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I built Three Sigma with a proper Pitot-static system, placing a Pitot-static tube (with Pitot and static ports) on a boom well in front of the left wing. Even though it is a heated Pitot-static tube, I included an "alternate static source" in the static line. This is simply a Curtis fuel sump drain (just like in my fuel tanks) teed into the static line under the instrument panel where I can reach it. Opening this valve vents the static source into the cockpit.
This past Sunday, my lovely wife Tuki and I were out doing some flight testing. On the card was to test the added error from opening the alternate static source. We were level at 118 KIAS at 6480 feet pressure altitude. I opened the alternate static source, and my airspeed instantly jumped to 131 KIAS and altitude jumped to 6690 feet. That's a change of 13 knots indicated airspeed and 210 feet of altitude. What can I conclude from that? The air pressure in the cockpit is lower than the freestream air pressure, at least at cruise conditions.
That's mildly interesting, and I need to look at how much difference there is at approach speed, because based on those results with the alternate static source open I would be flying slower than I think I am, which could put me in danger of stalling on approach.
The bigger issue I see is for those of you who followed Bob Barrow's lead and just left your altimeter and airspeed indicator open to the cabin, rather than putting in a proper static system. Bob is a great guy and designed a great airplane, but I have never agreed with him on that decision. Based on my results, if you did that you will always think you are flying significantly faster than you are, and looking at the ground speed on your GPS and wondering why you always seem to have an unforecasted headwind.
Unless you like telling your kids "Back in the day I had to fly my Bearhawk to Oshkosh, against the wind, both ways!" you might want to consider installing a proper static system. That is, a static port on the outside of the aircraft. My approach seems to work well, and gives reasonable airspeeds compared to GPS ground speeds over 5 years of looking at it. I think some others have had success with static ports on the side of the fuselage about halfway between the wing trailing edge and tail leading edge.
This past Sunday, my lovely wife Tuki and I were out doing some flight testing. On the card was to test the added error from opening the alternate static source. We were level at 118 KIAS at 6480 feet pressure altitude. I opened the alternate static source, and my airspeed instantly jumped to 131 KIAS and altitude jumped to 6690 feet. That's a change of 13 knots indicated airspeed and 210 feet of altitude. What can I conclude from that? The air pressure in the cockpit is lower than the freestream air pressure, at least at cruise conditions.
That's mildly interesting, and I need to look at how much difference there is at approach speed, because based on those results with the alternate static source open I would be flying slower than I think I am, which could put me in danger of stalling on approach.
The bigger issue I see is for those of you who followed Bob Barrow's lead and just left your altimeter and airspeed indicator open to the cabin, rather than putting in a proper static system. Bob is a great guy and designed a great airplane, but I have never agreed with him on that decision. Based on my results, if you did that you will always think you are flying significantly faster than you are, and looking at the ground speed on your GPS and wondering why you always seem to have an unforecasted headwind.
Unless you like telling your kids "Back in the day I had to fly my Bearhawk to Oshkosh, against the wind, both ways!" you might want to consider installing a proper static system. That is, a static port on the outside of the aircraft. My approach seems to work well, and gives reasonable airspeeds compared to GPS ground speeds over 5 years of looking at it. I think some others have had success with static ports on the side of the fuselage about halfway between the wing trailing edge and tail leading edge.
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