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humptybump

Well Known Member
My RV-8 originally had Aluminum tube from the Stock Vans designed pitot tube to the ASI. Other than a couple fittings to piece itvall together, it was a dedicated feed.

When I added the autopilot, I needed another feed. I also assumed I'd need at least one more later on for an EFIS. Thus, I installed a manifold and switched to the plastic tubing system from Stein.

Everything works but my airspeed is reading low. Stall speed dropped from 50 to 45. Cruise speed at altitude also appears to be lower by 3-5 kts. I have been looking for some calm air days to do a series of speed tests at various altitudes. Worse case, I'll have a CAS card when I'm done and update the POH.

I'm looking for suggestions to remove the error. Ideally, I'd like it to read what it did before.
 
If all you did was to install a manifold, then you have a leak somewhere. The airspeed indicator works on pressure, and pressure doesn't change with a manifold.
 
Glen,
I hear those push in fittings can be finicky if the tubing is not cut perfectly square. Did you get the cutter from Stein?
 
Looks like I'll be building the EAA pitot static test rig :)

As background ...

The curent system has the aluminum tubing from the pitot connection all they way to behind the firewall. Originally, this had a single AN male to AN male coupler and then aluminum tubing to the ASI.

Now, the aluminum tubing from the pitot connection uses the AN male to AN male coupler to connect to a female quick connect fitting and then plastic tubing to the manifold. The ASI and autopilot now have the quick connect fittings on them for both pitot and static lines and are connected using the plastic tubing.

The static system has always used the quick connects and plastic tubing. I did not change that other than to add in the autopilot into a spare port.
 
You could easily isolate it into two sections for a quick test.

Disconnect the plastic tubing at the entry to the manifold. Now all instuments are disconnected so you can pressurize the pitot to manifold tube and see if it holds pressure. If it leaks use the soapy water trick for bubble detection. With the sensitive instruments out of the loop you will have no excess pressure damage worries.

This will isolate it to either the long plastic run from the pitot, or if that OK, then the manifold to instrument tubing is at fault.
 
My homemade static tester is a turkey injecter syringe with medical tubing. Attach to the pitot tube and very slowly press up for a 200 knot indicated air speed. You can use soapy water to find the leak on the Pitot side.
 
Thanks all for the pointers!

Just a quick update ... It was definitely a leak at the tube going to the AP. Tests much better now :)

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