Mystery Airspeeds

I?m having trouble working this one out.

My Rocket has been flying fine for years, but I have just installed an AFS/Dynon ADHR supporting a single AF5600 display and the other equipment necessary in order to gain ADSB compliance. I decided to retain my standard ASI and altimeter as much for aesthetic reasons as any other. The old ASI is now plumbed to a new AoA capable Dynon pitot that is ?tee?d? to the ADHR?s in the rear of the airplane. Because this is my 4th system upgrade (!) it has worked out that the two systems are connected to two separate static ports---the standard airspeed to a forward set and the ADHR?s to a separate set of sources on the rear fuselage. Both airspeed indicators work but display speeds that in addition to being very low, are different from each other. I have some confidence at least in the accuracy of the static sources since both the standard altimeter and the AF5600 altimeter tape are in agreement and are connected to the same two separate static sources as the ASI?s.

The displayed speed differences vary with airspeed but at modest cruise power settings where I used to see 165+ knots or so, I?m now seeing 125 up front (standard ASI) and 115 or so on the AFS tape driven by the ADHR. At approach speeds of (an estimated) 80 knots, the ASI is reading closer to 50 knots and the AF5600 airspeed tape is even lower. Turning final at ?50 knots? is interesting!

An NTPS ground speed flight test and both GPS ground speeds (GNS530W and AFS5600) agree with each other at around 170 knots for the first configuration above (where the ASI is showing only 125 knots or so). The speed sourced from the ADHR can be ?tuned? to the correct number if need be, but the standard ASI still should not be that far off. The discrepancy between the two airspeeds can probably be explained at least in part by the two different static sources but that doesn?t explain why either or both of them are so low. Seems to me that all this means I must have a pitot (rather than/or in addition to, a static) leak somewhere OR, connecting two airspeeds to a single pitot tube is not workable in this particular configuration. Or, maybe it's something else entirely?

Before I take the whole thing apart again (which in the Rocket is a significant undertaking), does anything here stand out as a probable cause or course of action to anyone?

Sorry for the long post, building the airplane in the first place was way easier than this!


Lee...

Have you checked your static sources by flying at a constant altitude at different speeds? When I did this I flew several low approaches down the runway at a constant GPS altitude and noted the indicated altitude(s) at each speed. They were a little different. Doing that might tell you something. Good luck!

First item, perform a leak down test of the static system.

Lee, seems like you have a pitot leak somewhere, don't think your static system would cause this kind of error. See if you can find a way to pressure test the pitot system (balloon test, water manometer?).

Gil: Thanks for checking in. The problem is---I have two systems: The original one up front just behind the cowling for my original/classic pitot-static instruments and a new one on the rear fuselage in the more conventional location serving the ADHRs. I have an ASI, a standard altimeter, and a TruTrak Gemini autopilot/EFIS on that one. The second ADHR's one is completely separate. I was going by the fact that the ADHR's altimeter and the standard altimeter, each of which of course only receive the static signal, are reading the same.

Given that, I figure it's a pitot issue only (or mostly), but I'm also hoping someone can suggest additional testing or other courses of action that will preclude opening the airplane up. That's a couple of days in and a couple of days out, on this baby! I will do leak down testing on both static systems and also on the pitot system before I go much further. I'm hoping something will be obvious. It's amazing the watch the ground go whizzing by while only indicating a little over a hundred knots!

Snopercod: Thanks to you too: I will do as you suggest. I did check both altitude indications on one pass I did. Both of them "seem" about right. More testing to do there, I think.


Lee...

Any chance you could've gotten the pitot and AOA ports mixed up and you're actually feeding your airspeed indicators the pressure from the angled AOA port?

Thread drift, but I'm curious

if low passes down the runway to test altimeter readout at differing airspeeds would run into any "ground-effect" influences on the measured static pressure. How close would you have to fly before the downwash ground effect cushion (as I understand it) would alter (probably increase) the local static pressure on the fuselage?

logansc said:

Gil: Thanks for checking in. The problem is---I have two systems: The original one up front just behind the cowling for my original/classic pitot-static instruments and a new one on the rear fuselage in the more conventional location serving the ADHRs. I have an ASI, a standard altimeter, and a TruTrak Gemini autopilot/EFIS on that one. The second ADHR's one is completely separate. I was going by the fact that the ADHR's altimeter and the standard altimeter, each of which of course only receive the static signal, are reading the same.

Given that, I figure it's a pitot issue only (or mostly), but I'm also hoping someone can suggest additional testing or other courses of action that will preclude opening the airplane up. That's a couple of days in and a couple of days out, on this baby! I will do leak down testing on both static systems and also on the pitot system before I go much further. I'm hoping something will be obvious. It's amazing the watch the ground go whizzing by while only indicating a little over a hundred knots!

Snopercod: Thanks to you too: I will do as you suggest. I did check both altitude indications on one pass I did. Both of them "seem" about right. More testing to do there, I think.


Lee...

Click to expand...

Altimeter can read almost perfect even if it is not connected to anything. Do test the system.

Bill Boyd said:

if low passes down the runway to test altimeter readout at differing airspeeds would run into any "ground-effect" influences on the measured static pressure. How close would you have to fly before the downwash ground effect cushion (as I understand it) would alter (probably increase) the local static pressure on the fuselage?

Click to expand...

Ground effect won't change the static pressure.

I might guess that you flip flop'ed the AOA and Pitot lines. The AOA port will provide a decent amount of air pressure to register a sem-realistic airspeed, but will be lower than expected.

Larry

Bill Boyd said:

How close would you have to fly...

Click to expand...

I wasn't clear. By "low pass" I meant 100 feet or so - well out of ground effect.

Well, for sure thanks to everyone for their ideas. I especially like the suggestion that I may have connected the AoA and pitot tubes backwards...that would certainly explain everything and will be the first thing I check tomorrow. I checked the connections pretty carefully I thought, but I didn't get nor download an installation manual so I could have easily made a mistake. I sure hope so!

Thanks! I'll report back...


Lee...

I don't believe it is a pitot leak. When I had a pitot leak in my first panel, after two years of flying with a traditional airspeed indicator and a Dynon D100, they were both low by about the same amount.

You didn't say where the static ports are located. I suspect the location of the forward static port port may be the issue. Try disconnecting it and leaving it open behind the panel and see how you ASI reads.

The pitot and AOA-

hoses are 2 different colors, so mixing them would be somewhat difficult, but not not impossible. BUT---at the pitot tube is another story. We'll check it.
Tom

Well, rats! It turns out that the pitot and AoA hoses are in fact installed correctly at the pitot tube after all. So much for the potential easy solution suggested by a couple of you. Now working my way inboard now from the outside (at the pitot). Swapped the old pitot for the new one today and there was actually a slight increase in fidelity between the pitot system driven displays and what they should have been. Both the pitot driven TruTrak Gemini EFIS indicated a/s display and the standard ASI display were identical. Neither quite as low as before but still way low, and both were identical. I don't really know why that would have been. The ADHR's display was another 10 or so (depending on overall speed) knots slower even than them.

Did another low pass altimeter test and again both static system driven altimeters were in agreement. The ADHRs driven altimeter tended to be 50' lower. That can be adjusted out but could also be just the difference in the accuracy of pre-takeoff settings.

On the theory that the only thing we haven't eliminated so far is a pitot system leak, tomorrow I will begin working my way past the pitot tube itself and back through the system testing for leaks. Just what I didn't want to have to do!

Again, thanks for all the suggestions. I'm getting better and better at flying it using incorrect airspeeds!


Lee...

Lee, suggest you make up one of these:

http://www.iflyez.com/manometer.shtml

Simple, cheap, and accurately confirms your ASI. Allows you to check for leaks as well. You could borrow mine but you are a bit far away.......

Remember the Dynon pitot recall? We were performing a Pitot/Static re-certification after installing the replacement pitot & experienced similar results as you have. The new & old pitots looked identical externally but as it turned out, the two aluminum tubes coming out the top were reversed!
So my question - did you confirm the line connections VISUALLY? Or did you confirm which aluminum tube was ASI pitot line by PHYSICALLY blowing thru it (while disconnected!)?

Mark: I have ordered some of the materials already for a manometer as you suggested. I can source the rest locally...still (maybe foolishly) hoping the "investigation" won't have to go that far!

Ralph: Good point but yes, in fact we did ensure that the tubes seemed to be correctly connected. If you blow in the pitot tube, air only comes out of it. If you blow in the AoA tube, air comes out of both. Does that sound right?


Lee...

Success!

Thanks to the diligent efforts of Tom Swearengen, we tracked down the problem with my pitot system. As just about everyone guessed, it was a leak in the system about where we suspected. We spent a couple days trying to avoid having to open up the floorboards but in the end that's what we had to do. Disconnected all three hoses (at the pitot tube, at the ASI, and at the ADHR's). With two ends blocked (nothing connected to actual instruments), blowing in the free end produced an audible leak we could hear beneath (of course) the floorboards.

Replaced what appeared to be the perfectly good original "T" we had installed in order to split the signal between the ASI and the ADHR. Conducted a hurried test flight with serious weather showing on the horizon and voila, all appears well now. Time for a comprehensive test of the pitot and related/subsidiary systems tomorrow, but for now I'm declaring victory. Just have to clean up the battlefield a bit and we're done.

Thanks to Tom especially and also to all who provided the host of good ideas we checked out.


Lee...

WE can laugh now about that education and recreation thing, but its NOT easy working on, or getting in and out of an F1. After changing the pitot tube yesterday ( because it was easy) we pretty well had eliminated everything but the connecting tee, under the pilots floor. Not a simple fix. But found the culprit, replaced with a new one, retested and then put it all back together for a quick test flight.

Recreation? NAAAA
Education---well our troubleshooting theory was good, so just because it looks good, doesnt mean it is.
Lee getting to fly his Rocket--priceless!
Tom

"Ralph: Good point but yes, in fact we did ensure that the tubes seemed to be correctly connected. If you blow in the pitot tube, air only comes out of it. If you blow in the AoA tube, air comes out of both. Does that sound right?"

Just checked a new Dynon pitot I have on hand- there are 2 tiny vent holes on the bottom about 1" & 1.5" back from the leading edge of the unit- the AOA inlet is vented slightly thru the forward tiny hole, the Pitot side is vented slightly thru the aft tiny hole, so when doing our Pitot/Static re-cert (in our case every 2 years) we would have plug off the AOA port and both vents to get valid test results.
I don't know if other manufacturers also employ these vents.

Redneck Pitot pressure test

Here's the redneck setup I used to pressure check my Pitot system:

First, I modified a soft, plastic thread protector by drilling a hole
and using RTV to bond in a 1/4" hose barb:


Then I used electrical/duct tape to cover the Pitot drain hole and support the little condom thingie:


I used a 60cc syringe to CAREFULLY pressurize the system until the ASI read an intermediate value:


It wasn't pretty, but it worked. After pressuring the Pitot line, it wouldn't hold pressure. I used soapy water to leak check all the fittings and found that the nylon adapter at the top of the Pitot tube was cracked. I replaced it.

John---ours wasnt that elaborate. WE disconnected all the avionics from the tubing, the ADAHRS AND his round ASI---yep Lee likes round gauges. That way we had just the tubing runs in the plane. Some air pressure and EUREKA--. Pulled the floor, for those of you with F1's you know what I mean, retested to verify the bad tee. Replaced, and rechecked. Gee it holds pressure.

HUM---I think I know what I'm going to do on my 7--

Tom

Was the leaking tee one of those push-in kind, or one of the nylon fittings with the ferrule and nut?

Snoper: The leaker was one of the push in kind and so was it's replacement. I have several of them in my full system including the usual 2 or 3 behind the instrument panel. I like them and have never had one leak before, but this time was different. That particular "T" has now been disciplined and filed in one of the big hangar "round files". Bad boy.

BTW: Thats a nice setup you made up there. I need to "collect" up all the odds and ends you used to make it.


Lee...

I told Lee to spank it with a LARGE hammer or similar tool to make sure we didnt install it on something else. His round file solution is just as good.

Tom

For a while there, Lee had the slowest Rocket in the country with a landing IAS somewhere around 40 knots