Pitot Static location

[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.

[13brv3]

It's certainly easy enough to try. I'll test it on my next flight next week sometime.
Thanks,
Rusty

[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.

[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

[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.

[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

[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