Static port

[Slyfox]

listen to what I'm saying, when inside the cab the reading for airspeed was faster. when switched to outside static, it went down. the altitude chaged also, I just don't remember. Now that I think about it more it was more like 8 to 10 difference. Makes since with all kinds of people here saying there kitfox is real fast when it may not be because of where they are pulling their static.

[CDE2FLY]

My static port is external (pilot side just forward and below the leading edge of the horizontal stab) where building manual indicates. Similar to Slyfox, my airspeed is consistantly 8-10 mph lower than GPS groundspeed. True airspeed is often closer to GPS groundspeed. My altimeter (set to ATIS report pressure) is consistently 100 feet below the GPS indicated altitude.

[Dave Holl]

Hi
I am building my mk7 with a pressure head, ie both pitot and static in one head in the normal place on the left wing.has anyone else done this?
Dave

[Dave Holl]

here is a photo of my installation
dave

[av8rps]

My static port is external (pilot side just forward and below the leading edge of the horizontal stab) where building manual indicates. Similar to Slyfox, my airspeed is consistantly 8-10 mph lower than GPS groundspeed. True airspeed is often closer to GPS groundspeed. My altimeter (set to ATIS report pressure) is consistently 100 feet below the GPS indicated altitude.

That is EXACTLY what I found with my Model IV. Before I figured out it was off I was thinking how odd it was that everywhere I went I had a nice tailwind...(who has that luck?). Then one day when I knew I had a strong tailwind of nearly 30 mph and the GPS was showing 136 mph groundspeed, and the ASI was showing 86 mph, I figured something was wrong. Plus my gps altitude and my altimeter never made sense. So I disconnected the static port connection and just use the cabin for my static now. And I've verified my ASI and ALT are now quite accurate. The only time that changes is when I open the door in flight. But since I rarely fly with the doors open (have great vents) that's not an issue for me.

Also had the exact same problem with the Highlander I recently purchased. It used a static port on the side of the cowl just forward of the left door, and that ASI was off by about 10 mph also. Interestingly, this Highlander went through the hands of 3 different owners, and has always been equipped with a GPS, and all the owners complained the airplane was slow. Once I realized it was off and disconnected the static port and then tested it to see if it was accurate, my airspeed was much better. At 5500 rpm I can fly at about 110 mph with a G/A sensenich 2 blade composite prop, just like the factory says it will.

Ironically, at the low end of the ASI scale my slow speeds stayed essentially the same. So apparently the static doesn't affect it as much at the lower end? The good thing is that my stall speed still appears to match factory specs. But I have more testing to do yet to verify that accuracy.

So...I wonder how many Avid, Kitfox, Highlander guys are flying around with their ASI being off 10 mph, and thinking their planes are slower than they really are. And how many of them might be flying approaches too fast and getting a lot of float because of that, extending their landing distances considerably? (I know I was)

I personally think our planes have been getting a bad rap for being slow from owners that have never taken the time to verify their ASI accuracy. Add to that the fact that most every pilot you take for a ride will focus on the ASI and form his opinion of how fast the plane flies on that alone, assuming it correct.

[Slyfox]

that's why it's so important to do your stall test and all flight test with your asi and not anybody elses results. If you change your instruments, like your static location or the instrument itself, you need to redue all your lest. PERIOD.

[av8rps]

listen to what I'm saying, when inside the cab the reading for airspeed was faster. when switched to outside static, it went down. the altitude changed also, I just don't remember. Now that I think about it more it was more like 8 to 10 difference. Makes sense with all kinds of people here saying there kitfox is real fast when it may not be because of where they are pulling their static.

I'm no aerodynamacist, but the Kitfox fuselage is so short and bowed out at the cabin that I believe it causes an abnormal amount of disrupted airflow down the sides of the fuselage, causing the static port on the side of the fuselage to either be pressurized or in a vacuum, neither of which we want for ASI or Altimeter accuracy. Add to that issue the fact that most of us do not keep the ball centered the entire time we are flying and we could be getting some pretty wild static signals. I could be wrong, but I really think the combination of a short fuselage with sides that are far from flat, combined with an aircraft that suffers from a lot of adverse yaw, and you end up with it being nearly impossible to find an effective static port location. It would really be interesting to wind tunnel or at least tuft test a few of our airplanes. However, even then I think we'd find a huge amount of variables impacting our results due to things like bubble doors vs smooth doors, different turtle decks, small vs larger tires, wheel pants, round cowl vs smooth cowl, antenna locations, radiator scoops vs no scoop, etc, etc.

So unless you have a wind tunnel readily accessible, I would suggest for anyone insistent on using an outside static port to do so either by putting it out on the wing like the photo in this thread shows, or putting one on both sides of the fuselage and connecting them with a tee, so as to at least minimize the error caused by pressure or vacuum effects.

It is more than interesting that we all fly very similar aircraft and yet we have ASI / ALT errors that are significantly off both ways, and appear all relative to how we decide to locate our static port. Just more proof that we should all be thoroughly testing and verifying our ASI's and Altimeters as part of the 40 hour flight testing process. I've test flown a lot of airplanes over the years, and regardless of brand it is a rare case to find one accurate as originally built.

[Slyfox]

I think I'll just continue to just fly by the seat of my pants and look out the window.

[av8rps]

Yeah, isn't that the truth? Don't tell anyone, but there's times that I do an entire flight and don't remember looking at the airspeed

[Slyfox]

easy to do in these airplanes