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  #1  
Old 03-27-2021, 02:41 AM
Daida's Avatar
Daida Daida is offline
 
Join Date: May 2005
Location: Hannover, Germany
Posts: 165
Default Pitot & Static calibration, show me your solution

Hi Folks,

my IAS (indicated Airspeed) and IA (indicated Altitude) are off.
When slow, near Stallspeed, everything is good. Speed and Altitude is dead on.
When going faster, the IAS is 15 knots to low and the IA is 300 ft low.

I did the TAS calibration (flying squares in the sky) a couple of times. The error is linear from 100-150 KIAS (10 kts increments tested).
Altitude was crosschecked with GPS Alt which was my Alt reference.

Why That the error is linear and not exponentially with raising speeds is another question.

Here is my math.
Pitot & Static is a closed system.
Too low IA read out means higher pressure at the static port (higher than ambient static pressure).
As IAS is the result from total pressure (Pitot) minus static pressure, the too low IAS i see is the normal result.

Probable cause:
I figure that the static port is in a area of high pressure due to airflow around the fuselage.

Solution:
Build something (damms, vanes, obstacles, .....call is what you want) in front of the static ports to create a low pressure area behind.
Try size and form until i get it right and static port gets correct ambient pressure.


Question:
There are numerous threads about this. But to give me a headstart, i kindly ask to post pictures from your solutions. I try to get an idea of mounting position, height, material, how attached, ..........of what you did.

If you could post the changes (IAS, IA) it produced would be most excellent.

Thanks in advance!!

p.s. i think i got it figured out, but if i am totally off or on the wrong path here, i appreciate any directive hints.

p.p.s fixed pitch sensenich GA prop gave me 166 KTAS @ 2500RPM,
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Last edited by Daida : 03-27-2021 at 03:54 AM. Reason: more info
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  #2  
Old 03-27-2021, 06:20 AM
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 5,887
Default

Has the static system passed a leak test?

Can you post a picture if the port, and is it in the standard location for your RV8?

Are the two ports plumbed with a loop over the top?

Has the airplane been parked in the rain?
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  #3  
Old 03-27-2021, 06:37 AM
Carl Froehlich's Avatar
Carl Froehlich Carl Froehlich is offline
 
Join Date: Dec 2007
Location: Dogwood Airpark (VA42)
Posts: 2,943
Default

My first (8A) I had a similar problem. The simple fix was to drill a small hole through the head of a 3/16” round rivet, cut off the rivet head and epoxy it onto the flat static ports (these ports were aftermarket products that while pretty did not work well). This fixed the altitude error. On the second RV I just drilled a hole through a 3/4” long 3/16” rivet, make a backing plate and epoxied them in at the standard locations. On the third RV I used the static port kit from Dynon - works perfectly.

Next step is to verify the accuracy of the IAS instrument. The ones I got from Van’s have been as much as 7kts off.

Google “Manometer” and you’ll find simple but very accurate examples of how to make one, along with a template to translate inches of water to knots. If you find it off, they can be calibrated.

Carl
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  #4  
Old 03-27-2021, 06:56 AM
Fred.Stucklen's Avatar
Fred.Stucklen Fred.Stucklen is offline
 
Join Date: Oct 2006
Location: Brooksville, FL
Posts: 405
Default Static Ports

What are you using for static ports? If the statics port are flush with the skin, you will get this type of error. The static ports should protrude through the fuselage skin surface. Van's original static ports where just Pop Rivet bodies without a shank. These protruded off the fuselage skin surface just enough to eliminate the problem you are seeing. There are other static port devices available that do the same.
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  #5  
Old 03-27-2021, 10:39 AM
Daida's Avatar
Daida Daida is offline
 
Join Date: May 2005
Location: Hannover, Germany
Posts: 165
Default Thanks so far

First of all, i am glad that this issue is known in the -8 or RV world.

To your questions:
Leak test passed.
Both static ports are “T-eed” together.
No rain, always hangared.
The ports are flush, aftermarket products (they look good and cost a fortune..)


I will start with something that protrudes through the boundary layer of the fuselage to measure ambient pressure.

I will add pictures a.s.a.p.

Thank you all for your replies. Keep ‘em coming!
I’ll keep u posted.
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  #6  
Old 03-27-2021, 10:48 AM
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wirejock wirejock is offline
 
Join Date: Oct 2005
Location: Estes Park, CO
Posts: 4,379
Default Static port

You didn't mention what brand of flush port. Sounds a bit like a SafeAir.
Some builders, me included, use them with sljght modification.
Drill the flush port and the fuse hole for the Vans rivet static port.
Drill the flush fitting to attach to the inside of the fuse.
Proseal and rivet the flush port to the inside. You want it sealed to the skin.
Install the Vans static rivet on the outside. Proseal it as well.
The result retains the performance of the Vans rivet plus the fittings on the inside.
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  #7  
Old 03-28-2021, 04:26 AM
penguin penguin is offline
 
Join Date: Jan 2005
Location: England
Posts: 1,103
Default

Quote:
Originally Posted by Daida View Post
First of all, i am glad that this issue is known in the -8 or RV world.

To your questions:
The ports are flush, aftermarket products (they look good and cost a fortune..)
This is your problem, it has been demonstrated by many builders over the years, the 10kt error you found is typical. The Van's pop rivet solution looks a little hokey but it works. Can you drill an 1/8" hole through your flush ports and insert the Van's specified pop rivet (and knock out the mandrel)? Proseal is a over-the-top, any glue will be adequate.

Pete
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  #8  
Old 03-28-2021, 11:29 AM
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Daida Daida is offline
 
Join Date: May 2005
Location: Hannover, Germany
Posts: 165
Default Promised pictures

Here are the pictures of my static port.

During the built you get sometimes drawn away. The simple and cheap seems not good enough for your “piece of excellence”......and you buy the expensive stuff, just to figure out later that the 0,0256 c/$ pop rivet would have been the better choice after all.

It’s probably one of the things homebuilding an aircraft teaches you......

Anyways, i build different shaped covers put of delron that i will put over the static port with double sided tape. They have a hole in it which 8 will match with the hole in the static port, just that the whole thing is now protruding through the boundary layer of the fuselage.

I will testfly the different shapes and report back here.
If nothing works, i drill a hole through the static port and pull a pop rivet from inside out, remove the mandrel and see what i get.

It is experimental aviation after all!
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  #9  
Old 03-28-2021, 11:47 AM
penguin penguin is offline
 
Join Date: Jan 2005
Location: England
Posts: 1,103
Default

Good plan! The shape is the important thing, doesn't matter how you achieve it. Needs to have a slight step on the edge.
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