Pitot Static location
Posted: Thu Aug 11, 2022 12:51 pm
by 13brv3
Greetings,
Has anyone else suspected interference between the wheel pants and the stock pitot static tube location? My wheel pants are 1" wider, and that’s all to the outboard side, so it’s a bit closer than the stock wheel pants would be. The tube appears completely stock, and in the correct location.
There are times when the airspeed seems erratic, and it looks like it could be affected by the wheel pants. It doesn’t always seem that way though. Sometimes I’ll be sure it’s a problem, then it will be completely smooth. I’ve tried to cause problems by yawing to the right, but I can’t say I’ve ever been able to make it happen. I’ve thought of trying to make a new pitot static tube that is bent outboard and maybe extended farther forward to see if it makes any difference. Just curious.
Thanks,
Rusty
Re: Pitot Static location
Posted: Thu Aug 11, 2022 4:27 pm
by XenosN42
A tool to help verify static port location: https://jasflyer.com/FAQ/AN4.aspx
Re: Pitot Static location
Posted: Thu Aug 11, 2022 6:08 pm
by 13brv3
Thanks, but neither of those articles does much for me. One says don’t put the pitot tube where it can be affected by an aircraft structure. That’s exactly my question- is the stock location on a Onex too close to the wheel pants? It’s pretty darn close.
Rusty
Re: Pitot Static location
Posted: Thu Aug 11, 2022 6:25 pm
by GraemeSmith
SURE the lines and tube are clear? The slightest drop of water (or mud dauber) affects that pitot if it is sitting in the bend. I sit in tie downs and am subject to weather. Even with a pitot cover on - sometimes I get a tiny drop of water in the tubes and readings are erratic. I actually keep a spare pitot - dry in a bag - ready to swap out and then take the other home to run a pipe cleaner and airline through to get it absolutely dry. Maybe once every 8 months or so.
I also blow out the lines from inside the aircraft to out. (NEVER blow into the lines when connected to instruments - you will destroy the instruments.)
Re: Pitot Static location
Posted: Thu Aug 11, 2022 6:55 pm
by 13brv3
I’ve certainly never seen anything in the tubes, but it won’t hurt to take a closer look. They’re covered, and in a private hangar, so no particular risk of contamination.
Rusty
Re: Pitot Static location
Posted: Fri Aug 12, 2022 3:01 am
by Kai
Well,
If it is any comfort, you´re not alone!
Ages ago I had a chat with Kerry about this. He mentioned that on the factory prototypes they often just terminated the static port line under the instrument panel. Hm…….
I decided to give it a try- so far my own attempts had not met with any success. The result was that the asi closely corresponded with the one in a RV4 flying on my wing. Low down it was also comfortably close to the groundspeed indicated by the gps.
The only issue I could observe was that when in the pattern and raising the nose to bleed off airspeed for flap deployment, the asi pointer would for a short while not immediately slow down- before it again decided to behave.
Better than constantly indicating too much, I thought. So it has been like this for the last 5-6 years or so.
Re: Pitot Static location
Posted: Fri Aug 12, 2022 10:39 am
by 13brv3
Interesting thought about the static port. That would be easy to test, and I might even be able to do it in flight since I have quick connects that I could make reachable for the test. Leaving it open to the cockpit seems like it would change when you open or close air vents though?
I’m still not 100% sure it’s a problem. There’s no such thing as a smooth day in the summer afternoons when I fly, so it could all simply be turbulence related. I do notice it only in climb, which would cause more potential interference with the wheel pants. On the othe rhand, a careful look at the Dynon logs often do show a correlation between pitch and speed, so maybe it’s all normal. The good news is that it’s not significant enough to be a hazard, though it makes climb speed testing imprecise.
Rusty
Re: Pitot Static location
Posted: Fri Aug 12, 2022 10:47 am
by GraemeSmith
Don’t forget instrument error from slipping. Depending on which way you slip the static side is pressurized or not and false readings will occur. Probably similar to Kai’s point about flap deployment.
Re: Pitot Static location
Posted: Fri Aug 12, 2022 11:09 am
by 13brv3
One of the things I plan to look at today is the hole placement on the static probe. I can’t recall if there’s one, or two, or even where it’s located on the probe. Van’s static port was similar to most certified planes, where there was a port on each side of the aft fuselage that were connected together by a Tee. That might be better, but maybe not enough to notice.
Re: Pitot Static location
Posted: Fri Aug 12, 2022 11:27 am
by Panther16
On my Panther build I installed static ports on each side of fuselage. After flying a 3 leg speed course found it to be reading 8 mph low. Installed a T in the static line to the ports and flew the speed course again with the Bottom of the T open. Now reads 1 mph slow, been flying it that way for 260+ hours. Opening a vent causes a little bump in IAS and then settles to previous reading. Cabin heat has no effect.
Re: Pitot Static location
Posted: Fri Aug 12, 2022 11:42 am
by Bryan Cotton
My static port is vented to the cabin through an orifice. Seems to work well. The certified Cessna C120/140 also have their static ports in the cabin per a Cessna service letter. It’s a very simple solution.
Re: Pitot Static location
Posted: Fri Aug 12, 2022 4:10 pm
by 13brv3
It’s certainly easy enough to try. I’ll test it on my next flight next week sometime.
Thanks,
Rusty
Re: Pitot Static location
Posted: Sat Aug 13, 2022 10:18 am
by Scott Todd
I’ve had this discussion with many people over the last 35 years of flying. Go look up the definition of Static pressure. Its basically the pressure you would feel if the air was not moving. Well its not usually moving much in the cockpit. For a non-sealed (pressurized) airplane doing less than 200 mph, its fine. When you start sealing all those little leaks and going fast, it starts to make a difference. Its not really a good idea on your Lancair or Glasair but could be an acceptable emergency source if calibrated properly. For our leaky as heck Sonex’s with all those rivet holes EVERYWHERE, and flying typically less than 150, its perfectly acceptable.
I usually use a small piece of hose or brake line with several tiny holes (1/16) drilled around it over 8 inches or so and the end capped. Then any slight breeze in the cockpit gets canceled put by the other holes and the holes are small enough to prevent critter homes from forming.
Re: Pitot Static location
Posted: Sat Aug 13, 2022 3:44 pm
by 13brv3
I’ve tested a number of static options over the years myself. I’ve tried multiple holes, a small needle valve to vary the opening, foam over the tube. I’m not sure the static will be the only issue with how close that pitot/static tube is to the wheel pants though.
Rusty
Re: Pitot Static location
Posted: Sun Aug 14, 2022 10:47 am
by Scott Todd
Rusty has a good point. The wheel pant will accelerate the local air and the effects could go as far out as the Pitot tube. But the effect will be an overall change in accuracy and probably minimal. In college, it was always a fun experiment to put a Pitot tube in a wind tunnel and watch the effects as the angle off attack varied. It takes a LOT to start showing. We also need to accept the fact that the airspeed indicator is not giving us an exact number. So what if it shows 73 and we are actually doing 76? The airplane performance figures are based on the indicated airspeed and will be different for every airplane. Sure Certified airplanes try to be accurate and consistent but we are flying one-off Experimental airplanes. The performance should be based on flight testing for that particular airplane. I tell anyone buying a used Experimental airplane that they need to go back thru some basic flight testing to verify the performance numbers.
Re: Pitot Static location
Posted: Wed Aug 17, 2022 8:15 pm
by 13brv3
I’ve made a couple flights with the static source open behind the panel. I’ve got a piece of open cell foam over the end, and I can’t see any change when opening and closing my vent. Of course the vent isn’t that great either. The ASI and VSI do seem a bit more stable.
At max cruise speed, it doesn’t seem to have changed the ASI reading. I did a GPS averaged speed test at 90 mph also, which i didn’t do before, and it’s probably as accurate as I can hope for.
One speed that changed a lot is the stall speed. Before, my stall speed was unrealistically low, and it was always on my to-do list to figure out why. For example, I would see 38 mph with full flaps, when the spec is probably optimistic at 45 mph. Seeing as I have the short wingtips, and I’m 20 lbs over the ideal empty weight, I always figured I should realistically see closer to 50. Well now I see 53 mph. I have to believe that’s more realistic, though more testing is needed. I had to add 10 mph to my pattern and landing speed 
I think I’ll call it good for now, and move on to other items on my list.
Thanks,
Rusty
Re: Pitot Static location
Posted: Thu Sep 08, 2022 9:31 am
by 13brv3
I ended up removing the pitot/static tube since the static wasn’t being used anymore, and I borrowed Van’s idea for a simple pitot tube. I tried a couple heights, and ended up with one about 2 inches shorter (height) than the original, which moves it about an inch from the wheel pants. It didn’t make any difference other than being easier to replace if it gets bent
Rusty