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  #1  
Old 02-17-2020, 02:29 PM
RV7A Flyer's Avatar
RV7A Flyer RV7A Flyer is offline
 
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Default IAS Error

Flying next to a friend in his new-to-him RV-6A (I believe he has an O-320 FP prop), I asked why we were going so damned slow...I was showing 100-110 KIAS, and he was showing 20 knots or greater *higher*. I *know* my airspeed is correct, by comparison with numerous other RVs in formation flights, at all sorts of altitudes and power settings. So this is a problem with his airspeed. But what?

BOTH his mechanical ASI and his D-10A were showing the same IAS. He has just had a pitot/static/XPDR cert done and passed, but I know this won't detect installation error that can only be found in flight.

He performed some stalls in order to find the *indicated* stall speed, for safety of flight on T/O and LDG, and says he stalled at around 65-70 KIAS, clearly way too high.

What could cause this? FWIW, his static ports are flush rivets, but I've only heard of problems with flush rivets causing an IAS that is too *low*, and it's almost impossible to think of an installation error with the *pitot* tube that would make it read that high.

Anyway, looking for troubleshooting advice to help out a buddy...TIA!
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  #2  
Old 02-17-2020, 03:35 PM
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static ports should be dome-head, not flush. I don't know why, but that is the standard plans configuration.
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  #3  
Old 02-17-2020, 03:49 PM
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Quote:
Originally Posted by fl-mike View Post
static ports should be dome-head, not flush. I don't know why, but that is the standard plans configuration.
That's what I've read numerous times over the years here, although some folks will claim successful installations without the dome-head type of ports.

Before changing them out, is there anything else that could cause such a large error that should be checked? Could a static leak cause such a gross error?
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  #4  
Old 02-17-2020, 04:23 PM
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A static leak into the cabin will cause airspeed to read high by maybe 10 knots (I’ve seen as much as 15) at cruise speed in RVs.
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  #5  
Old 02-17-2020, 04:30 PM
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Default

Yep - similar problem on my first RV (most prominent indication was an altitude error).

Simple test, take a 1/8? round head rivet, drill a 1/16? hole through the center, then cut the head off with a Dremel wheel. Use JB Weld to epoxy this onto the flush static port. Use a trimmed toothpick to hold the rivet head to the port while the epoxy sets (also keeps the port clear of the JB Weld.

Moving the port location out of the boundary layer fixed the problem for me - and these rivet heads are now going on 18 years in place.

Carl
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  #6  
Old 02-17-2020, 04:38 PM
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Quote:
Originally Posted by Ironflight View Post
A static leak into the cabin will cause airspeed to read high by maybe 10 knots (I?ve seen as much as 15) at cruise speed in RVs.
Wouldn't his recent pitot/static certification have caught that? Every time I've had that done they did a pressure decay test on the static system.
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Old 02-17-2020, 04:41 PM
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On my first flight, my IAS was reading 10 Kts low as well. I floated 4,000' down the runway before touching down on my first landing LOL! My solution was to build a little "dam" in front of each static port. Now my IAS is right on the money.
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  #8  
Old 02-17-2020, 05:27 PM
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Quote:
Originally Posted by Ironflight View Post:
A static leak into the cabin will cause airspeed to read high by maybe 10 knots (I?ve seen as much as 15) at cruise speed in RVs.

Quote:
Originally Posted by snopercod View Post
Wouldn't his recent pitot/static certification have caught that? Every time I've had that done they did a pressure decay test on the static system.
Ironflight nailed it. Had an Faa repair station that included instrument. Not a rare occurrence. In my opinion, only the static system can give that much + error. Looks like cabin static - actually anywhere in the fuselage if the static sys is open.

Re: boundary layer. It is 1/2 inch or more thick at that aft fuselage position - turbulent/attached. There is virtually no pressure difference from surface to full Q. A button vs surface port would tend to reduce pressure, I should think, and make a slightly higher AS reading due to a slight acceleration of the flow over the rivet bump. Is there some other phenomenon I am not aware of that would produce the opposite effect?

ron
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  #9  
Old 02-17-2020, 05:35 PM
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Quote:
Originally Posted by snopercod View Post
Wouldn't his recent pitot/static certification have caught that? Every time I've had that done they did a pressure decay test on the static system.
Always look at the last thing that was done to the airplane.....I have seen static test ports left open after a ?perfect? test.
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  #10  
Old 02-17-2020, 05:47 PM
Cumulo Cumulo is offline
 
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Quote:
Originally Posted by Ironflight View Post
Always look at the last thing that was done to the airplane.....I have seen static test ports left open after a ?perfect? test.
Yep. Don't ask me Why I know.

ron
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