hardest-working and most-ignored systems in any aircraft is the exhaust. Existing in a high-vibration environment, it must put up with extremes of heat that would set fire to most components you’ll find forward of the firewall. Exhausts are often thought of as add-ons to a powerplant, yet poor designs can rob those engines of enormous amounts of hard-won horsepower. In the early days of homebuilding, most exhaust systems were custom-made by the aircraft builder, or the aircraft cowling was built around a system taken from a certified aircraft. Today’s builders have the comparative luxury of choosing from a variety of systems designed specifically for their kit or plansbuilt aircraft. One of the major suppliers of pre-built systems to the Experimental market is Vetterman Exhaust, Inc. of Hot Springs, South Dakota. KITPLANES paid a visit to learn about how exhaust systems are made, and what is important in choosing a system for your project.

Like many small businesses that provide parts to the Experimental aircraft community, Vetterman Exhausts began when a builder (Larry Vetterman) needed an exhaust system for his own project. Dissatisfied with the available choices for a number of reasons, Larry bought some tubing and began to bend and weld. A veteran Army and public service pilot, Vetterman learned about what makes a good exhaust system by studying, building, and trying out the results. The science of exhaust system design is as old as the internal combustion engine, and while there are some well-known rules about how to get more and better power, there is also an art to the process. Getting just the right length and bend radius on each pipe can be the difference between gaining or losing several (or more) horsepower. And field experience is essential in building systems that will survive without melting, breaking, or cracking in the harsh environment of the cowling.

Vetterman now has thousands of his designs flying around the world, and very few of them come back for repair-a testament to good, solid design and solid performance. While Vetterman himself is now somewhat retired from the business (now run by Clint Busenitz, a long-time employee, expert fabricator and aircraft builder), it was hard to tell that during our visit. He clearly enjoys the world of Experimental aircraft and enjoyed showing us the machines and processes that go into fabricating the components of each exhaust system. We followed the process as it went from straight tubing to a system ready to ship, and learned a lot along the way.

During our visit, production was split between two locations in the Hot Springs area-one at Vetterman’s home in the hills overlooking the Angistura Reservoir, and the other near the Hot Springs Airport where Busenitz has a small farmstead. It doesn’t take a lot of real estate to produce exhaust systems, but both facilities are well-equipped with machines and the necessary space for design, fabrication, and storage. Each has space for several workers for simultaneous production of numerous systems, and their production can be ramped up or down as demand requires. Vetterman also maintains two hangars at the Hot Springs Airport where he can quickly install and fly prototypes for testing on his own aircraft. Vetterman won’t sell designs that he hasn’t flown-he wants to know what they will do before he provides them to a customer.

It Starts With Tubing

Vetterman fabricates most of the components for their systems from raw stainless steel tubing. They’ve got a large stock of 1.5-inch, 1.75-inch and 2-inch 321 stainless steel tubing (as well as 3-inch for muffler casings) in a rack at the front end of their production facility. These sizes of tubing constitute the majority of the material that goes into their systems. Some parts-exhaust flanges for example-are laser cut elsewhere to Vetterman’s specifications. This cuts down on the total amount of machinery they need to keep in house and maximizes efficiency in overall production. The original flanges Vetterman used in his early days were cast-the newer laser-cut units are simpler, stronger, and far less expensive to produce. All components and material are checked upon receiving to make sure they match specifications. For instance, samples of each shipment of tubing are bent to check their spring-back characteristics so that production can be tweaked to make each bend the exact number of degrees.

Most exhaust systems are produced from “stock bends”-the first step in the Vetterman process being to produce these bends. Racks and racks of pre-bent tubes are stacked at both facilities-the products of the first steps of the process. Each shelf hold stacks of bends of varying degrees: 43, 45, 46, etc. Some extreme bends (those beyond the capability of Vetterman’s machines) are procured from other suppliers as an efficiency measure. A good example were some 180-degree bends piled in a box to be checked for quality before being added to the stock shelves. The stock bends are eventually cut to the necessary lengths during fabrication of a particular system to build up each pipe. One trick to eliminate waste it to cut the raw stock for the bends as short as possible.

The initial cutting from long, straight lengths is not done with a saw-it is done with a powered, mechanical tubing cutter. This eliminates the need for deburring each cut and wastes less material. The process does, however, neck the tube down a little at each cut, so another simple machine restores the end of the tube to its full, original diameter. None of these machines are complicated-their simplicity is impressive. One person can cut and dress many tube bits in short order. The stock shelves tell Vetterman when to make more of each bend, and his notes tell him how long to cut each piece to make the necessary bends.

Bending the Easy Way

Any builder who has worked with aluminum fuel lines must wonder how in the heck you can bend 2-inch stainless tubing without crimps or ripples in the finished product. If they have bent tubing by hand, they must wonder how such tubing can be bent at all! The answer is in a medium-sized hydraulic, computer-controlled bending machine, probably the most complex machine in Vetterman’s shop. The necessary bend angel is programmed into the machine, and with the appropriate dies installed for the size tubing being used, production is a matter of lubing up a mandrel, installing the pre-cut straight length of tubing, getting out of the way, and pressing the start button. Staying out of the way is important-the forces used in the bending process are considerable, and a soft human body part in the wrong place wouldn’t even slow the machinery down.

The key to bending the tube without collapsing it is the mandrel-a brass device with a flexible end that is inserted in the tubing before bending, and is hydraulically withdrawn as part of the bending process. Drawing this device through the tube restores the original diameter throughout the bend, a motion helped with a brushing of water-soluble lubricant applied to the mandrel. A bend takes just a few seconds, and the resulting piece is of constant-diameter throughout the bend radius. After bending a box of shorts tubes, Vetterman takes the components to a wash tank filled with Dawn dishwashing detergent to clean off the lubricant, preparing them for the welding process. He emphasized just how important it was for the parts to be clean and grease-free, or the welds would have voids that could lead to failure.

Bending appears to be a batch process-whenever the stock shelves are running low of a particular bend, someone fires up the big hydraulic machine and cranks out enough of that bend to refill the shelf. Dies need to be changed when tubing size changes, and this takes some time so production planning is important.

Pattern Boards and Engine Mockups

The key to building exhausts for different engines and airframes are the pattern boards-pieces of plywood with the shapes of the various components for each tube drawn on their surface. Wooden blocks are glued or screwed to the surface to help the fabricator locate each piece. These two-dimensional fixtures create the components that are then fit to the actual engine mockup to create a three-dimensional exhaust system. Vetterman has many different pattern boards because he builds exhausts for many different airframes-and many different engines. Everyone knows the old joke about the mechanic that thought he had found two identical Lycoming engines-but then realized he was mistaken. Well, the exhaust system for each variant of an engine can vary enough to fit around carburetors and sumps that it is important to keep patterns for everything they have done.

Engine mockups are equally important because each exhaust system that comes out of the shop is essentially hand built in place on an engine-or more accurately, what is left of an engine. Vetterman keeps a number of crankcases and cylinders around-I counted ten without poking around in corners-and a surprising variety of sumps as well. None of them are even close to airworthy-holes in cases abound-but enough of the cases are there to hold the cylinders in the right positions and mount the wide variety of sumps they run across. Both Lycomings and Continentals are represented in the Vetterman shop, and more types may appear as they expand their customer base.

Actual engine mockups are important as the fabricator builds up the exhaust system in three dimensions out of components that were started on the pattern boards. Once all of the bits and pieces are in place and not interfering with engine parts, they are tack welded together at each joint so that they can be removed and finish welded on the bench. Before tack welding, places where pipes join at odd angles must be trimmed-the resulting shapes are much more elaborate than the fish-mouths used when welding up a steel-tube fuselage. There is clearly an art to making these complex joints on compound curves, and experience is evident in the speed with which Busenitz works. When he had completed the trimming, he used standard hose clamps to hold these joints together for the spot welding necessary to keep them in alignment for finish welding.

Welding-the Art and Science

MIG welding is used to build the exhaust systems at Vetterman, and the precision with which Busenitz can join and blend the various tubes is fascinating to watch. This is not what I call “farm welding”-joining big chunks of steel to make heavy-duty (and possibly ugly) assemblies to take heavy loads. This is fine-finish welding with beautiful beads laid down by hand. One hand controls the electrode, the other the rod. A foot pedal controls the current, and the entire operation reminded me of watching a pianist playing a concerto. Vetterman and Busenitz have special tools that hold the exhaust components and allow the parts to turn and spin as they do circumferential welds. The results are uniform and precise-and hand done.

Argon gas is used to isolate the melted puddle of steel from the surrounding atmosphere-particularly the oxygen-that can create porous or pocketed welds. The tip of the electrode is constantly bathed in argon, and when welding a section of pipe, the interior is purged with argon so that the heat penetrating through to that side will also not cause problems with the weld.

Surprising to those familiar with current production technology, there are no welding robots at Vetterman’s shop-all fabrication and assembly is done by hand, the same way it has always been. The varied nature of engine models and low overall market size simply could not sustain robotic manufacturing, and there is considerable tweaking of each system in order to fit around different engines-so one-at-a-time manufacturing is the rule.

On Their Way

Once a system is fully assembled and checked for quality, it is boxed up by Busenitz’s wife, and the boxes for the day’s shipments are lined up for the local UPS driver. Again, there are no loading docks, no forklifts, and no stockmen handling thousands-or even hundreds-of packages. This is a one-at-a-time, family-run manufacturing facility, and the local UPS man knows to stop. Vetterman employees have his cell phone number saved in their own phones and text him with the planned number of packages before he arrives. Western South Dakota is like that-informal and friendly.

Exhaust systems are available directly from Vetterman or through kit companies for whom Vetterman is a primary supplier. Either way, the customer is getting a hand-built, quality unit that is backed by the folks who build them.

Custom Design and Experimentation

Larry Vetterman has been experimenting with exhaust system design since the first time he welded up a system for his own airplane. At first, it was about the fit to the engine and cowl. Early exhaust systems for the RV line of aircraft worked to conduct hot gases from the cylinders to the outside of the cowl, but performance was not always optimum, and things like allowing access to oil drain ports and providing good, long-lasting attachments weren’t always at the top of the list. But that was early homebuilding, and we have evolved since then.

Vetterman recounted numerous stories of testing various exhaust designs on his own airplanes, including one instance where the system was developing so much backpressure that he barely had enough power to make it around the pattern and land. Pipe lengths must be designed so that the backpressure seen at the cylinder’s exhaust port is just right-not too low, not too high. Too high and the cylinder can’t exhaust its spent charge; this leads to a contaminated charge for the next firing and lower overall power. But too little backpressure can allow unburned, fresh charge to flow from the intake valve and out the exhaust, again resulting in a loss of power. Lycoming wants to see just a little backpressure-but not too much.

Many early homebuilts, with cylinders sticking out the sides of cowls, got away with short-stack, individual pipes. Later designs with full cowls used four-pipe systems that brought the exhaust all the way down to the bottom of the fuselage before sending the gases overboard. Both systems are still in use, providing good power and good service. Newer designs, those that connect two cylinders on opposite sides of the engine (and hence called crossover exhausts), help the scavenging by using pressure or suction from one cylinder to help charge or evacuate its partner. Pipe length is critical when it comes to these systems, and that is where experimentation comes in. Vetterman noted that it’s not just the length of the pipe that matters, but the radius of curvature and the diameter of the pipe that contribute to the calculations-and in the end, what matters is how the system performs on the dynamometer.

As mentioned earlier, good design also allows for maintenance and inspection. Vetterman is careful to design his systems so as not to block oil drains and oil screen access on the engines he marries them with. He also provides slip joints in key places to allow for expansion and contraction-this prevents cracking down the line and makes for a long-lasting system. A unique pin and hole bracket keep the systems together while allowing for relative motion, and very few customers have reported problems with longevity. Vetterman provides mounting hardware that allows the builder to hang the rear of the system from the engine mount or the sump as they prefer-a matter of considerable debate on Internet forums, with evidence abounding that both methods work.

Vetterman’s latest systems include mufflers to satisfy a growing demand for quieter flight, and he has been able, through careful design, to build systems that are providing equivalent power to the unmufflered systems that they have traditionally sold. Good design is also a key to providing consistent exhaust gas temperatures across all cylinders, making for a more balanced and smoother powerplant.

Not Just an Afterthought

Modern exhaust systems are not simply a way to get the products of combustion out of your cowl without catching anything on fire. They can enhance engine power, provide for smooth operation, and, of course (with heat muffs), give us cabin and carburetor heat. Good design is important for performance and long life-both matters of interest to those who fly and maintain their own airplanes. Certified airplane owners have, for years, found muffler and exhaust pipe cracks at annual inspection time-I know, I used to be one of them. Yet exhaust system problems have decreased in the Experimental community over the years, primarily because of designers and builders like Vetterman.

While there are still builders who will want or need to build their own exhaust system-due to the desire for adventure or because of a very unique aircraft design-it is easy and inexpensive to buy one that will meet most needs today and in the future. As we learned from our visit to Vetterman Exhaust, building your own is not impossible for those with the welding and fabrication skills. But with systems this good, it is going to be hard to beat store-bought in today’s marketplace.