Power

This site was last updated on February 15, 2008.

To achieve best results (speed), we try to run our engines at the RPM that gives the highest horsepower. Typically, this is achieved by selecting a prop that allows the engine to achieve this RPM for your given airplane. But first you need to know the target RPM.

We can compare two engines on the bench by running them with the same prop (or perhaps a range of props) and measuring the RPM for each run. For example, I once tested one of my Fox Mk 6 engines, using an APC 7.8 x 6, cut down to 6.8 inches. On this particular day, it ran about 22,700 rpm. Using the same prop on a Jett 36, I observed it running about 24,000 rpm. Using the equation

P = Cp x R^3

(where P stands for Power absorbed by the prop, Cp is a coefficient dependent on the prop, and R is RPM), we can do some algebra to show that my Jett was

Cp x 24000^3   24000^3   
------------ = ------- = 1.18
Cp x 22700^3   22700^3

times as powerful (or a 18% improvement). Notice that using the same prop in both tests allows us to ignore the value of Cp. Using a second equation

T = Ct x R^2

(where T is the thrust produced by the prop), we find that the Jett (running this prop) would be producing about 1.12 times as much thrust as the Fox and, since speed varies with the square root of the thrust, we could expect about 1.057 times the speed (perhaps 119 mph for the Jett versus 113 mph for the Fox).

Of course, a single measurement doesn't tell the whole story. To fully understand the performance of an engine, we need to test the it over a range of RPMs. One approach is to use an inertial dynomometer which can do the complete test in a single run. If you don't have such a beast, you can do several runs, using a range of calibrated test props.

A calibrated prop is simply a prop where we know D and Cp (the diameter and the coefficient of power). Given those two values, we can measure the RPM the engine gets with the prop, do a little math, and we'll have the power for that RPM (ideally, you would also pay attention to temperature, altitude, and humidity). And by using a range of calibrated test props, we'll be able to get the power over a range of RPMs.

And how do we get a set of these calibrated props? Apparently Kavan used to sell such a thing, but you'll probably have to make your own -- which leads us back to using a dynomometer.