Gearbox Vent Modifications

      Why a whole web page about the gearbox vents? All of the gearboxes on the JetExec have vents that keep the gearboxes at atmospheric pressure when they heat and cool, or as atmospheric pressure changes. The KISS conversion kit includes vent filters to be installed on the turbine, Rotax, main rotor and tail rotor gearboxs and even for the fuel tank too.

      I experienced problems with several of these gearbox vents purging copious volumes of oil all over the mechanical areas inside the helicopter during startup. And I spent many hours disassembling the helicopter, cleaning up the oil and putting everything back together. You don't want to do this.

      I highly recommend during startup, rather than using the vent filters provided, that you instead temporarily install clear PVC (vinyl) tubing from each vent location and run it down to the nearest point somewhere beneath the helicopter. If for some reason the vent starts puking oil while you are trying to get in your first 15 or 20 minutes of flight, you won't have to clean it out of the helicopter. Once you have a few hours and you see that all of the vents are O.K., you can discard the tubing and install the vent filters on each gearbox if you wish.

      This is the T/R gearbox vent filter that I tried to build and install exactly as per the construction manual. The first time I spun up the tail rotor though, the gearbox heated up and pushed oil up the pick-up tube, out the vent filter and all over down the inside of the tailboom. Oil splashes and foams inside the small gearbox area and it is difficult to keep it out of the tube. I tried modifying the tube some without success.

      I finally realized that the only oil-free area in the gearbox was inside the hollow T/R drive shaft, which spins all the oil out like a centrifuge when it is running. I could see it through the gearbox side window.

      I built this small pick-up tube from a 1-1/2" piece of 5/32" brass tubing soldered inside a 3/16" 90° brass hose barb fitting. The tube inserts through the gearbox side window into the hollow T/R driveshaft.
      I was able to install this without removing the gearbox. I drained the oil and drilled a 13/64" hole in the center of the plastic side window, trying to not get any plastic shavings inside the gear box. I cleaned the hole with lacquer thinner, then epoxied and pressed the hose barb fitting securely into the side window. The 5/32" brass pick-up tube extends an inch or so inside the T/R shaft and bends slightly upward, without contacting the shaft wall.

      (Note: You can click on these pictures to view in higher resolution.)

      A short length of 1/4" O.D. (3/16" I.D.) clear PVC tubing runs from the hose barb fitting down and out through a rubber grommet in the bottom of the tailboom. The tubing will overflow oil if the gear box is filled more than 1/2 full, but it will not leak oil when the gearbox is running! I've had no problems with oil leaks from the T/R gearbox since.
      I deviated from the original KISS JetExec design when I rotated the Rotax C gearbox upward to allow me to lower the turbine. This is how I originally installed the vent filter, on the oil level sight tube. Not a good spot, it began to puke oil as soon as I started running the gearbox.

      Unfortunately, the gearbox only has one other port higher when rotated in this position, where the oil return line connects, in the very top of the picture here.

      I had to get a little creative here. I ordered a steel -6 male o-ring boss x 3/8" female pipe thread fitting, center. I drilled and tapped a 1/8" female pipe thread in the side, drilled out the bottom end for the largest diameter of brass tubing I could fit in there (17/32") and epoxied a ~1.25" long piece of the 17/32" tubing into the bottom.

      The fitting on the left is a 1/8" MPT x 1/4" 90° brass hose barb.

      The fitting on the right is an Earls 3/8" MPT x 6AN hose connector. I drilled it 13/32" through, flared one end of some 13/32" brass tubing and dropped the tubing down through the connector.

      Here is the combined oil return line and vent fitting assembled. The 13/32" brass tubing oil return tube extends down through the inside of the steel fitting to within about 1/4" of the end of the larger brass tube.

      The brass hose barb fitting connects through to the annular space between the two brass tubes. The outer brass tube forms a large open drip lip and shields gearbox oil from splashing up into the vent space between the two tubes.

      Here is the fitting installed on top of the Rotax gearbox, with the oil return line attached. Down beneath it, I capped the top of the 1/4" brass sight tube fitting where the vent filter was originally located.
      Originally I ran about 4' of 3/8" clear PVC tubing from the new vent fitting down to beneath the helicopter. After running for several hours without any leaks or visible oil in the vent tubing, I shortened the 3/8" tubing and stuffed it through the side of the 1" PVC fuel tank vent hose, up by the hood bracket.
      This leaking vent filter on the M/R gearbox was my fault too. I installed the oil vent right next to the pinion gear, where oil was sure to be slung. The first time I pulled pitch and the gearbox heated up, it pushed about a cup of oil out all over the gearbox, belts and the entire mechanical area between the seat back and turbine and down into the bottom tub. It took me two full days to disassemble everything and clean up.
      Fortunately, the fix was simple. I moved the oil return line and fill plug one port to the left, and moved the vent fitting all the way to the right as far away from the pinion gear as I could.

      Initially I ran about 4' of 1/2" clear PVC tubing from the new vent location down to beneath the helicopter.

      After running for several hours without any leaks or visible oil in the vent tubing, I replaced it with a short length of 3/8" tubing and poked it through the side of the 1" PVC fuel tank vent hose up by the hood bracket, similar to the Rotax C gearbox vent tubing beneath.
Last Updated: June 15, 2014
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