Sunday, September 5, 2021

Probe nose testing at Wichita State

Airball collaborator Matthew Schmid at Wichita State University did some new testing of two probe nose concepts. Here are some quick videos of the testing in progress:

The experiment is best described in the Github repo, so please first check out the following:

First let's analyze the "2-hole" head. This is a plot of the pressure coefficients from all the holes, for the 5 psf test:

First, all 5 holes from the probe nose show really good correspondence of the data (orange dots) with theory (blue dots). There is some difference, which is proportionate. This is great.

Notice, however, how the static pressure signal is really poor. It is highly asymmetric and does not seem consistent. The "2-hole" concept relies on having two controlled orifices, on the left and the right, feeding pressure into a common plenum and thus averaging out the pressure in a predictable manner. But in our setup, we just plumbed 1/16" passages through windy 3D printed matter, with a right angle somewhere there, creating an unknown and inconsistent resistance to cross-flow.

If we want to try again, we should probably consider creating two, precisely drilled, tiny "orifice plates" out of metal to create the two side holes, and connecting them with a large plenum. Not unsurprisingly, this setup sounds very much like the static source on the side of an actual airplane!

Having basically dismissed the 2-hole static source as currently implemented, let's take a look at the case with the explicit static tube. Here again is the 5 psf test:

There the static pressure signal looks very symmetrical and useable. There is one interesting factor though. Look at the way it varies with alpha:

At positive alpha, the static probe is "shadowed" behind the probe nose, and so it reads a lower pressure. We decided to put the static probe tube above the nose so as not to interfere with the angle of attack reading -- but now we realize that the nose interferes with it! This is okay. We can use this calibration to get a reasonable system.

If we combine the static probe head data for 5 psf and 20 psf, we can see the effect of Reynolds number on our results. At least in the range tested, the pressure coefficients are consistent (not an outlier row of data around beta = 24 degrees, which we will eliminate).

The next steps are to reproduce the static probe geometry in a manufacturable and durable manner. In particular, the problem we were having is that the probe tube would rotate freely in the 3D printed nose, and just adding a set screw was not enough to constrain it. Our current approach is to use stainless steel capillary tubing and silver-solder on a small mounting "foot" to constrain it better.

Stay tuned for details.

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