Tip: Alternate Static Air Valve

Pat Hatch · #1 02-16-2011, 11:26 AM

I?m not recommending that anyone actually do this, but it worked for me. It?s not something I would have set out to do?it just sort of ended up this way.

I started my RV-6 project with a serious IFR setup in mind?not that I intended to actually fly it in hard IMC weather, but you never know, and, besides, I needed to stay IFR current in my retirement (and absent the 6-month sims that I did regularly in my flying job).

So against my better judgment, I planned a static system that involved static ports on a heated pitot tube. I didn?t know what kind of static position errors I would encounter with this setup, but I had read up on it on Kevin Horton?s web site and knew it probably wasn?t my best choice but decided to take my chances with it anyway.

When I got my RV-6 flying, I was pleasantly surprised to find my airspeed error was just a few knots? I didn?t keep a written log of my results but I do remember flying down the runway of a known elevation at max power and high speed and noted my altitude error was just a few feet (less than 50? as I recall). But, and this is a big but, I was encountering an unacceptable porpoise at altitude with altitude hold engaged on the autopilot. I vented the altitude hold transducer to cabin pressure and this fixed the problem, but I had to be very careful about changing the airflow into the cabin?either with cabin heat or adjusting the wemac flow. My thought was that the porpoising was an instability generated by the top and bottom static holes on the pitot tube caused by a change in angle of attack. So, fast forward to the present:

While performing this year?s condition inspection, I thought it would be a good time to add static ports to the side of the fuselage in Van?s recommended position for the RV-6. I had the original static kit that came with my RV-6 so I just got a few of the new push-on connectors from Stein and tapped the line into the existing static system. [I also used Darrell Reiley?s new machined static ports, nice!] I actually disconnected the wing static line and used a union connector to connect the new static line to the instruments. But then I got to thinking: What if I get a 3-way valve and use the pitot tube static ports as an alternate static air system? A heated backup static source: cool!

So I sent off for the Clippard FTV-3P pneumatic valve. About $16 plus shipping and handling. I was kind of skeptical after I got the valve because it just seemed kind of old tech and clunky. Also, it?s made of brass and kind of heavy. I was thinking that this thing is just going to be another source of leaks and problems, so I set off to prove to myself that this thing would work, or not. I used my fuel tank test rig that has a schrader tire valve epoxied on to a ?? aluminum tube. I use this to pressurize my fuel tanks for the balloon test. I added a couple of Stein?s connectors and some of the static tubing that Van?s supplies as a proof of concept. I set it up last night with a couple of balloons and was really impressed with the efficacy of this little valve. Flip is up and you fill one balloon, flip it the other way and it fills the other one. OK, but how about leaks? So this is what it looked like today after 24 hours, there is no discernible reduction in the size of the balloons. No bubbles anywhere with soapy water either. So I?m satisfied that this valve is going to work and that the push-on fittings are air tight even with Van?s tubing. Here are the photos:

So now I?m off to test fly this rig and to check out the position error of Darrell?s new static ports. I?ll post my results in a few days.

CharlieWaffles · #2 03-16-2011, 05:10 PM

How did the tests go?

RKellogg · #3 03-16-2011, 05:52 PM

To create a simple alternate static air source, one could mount an AN832-4 bulkhead union through the instrument panel (adjacent to the backup airspeed indicator), tee it into the static line behind the panel, and put an AN929-4 cap on the front, finger tight. Loosening / removing the cap connects the static system to cockpit pressure. Simple, light weight and robust without adding additional failure modes. Total cost about $4.
- Roger Kellogg

Pat Hatch · #4 03-16-2011, 06:01 PM

Charlie, I have only been able to obtain data points at the upper end of the IAS scale because I am breaking in new rings on a cylinder and have to keep my power up. So I have not been able to plot the Position Error curve throughout the entire airspeed range. What I have for the upper airspeeds is an indicated airspeed error of -9.5 knots and an indicated altitude error of -139 feet. This is roughly the same as what I had with the static ports located on the pitot tube. I suspect that my error will be somewhat less at the lower airspeeds, but we'll see. I will post my results when I can slow the airplane down enough to run the data at the lower end.

Bavafa · #5 03-16-2011, 06:02 PM

I bought one of those pneumatic valve (different then pictured above) but it was leaking like crazy. After taking it a part to see what/how it is made and seals, it proved to be made that works under positive pressure and not negative. So if I was pumping air thru it, it would hold great but if I was sucking air thru it, then no good.
So I chose a cheap and simple method in place of it.

Pat Hatch · #6 03-16-2011, 06:45 PM

Quote:

Originally Posted by Bavafa

I bought one of those pneumatic valve (different then pictured above) but it was leaking like crazy. After taking it a part to see what/how it is made and seals, it proved to be made that works under positive pressure and not negative. So if I was pumping air thru it, it would hold great but if I was sucking air thru it, then no good.
So I chose a cheap and simple method in place of it.

I agree, the valve probably adds more complexity and failure modes to the equation. However, I don't really think we're dealing with significant negative pressures (a vacuum) here. Keep in mind that the pressure of air at sea level is about 14.7 psi--and even at 18,000 feet it is still about half this. Moreover, what you are guarding against is a leak caused by differential pressure, i.e., the difference in pressure inside the airplane versus the pressure outside the airplane where the static ports are. I suspect this differential is pretty minute. However, you are right, it could indeed be a negative differential.

Playing around with this valve is more of a learning experience for me, and a way to compare entirely different static port locations on the airplane. And it's kind of fun.

Whether or not it works out as a good alternate static source valve in the long run remains to be seen.

Noah · #7 03-17-2011, 11:01 AM

Hi Pat, I have an identical setup with a heated pitot/static tube and also standard static ports on the aft fuse, with a 3 way clippard valve. Just wondering if you considered the valve I'm using, which is a little different (and maybe a little lighter):
http://www.clippard.com/store/displa....asp?sku=MTV-3

Since I'm not flying yet, I'm patiently awaiting your results!

Pat Hatch · #8 03-17-2011, 08:23 PM

Quote:

Originally Posted by Noah

Hi Pat, I have an identical setup with a heated pitot/static tube and also standard static ports on the aft fuse, with a 3 way clippard valve. Just wondering if you considered the valve I'm using, which is a little different (and maybe a little lighter):
http://www.clippard.com/store/displa....asp?sku=MTV-3

Since I'm not flying yet, I'm patiently awaiting your results!

Noah, the Clippard valve I got was the one the engineer I spoke to recommended for my application, but I did mention that I was using 1/8" NPT fittings. How are you going to connect to the 10-32 ports? I planned on using the quick disconnects, so I guess my valve is larger because of that. I think your valve will work fine if you have picked out the connectors for the 10-32 ports.

Hang in there, I'll have the PEC curves as soon as I can and I'll post them for you.

Bavafa · #9 03-17-2011, 08:39 PM

Quote:

Originally Posted by Pat Hatch

Keep in mind that the pressure of air at sea level is about 14.7 psi--and even at 18,000 feet it is still about half this. Moreover, what you are guarding against is a leak caused by differential pressure, i.e., the difference in pressure inside the airplane versus the pressure outside the airplane where the static ports are. I suspect this differential is pretty minute. However, you are right, it could indeed be a negative differential.

I agree that the differential pressure between the cabin and outside is not significant enough to create enough of a vacuum, however my concern was for passing the test for certifying it for IFR. My own test on the valve which I was putting 1" of negative pressure and hoping to hold for one minute failed each time.

Pat Hatch · #10 03-17-2011, 08:46 PM

Quote:

Originally Posted by Bavafa

I agree that the differential pressure between the cabin and outside is not significant enough to create enough of a vacuum, however my concern was for passing the test for certifying it for IFR. My own test on the valve which I was putting 1" of negative pressure and hoping to hold for one minute failed each time.

Just curious, was it a Clippard?