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n1cxo320(at)salidaco.com
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 Posted: Mon Sep 25, 2006 12:48 pm Post subject: Airspeed Indicator Correction? |
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I have a true airspeed indicator...you set your altitude opposite the temperature and the rotatable portion moves to correct for non-standard conditions.
A manometer shows that the stationary part of the indicator is accutate - that's up to 140mph. The moveable section, over 140mph, always seems a bit high when "properly" set.
Rough and ready comparisions with GPS confirms an error.
Before I tear up the instrument panel, I need to ask: Is there some screw slot or some-such that allows a mechanical correction to be applied, or do you have to live with these things?
[quote][b]
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gyoung
Joined: 09 Jan 2006
Posts: 211
Location: Republic of Texas
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| Posted: Mon Sep 25, 2006 1:35 pm Post subject: Airspeed Indicator Correction? |
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That moveable portion is just circular slide rule. Unless the mask they used to print it was off it will be correct. Just like your E6B, there is no adjustment. Parallax can induce apparent errors as well. Run the same IAS and temp thru your E6B - should be the same.
Greg Young
[quote] From: owner-rv-list-server(at)matronics.com [mailto:owner-rv-list-server(at)matronics.com] On Behalf Of John Fasching
Sent: Monday, September 25, 2006 3:48 PM
To: rv-list(at)matronics.com
Subject: Airspeed Indicator Correction?
I have a true airspeed indicator...you set your altitude opposite the temperature and the rotatable portion moves to correct for non-standard conditions.
A manometer shows that the stationary part of the indicator is accutate - that's up to 140mph. The moveable section, over 140mph, always seems a bit high when "properly" set.
Rough and ready comparisions with GPS confirms an error.
Before I tear up the instrument panel, I need to ask: Is there some screw slot or some-such that allows a mechanical correction to be applied, or do you have to live with these things?
[b]
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khorton01(at)rogers.com
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| Posted: Mon Sep 25, 2006 1:50 pm Post subject: Airspeed Indicator Correction? |
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On 25 Sep 2006, at 16:48, John Fasching wrote:
| Quote: | I have a true airspeed indicator...you set your altitude opposite the temperature and the rotatable portion moves to correct for non-standard conditions.
A manometer shows that the stationary part of the indicator is accutate - that's up to 140mph. The moveable section, over 140mph, always seems a bit high when "properly" set.
Rough and ready comparisions with GPS confirms an error.
Before I tear up the instrument panel, I need to ask: Is there some screw slot or some-such that allows a mechanical correction to be applied, or do you have to live with these things?
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I don't think there is any provision for a correction at the ASI.
You say that a manometer shows the stationary scale is accurate to 140 mph. Did you test it with the manometer above 140 mph?
There are several possible sources of your error:
1. ASI instrument error - check the ASI against a good pressure source over the full range of speeds.
2. Pitot or static system leaks.
3. Errors in the OAT source you are using to set the moveable scale on the ASI. Some people put their OAT sensor in locations where they are known to read high (e.g. in a NACA duct on the forward fuselage).
4. Temperature scale on the ASI not accurate (i.e. it provides an inaccurate correction from IAS to TAS). To check this, note the IAS, altitude, temperature and TAS the next time you are trying this feature out. Do a manual conversion from IAS to TAS after you get back on the ground to see if you come up with the same TAS as your ASI did.
5. Inaccurate method to calculate TAS from GPS data. Many people seem to think that if you average the GPS ground speeds from several different headings that the result will be TAS. This is only true if the wind is zero. If there is any wind, the average of several GPS ground speeds will be greater than the TAS.
6. Static system position error. Every static system has some error. Some people have found that the nice looking flush static ports have larger errors than the pop rivet static port that Van calls for. The flush static ports result in a sensed static pressure that is too high, and thus the ASI reads too low.
I've got a bunch of links on pitot-static systems on my web site that address many of the above areas:
http://www.kilohotel.com/rv8/phplinks/index.php?&PID=48
http://www.kilohotel.com/rv8/phplinks/index.php?&PID=49
Kevin Horton
Ottawa, Canada
[quote][b]
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