Turbocharging makes sense for the Seawind!
To approach sea-level performance at a high mountain lake is nothing short of a great idea. In fact it's such a good idea that even SNA* had considered developing the modifications necessary for adapting a turbocharger to their kit. One of the leaders in adapting this technology to Seawinds was ISPA member Scott Devlin at Devlin Aviation* in the Pacific Northwest. Tragically, Scott Devlin lost his life in a mid air collision.
An engine that uses intake air at atmospheric pressure is said to be normally aspirated. The amount of air a normally aspirated engine can use is limited by the air density (barometric pressure) in which the engine is operating, and pressure losses in the intake system. As most pilots know, under normal conditions, air density decreases with altitude, resulting in reduced power output. With a normally aspirated engine, most airplanes (that are also equipped with a constant speed prop) have a manifold pressure gauge. At full takeoff power, at sea level, on a standard day the manifold pressure gauge would indicate a MP reading of approximately 29" of Hg. Takeoff power at 5,000 ft. density altitude airport would read about 24" MP. The normally aspirated engine uses atmospheric pressure and is thereby altitude limited.
Dean Rickerson's twin-turbocharged Seawind being completed at Graham Wood's shop in 2003.
In order to get more air into an engine, small compressors are sometimes used to pressurize the intake air. The amount of pressurization is referred to as "boost." If the compressor is driven off the engine crankshaft, the process is called supercharging. Another way to power the compressor is to put it on a common shaft with a turbine driven by the engine's exhaust gas. This process is called turbocharging. For most aircraft engines, turbocharging is preferred since it extracts energy from exhaust gases that would otherwise be wasted, so it can be more efficient than supercharging, which takes energy from the crankshaft. In some cases, two turbochargers are installed on a single engine, one in the exhaust ducting on each side of the engine.
Recently, turbo-charging has become rather common in the automotive industry, but because turbochargers can augment combustion air by boosting the intake manifold pressure at higher altitude, they have been been used in aircraft applications for several years.** Turbocharging can provide greater utility to the piston engine by providing sea-level horsepower, in some models, as high as 20,000 feet; or it can be used to add horsepower to the engine specifically for takeoff. The faster the engine runs, the more air the turbocharger can pack into the cylinders to compensate for the thin air of altitude, or to increase the horsepower.
To maintain boost pressure at a relatively constant amount over a wide range of engine speeds, some sort of pressure regulation is needed. This pressure regulation is usually accomplished by using a variable waste-gate that bypasses excess pressurization from the engine intake. There are also turbochargers that use fixed, or non-variable waste-gates as well. As expected, they are not quite as efficient as the controllable versions.
Some precautions are required for turbocharged engines. Over-boosting, or over-pressurization, especially during take-off, can severely damage an engine. Turbocharger control systems may include linkages or controls to prevent over-boost, but if not, strict operational guidelines must be followed.
Turbocharged engines as manufactured by Lycoming consist of a turbocharger unit with a small turbine wheel attached by a common shaft to a compressor wheel. They utilize the engine exhaust gas by directing it over the turbine wheel to drive the compressor. The horsepower loss in operating the turbocharger is negligible.
When turbocharging is used with a fuel injected, opposed Textron Lycoming engine with a 540 cubic inch displacement, it is designated as a TIO-540 model. "T" represents the turbocharging, "I" represents fuel injection, and "O," the opposed cylinder configuration.
Lycoming makes a TIO-540 that is perfectly suited to the Seawind. Most of the turbocharged Lycoming engines have cooling schemes that are different than our standard K1H5. These engines are most commonly used on Piper Navajos. The airflow direction is from the bottom side of the cylinders up through the top, opposite that of the engine usually used on the Seawind. The exhaust ports are also located on top of these engines. Next time you see a Navajo, take a look at the cooling inlets and outlets. Of course, turbochargers have been used with the bottom exhaust Lycomings as well.
Depending on the turbocharger/engine configuration being used, adapting a turbocharged engine to a Seawind may require various modifications. Usually the cowling and exhaust systems require modification, and sometimes the engine mount as well. (Devlin Aviation's turbocharger application used the stock Seawind engine mount provided by SNA with the Seawind kit.)
Not only is high altitude performance enhanced by turbo-charging, but the rated horsepower of the most common turbocharged Lycoming is 350 HP. If you are a prospective Seawind owner / builder, you may want to spend some time those who build, own, or fly the turbocharged versions. One very successful turbo charged conversion was created in Graham Woodd's shop outside of Seattle, Washington. This twin-turbo creation is said to produce over 400HP.
There are a few turbocharged Seawinds flying. The ISPA is seeking content for this page. Please submit photos and articles to the ISPA editor. Thank you.
*The ISPA has no formal affiliation with Devlin Aviation, SNA Inc., nor any of those above.
**Around 1920, Dr. Sanford Moss of the General Electric Co. was spearheading turbocharger research in an effort to maintain sea-level performance in aircraft engines in the thinner air of high altitudes. Dr. Moss eventually become known as the "Father of Turbocharging."