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Airspeed indicator failure

B

Bob Axsom

Well Known Member
After test runing the Lone Star Air Rally Course this evening my airspeed indicator was reading well over 200 kts indicated even though I was preparing to land. The little panel display on the 695 GPS was comforting. Now sitting out on the ramp with all pneumatic ports open the indicator is displaying approximately 100 kts. It seems that "the guts are busted." Never experienced this before.

Bob Axsom
 
I would take the lines off the back of the instrument before condeming it, a plugged pitot line is a possibility.
 
Bob Axsom said:
After test runing the Lone Star Air Rally Course this evening my airspeed indicator was reading well over 200 kts indicated even though I was preparing to land. The little panel display on the 695 GPS was comforting. Now sitting out on the ramp with all pneumatic ports open the indicator is displaying approximately 100 kts. It seems that "the guts are busted." Never experienced this before.

Bob Axsom
Click to expand...

You done wore it out with all your racing, Bob. :)
 
The race went well

I'm guessing I did around 210 mph. I saw 182 kt ground speed a lot. Gary Shelley beat me but I'm hopin that was all.


Bob Axsom
 
Well Shoot!

I had no time to do it right and I still don't. I bought a new airspeed indicator and it came in the mail early thursday morning. I just installed it and flew to Jasper it seemed OK but after flying with one configuration for 8 years the new one seemed well, less quality, with a smaller speed range, MPH on the outer ring and Kts on the inner ring ... it was kind of like reading pig latin - you can figure it out but you are always translating.

I figured I would get used to it eventually but after the race I experienced it go way high in airspeed indication after the race at Jasper when I was going through the pull up, slow down and enter the landing sequence. I think this one only goes up to 230 mph and I'm sure I blew right through that during a descent leg in the race. I have gone through the mental gymnastics of differential pressure driving a needle movement and nothing but mechanical over stress seems to fit the symptoms I am seeing. I did some cursory inspecting before I bought the new gauge and saw nothing amiss with the pitot or the static ports. I can kind of imagine a check valve like obstruction in the pitot line but I hate to waste time if I have just over stressed another gauge and need one with a broader scale (higher top end).

Enlighten me in the failure modes of airspeed indicator systems at your leisure - I will be working the problem but I welcome tales of experience with these little beasts. I have never had one fail before.

Bob Axsom
 
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Current state

PaigeHoffart said:
Geez Bob, what's your VNE?
Click to expand...
I haven't found it yet but I keep experimenting and testing - It's a short tailed 6A. In a time trial at Courtland, AL in the northwest pass through the trap at 500 ft AGL on the video I heard my ground speed reported as 231 mph. Since that was the "into the wind" pass the TAS would have been higher. I have seen over 205 kts indicated which is ~235.9 mph but that is far above the top end of my the TAS window in even the original ASI which I think you are looking for.

We are planning the increase the power of the engine after the end of this year's racing season to increase the speed and level the playing field with the 200 hp IO-360 RV-8 competition. One thing that is a concern is the play in the manual elevator trim tab. It is the vernier green cable manual system. There never has been even a hint of a problem but my co-worker and expert engine man has expressed concern. I believe the "play" is in the green cable itself. I think a friction device at the tail end of the control cable is one approach. Ideas on how to do this are welcome.

On the original failure question I called United Instruments, Inc. the original instrument manufacturer. The lady I talked to said overpressurization is one of the failure modes of the ASIs. Her boss is away at a show this week and she asked me to call back next week and talk to him. I do not want to just keep over pressurizing these things so it seems to me that I need an instrument with a greater operating range.

This is not a normal cruise around the pattern application. It is experimental and the airplane is being modified continuously (including a 1.5 ft wingspan reduction by installing 3" streamlined tips) to increase the speed for racing. It should not be considered safe for your airplane or any other airplane. I have fabricated flat metal zero span tips with sharp 90 degree corner edges and installed them but tests indicate that the 3" streamlined tips are faster.

And as Sunny used to say "the beat goes on."

Bob Axsom
 
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