Saturday, September 26, 2020

Sensor supply chain and COVID-19 calibration stoppage miseries

We are really close to getting our probes to a "done"-ish stage. Really really bloody close. So close. And yet.

First problem: Calibration is blocked by COVID-19

The first misery we're dealing with, along with everyone else of course, is COVID-19. This has caused all the wind tunnel facilities nearby to close down. And there with it goes any hope of real calibration. Now a good friend (not outing him in case he wishes to remain anon) offered use of his own homebuilt tunnel, but that's far away and would require a lot of setup and preparation, and honestly my day job is overwhelming at the moment. (With a nearby wind tunnel, I can afford to cheese my setup a little bit and, if it does not work, I can always go back the next weekend.) It is possible to imagine complicated devices I could fly on my plane to achieve the effect of a "wind tunnel" -- but that would require even more R&D time.

Second problem: Pressure sensor stock is limited

You will recall that, after much tribulation, we decided to go with 3.3V SMT mount SPI Honeywell TruStability pressure sensors. They are reliable, name brand, and easy to use. But the problem is that the range of pressures in stock is limited. You can get what you want but with terms like, minimum quantity 100 and lead time 7 weeks. Yikes. And this is in turn a problem because my "new" probe design, that eliminates the static probe "stick", tends to amplify the pressures. This is good, right? But this means we need at least one of the sensors to be a wider range. And this is difficult.

Look at this plot of dynamic pressure (q) versus IAS:

For a Cessna 172 and thereabouts, you can figure q < 0.5 psi. For a Cirrus or Mooney, we're getting into the range of q < 1.0 psi, in other words, the dynamic pressure doubles.

In the old probe design, with the static pressure stick, two of the measured pressures are in the range [-q, q], and one of them is in the range [0, q]. So for a Cessna 172, with VNE = 160 kias corresponding to q = 0.57 psi, a +/- 1 psi sensor, which is a very common part, is adequate for everything.

In the new probe design, which "amplifies" the pressures, two of the pressures remain in the range [-q, q] while the third is jacked up to the range [0, 2.3q].  At VNE with a Cessna 172, this would mean [0 psi, 1.3 psi]. So this means that, even for the humble Cessna, a +/- 1 psi sensor is really pushing it. Close but pushing it.

I could go out and order 100 sensors of the appropriate size and wait 7 weeks. That would be between $3,500 and $4,500 of parts, and I would be doing this on a hope and a prayer having done no wind tunnel calibration. That is a bit -- um -- let's say, gutsy?
In making the next round of probes, I have to use the 1 psi sensors that G-d has graciously provided us. There is no getting around that. Until I do a serious calibration, I'm not going out and buying expensive crap.

Right now, I have a probe board sitting on the bench that has three +/- 60 millibar (+/- 0.87 psi) sensors. These were purchased when I was using the "old" probe nose design. Might as well use them and, if they saturate, they saturate. We will at least get some partial data out of them.

Moving forward, depending on how things go with the new probe in flight, I might decide to go back to the old probe with the static "stick", or standardize on +/- 1 psi sensors and hope for the best.

In this case, "hoping for the best" means that, for the pressure sensor reading that is in the range [0, 2.3q], we would saturate at q = (1 psi / 2.3) = 0.43 psi. This corresponds to an indicated airspeed of 138 kias. This is a reasonable compared to the VNE for our tiny little LSA.

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