REFLECTOR: Altimeter v. Airspeed

[Chuck Jensen]

Steve,
 
My statement that I believed the altimeter to be accurate at altitude and cruise is a reliance on comparison of the altimeter reading and the CNX-480 GPS calculated altitude indication.  The two are within <50' differential most times though the differential is a bit over 100' on occasion, but this was true even when the static ports were being used, so I assumed the altimeter at cruise is quite close, if not dead one.  Am I missing something...as usual?
 
Ron, my static ports used to be under the door and worked well but the embedded static line or connections apparently developed a leak and the static ports were moved to the side of the fuselage about a foot below and behind the canard; not a good place.  In turbulence, the area is apparently prone to pressure waves or disturbances.  In turbulence, the altimeter would jump 50'-75' in a second or two, even though the plane itself had changed little in altitude.  Jim Younkin referred to it as a "noisy altimeter".  When I disconnected the static line from the external pitot ports, so the static system was communicating with the cabin pressure only, the altimeter calmed down and behaved consistent with the actual altitude changes of the plane itself.
 
What I'm trying to determine is 'should I bother repositioning the external static ports or simply let the static system reference the cabin pressure if it is, as it seems, representative of the ambient pressure.  One benefit of venting to the cabin is the port is unlikely to ice over!!  Therein lies my dilemma; to move the external static ports or leave them disconnected
 

Chuck Jensen 

-----Original Message-----
From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On Behalf Of steve korney
Sent: Sunday, January 06, 2008 2:18 PM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: Altimeter v. Airspeed


Chuck.....

How do you know your altimeter is correct.....?

Best... Steve

> Date: Sun, 6 Jan 2008 08:49:21 -0500
> From: cjensen at dts9000.com
> To: reflector at tvbf.org
> Subject: Re: REFLECTOR: Altimeter v. Airspeed
> 
> Tom,
> 
> It's understood that cabin pressure can be affected by opening vents, changing aircraft configuration, such as putting down the gear or opening the alt-air valve in the cabin. But if at cruise, the altimeter is reading correct, is that not an indication that the pressure in the cabin is near ambient and thus the IAS will also be correct.
> 
> Al, my pitot is on the pointy nose so its outside of the influence of a pressure wave from either the canard or wing (unlike my static port, which is the reason I'm doing this test), so the starting assumptions is the IAS is accurate if the static system pressure is correct. But you're right, the final word is to do the triangulation flight and input the data into any of the several spreadsheets out there on the web---which to be really accurate, are a bit more complex than one would think at first.
> 
> Chuck Jensen
> 
> 
> -----Original Message-----
> From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org]On
> Behalf Of Tom
> Sent: Sunday, January 06, 2008 6:23 AM
> To: reflector at tvbf.org
> Subject: REFLECTOR: Altimeter v. Airspeed
> 
> 
> You can also have variations in pressure when static is ported to cabin. 
> Flew a neighbors MustangII last week and I had his trutrak a/p on which 
> is vented to cabin. I opened the air vents and the a/p read the sudden 
> pressure increase in cabin as a quick descent and shot to the sky.
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