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How I instantly gained 13 extra knots of airspeed

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  • How I instantly gained 13 extra knots of airspeed

    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.
    Russ Erb
    Bearhawk #164 "Three Sigma" (flying), Rosamond CA
    Bearhawk Reference CD
    http://bhcd.erbman.org

  • #2
    Have you hooked your system to a calibrated pitot-static test system, and if so, what were the results?
    Dave Bottita The Desert Bearhawk
    Project Plans #1299
    N1208 reserved www.facebook.com/desertbearhawk/

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    • #3
      Hi Russ, I am planning on using the same style pitot/static probe on my bird and I am pretty sure I had seen the answer to my next question in your cd but can't find it...how far out from the leading edge does your probe stick out? Also, did you use 6061t6 aluminum for the mast or did you go with steel tubing? Thanks!
      Joe
      Scratch-building 4-place #1231
      Almost Wyoming region of Nebraska

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      • #4
        Russ, you might remember that I called you when my airspeed was reading about 160 MPH cruising around on my first flights. You suggested trouble shooting the static system. I ended up doing a RV like static system with two ports on the fuselage sides about 1/2 way back to the tail. It fixed the problem. But I was reading high in speed and altitude as well. Mark

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        • #5
          Originally posted by DesertBearhawk View Post
          Have you hooked your system to a calibrated pitot-static test system, and if so, what were the results?
          We had ours tested with certified equipment (it's a requirement here to receive a Certificate of Airworthiness - Special Category). ‚ÄčEverything worked to within about 10ft altitude and a couple of kts airspeed.

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          • #6
            Originally posted by DesertBearhawk View Post
            Have you hooked your system to a calibrated pitot-static test system, and if so, what were the results?
            Yes, I did. It passed the VFR static system check required by FAR 91.411. However, that means nothing in this case. What that test evaluates is instrument error. The question here is about position error, which has to do with the distortion of the flow field around the aircraft, and only exists in flight. You can't test for it on the ground (unless, perhaps, you had a big wind tunnel).

            Russ Erb
            Bearhawk #164 "Three Sigma" (flying), Rosamond CA
            Bearhawk Reference CD
            http://bhcd.erbman.org

            Comment


            • #7
              Originally posted by Mark Goldberg View Post
              Russ, you might remember that I called you when my airspeed was reading about 160 MPH cruising around on my first flights. You suggested trouble shooting the static system. I ended up doing a RV like static system with two ports on the fuselage sides about 1/2 way back to the tail. It fixed the problem. But I was reading high in speed and altitude as well. Mark
              I don't really remember that (hazard of getting old), but I'm glad I gave you the right advice! Your results sound very consistent with what I saw. A static system like yours would probably be much easier to retrofit than my system. Thanx for confirming the static port placement.
              Russ Erb
              Bearhawk #164 "Three Sigma" (flying), Rosamond CA
              Bearhawk Reference CD
              http://bhcd.erbman.org

              Comment


              • #8
                Originally posted by bestbearhawk1231 View Post
                Hi Russ, I am planning on using the same style pitot/static probe on my bird and I am pretty sure I had seen the answer to my next question in your cd but can't find it...how far out from the leading edge does your probe stick out? Also, did you use 6061t6 aluminum for the mast or did you go with steel tubing? Thanks!
                Look on your CD under "Wing" and "Pitot-Static Installation". You will see that my mast is 1-1/4 x .035 4130 steel tubing. Upon reviewing my copy of the CD, I see that I failed to mention how long it was. My Dad and I looked at a bunch of aircraft with similar installations and determined the typical length compared to the wing chord. Based on this, the mast tube sticks out 18 inches in front of the leading edge. That is, of course, the distance to the back end of the AN5816 Pitot-static tube.

                BTW, I finally came across an old Army training film that described the purpose of the shark fin. It is a moisture trap to keep water out of the Pitot and static lines. See http://youtu.be/CATOBFrqGxg or search for "diaphragm instruments".
                Russ Erb
                Bearhawk #164 "Three Sigma" (flying), Rosamond CA
                Bearhawk Reference CD
                http://bhcd.erbman.org

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                • #9
                  Nice find on the film, I've got the same style pitot as you. Felt very special opening the finely packed quality instrument after it had been sealed during WWII. Did you do anything to reduce the current draw? Seems to me it was about 20 amperes and I won't be traveling over 200k like it was designed for. Contemplating a heating resistor in series???

                  Mark J

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                  • #10
                    No, I didn't do anything to reduce the current draw. If you put a resistor in series it's going to need to be a high wattage resistor, and then you won't know if it heats up enough to melt the ice. The ice doesn't really care how fast your are going. It cares about humidity and temperature. I'd leave it as designed.

                    From the CD archives:

                    Anyway, the answer was (for any of you who may decide to use this tube) steady state current was 15.12 amps, with a maximum inrush current of 28 amps. The maximum inrush current occured immediately after applying power to the heater, and the current dropped as the heater warmed up.
                    Russ Erb
                    Bearhawk #164 "Three Sigma" (flying), Rosamond CA
                    Bearhawk Reference CD
                    http://bhcd.erbman.org

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                    • #11
                      Hi I have the same setup on my BH, but no alt source. A

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