Pat Hatch

Pat Hatch

Well Known Member
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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:

DSC_0020pp.jpg


DSC_0005pp.jpg


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.
 
Simple alternate static source

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
 
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.:)
 
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.
 
Bavafa said:
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.
Click to expand...

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.
 
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/display_details.asp?sku=MTV-3

Since I'm not flying yet, I'm patiently awaiting your results!
 
Noah said:
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/display_details.asp?sku=MTV-3

Since I'm not flying yet, I'm patiently awaiting your results!
Click to expand...

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. :)
 
Pat Hatch said:
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.
Click to expand...

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.
 
Bavafa said:
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.
Click to expand...

Just curious, was it a Clippard?
 
Bavafa said:
Yes it is.
Click to expand...

Given your comments, I think I will perform another test. There is a companion thread about static leak checks that uses a hypodermic needle to create a vacuum sufficient to increase the altimeter reading by 1,000'. I think I will try this with and without the valve and see what I get. I'll post the results.
 
Pat Hatch said:
Given your comments, I think I will perform another test. There is a companion thread about static leak checks that uses a hypodermic needle to create a vacuum sufficient to increase the altimeter reading by 1,000'. I think I will try this with and without the valve and see what I get. I'll post the results.
Click to expand...

I will be interested to know about the result. When I disassembled the valve to see if I can do any thing to fix/improve it, I saw that it has a valve similar of an inner tube/tire and when there is pressure on one side (inside to outside) it pushes it in to seal but nothing in the other direction which would constituted a negative pressure
 
Pat Hatch said:
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. :)
Click to expand...

Thanks Pat!

I used the tiny 10-32 to 1/4 air tubing quick disconnect fittings from McMaster Carr, in this instance the P/N is 5779K246. Cost is $2.95 ea in case anybody else is going this route. McMaster.com sells all kinds of these quick disconnect fittings (they call them "instant", or "push to connect" fittings), which are sold by some of the aftermarket RV suppliers. Most of my pitot-static system came from them.

Overall this valve assembly all-told was heavier than I wanted, but I think it is at least minimized for what it needs to do. I don't think the small bore will be an issue for a pitot-static system where there is little flow and pressure is just being transmitted elsewhere on the airframe, but time will tell! Reading this and that other P-S test thread has convinced me that I should test this sytem now when everything is accessible rather than wait until its all buttoned up!
 
I used a similar Clippard valve for my alternate static air. I just had my system recertified a couple of months ago and it is doing great. The technician commented that I had a nice tight system.
 
Bavafa said:
I will be interested to know about the result. When I disassembled the valve to see if I can do any thing to fix/improve it, I saw that it has a valve similar of an inner tube/tire and when there is pressure on one side (inside to outside) it pushes it in to seal but nothing in the other direction which would constituted a negative pressure
Click to expand...

OK, I dug into these valves a little more. Hope this information helps. I just got off the phone with one of the engineers at Clippard and explained what we were trying to do with their valves and how negative pressures might affect their operation. His response was that this particular valve is designed to handle any kind of pressure, positive or negative, because they are "fully ported." He said to make sure you specify a fully ported valve, i.e., any that began with the letter "F." There are other valves that they make that may not work for a vacuum.

In any event, the Part 23 static system test would seem to be a good standard to test to, not only for your system but for these valves as well. It is hard to imagine a cockpit-to-static-port pressure differential in a non-pressurized airplane that would exceed the 1" of mercury that they talk about (roughly a 1,000' altitude increase). So it's going to be interesting to see how the whole system holds up to this test. Hope to go out tomorrow and try it.
 
Pat Hatch said:
OK, I dug into these valves a little more. Hope this information helps. I just got off the phone with one of the engineers at Clippard and explained what we were trying to do with their valves and how negative pressures might affect their operation. His response was that this particular valve is designed to handle any kind of pressure, positive or negative, because they are "fully ported." He said to make sure you specify a fully ported valve, i.e., any that began with the letter "F." There are other valves that they make that may not work for a vacuum.
Click to expand...

That's very interesting, thanks for digging that up. The valve I currently have in my panel is a MTV-2P, picked mostly due to the fact that it had the right number of ports and was sized right to fit in the available panel space. But based on your new info, it sounds like I would be better off with something like a FTV-3FP. Do I have that right?

mcb
 
mburch said:
That's very interesting, thanks for digging that up. The valve I currently have in my panel is a MTV-2P, picked mostly due to the fact that it had the right number of ports and was sized right to fit in the available panel space. But based on your new info, it sounds like I would be better off with something like a FTV-3FP. Do I have that right?

mcb
Click to expand...

Right, Matt. Mine is an FTV-3P, which is for 1/8 NPT fittings. Yours has the plastic toggle. There are many other combinations. :)
 
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Pat Hatch said:
Right, Matt. Mine is an FTV-3P, which is for 1/8 NPT fittings. Yours has the plastic toggle. There are many other combinations. :)
Click to expand...

Thanks Pat. You're right, the FTP-3P is the one with the nice metal toggle. And, I guess if I'd read the catalog pages carefully, I would have noticed that the FTV valves are the only ones with a minimum pressure rating of vacuum.

Well, there goes another $16 for a new valve, and about the same amount for shipping! :rolleyes:

mcb
 
In the end unless you have a pressurized aircraft it really shouldn't make any difference...there shouldn't exist a traditional "vacuum" on that line anyway (with the exception of a pitot/static check, which is rather easy to manage). I can't see it being of any huge concern unless there is a huge cabing pressure differential.

My 2 cents as usual!

Cheers,
Stein
 
F type?

Stien,

Just to be clear, the valve I bought from you is the F type or not?

thanks

Bevan
 
Thanks Pat for finding the difference and the right part.
After your post, I bought one (FTV-3FP) on ebay and it is on the way right now. I will be testing and if it does not leak like the other one that I have which is not the "F" model, I will be installing it.
 
Thanks

Pat,

Thanks for this thread. I just ordered a valve like the one you are using from ebay...9.99 delivered.

Where are you mounting yours? I had considered the left sind of the cockput where the static line runs past the pilots seat. Is there a reason to mount it on the panel?

Tony
 
tonyjohnson said:
Pat,

Thanks for this thread. I just ordered a valve like the one you are using from ebay...9.99 delivered.

Where are you mounting yours? I had considered the left sind of the cockput where the static line runs past the pilots seat. Is there a reason to mount it on the panel?

Tony
Click to expand...

Tony, I currently have mine in the same area, down by the left side forward of the spar. This area gets covered with an upholstery panel, which I have off at the moment until I decide where I'm putting the valve (I'll probably mount it on the upholstery panel somewhere). I definitely am not putting it on the panel, too complicated. I might have if I was starting over, but too late for this panel.

I discovered a static leak today somewhere behind the panel, so that has kind of taken priority until I resolve it. Right now I suspect a bad instrument because the lines all seem to be secure. I used the syringe to try to get the altimeter up a 1,000 feet but it wasn't going anywhere, so basically it appears as if my instruments are vented to cabin pressure for some reason. I'm probably going to have to pull the panel to really get at everything and troubleshoot properly. Bummer. :(
 
Good day today... found my static system leak! :) This one was tough. I had to pull every instrument out of the panel and check every fitting and piece of hose before I started eliminating the instruments themselves. The last instrument I checked turned out to be the culprit: The RMI ?Encoder. I built it myself about 9 years ago, but what happened was that an internal piece of tubing that connects the external bulkhead fitting to the altitude sensor came off the barb inside the unit. So not only was the ?Encoder vented to cabin pressure, but it caused all of the other instruments to be vented to cabin pressure too. Insidious, huh? The fickle finger of fate got me good. BUT, I found it and now everything checks out perfectly. I ran my altimeter up a 1,000' and it held steady for about 10 minutes. My panel is a mess, but tomorrow should be able to get it all back together.

Lessons learned: recurring static system checks are important! Lots of ways to get a static leak. Take a pillow to put up against the rudder pedals!

Now I feel pretty good about my upcoming FAR altimeter/static checks!
 
Results of the static system position error tests...

I flew the airplane last night with the newly restored static system and sporting Darrell Reiley's new static ports. I wanted to make sure there was not a significant position error due to the slimness of the machined static ports (they sure look nice, but do they perform?) The static ports are located in the Van's-recommended aft-fuselage location.

My technique was to fly sets of four legs on cardinal headings and record pressure altitude, indicated airspeed, indicated OAT, GPS track and ground speed, for various indicated airspeeds. The 4 legs give you an accurate TAS for each data point. I fed all of the data into the spreadsheet (available on Kevin Horton's web site). The results were very good: at the high end of the airspeed curve my altimeter error is -32 feet and airspeed position error is -2.3 knots (both are indicating slightly high). Not bad. At the low end (around 100 knots indicated), I'm getting position errors of -21 feet and -2.1 knots. I am happy with these results!

Interestingly, when I performed the tests with the leaky static system my errors were significantly higher, which tells me I was getting lift over the canopy, lowering the pressure inside the cockpit and causing the airspeed and altimeter to read higher, significantly higher.

For anyone who has an RMI µEncoder, I would caution you that the internal tubing can fail and this will vent your static system to cabin pressure. More importantly, there is also a short piece of pitot tubing inside. This was just about to fail in my unit, and if it had, it would have caused my airspeed to go to zero. The tubing was just crumbling in my fingers when I took it out, totally rotted. So I switched over to a good quality Tygon instrument tubing which should last a long time.

Darrell's static ports: Highly recommended! :)

[Here's a tip I learned during this experience: if you're flying along and pull your cabin heat all the way out, look at your VSI. If you get a change, you have a static leak!]
 
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I received my new FTV-3P valve and it is a chunck of metal. I think I am going to forego its installation as it weight nearly a half a pound. I would need to put a doubler any where I want to install this :(
 
I used a MTV-2P valve in my Rocket panel as an alternate static source. Works great, and it's a lot lighter than the 3-way versions. I believe the only time you'd get into trouble with leakage is if there was a significant reverse pressure difference between cabin and static, which would only be possible with a pressurized airplane, and maybe not even then. In fact, in an unpressurized airplane isn't the cabin usually lower pressure than the static source? In this case the pressure differential is in the direction the valve is meant to handle.

Mark Olson RV-7A F1-EVO Rocket
 
I finally got my new valve FTV-3P installed and it is working great, thanks to Pat for his suggestion on the valve. The valve is rather heavy but I was able to drill and cut some portion of the metal that serves no propose and lighten it by 2.5 ounce. Got to test it last night in the air, invoking it gives a very sudden 200' of altitude (to our disadvantage) and closing it will give that 200' back instantly. If my RV could climb that fast :)

Here are some pix of the valve and its location.

DSC_6145 by bavafa1, on Flickr


DSC_6143 by bavafa1, on Flickr


DSC_6148 by bavafa1, on Flickr

DSC_6147 by bavafa1, on Flickr
 
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sstellarv10 said:
Mehrdad
Where did you get the switch with the safety cover on it?
Click to expand...

Steve,
The switch is the Clippard FTV-3P valve and I got it from ebay for $10. The cover is from and electrical (On/off) switch from Harbor Freight and just took the cover off and mounted on the valve. Closing the cover will close the valve, so ensures a closed position.
 
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