Tuesday, May 19, 2020

Wing Inspection Holes

May 19, 2020:

I decided to conduct an experiment to see if I can reduce RFI from my LED strobes. This required gaining access to the unshielded power wires inside my wings, in order to replace them with two-conductor, shielded cables (100% foil wrap with drain wire). This entailed the addition of an inspection hole in the underside wing tip, so that I could untie the individual power wires from the wingtip rib and more easily wire up the wing tip nav strobes (including the addition of two ferrite chokes as close as possible to the lamps).

For the tail strobe, I replaced it's 4-conductor, shielded cable (which I did not ground, for some unknown reason) with a 2-conductor and grounded the shield, connected to the negative side of the terminal strip inside the instrument panel.


Standard 4-3/4" diameter inspection hole rings and recessed covers from Aircraft Spruce.


I glued the rings to the painted fabric with 3M 30NF contact cement (same stuff the Stewart System uses), then overlaid it with a circular "patch" of dacron/ceconite. I used the tip of my aircraft iron to "activate" and bond the fabric along the inner and outer circumferences of the ring, then covered the patch with glue, wiping off the excess, standard Stewart System style.


It took five thin coats of exterior semi-gloss latex paint to cover the glued patch. When dry, I cut out the hole with a razor knife. Precision not required.


Strobe power wire replacement complete. On go the painted covers.
May 22, 2020:

The experiment didn't work. RFI noise is exactly the same as before (squelch breaking level 7 in certain areas of the sky where terrestrial RF transmitters are present). This must mean RFI is being radiated directly from the LED strobes themselves and not from or through the +V power lines. The only way to verify that theory is to power the strobes from an external DC power source, in order to 100% isolate them from the instrument panel and radio.


May 23, 2020:

I did some more investigation today by using the radio, handheld with its rubber ducky antenna, to sniff around the trailered plane. I powered the instrument panel with a 14 volt DC power supply. Turns out that the wing tip LED strobes are not emitting RFI from the new shielded power cables and only very weakly from the lamps themselves. However, the tail strobes very much do, despite the presence of THREE ferrite chokes and a 0.1 uF capacitor installed on the leads coming out of the lamps. Then I discovered that the new fuel level sender was putting out a constant whine, so I added two clam shell chokes on the leads coming out of the sender, along with a 0.02uF cap across +V and gnd. This cleaned up that noise nicely, though it wasn't very strong to begin with. This left the puzzling reason of why the pulsing tail strobe RFI was coming down its shielded power cable that runs the length of the boom. I found a strong node of radiation coming from it right above my head,  so I put two clam shell chokes on the cable at that location. This significantly reduced the noise. Now to test it all again with the engine powering everything, which adds around 2 squelch "units" of its own hashy noise to the mix.


June 7, 2020:

I was able to get back in the air this morning. I'm happy to report that the "RAP (RFI Abatement Procedures) above were a SUCCESS! Squelch level is now down to 4, which is its normal value (out of 9) for the noise floor with the engine running. No more whine, hash, and strobe chaa-chaa-chaa.




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