REFLECTOR: Altimeter v. Airspeed

[Ron Brown]

Chuck,

I would also suggest that you try a high speed pass over the runway at 50 
feet above the runway to check your static versus actual altitude.  Just be 
careful and don't try to get too low.  50 feet is plenty to verify that your 
static port (either external or cabin pressure) is close to being correct. 
And, yes, if the static port is correct, your indicated airspeed should be 
accurate.  If low, suspect a leaking pitot line.  If high, then it sounds 
like your airspeed instrument(s) are not calibrated correctly.  You can 
check them using a water manometer.

                     Knots             MPH      " Water
      50 57.5  1.63
      70 80.5  3.2
      100 115  6.56
      110 126.5  7.95
      120 138  9.48
      130 149.5  11.14
      150 172.5  14.87
      200 230  26.71


Ronnie


----- Original Message ----- 
From: "Chuck Jensen" <cjensen at dts9000.com>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Sunday, January 06, 2008 8:49 AM
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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