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CVT PSRU?

Continuously variable transmission propeller speed reduction units what do you think?

By Vince Homer, EAA 162674, for Experimenter

The following article is merely a discussion on the potential of utilizing the technology used in the continuously variable transmission found on some all-terrain vehicles, snowmobiles, and a small number of automobiles. The idea is similar to a constant speed propeller except the engine speed (and horsepower) is increased, or otherwise variable, with the prop speed staying relatively constant. Some may already see a flaw in this concept. However, read the article and share your thoughts at EAA Forums.

Propeller speed reduction units (PSRUs) are a fact of life for almost all builders using alternate engines for experimental aircraft. With the exception of the Volkswagen, Corvair, and the Model A/T Ford (and maybe a few lesser-known auto conversions), automotive engines develop power at revolutions that are far beyond the usable range for common propeller diameters.

This article presents the concept of using a continuously variable transmission (CVT) as a PSRU. The graph below is a summary of an Excel exercise on the performance of a propeller and automotive engine with fixed ratio and a CVT PSRU. The engine data is from a GM-published horsepower/torque versus rpm graph, and the propeller results are attained from a freeware propeller calculation program. The design max speed for the propeller is 136 mph at standard conditions. It must be emphasized that this is a paper study and not based on any real data from a real hardware engine/propeller/CVT test program.

The curves on the left side of the graph represent the performance of a standard, fixed-ratio PSRU of 2.14:1 and 1.82:1 used with a GM Vortec in-line six-cylinder engine. The table below represents the rpm/power results for the fixed-ratio PSRU:

PSRU
ratio

Engine
rpm

Propeller
rpm

Engine
max hp

Propeller hp
absorbed

Nonavailable
hp

2.14:1

4,640

2,150

240

125

115

1.82:1

3,950

2,150

195

125

70

From this table and the graphical representation (toward the end of this article), one can see that the PSRU ratio only provides a match between engine power available and propeller power absorbed at one rpm. This rpm will be the maximum that the engine will drive the propeller. Beyond the match point the engine will not develop sufficient power to drive the propeller faster.

Although the whole problem is simplified in this study, bear with me as this is just a concept to be considered. Maybe someone out there has already tried this approach or will do so as a result of reading this article.

The horizontal line between the engine and propeller curves shows examples of the PSRU ratio at various operation points. Suppose a way could be found to vary the ratio of the PSRU to match these ratios shown on the graph. The result would be that the engine curve would lie on top of the propeller curve, and the power available would match the propeller power absorbed through the entire engine operation range.

CVT

The CVT concept has existed for a very long time. In 1490, Leonardo da Vinci conceptualized a stepless continuously variable transmission, even sketched one in his notebook. They have developed in the last few decades to very robust units. Among the biggest users are recreational vehicles such as snowmobiles and four-wheeled off-road vehicles known as quads. Many current snowmobiles develop nearly 200 hp in the standard configuration and considerably more for the racing versions. In addition, some automobile manufacturers, Subaru for example, are offering a CVT as an option or in some cases as standard equipment. The recreational vehicle versions use polymer belts, and the automobile ones use metal link belts.

An example of a typical snowmobile CVT is shown below along with an illustration of the operation principle:

CVT
Courtesy Ski-Doo Snowmobiles    

CVT

These units are available with a speed ratio range from less than 0.8:1 to over 3:1.

Essentially the CVT is a constant torque device. The torque, or pull on the belt, tries to separate the driven pulley, and this separation is resisted by springs. The spring force may be adjusted to match the torque of the engine used with the intent of keeping the engine operating at or near the desired operating rpm. The effect of the CVT is to match the engine torque with the torque required to drive the propeller.

CVT

The nonlinearity of the engine hp/rpm curve at the high end (shown in the circled area in the graph above) might be difficult to match with a CVT. One fix for this would be to sacrifice the last 500 rpm, in this case, from 6,000 to 5,500 rpm. This would reduce the horsepower available by about 15 but might contribute to engine longevity. It may be possible to accommodate this nonlinearity by the use of a more complex spring geometry and regain the last 15 hp.

Due to the operational nature of CVTs, their friction losses would be higher than a fixed-ratio PSRU. As a result, it might be necessary to provide provisions for forced-air cooling. After all, snowmobiles operate in a much different environment than an aircraft PSRU.

Is there anyone out there thinking about trying this? Does anyone see a fatal flaw in this concept?

Write in and tell us your thoughts at EAA Forums.

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