Dynamometers

This site was last updated on February 15, 2008.

I've done a limited number of experiments with the fan-brake dynomometer distributed by

American Hobby Products
12 West Hill Circle
Reading, Massachusetts 01867

It's pretty cool.

In the picture above, we can see the dyno mounted on a portable work table. In the center, the engine is mounted on an adjustable table that pivots clockwise in reaction to the engine's torque. The various weights are used to balance different engines and allow the dyno to accommodate a wide range of engines. The black tube on the right holds a bladder, connected to a remote needle valve, and thence to the engine. Torque measurements can be read from the large dial in the upper left.

Using the torque reading with the measured RPM (you'll need a tach), you can compute horsepower using the formula

     T x R
P = -------
    1008406

(where T is measured in ounce-inches). The documentation discusses how to correct for temperature and humidity variations, so I won't go into it here. The discussion is adapted from a series of articles by Dave Gierke, published in 1973 by Flying Models, in the June, July, and August issues.

The dyno isn't completely ideal. It uses a form of mechanical damping, which may interfere with accuracy. It's also possible that the prop swirl will interfere with bars supporting the balance weights, affecting the results. Finally, a calm day is required, since any wind will interact noticably with the prop. I think many tests will be required for each prop to get a reasonable feeling for its associated Cp.

Here's the result of 4 runs of a Nelson 36.

Prop RPM Torque Power Cp

7.8 x 6 22200 85.2 1.88 1.714e-13

7.3 x 6 22400 86.4 1.92 1.708e-13

6.8 x 6 24200 80.4 1.93 1.361e-13

6.3 x 6 26700 75.6 2.00 1.052e-13

where torque is in ounce-inches, power is in horsepower, and the props are cut down from APC 7.8 x 6 (7.8" diameter, 6" pitch). The values for Cp are expressed in a kind of scientific notation: 1.709e-13 means 1.709 times 10 raised to the -13th power. Remember that props will vary, even from a single manufacturer, so don't be tempted to use my results on your props. Remember also that the results for the props will vary with the air density (which is related to temperature, barometric pressure, and relative humidity) as will the power of each engine.

Looking at the results, I'd say the first 2 props look unreasonably close and I'd like to see more tests. I also wonder at the repeatability of all these results. Of course, the way to find out is to repeat the tests, preferably many times for each prop (though the engine could vary). I'd also like to see a greater range of props.

There are other, more expensive and hopefully more accurate, ways to build dynos. Check out Lothar Hentschel's test stand and Brian Callahan's inertial dyno.