Engine testing on a stand is usually accompanied with additional instrumentation and a bellmouth for the measurement of engine airflow. These bellmouths are built to known dimensions and had previously been calibrated such that the airflow is known for a given delta pressure reading. Utilizing a given bellmouth geometry, pressure readings can be used to calculate airflow. I made a small bellmouth for the SJ10 engine and it is shown in the following pictures:

The picture on the right shows the gauge I used to measure bellmouth static pressure. The static pressure could also be measured by a DIY water manometer (a lot cheaper).

The data reduction table above shows the SJ10 performance data with and without exhaust nozzle. The exhaust nozzle throat diameter = 1.50” and the Inducer diameter = 1.68”. I used this to plot operating lines on the compressor map for the run with the exhaust nozzle on and the run with the exhaust nozzle off. That way I can see the effect of additional back pressure on the engine. These plots are shown below:

For an explanation of corrected values see the section below this one. The difference in engine values can be readily seen by comparing the appropriate charts. For instance, the difference in EGT is a couple of hundred degrees F. This data was taken on the SJ10 with an inducer diameter of 1.70”

Please note that the static tap holes should be deburred but still have sharp edges. The design of an ASME bellmouth is shown below:


The drawing of the SJ10 bellmouth is shown below:

I have a manual lathe so when I machined the bellmouth I needed a way to cut the one-quarter ellipse. The method I came up with is the “step” method. This cutting the curve in a series of 0.001” to 0.003” steps, putting bluing on the steps, and using fine sandpaper to sand the blue off which gets rid of the steps. Do you see any steps in the photos?

The other consideration is that there are over a hundred steps that must be calculated. The ellipse portion has a well defined equation, but that would take a lot of calculations for each step; therefore I used a spreadsheet to calculate all the steps at once. The program is available below for downloading.

Download Excel Spreadsheet: SJ10 Bellmouth Coordinates 6-7-13

Once you have the bellmouth machined and installed on your SJ10 and run some data, you will need a method of turning the inches of water reading into corrected airflow. Some information is needed such as air temperature, elevation of test site, and barometric pressure, as well as engine test data. This information is put into the data reduction and correction program which is available below for downloading.

Download Excel Spreadsheet: SJ10 Airflow Data Reduction and Correction 6-9-13

Any questions, please contact me and I’ll try to answer them.